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<html>
<head>
<title>netrik hacker's manual: layout engine</title>
</head>
<body>

<h1 align="center">netrik hacker's manual<br />>========================&lt;</h1>

<p>
[This file contains a description of the layouting module. See hacking.txt or
<a href="hacking.html">hacking.html</a> for an overview of the manual.]
</p>

<h2>0. Overview</h2>

<p>
The whole layouting is split up into several, fairly simple passes, which are
executed one after the other. See the 
<a href="hacking.html#notes">notes in hacking.*</a> for a discussion of
this approach.
</p>

<p>
The first pass is the <a href="#parseSyntax">parse_syntax()</a>
function, which creates a <a href="#syntaxTree">Syntax Tree</a> of
the document. This tree contains all HTML elements and their content, but the
elements have no special meaning yet.
</p>

<p>
<a href="#dumpTree">dump_tree()</a> can be used to output the syntax
tree.
</p>

<p>
In the next pass (<a href="#parseElements">parse_elements()</a>),
all element and attribute names are looked up in tables and stored as enums to
facialiate further processing.
</p>

<p>
If not compiled with -DXHTML_ONLY, an additional pass is inserted after element
parsing: In <a href="#sgmlRework">sgml_rework()</a>, the
syntax tree is modified to fix the wrong element nesting caused by missing end
tags in SGML documents.
</p>

<p>
dump_tree() can be used again to dump all element and attribute types as found
in the lookup, and the possibly modified tree structure.
</p>

<p>
The third pass is the central processing step.
<a href="#parseStruct">parse_struct()</a> interprets the elements
and their attributes, and creates a <a href="#itemTree">Structure
Tree</a>, which contains all the items that will be visible on the
output page.
</p>

<p>
The fourth pass prepares the page for rendering. In
<a href="#preRender">pre_render()</a>, all items created in
parse_struct() are assigned actual sizes and positions in the output page. Also,
a structure <a href="#pageMap">"page_map[]"</a> is created, needed for
fast lookup what items are present in any given line.
</p>

<p>
All of the passes mentioned above are necessary to prepare the rendering, and
are executed from <a href="#layout">layout()</a>.
</p>

<p>
The actual rendering is done in render.c. However, this isn't done for the
whole page like the other layouting passes. Instead, every time some region of
the output page needs to be displayed,
<a href="#render">render()</a> is called to render exactly that
region.
</p>

<p>
Alternatively, The whole page can be dumped to the terminal line by line, using
<a href="#dump">dump()</a>.
</p>

<p>
The third function in render.c is <a href="#dumpItems">dump_items()</a>.
This is not really a rendering function; it only dumps the item tree, including
the (coloured) text.
</p>

<h2>1. layout.c</h2>

<p>
This file forms the framework for the layouting process. It contains functions
to load a file and prepare it for rendering, but also to free the memory used
by a document when it is no longer needed.
</p>

<a name="layout" id="layout">

<h3>layout()</h3>

<p>
layout() is given a URL of a file or web resource to load, and does all
actions necessary to be able to render the corresponding page. 
</p>

<p>
Before starting any of the loading or layouting operations, a descriptor is
allocated where all the data structures created inside layout() will be stored.
</p>

<p>
The descriptor is a "struct Layout" pointer. It contains the following data:
</p>

<ul> <li>
 A pointer to the input ressource descriptor ("input")
</li> <li>
 An additional pointer "url" to the effective page URL, necessary to hold the
 URL after the input resource descriptor is freed
</li> <li>
 Pointers to all data structures necessary for the layouting ("syntax_tree",
 "item_tree", "page_map[]")
</li> <li>
 Pointers to the "<a href="hacking-links.html#linkList">link_list</a>" and "<a href="hacking-links.html#anchorList">anchor_list</a>" data structures
</li> </ul>

<p>
After allocating the descriptor, layout() first opens the resource with
<a href="hacking-load.html#initLoad">init_load()</a>. (Described in
hacking-load.*)
</p>

<p>
Afterwards, <a href="#parseSyntax">parse_syntax()</a>,
<a href="#parseElements">parse_elements()</a>,
<a href="#sgmlRework">sgml_rework()</a>,
<a href="#parseStruct">parse_struct()</a>, and
<a href="#preRender">pre_render()</a> are called in sequence.
These functions are responsible for preparing the page for rendering.
</p>

<p>
The file loading itself is done inside parse_syntax(), which uses the
<a href="hacking-load.html#load">load()</a> function from load.c (see
<a href="hacking-load.html">hacking-load.*</a>) to read a data block
every time the input buffer is empty. It processes the data in the buffer
character by character (keeping track of the current read position by
"input->buf_ptr"), and when it reaches the end it calls load() again to get the
next data block.
</p>

<p>
After parse_struct(), the syntax tree is no longer needed. It is freed by
<a href="#freeSyntax">free_syntax()</a>.
</p>

<p>
At this point also the "link_list" and "anchor_list" data structures are
created using
<a href="hacking-links.html#linkList">make_link_list()</a> and
<a href="hacking-links.html#anchorList">make_anchor_list()</a>. (See
<a href="hacking-links.html">hacking-links.*</a> )
</p>

</a>    <!-- layout -->

<a name="freeLayout" id="freeLayout">

<h3>free_layout()</h3>

<p>
When a page is unloaded (usually before loading a new page), this function is
called to free all the data structures created by the layouting process to
allow rendering (Item tree, page map, link list, anchor list), i.e. all data
stored in the "Layout" descriptor, except for "input" and "syntax_tree", which
are already freed during the layouting process. (s.a.) The descriptor itself is
also freed.
</p>

</a>    <!-- freeLayout -->

<a name="resize" id="resize">

<h3>resize()</h3>

<p>
The resize() function is somewhat similar to
<a href="#layout">layout()</a> -- it calls the same subfunctions
to create a combination of an item tree with assigned coordinates and a page
usage map. The difference is that resize() does not start from scratch, but
only repeats the steps necessary to adapt to a new screen width; the properties
determined by the document itself (i.e. the item tree) are kept.
</p>

<p>
So we actually just call pre_render() (see <a href="#preRender">7.
pre-render.c</a>) again. (Note that the minimal item sizes calculated
in <a href="calcWidth">calc_width()</a>could be kept also; for
simplicity, we just pre-render completely again anyways -- this shouldn't be
too big a loss, we believe. When implementing rendering of incompletely loaded
pages, we will have to create some mechanism to skip such unnecessary
re-calculations per item anyways.)
</p>

<p>
As resize() starts from a "ready" page, not from scratch, it has to free the
old data structures (page map) before creating new ones.
</p>

</a>    <!-- resize -->

<a name="parseSyntax" id="parseSyntax">

<h2>2. parse-syntax.c</h2>

<p>
The first thing to be done when layouting is parsing the syntax of the input
file. This is done by parse_syntax(). This function creates a syntax tree. The
pointer to the head of this tree is returned to layout() and stored as
"layout->syntax_tree" there.
</p>

<h3>Syntax Tree</h3>

<p>
Every node of "syntax_tree" is a structure of the type "Element" (defined in
"syntax.h") and corresponds to one HTML element. (An element in an HTML
document is represented by an HTML-tag, and the corresponding end tag, if any.)
End tags do not create tree nodes, as they only close elements already stored.
</p>

<a name="testHtml" id="testHtml">

<p>
For the supplied "test/0.html":
</p>

<p><pre>
   header text
 
   &lt;html>&lt;head> &lt;/head>
   &lt;body>
      &lt;h1> heading &lt;/h1>
      &lt;p>
         first paragraph of text; 
         includes  multiple spaces and newlines,
         &lt;em> emphasized text &lt;/em>and 
         &lt;strong> strong text &lt;/strong>
      &lt;/p>
      &lt;p>
         &lt;center>starting with an evil center tag,&lt;/center>
         this very long second paragraph contains some special characters (including a simple space...):
         &amp;amp;; &amp;lt;&amp;gt;&amp;quot;=/ plus a big gap&amp;nbsp;&amp;nbsp;&amp;nbsp;and two unicode escapes
         (decimal: &amp;#161; and hexal: &amp;#xbf;)
         but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
         (this anchor also is the only tag with parameters);
         and finally a blank row &lt;br /> (a single tag)
      &lt;/p>
   &lt;/body>
   &lt;/html>
</pre></p>

</a>    <!--testHtml-->

<p>
the syntax tree looks like this:
</p>

<a name="syntaxTree" id="syntaxTree">

<p><pre>
  ++>NULL
  +
+---+
| ! |-. &lt;++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+   &lt;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
        | head |->| body |-. &lt;++++++++++++++                                              +
        +------+  +------+ | &lt;+            +                                              +
                           v  +            +                                              +     &lt;+++++++++++++++++++++++++++++++++++++++++++++++++
                        +----++          +---+     &lt;++++++++++++++++++++                +---+   &lt;++++++++++++++++++++++++++++++++                +
                        | h1 |-. &lt;+   ,->| p |---. &lt;++                 +             ,->| p |-. &lt;++++++++++++++++++             +                +    (back
                        +----+ |  +   |  +---+   |   +                 +             |  +---+ | &lt;+                +             +                +    to top)
                               v  +   |          v   +                 +             |        |  +                +             +                +          ^
                          ,--------.  |  ,------------------.       ,----.           |        |  +          ,----------.   ,---------.   ,---------------.  |
                         (" heading") | (" first...newlines,")     (" and")          |        v  +         (" this...em") ("ded...row") (" (a single tag)") |
                          `--+---+-'  |  `-----+----+-------'     +-+----+-+         |   +--------+         `---+---+--'   `--+----+-'   `-----+---+-----'  |
                             | ? |----'        | em |-. &lt;++    ,->| strong |-. &lt;++   |   | center |-. &lt;++    ,->| a |-. &lt;+ ,->| br |---------->| ? |--------'
                             +---+             +----+ |   +    |  +--------+ |   +   |   +--------+ |   +    |  +---+ |  + |  +----+           +---+
                                                      v   +    |             v   +   |              v   +    |        v  + |
                                           ,----------------.  |     ,------------.  |   ,----------------.  |      ,---.+ |
                                          (" emphasized text") |    (" strong text") |  (" starting...tag,") |     ("bed") |
                                           `--------+---+---'  |     `-----+---+--'  |   `--------+---+---'  |      +---+  |
+++> "parent"                                       | ? |------'           | ? |-----'            | ? |------'      | ? |--'
---> "list_next"                                    +---+                  +---+                  +---+             +---+
</pre></p>

</a>    <!--syntaxTree-->

<p>
(I'm really curious if anyone can read this ;-) )
</p>

<p>
The "Element" structure includes:
</p>

<ul> <li>
 The pointers "list_next" and "parent" describe the tree structure.
 "parent" points to the element (node) which contains this one in its content
 (text) area. "list_next" points to the next element as it appears in the
 input stream.
</li> <li>
 The "closed" flag is a helper flag for <a href="#sgmlRework">sgml_rework()</a> and has
 no meaning outside of it.
</li> <li>
 The union "name" describes what kind of element this node represents. ("html",
 "head" etc.) It can store the element name either as a pointer to a string
 (as appears in the input stream), or as an enum number.
</li> <li>
 "attr_count" stores the number of attributes of this element. (Attributes
 are the parameters of an element, which appear inside the start tag, like:
 href="foo" etc.)
</li> <li>
 "attr" points to an array of "Attr" structures. Each of these structures
 contains the data for one attribute; it consists of a union of type
 "Attr_name", which, like "Element_name", stores the attribute name either as a
 string or as an enum; and a union of type "Attr_value", which stores the
 value of the attribute. (String or number.)
</li> <li>
<a name="elementText">
 The "content" string stores the content. (The text between the tags.) Every
 element stores the content between the previous tag and the start tag of this
 element. Thus it does not store the content of the element itself, but part
 of the content of the *parent* element. This simplifies processing a bit,
 because this way no facility for storing content blocks divided by
 sub-elements is needed -- the sub-elements store the content themselves. The
 caveat is that a lot of dummy elements are needed to store the content if no
 further sub-element follows them. This is quite a big inefficiency, as nearly
 every real element also needs a dummy element to store its content. This
 should change in the future -- if we won't drop the syntax-tree in its
 present form at all... Which we will :-)
</a>    <!--elementText-->
</li> </ul>

<h3>Initialization</h3>

<p>
Before starting parsing, we have to create the tree top. (We call it the global
element.) This is done by setting "cur_el" to NULL and calling add_element().
</p>

<a name="addElement" id="addElement">

<h4>add_element()</h4>

<p>
This function creates a new node and inserts it into the syntax tree; thus it
has to set the "parent" and "list_next" pointers too, and adjust some
pointes of other nodes to point to this one.
</p>

<p>
"parent" is set to "cur_el", as any new tag is created while parsing the
content area of its parent. "list_next" is set to NULL, as the new node is
always the last one in the list. "list_next" of "last_el" (the last node in the
list up to now) is set to point to the new node; this is omitted if "cur_el" is
NULL, indicating that there are no other nodes yet.
</p>

</a>    <!--addElement-->

<a name="parsing" id="parsing">

<h3>Parsing</h3>

<p>
The parser itself works in a very simple way. It is some kind of state machine.
For every input character, one action is taken, selected by a dispatcher
depending on the current state (stored in "parse_mode") and the input char
itself. Several combinations (e.g. tag start) change the current state, thus
the following character(s) are parsed in a different mode. (Other actions are
taken.)
</p>

<p>
Sometimes a character that causes a mode switch has to be parsed in the new
mode itself. In this case the flag "recycle" is set after the mode change,
causing the dispatch to be repeated for the same char, but in the new mode.
</p>

<p>
Again, the parsing is not very efficient in the present implementation. (In
fact, it is by far the most time consuming part of the whole layouting.)
Especially the huge switch is quite slow. (Good compilers have a fairly
efficient implementation of the switch itself; however, it still causes many
unpredictable branches.) There are some possibilities to optimize this. The
bigger problem is that the inner loop is quite big, and may not fit into the
processor's instruction cache, thus making it terribly slow. Maybe splitting
the parsing into several simpler passes would help. However, we are planning to
switch to a completely different, (hopefully) much more efficient parser system
in the next major release...
</p>

<p>
The default parsing mode is "PM_CONTENT", which is the mode for parsing element
content. Any normal character encountered in this mode is simply added to
"text_buf" by "buf_add_char()". A ' ', '\t', '\n', '\r' or '\f' aren't stored;
we switch to "PM_BLANK" instead. Any following blank space is ignored. As soon
as a normal character occurs again, we store a single ' ' and swich back to
"PM_CONTENT".
</p>

<p><pre>
input:
         first paragraph of text; 
         includes  multiple spaces and newlines,
                      ^
                      file position
 
text_buf: " first paragraph of text; includes mul"
</pre></p>

<h4>&lt;pre> Blocks</h4>

<p>
After a &lt;pre> tag, the mode isn't switched back to "PM_CONTENT", but to
"PM_PRE". In this mode all blank space characters are stored to "text_buf"
as non-breakable spaces, except newlines which are stored directly.
</p>

<p>
The mode is ended and switched to "PM_CONTENT" again when a closing "&lt;/pre>"
tag is encountered.
</p>

<p>
PM_PRE is also (mis-)used for &lt;textarea>: The content of a &lt;textarea> is used
as the initial value; and this is plain text, so it has to be treated
literally, without messing with the blanks. Thus it can be handled similar to
&lt;pre>, except that blanks are really stored directly, not even converted to
&amp;nbsp; . The "textarea" flag indicates we are in a &lt;textarea> not a real &lt;pre>,
and this exception needs to be applied.
</p>

<p>
Note that this is quite a dirty hack, which may not work in all situations.
(&lt;textarea> inside &lt;pre>...) However, the new parser in 2.x will handle this
totally diffent anyway, so it's not worth more effort with the old parser.
</p>

<h4>References</h4>

<p>
An '&amp;' indicates a character reference (unicode escape) or entity reference
(named escape), and starts the reference parsing mode "PM_AMP". On entering
this mode, the current write position in "text_buf" is saved to "amp_pos".
</p>

<p><pre>
input: [...] &amp;lt; [...]
             ^
text_buf: "[...] &amp;"
                 ^^
                  text_buf_len
                 amp_pos
</pre></p>

<p>
There are several submodes in the PM_AMP family, keeping track of the reference
syntax -- this is necessary in SGML mode, as there is no other method to
reliably discover the end of the reference or a '&amp;' character which isn't
actually a reference.
</p>

<p>
Nonetheless, all characters occuring in any of these submodes are added to
"text_buf"; actually evaluating the reference is done only when it's end is
encountered. The mode is then switched back to the previous parsing mode before
the reference occured (saved in "prev_mode_amp") -- references can occur in
content and in attribute values.
</p>

<p><pre>
input: [...] &amp;lt; [...]
                ^
text_buf: "[...] &amp;lt;"
                 ^   ^
                     text_buf_len
                 amp_pos
</pre></p>

<p>
The text between the saved start position of the escape sequence and the
current positon is converted then, depending on the type of the reference.
</p>

<p>
If it's a symbolic (named) reference, the string is looked up in "ref_table[]",
which is a table of named characters, defined in facilities.c.
</p>

<p>
For numerical character references, the integer value is extracted, using
decimal or hexadecimal conversion, depending on whether the number starts with
'x'. (Note that we have different parser states for decimal and hexal; however,
testing for the 'x' instead of the parser state saves us one condition, as we
would have to test the 'x' anyways -- the parser treats every alphanumerical
sequence starting with a letter as a hex number.)
</p>

<p>
If a replacement char was found either in the table or by the number
conversion, the reference is removed from "text_buf" and the replacement char
is inserted instead.
</p>

<p><pre>
text_buf: "[...] &lt;"
                  ^
                  text_buf_len
</pre></p>

<p>
If no replacement was found, the string is left unchanged. Probably we will
mark unknown escapes with some visible attribute in the future.
</p>

<h4>Tags</h4>

<p>
A '&lt;' starts tag parsing. There is a a whole bunch of tag parsing modes. The
one entered after the '&lt;' is "PM_TAG_START", which indicates that the tag name
should follow next.
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                  ^
text_buf: "[...] but also an anchor em"
tree:
     +
     +     
   +---+   
-->| p |-.     &lt;== cur_el
   +---+ | &lt;+         
         |  +         
         |  +         
         v  +         
    +--------+        
    | center |-. &lt;++  
    +--------+ |   +  
               v   +  
    ,----------------.
   (" starting...tag,")
    `--------+---+---'   &lt;-- last_el
             | ? |->NULL
             +---+
</pre></p>

<h5>Start Tags</h5>

<p>
If the following character is an normal char, it's a start tag. (Or a single
tag, which is treated the same way for now.) "PM_TAG_NAME" is entered. A new
element node is created by
<a href="#addElement">add_element()</a>. Any content in front of
this new element, which was stored in "text_buf" up to now, is stored to the
new node's "<a href="#elementText">text</a>" field by "insert_buf".
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                   ^
text_buf: ""
tree:
    +    
  +---+
->| p |-. &lt;++++++++++++++++++
  +---+ | &lt;+                +
        |  +  &lt;==           +
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'    &lt;--
   | center |-. &lt;++    ,->|   |->NULL
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       
</pre></p>

<p>
Normal characters encounterd in "PM_TAG_NAME" mode (including the one that
started the mode) are stored to "text_buf".
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                   ^
text_buf: "a"
</pre></p>

<p>
A blank space character ends "PM_TAG_NAME" and switches to "PM_TAG", which indicates that
attributes may follow. "text_buf" is stored as the element name.
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                    ^
text_buf: ""
tree:
    +    
  +---+
->| p |-. &lt;++++++++++++++++++
  +---+ | &lt;+                +
        |  +  &lt;==           +
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'    &lt;--
   | center |-. &lt;++    ,->| a |->NULL
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       
</pre></p>

<p>
A following normal char is the beginning of an attribute name, and switches to
"PM_ATTR_NAME". Characters encounterd in "PM_ATTR_NAME" mode are also stored to "text_buf".
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                        ^
text_buf: "name"
</pre></p>

<p>
The attribute name ends with an '=' or a blank char. A new entry is created in the "attr[]"
array, and "text_buf" is stored as the attribute name. Mode is switched to
"PM_ATTR_NAME_END" first, which indicates that the attribue value should follow.
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                         ^
text_buf: ""
attr:   name:   data:
        "name"  ""
</pre></p>

<p>
If the attribute name was ended by an '=', mode is switched immediately to
"PM_ATTR_VALUE", otherwise as soon as an '=' is encountered. (After any amount
of whitespace.)
</p>

<p>
White space in "PM_ATTR_VALUE" mode (after the '=') is ignored too.
</p>

<p>
Next char must be either a '"' or a '\'', and switches to "PM_ATTR_DATA_QUOT"
or "PM_ATTR_DATA_APOS", respectively. In this modes characters are stored to
"text_buf" again.
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                                ^
text_buf: "anchor"
attr:   name:   data:
        "name"  ""
</pre></p>

<p>
A second '"' (or '\'', respectively) ends this mode. "text_buf" is stored as
the attribute value for the new "attr" entry.
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                                 ^
text_buf: ""
attr:   name:   data:
        "name"  "anchor"
</pre></p>

<p>
Mode is swiched back to "PM_TAG". Now blank space may follow (which is
ignored), followed by another attribute.
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                                   ^
</pre></p>

<p>
In "PM_TAG" mode also a '>' may occur, ending tag parsing and switching back to
the mode before tag parsing had begun. (PM_CONTENT or PM_BLANK.) In this case,
"cur_el" is set to "last_el"; this means descending in the syntax tree to the
newly created node.
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                                          ^
text_buf: ""
attr:   name:   data:
        "name"  "anchor"
        "href"  ""
tree:
    +    
  +---+
->| p |-. &lt;++++++++++++++++++
  +---+ | &lt;+                +
        |  +                +
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'     &lt;==
   | center |-. &lt;++    ,->| a |->NULL  &lt;--
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       
</pre></p>

<p>
A '>' may also occur in "PM_TAG_NAME" mode, meaning the element has no
attributes.
</p>

<p><pre>
input: &lt;html> &lt;head> [...]
            ^
</pre></p>

<p>
In this case creating the new node and storing the name, and descending into
the element are done in one step. (By "recycle".)
</p>

<h5>End Tags</h5>

<p>
If the first character after the '&lt;' is a '/', the tag is an end tag, and we
switch to "PM_END_TAG_START", and then to "PM_END_TAG_NAME" on the first
letter.
</p>

<p>
If any text was pending in "text_buf" before the tag, we have to store it
somewhere. As an end tag normally does not create a new element node, we have
to create a dummy node for this. (Very inefficient, s.a.)
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                                               ^
tree:
    +
  +---+
->| p |-. &lt;++++++++++++++++++
  +---+ | &lt;+                +
        |  +                +
        |  +          ,----------.
        v  +         (" this...em")
   +--------+         `---+---+--'    &lt;==
   | center |-. &lt;++    ,->| a |-. &lt;+
   +--------+ |   +    |  +---+ |  +
              v   +    |        v  +
   ,----------------.  |      ,---.+
  (" starting...tag,") |     ("bed")
   `--------+---+---'  |      +---+        &lt;--
            | ? |------'      | ? |->NULL
            +---+             +---+
</pre></p>

<p>
Normal chars in "PM_END_TAG_NAME" mode are stored to "text_buf", too.
"PM_END_TAG_NAME" can be ended immediately by a '>', or by blank space
(switching to "PM_END_TAG_SPACE") followed by '>'.
</p>

<p>
The tag name extracted to "text_buf" is compared against the element name of
the current element, to see if the end tag matches, and then abdannoned. The
element is closed by ascending to the parent. 
</p>

<p><pre>
input: [...] but also an anchor em&lt;a name="anchor" href="">bed&lt;/a>ded inside a word
                                                                 ^
tree:
    +
  +---+
->| p |-. &lt;++++++++++++++++++
  +---+ | &lt;+                +
        |  +    &lt;==         +
        |  +          ,----------.
        v  +         (" this...em")
   +--------+         `---+---+--'
   | center |-. &lt;++    ,->| a |-. &lt;+
   +--------+ |   +    |  +---+ |  +
              v   +    |        v  +
   ,----------------.  |      ,---.+
  (" starting...tag,") |     ("bed")
   `--------+---+---'  |      +---+        &lt;--
            | ? |------'      | ? |->NULL
            +---+             +---+
</pre></p>

<h5>Single Tags</h5>

<p>
If a '/' appears instead of an attribute name in "PM_TAG" mode, "parse_mode" is
set to "PM_SINGLE_TAG", indicating an (XML) single tag.
</p>

<p><pre>
input: [...] and finally a blank row &lt;br /> (a single tag)
                                         ^
</pre></p>

<p>
The '/' can also immediately follow the element name. (In "PM_TAG_NAME" mode.)
</p>

<p><pre>
input: &lt;hr/>
          ^
</pre></p>

<p>
In this case, creating the node and switching to a single tag are done in one
step by "recycle".
</p>

<p>
In any case, a '>' has to follow, and switches back to normal mode just like in
a start tag, only it does not descend (set the new node as "cur_el") -- a
single tag has no content area; the content following a single tag still
belongs to the parent.
</p>

<p><pre>
input: [...] and finally a blank row &lt;br /> (a single tag)
                                          ^
tree:
    +
  +---+   &lt;++++++++++++++++++++++++++++++++
->| p |-. &lt;++++++++++++++++++             +
  +---+ | &lt;+                +             +
        |  +   &lt;==          +             +
        |  +          ,----------.   ,---------. 
        v  +         (" this...em") ("ded...row")
   +--------+         `---+---+--'   `--+----+-'      &lt;--
   | center |-. &lt;++    ,->| a |-. &lt;+ ,->| br |->NULL
   +--------+ |   +    |  +---+ |  + |  +----+   
              v   +    |        v  + |
   ,----------------.  |      ,---.+ |
  (" starting...tag,") |     ("bed") |
   `--------+---+---'  |      +---+  |
            | ? |------'      | ? |--'
            +---+             +---+
</pre></p>

<h4>Comments</h4>

<p>
In "PM_TAG_START" mode (after the '&lt;'), also an '!' can follow, indicating that
we have not any tag at all, but either a comment, a DOCTYPE declaration, or a
CDATA section. "parse_mode" is set to "PM_EXCLAM" in this case.
</p>

<p><pre>
input: some text &lt;!--a test-comment--> and more text
                  ^
</pre></p>

<p>
If a '-' follows, it's a comment. Mode is switched to "PM_COMMENT_START".
</p>

<p><pre>
input: some text &lt;!--a test-comment--> and more text
                   ^
</pre></p>

<p>
Now a second '-' has to follow, switching to "PM_COMMENT". In this mode any
characters but a '-' are simply ignored.
</p>

<p><pre>
input: some text &lt;!--a test-comment--> and more text
                          ^
text_buf: "some text"
</pre></p>

<p>
A '-' switches to "PM_COMMENT_END1", which means that it *may* be the comment end.
</p>

<p><pre>
input: some text &lt;!--a test-comment--> and more text
                           ^
</pre></p>

<p>
However, if it is followed by any other char than a second '-', mode is
switched back to "PM_COMMENT".
</p>

<p><pre>
input: some text &lt;!--a test-comment--> and more text
                            ^
</pre></p>

<p>
A second '-' in "PM_COMMENT_END1" switches to "PM_COMMENT_END2", which means
that now the comment really ends.
</p>

<p><pre>
input: some text &lt;!--a test-comment--> and more text
                                    ^
</pre></p>

<p>
Now the '>' has to follow, and switches back to parsing mode before the
beginning of the comment ("prev_mode_tag").
</p>

<p><pre>
input: some text &lt;!--a test-comment--> and more text
                                     ^
</pre></p>

<h4>DOCTYPE Declarations</h4>

<p>
If a normal char occurs in "PM_EXCLAM", we assume it is the "D" in "&lt;!DOCTYPE".
</p>

<p><pre>
input: garbage &lt;!DOCTYPE somedoc> more garbage
                 ^
</pre></p>

<p>
We treat DOCTYPE declarations as comments. Any characters but '>' are ignored.
</p>

<p><pre>
input: garbage &lt;!DOCTYPE somedoc> more garbage
                               ^
text_buf: "garbage"
</pre></p>

<p>
A '>' returns to normal mode.
</p>

<p><pre>
input: garbage &lt;!DOCTYPE somedoc> more garbage
                                ^
</pre></p>

<p>
This isn't a very reliable detection, as according to the grammer, an unescaped
'>' may appear in some system literal inside the declaration. However, we
assume that this won't happen... (We would have to parse the whole declaration
otherwise.)
</p>

<h4>CDATA Sections</h4>

<p>
A '[' in "PM_EXCLAM" mode starts a CDATA section, indicated by
"PM_CDATA_START". If there is a pending blank ("prev_mode_tag" is "PM_BLANK"),
it has to be stored *before* the CDATA.
</p>

<p><pre>
input: some text &lt;![CDATA[a tricky ]> CDATA section]]> and more text
                   ^
text_buf: "some text "
</pre></p>

<p>
Following normal chars (should) belong to the "CDATA" string, and are ignored.
</p>

<p><pre>
input: some text &lt;![CDATA[a tricky ]> CDATA section]]> and more text
                        ^
text_buf: "some text "
</pre></p>

<p>
A second '[' in "PM_CDATA_START" mode switches to "PM_CDATA", indicating that
the actual data will follow.
</p>

<p><pre>
input: some text &lt;![CDATA[a tricky ]> CDATA section]]> and more text
                         ^
</pre></p>

<p>
Any characters in "PM_CDATA" mode but '>' are stored directly to "text_buf".
</p>

<p><pre>
input: some text &lt;![CDATA[a tricky ]> CDATA section]]> and more text
                                   ^
text_buf: "some text a tricky ]"
</pre></p>

<p>
When a '>' occurs, the previous two chars (in "text_buf") are tested against
"]]". If they do not match, the '>' is simply stored just as any other
character.
</p>

<p><pre>
input: some text &lt;![CDATA[a tricky ]> CDATA section]]> and more text
                                    ^
text_buf: "some text a tricky ]>"
</pre></p>

<p>
If they match, the last two characters are removed from "text_buf" (they belong
to the CDATA terminator), and mode is switched back to "PM_CONTENT". (It
doesn't need to be switched back to the mode before the CDATA section, as any
pending blanks already have been stored, and a CDATA section can't start in
other modes than "PM_TEXT" or "PM_BLANK".)
</p>

<p><pre>
input: some text &lt;![CDATA[a tricky ]> CDATA section]]> and more text
                                                     ^
text_buf: "some text a tricky ]> CDATA section]]"
</pre></p>

<p><pre>
input: some text &lt;![CDATA[a tricky ]> CDATA section]]> and more text
                                                      ^
text_buf: "some text a tricky ]> CDATA section"
</pre></p>

<h4>Processing Instructions</h4>

<p>
The '&lt;' may also be followed by a '?', indicating a processing instruction.
Mode is switched from "PM_TAG_START" to "PM_INSTR".
</p>

<p><pre>
input: some text &lt;?a fake? processing instruction??> more text
                  ^
</pre></p>

<p>
Processing instructions are also treated as comments. Any chars but '?' are
ignored in "PM_INSTR".
</p>

<p><pre>
input: some text &lt;?a fake? processing instruction??> more text
                        ^
text_buf: "some text"
</pre></p>

<p>
A '?' switches to "PM_INSTR_END", indicating this *may* be the end of the
processing instruction.
</p>

<p><pre>
input: some text &lt;?a fake? processing instruction??> more text
                         ^
</pre></p>

<p>
If a normal char follows the '?', mode is switched back to "PM_INSTR".
</p>

<p><pre>
input: some text &lt;?a fake? processing instruction??> more text
                          ^
</pre></p>

<p>
If a second '?' follows, "PM_INSTR_END" is kept, as the first one isn't the end
of the processing instruction, but the new one could be.
</p>

<p><pre>
input: some text &lt;?a fake? processing instruction??> more text
                                                  ^
</pre></p>

<p>
A '>' in PM_INSTR_END really ends the processing instruction, and switches to
"prev_mode_tag".
</p>

<p><pre>
input: some text &lt;?a fake? processing instruction??> more text
                                                   ^
</pre></p>

<h3>SGML Mode</h3>

<p>
When compiled without the "-DXHTML_ONLY" option, a few cases more are possible.
</p>

<h4>Unclosed Tags</h4>

<p>
In SGML, not every element has to have an end tag.
</p>

<p>
When an end tag is encountered, we ascend in the syntax tree not only once, but
until an element is found that matches the end tag. Thus, all elements in
between are automatically closed.
</p>

<p><pre>
input:
&lt;body>
   &lt;p>
      some text
      &lt;hr>
   &lt;/p>
   ^
 
+------+
| body |-.&lt;+
+------+ | +
         v +
       +---+
       | p |-.&lt;+
       +---+ | +
             v +
        ,-------.
       (some text)
        `-+----+'
      --> | hr |->NULL
      ==> +----+
</pre></p>

<p><pre>
input:
&lt;body>
   &lt;p>
      some text
      &lt;hr>
   &lt;/p>
      ^
 
+------+
| body |-.&lt;+   &lt;==
+------+ | +
         v +
       +---+
       | p |-.&lt;+
       +---+ | +
             v +
        ,-------.
       (some text)
        `-+----+'
      --> | hr |->NULL
          +----+
</pre></p>

<h4>Unquoted Attribute Values</h4>

<p>
When a normal char occurs in "PM_ATTR_VALUE" mode, "PM_ATTR_DATA_NOQUOTE" is entered.
</p>

<p><pre>
input: &lt;sometag someattribute=somevalue minimized third="nothing">
                              ^
</pre></p>

<p>
This mode is just like  "PM_ATTR_DATA_QUOT" or "PM_ATTR_DATA_APOS", only it
is ended by a blank or the tag end.
</p>

<p><pre>
input: &lt;sometag someattribute=somevalue minimized third="nothing">
                                       ^
</pre></p>

<h4>Mimimized Attributes</h4>

<p>
In SGML, attributes without a value are possible. This is recognized when a
normal char or the tag end occurs in "PM_ATTR_NAME_END" mode instead of the
'='.
</p>

<p><pre>
input: &lt;sometag someattribute=somevalue minimized third="nothing">
                                                  ^
</pre></p>

<p>
The attribute is ended immediately. "text_buf[]" (which is empty in this case)
is stored just like at the end of an unquoted attribute value. Mode is set to
"PM_TAG", and the current character (the tag end or beginning of next
attribute) is processed in this mode.
</p>

<h4>SGML Comments</h4>

<p>
Comments also allow more complicated syntax. For one, blank space is possible
between the "--" ending the comment string and the '>' ending the declaration.
Thus, blank space in PM_COMMENT_END2 is ignored.
</p>

<p>
Moreover, another comment string may follow the end. Thus, a '-' in
PM_COMMENT_END2 switches back to PM_COMMENT_START, similary to the '-' after
the "&lt;!".
</p>

<p><pre>
input: &lt;!--comment start--  --second comment string in same declaration--  >
                            ^
</pre></p>

<p>
Finally, SGML also allows empty declarations ("&lt;!>"), which are also a kind of
comment. Thus a '>' in PM_EXCLAM switches immediately to PM_COMMENT_END2 and
recycles.
</p>

<p><pre>
input: &lt;!>
         ^
</pre></p>

<h4>Unclosed Tags</h4>

<p>
In SGML, tags needn't be closed by '>', if the tag end can be deduced from the
context. In practise, this means that a tag can also be ended by a '&lt;'
character, which may be the beginning of a following tag. Thus we have to
handle this in all situations where a '>' could also occur.
</p>

<h4>Unhandled Constructs</h4>

<p>
SGML also allows some constructs that aren't recognized by any browser we know
of. These include empty tags (&lt;> and &lt;/>) and "net mode".
</p>

<p>
Netrik recognizes these constructs and prints a warning, but doesn't handle
them either -- there is no point in this, as autors couldn't use them anyways
due to lack of support in other browsers. Handling them correctly would
actually even break some pages, because it would behave different than all
other browsers.
</p>

<h4>Loose '&amp;' and '&lt;' Chars</h4>

<p>
If some illegal char occurs in a entity/character reference, it's not really a
reference, but an unescaped '&amp;'. We keep the whole sequence literally and
switch back to "prev_mode_amp".
</p>

<p><pre>
input: x = a &amp; b
              ^
text_buf: "x = a &amp; "
</pre></p>

<p>
Similar for illegal characters in "PM_TAG_START" (and some other PM_TAG*
modes), which indicate an unescaped '&lt;'. We store a '&lt;' and switch back to
"prev_mode_tag".
</p>

<p><pre>
input: if(a &lt; b)
             ^
text_buf: "&lt; "
</pre></p>

</a>    <!-- parsing -->

<h3>Finishing</h3>

<p>
Parsing is ended by EOF. This should only appear in "PM_CONTENT" or "PM_BLANK"
mode (not inside some tag, comment, CDATA section or chracter/entity
reference), and only if the current element is the global one (not while
parsing some element's content).
</p>

<p>
The "list_next" pointer of the last node is set to point back to the tree top.
This faciliates easier processing in the following steps.
</p>

<h3>Error Handling</h3>

<p>
When using -DXHTML_ONLY, every syntax error encountered causes netrik to print
an error message and immediately quit. (The XML standard requires this.)
</p>

<p>
Without -DXHTML_ONLY, netrik is more tolerant.
</p>

<h4>Workarounds</h4>

<p>
Netrik uses simple workarounds for some of the most common cases of broken
HTML.
</p>

<p>
Most notable is comment parsing: As SGML comments have a quite complicated
syntax, reasonable error handling is also quite complicated.
</p>

<p>
If someting else then '>' (end of comment declaration), '-' (beginning of
second comment string), or blank space follows in PM_COMMENT_END2 (after a
"--"), then the "--" was probably not intended to have any special meaning, but
simply to be part of the comment. Thus, mode is switched back to PM_COMMENT.
</p>

<p><pre>
input: &lt;!-- some broken -- comment -->
                           ^
</pre></p>

<p>
The same is done for unexpected characters in PM_COMMENT_RESTART mode, which is
most common for "---" inside a comment.
</p>

<p><pre>
input: &lt;!-- some broken --- comment -->
                            ^
</pre></p>

<p>
There is one exception to this, however: If a '>' follows in PM_COMMENT_START
mode, and it was preceeded not only by one '-' (the one which started
PM_COMMENT_START) but two or more, then the the '>' together with the last two
'-' was probably intended as as an XML-like "-->" comment end.
</p>

<p><pre>
input: &lt;!--- anything --->
                        ^
parse_mode: PM_COMMENT_START
dash_count: 3
</pre></p>

<p>
The "dash_count" variable keeps track of how many dashes have been encountered
in a row; it is incremented every time a '-' apprears in some of the comment
parsing modes, and is reset to 0 every time some other character is
encountered.
</p>

<p>
"dash_count" is also used in another situation: If a '>' follows in PM_COMMENT
or PM_COMMENT_END1, normally it is part of the comment. The '>' is ignored and
mode stays PM_COMMENT. (Or is switched back from PM_COMMENT_END1.)
</p>

<p><pre>
input: &lt;!-- comment with > and -> in it -->
                                ^
</pre></p>

<p>
However, if there were two or more dashes in front of the '>', this "-->"
combination was probably also intended as a comment end. A comment consisting
of a series of dashes is a typical example:
</p>

<p><pre>
input: &lt;!------>
              ^
parse_mode=PM_COMMENT
dash_count=6
</pre></p>

<p>
However, only a little warning can be printed in this case -- this is valid
SGML, and *has* to be treated as part of the comment, even if it's probably not
what the page author intended! Printing an error and using a workaround would
mean deliberately to violate the standard in favour of broken pages, which is
probably not a very good idea...
</p>

<p>
There is another trick however, which contervails this in most situations: As
soon as any clear error is detected, a "broken" flag is set for the time of
this comment. If the above situation occurs afterwards, we treat it as an error
and abort the comment -- as we are sure that the comment has errors, there is
no point in continuing as if the comment was correct.
</p>

<p>
There are also a couple of specific workarounds for tags:
</p>

<p>
Spurious quotes inside the attribute value are quite common when the autor
forgets the opening quote but not the closing one. These have to be ignored. (A
warning is printed, but they aren't stored as part of the value or handled
otherwise.)
</p>

<p>
Very often we find illegal characters in unquoted attribute values. (According
to the standard, only name characters are allowed here.) These produce a
warning, but are otherwise handled like legal chars -- as long as there are not
ambigious. (A '&lt;' is always an error for example, as it usually indicates
another tag start.)
</p>

<p>
Other unexpected characters in tags (e.g. "a&lt;b =" or something the like) are
handled by immediatly aborting tag parsing and returning to normal mode. This
seems the surest bet, because such a situation usually indicates that the
construct wasn't intended as a tag at all, only looked similar by incident. By
bailing out as soon as possible, we try to limit the damage -- staying in tag
mode might produce more critical problems, like hiding or misinterpreting
considerable parts of the remaining document.
</p>

<p>
Note that even better would be storing the whole preceeding part of the
presumed tag literally as content. it would be much more complicated however;
we haven't bothered to implement this.
</p>

<p>
Unexpected characters in any other mode are simply ignored, hoping for the
best.
</p>

<h4>html_error()</h4>

<p>
Whenever some syntax error is detected (no matter whether workarounds are
available), html_error() is called, with several parameters describing the
error. This function is responsible for everything that needs to be done when
an error occurs.
</p>

<p>
Before taking any action, the requested error message is tested against an
array with all errors printed so far. Only if the message is new, the function
proceeds; otherwise, an "ignored"-counter is incremented, and the function
returns early.
</p>

<p>
Only now html_error() starts it's normal operation: First, it prints an the
message. The message text is passed from parse_syntax(), and used as the format
string for printf(). If the error message requires additional arguments, they
are passed at the end of the parameter list when calling html_error().
</p>

<p>
If the parsing mode requires that, html_error() quits immediately afterwards.
The mode is determined by the config variable "cfg.parser", which is an "enum
Parser_mode", with the possible values FUSSY_HTML, CLEAN_HTML, VALID_HTML,
BROKEN_HTML and IGNORE_BROKEN. The parser quits only in FUSSY_HTML mode, or
when -DXHTML_ONLY is enabled. If the input resource from which the page is
loaded is a pipe from wget (see
<a href="hacking-load.html">hacking-load.*</a>, the pipe is closed
before quitting to assure a cleaner exit.
</p>

<p>
In all other modes, an additonal message passed from parse_syntax() is printed
afterwards, informing in which way netrik will handle the error. (workaround,
ignore etc.)
</p>

<p>
Finally the error level passed from parse_syntax() is compared against the
highest error level up to know, and the new higest level is returned.
</p>

$$<a name="warn" id="warn">

<h4>Warning messages</h4>

<p>
parse_syntax keeps track of most severe syntax error that was found while
parsing the page in "err_level", which is of type "enum Syntax_error" and can
have the following values:
</p>

<ul> <li>
 SE_NO: No errors were found
</li> <li>
 SE_BREAK: The user issued an interrupt (SIGINT) while loading the document.
 This isn't really an error, but can be handled very convenient this way...
</li> <li>
 SE_DISCOURAGED: Some constructs were found that are strictly speaking valid
 SGML, but explicitly discouraged in the HTML standard. These may be handled
 differently by other browsers -- especially comments.
</li> <li>
 SE_UNIMPLEMENTED: Also valid SGML and discouraged in HTML, but not handled
 correctly by netrik nor any other popular browser.
</li> <li>
 SE_WORKAROUND: Real errors were found, but workarounds could be applied that
 work in most cases.
</li> <li>
 SE_CRITICAL: Something went terribly wrong: We have an error situation which
 we can not make out, and thus no useful workaround could be applied. The page
 almost certainly will look broken, often with considerable parts or the
 content missing. (e.g. a misinterpreted comment or missing closing quote)
</li> <li>
 SE_FAIL: This isn't really a syntax error. It is not used inside
 parse_syntax() itself; it's only set before returning when a file loading
 error was detected, for the sake of the calling function.
</li> <li>
 SE_NODATA: Similar to SE_FAIL. This is set if EOF is returned by load()
 before *any* data has been read.
</li> </ul>

<p>
After the whole page is parsed, a warning message is printed if some error was
found. The message text depends on the error level. The error level is also
passed back to main(), which then waits for a keypress before starting the
pager, so the message will be seen.
</p>

<p>
In IGNORE_BROKEN mode the warning is suppressed, and "err_level" is reset. In
BROKEN_HTML mode, all but SE_CRITICAL errors are suppresed; and in VALID_HTML,
all but SE_CRITICAL and SE_WORKAROUND.
</p>

<p>
SE_BREAK is set if EOF is returned, but at the same time "input->user_break"
has been set, indicating that it's not really EOF, but transfer was interrupted
by the user. Other errors are supressed in this mode, as a user break during
loading might cause several syntax errors (unclosed elements etc.) with the
page itself being not to blame for.
</p>

<p>
SE_NODATA is set if EOF is returned by load() before any data has been read.
(This can be caused by failure to open the resource, but also by an empty
file/http response.) It's handled like a normal syntax error; the only
difference is that it can't be masked even by IGNORE_BROKEN. The syntax tree
consists only of the global element; it will be correctly rendered to an empty
page.
</p>

<p>
SE_FAIL isn't set during parsing. Before returning, parse_syntax() checks
whether "input->type" is RES_FAIL; if it is, an error message is printed, and
SE_FAIL is set (so main() knows an error occured). However, this test is only
necessary if SE_NODATA isn't set; otherwise, an error message has already been
printed and an error code would be returned anyways. As EOF is returned by
load() also when an error occured, SE_NODATA is already set for most errors;
SE_FAIL is only used if the error occurs after some data could be read.
</p>

</a>    <!-- warn -->

<a name="freeSyntax" id="freeSyntax">

<h3>free_syntax()</h3>

<p>
This function is responsible for freeing the memory used by the syntax tree
when it is no longer needed.
</p>

<p>
The whole tree is traversed by "list_next", and the element nodes are freed one
by one.
</p>

<p>
As the "list_next" pointer is necesary to find the next node, but not longer
available after freeing the current node, it is saved in "next_el" before
freeing. At the the beginning of the next iteration this is copied to "cur_el".
</p>

<p>
Before freeing the element node itself, all dynamic data belonging to the node
has to be freed.
</p>

</a>    <!--freeSyntax-->

</a>    <!--parseSyntax-->

<a name="dumpTree" id="dumpTree">

<h2>3. dump-tree.c</h2>

<p>
dump_tree() is primarily used for dumping the syntax tree generated by
parse_sytax() for debugging purposes. The reason it resides in an own file is
that it could be easily modified to be a really useful function for dumping a
HTML document's structure. This may be implemented in the future, if someone
shows interest...
</p>

<p>
The implementation of dump_tree() is quite straightforward, as the function
only needs to print every node in the order it occured in the HTML file.
</p>

<p>
For every node, first the text is printed. (If "dump_content" is given.) The
reason it is printed in front of the node itself is because it's also the
content in front of the element in the original HTML file.
</p>

<p>
Next, the current tree depth is shown by a number of '|'. The current depth is
always stored in "depth".
</p>

<p>
Afterwards the element name is printed, and all attributes with their values.
Depending on "elements_parsed", either the raw values extracted from the
document are printed, or the transformed values generated by parse_elements().
</p>

<p>
The next node is reached by the "list_next" pointer of the current node. Before
going to the next node the tree depth of the new node needs to be calculated.
This is done by assuming that the next node is below the current one, and than
going up, until we find the parent of the new node. This idea is explained in
more detail in <a href="#parseStruct">6. parse-struct.c</a>.
</p>

</a>    <!--dumpTree-->

<a name="parseElements" id="parseElements">

<h2>4. parse-elements.c</h2>

<p>
parse_elements() is responsible for making out the elements and attributes from the
syntax tree. (Extraced by parse_syntax().)
</p>

<p>
All elements but the first one (which is always ELEMENT_GLOBAL) are processed one
by one; the tree is traversed via "list_next". For every element, the name is
looked up in "element_table[]" by comparing to all entries, in a loop.
</p>

<a name="elementTable" id="elementTable">

<p>
"element_table[]" contains all names of ordinary elements, then the "?"
representing ELEMENT_NO, and finally "!" representing ELEMENT_GLOBAL. (It also contains
other properties of the elements; more on this in
<a href="#parseElementsProcessing">Processing</a> in
<a href="#parseStruct">6. parse-struct.c</a>.)
</p>

</a>    <!--elementTable-->

<p>
The last two aren't checked against the element name.
</p>

<p>
As soon as a match is found, the loop is left and the entry number is stored to
"syntax_tree" in place of the string. The entry number is an "enum
Element_type", defined in syntax.h; it tells the element type in the following
processing passes.
</p>

<p>
If none of the ordinary entries matched, the entry nuber, which now is ELEMENT_NO
(as this one follows after the ordinary entries), is stored anyhow, indicating
that the element is unknown. If no element name was stored ("cur_el->name.str"
is NULL), indicating a dummy tag, ELEMENT_NO is set also.
</p>

<p>
After the element name, all attribute names are processed in a loop. They are
looked up in "attr_table[]" the same way the element name is.
</p>

<p>
The attribute value isn't processed at all yet.
</p>

</a>    <!--parseElements-->

<a name="sgmlRework" id="sgmlRework">

<h2>5. sgml_rework()</h2>

<p>
Before the syntax tree is further processed, sgml_rework() (from sgml.c) is
applied. (Unless compiled with -DXHTML_ONLY.)
</p>

<p>
This function is responsible for fixing the problems arising from the fact that
SGML allows certain end tags to be left out; thus the syntax parser doesn't
recognize the elements' ends, and stores all following elements as children,
even if they should actually be at the same level. (e.g. the list items in a
list.) sgml_rework() goes over the complete (broken) tree, finds such
situations, and unnests the elements, thus creating a correct syntax tree.
</p>

<p>
It won't be covered in too much detail here, as this is only a temporary
solution; it will become obsolete with the planned new parser(s).
</p>

<p>
The recognition of the missing element ends is done by the "element_group" enum
in "<a href="#elementTable">element_table[]</a>". This Enum has the
values GROUP_SINGLE for single tag elements (elements which mustn't have any
content), GROUP_OBLIGATE for all elements where the end tag can't be left out,
and several others for various kinds of elements with optional end tag.
</p>

<p>
The whole tree is scanned element by element (using "list_next"), and each one
is tested to fullfill one of the offending conditions. Two things have to be
handled: Unclosed single tag elements, and unclosed optional end tag elements.
</p>

<p>
The second situation is more complicated. If the element is of some type with
optional end tag, it could terminate a previous (unclosed) element from the
same group; e.g. a &lt;li> will terminate the previous &lt;li>. It doesn't terminate
elements from other groups, though; a &lt;td> inside a &lt;tr> doesn't terminate the
&lt;tr>, for example. Thus, the group of the current element needs to be tested
against the group of the parent (all elements following an unclosed one are
stored as its children by the parser!); if they are the same, the parent is
actually an element that should be terminated at the position where the child
starts, and the child should follow it, at the same depth. This means that the
child has to be "lifted" out of the parent. However, we don't do that
immediately; we only set the "closed" flag of the parent element for now, and
the lifting will be done later.
</p>

<p>
However, it's not enough to test only the immediate parent: The element may
follow some other unclosed element, and thus be a child of it, e.g. a &lt;tr>
following a &lt;td>, which is inside the previous &lt;tr>. This also needs to be
recognized, and *both* the previous &lt;tr> and the &lt;td> have to be closed. Thus,
not only the immediate parent's group is compared, but all ancestors are
scanned. The scanning only stops on an element with obligate end tag -- as the
element's end is always known for these, nothing will be ever stored inside
that element that doesn't belong there, and nothing should be lifted out. (In
nested tables for example, the &lt;tr>s and &lt;td>s of the inner table shouldn't
mess with the ones of the outer table -- this is ensured by the scanning of the
inner table's rows and columns stopping at the inner &lt;table> element.) The
"closed" flag is set for all the closed ancestors.
</p>

<p>
Some more handling is necessary due to the fact that an element node always
stores the content which appears *before* the element. When an element is
lifted, the content musn't be lifted also -- it appeared *before* the element,
and thus also before the previous element's end, so it has to stay where it is.
We have to create a new dummy tag inside the closed element therefore, taking
the place of the lifted element and storing its content.
</p>

<p>
However, this isn't done when the parent was already closed. This happens if
the parent is a single tag element. These elements end right where they start,
not at the beginning of the next element; the content also has to be lifted
out. (Nothing is allowed to stay inside a single tag element!)
</p>

<p>
The actual lifting is done after processing the element: If the parent is
closed, we have to "leave" it. (This is done by setting the "parent" to the
previous grandparent -- this way, the element is no longer a child of the old
parent, but a sibling.) Thus the element that closed it's parent is lifted
right afterwards; all following elements of the parent will be lifted also,
after being processed. Any preceeding elements (as well as the possibly created
new dummy) won't be lifted on the other hand, as they won't be processed
anymore.
</p>

<p>
Single tag elements are handled more or less the other way round: They are not
closed by some child (which turns out to actually be a sibling), but close
*themselfs*, as soon as they are encountered. This way all children will be
lifted out, no matter what.
</p>

<p>
No other processing is necessary for single tag elements, as they won't ever
terminate some other element.
</p>

</a>    <!-- sgmlRework -->

<a name="parseStruct" id="parseStruct">

<h2>6. parse-struct.c</h2>

<p>
After the syntax tree was generated by parse_syntax(), we have to "understand"
it. This is done by parse_struct(), which is the central pass of the layouting
process. In this function the syntax tree, which contains a nearly 1:1
reproduction of the HTML file, is converted to an item tree, which contains a
representation of what will be actually shown as the output of the browser --
text blocks, blank rows, boxes grouping severel other items.
</p>

<h3>Structure Tree</h3>

<p>
For 0.html, we have to convert the syntax tree:
</p>

<p><pre>
  ++>NULL
  +
+---+
| ! |-. &lt;++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+   &lt;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
        | head |->| body |-. &lt;++++++++++++++                                              +
        +------+  +------+ | &lt;+            +                                              +
                           v  +            +                                              +     &lt;+++++++++++++++++++++++++++++++++++++++++++++++++
                        +----++          +---+     &lt;++++++++++++++++++++                +---+   &lt;++++++++++++++++++++++++++++++++                +
                        | h1 |-. &lt;+   ,->| p |---. &lt;++                 +             ,->| p |-. &lt;++++++++++++++++++             +                +    (back
                        +----+ |  +   |  +---+   |   +                 +             |  +---+ | &lt;+                +             +                +    to top)
                               v  +   |          v   +                 +             |        |  +                +             +                +          ^
                          ,--------.  |  ,------------------.       ,----.           |        |  +          ,----------.   ,---------.   ,---------------.  |
                         (" heading") | (" first...newlines,")     (" and")          |        v  +         (" this...em") ("ded...row") (" (a single tag)") |
                          `--+---+-'  |  `-----+----+-------'     +-+----+-+         |   +--------+         `---+---+--'   `--+----+-'   `-----+---+-----'  |
                             | ? |----'        | em |-. &lt;++    ,->| strong |-. &lt;++   |   | center |-. &lt;++    ,->| a |-. &lt;+ ,->| br |---------->| ? |--------'
                             +---+             +----+ |   +    |  +--------+ |   +   |   +--------+ |   +    |  +---+ |  + |  +----+           +---+
                                                      v   +    |             v   +   |              v   +    |        v  + |
                                           ,----------------.  |     ,------------.  |   ,----------------.  |      ,---.+ |
                                          (" emphasized text") |    (" strong text") |  (" starting...tag,") |     ("bed") |
                                           `--------+---+---'  |     `-----+---+--'  |   `--------+---+---'  |      +---+  |
                                                    | ? |------'           | ? |-----'            | ? |------'      | ? |--'
                                                    +---+                  +---+                  +---+             +---+
</pre></p>

<p>
to this structure tree:
</p>

<a name="itemTree" id="itemTree">

<p><pre>
***> "string"                                                                             (back  &lt;+++++++++
xxx> "first_child"                                                                      to first          +
+++> "parent"                                                                             item)        +-----+
===> "next"                                                                                         ,->| box |-->NULL
---> "list_next"                                                                                    |  +-----+&lt;==#
                                                                                                    |  x  ^  #===#
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx  +
  x                                                                                                 |     +
  x  ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  x  +         +           +          +           +          +           +          +          +    |
  v  +         +           +          +           +          +           +          +          +    |
+------+   +-------+   +------+   +-------+   +------+   +-------+   +------+   +------+   +------+ |
| text |-->| blank |-->| text |-->| blank |-->| text |-->| blank |-->| text |-->| text |-->| text |-'
+----*-+==>+-------+==>+----*-+==>+-------+==>+----*-+==>+-------+==>+----*-+==>+----*-+==>+----*-+==>NULL
  x  *         x         x  *         x         x  *         x         x  *       x  *       x  * 
  v  *         v         v  *         v         v  *         v         v  *       v  *       v  *   ,--------------.
NULL *       NULL      NULL *       NULL      NULL *       NULL      NULL *     NULL *     NULL **>("(a single tag)")
     v                      v                      v                      v          *              `--------------'
 ,-----------.          ,-------.           ,------------.       ,---------------.   *   ,----------------.
("header text")        ("heading")         ("first...text")     ("starting...tag,")  **>("this...blank row")
 `-----------'          `-------'           `------------'       `---------------'       `----------------'
</pre></p>

</a>    <!--itemTree-->

<p>
which, in turn, is a representation of this output page:
</p>

<a name="itemPage" id="itemPage">

<p><pre>
+-------------------------------------------------------------------------------------------------+
|+-----------+                                                                                    |
||header text|                                                                                    |
|+-----------+                                                                                    |
|                                                                                                 |
|+-------+                                                                                        |
||heading|                                                                                        |
|+-------+                                                                                        |
|                                                                                                 |
|+-----------------------------------------------------------------------------------------------+|
||first paragraph of text; includes multiple spaces and newlines, emphasized text and strong text||
|+-----------------------------------------------------------------------------------------------+|
|                                                                                                 |
|+---------------------------------+                                                              |
||starting with an evil center tag,|                                                              |
|+---------------------------------+                                                              |
|+-----------------------------------------------------------------------------------------------+|
||this very long second paragraph contains some special characters (including a simple space...):||
|| &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  and hexal: ) but also an anchor||
|| embedded inside a word (this anchor also is the only tag with parameters); and finally a blank||
|| row                                                                                           ||
|+-----------------------------------------------------------------------------------------------+|
|+--------------+                                                                                 |
||(a single tag)|                                                                                 |
|+--------------+                                                                                 |
+-------------------------------------------------------------------------------------------------+
</pre></p>

</a>    <!--itemPage-->

<p>
Note that there are no actual sizes or postions for the items, and no line
breaks inside the text items; this is all done in a later processing step
(pre-render.c). The item tree at this point only represents the structure of
the output page. (The line breaks in the fifth text block aren't really there;
we have inserted them in the figure because the text block is a bit too long to
put in a single line...)
</p>

<p>
The item tree looks complicated at first, but it's a quite trivial example when
taking a closer look. (This is becuase at the time of creating the 0.html file
used here, netrik wasn't able to do anything more complicated...) However, it
should be sufficient to get the idea...
</p>

<p>
Every node of the item tree consists of an "Item" structure. This structure is
declared in "items.h". It contains:
</p>

<ul> <li>
 The pointers "list_next", "next", "parent", and "first_child" connect the
 nodes inside the tree. "list_next" points to the next node in the order they
 are generated. Note that in contrast to the element tree, in the item tree
 any children are generated *before* the parent. "next" points to the next
 item at the same tree depth and in the same branch, i.e. the next sibling.
 "parent" points to the parent item, and "first_child" to the first sub-item.
 More pointers are necessary than for the element tree, because the item tree
 is traversed in several differnt ways while processing.
</li> <li>
 The "center" flag indicates whether the item is centered. As not all items
 use this, and the exact meaning varies between different item types, it may
 be reasonable to move this to the item specific data. We'll decide on this as
 soon as enought HTML facilities are implemented.
</li> <li>
 "x_start", "x_end", "y_start" and "y_end" define a square area inside the
 layouted page, in which the item is displayed. In some processing steps
 "x_end" end "y_end" are also "abused" to store the minimal size of the item.
</li> <li>
 "type" is an enum storing what kind of item this node represents. (Currently
 ITEM_TEXT, ITEM_BOX, ITEM_FORM, ITEM_BLANK, ITEM_BLOCK_ANCHOR or
 ITEM_INLINE_ANCHOR.)
</li> <li>
 "data" is an union storing all data specific to different item types.
 Currently this can be the pointer to a text string for text items, to an
 (block or inline) anchor struct, or to a form paramters struct.
</li> </ul>

<h3>add_item()</h3>

<p>
New items are created by add_item(). This function allocates a new "Item"
structure, and sets some pointers. The item isn't inserted into the tree
directly; it's only inserted into a list of all items at the current tree
depth. This is a single linked list maintained by the "first_item" and
"last_item" pointers of "state" (there is one such list for each tree depth),
and the "next" pointers of the item structures. The only other pointers set are
"list_next". The "parent" and "first_child" pointers aren't set; this is done
later when the items are actually inserted into the tree, while ascending from
the current depth. (For the first item in every tree depth the "parent" is
explicitly set to NULL, indicating that there is no parent yet.)
</p>

<p>
This function is called directly to create box items, and from add_string() to
create text items. Under certain conditions it also creates a blank item before
the actual text item or box item; more on this later, under
<a href="#blankLines">Blank Lines</a>.
</p>

<p>
When called with the "virtual" flag, this function behaves slightly different:
No line break/blank line handling is done; the status remains unchanged. This
is for creating the <a href="#virtual">Virtual Boxes</a> used for
anchors.
</p>

<a name="string" id="string">

<h3>String</h3>

<p>
The actual text data of text items is stored in a different place. Every text
item points to a "String" structure (also declared in "items.h"). This
structure consists of a normal C-string containing the text itself, and an
array of "Div" structures, containing all attribute information. (Color etc.) A
"String" can consist of several divisions with different attributes. Every
"Div" structure stores the attributes for one such division, and the ending
position of the division inside the string. (More exactly: the position *after*
the end of the div -- which is the starting position of the next div.) The end
of the last division is also used to find out the string length. This is quite
inefficient...
</p>

<p>
The "String" structure also contains "line_table[]", which holds the positions
of all line breaks inside the string; more on this in <a
href="#preRender">7. pre-render.c</a>.
</p>

<p>
Finally, it contains an array of
"<a href="hacking-links.html#linkStruct">Link</a>" structures, which
describe all the links (and form elements) inside this text block. See
<a href="hacking-links.html">hacking-links.*</a> for this.
</p>

</a>    <!-- string -->

<a name="parseElementsProcessing" id="parseElementsProcessing">

<h3>Processing</h3>

<p>
Many properties of the various element types are data-driven. This presently
includes the line break/blank line handling, elements creating a box around all
children, and elements whose content is not to be rendered. This properties are
stored to the same "<a href="#elementTable">element_table[]</a>" as
the name strings used by parse_elements(). In future probably more properties
will be data driven, making the code simpler, and also necessary for handling
style sheets, which allow changing of almost all formatting properties.
</p>

<p>
The item tree is generated while traversing the element tree. Processing of
each element is done in two steps: One step is done before entering an element
(descending in the element tree), and the second step is done after leaving the
element (ascending). Between those two steps, the same is done for all
sub-elements. You guess it: This is a recursive algorithm. Only we haven't
implemented it recursively, as mentioned in chapter 0. In this function the
pseudo-recursive implementation is most evident; we even have to use a
pseudo-stack.
</p>

<p>
The processing could also be split into a couple of much simpler passes using
some temporary data structures, e.g. one generating the "normal" items, one
generating blank items, one generating the strings, one "optimizing" the tree
(lifting items where possible). This would be much easier to understand, and
probably it would have been a good idea for the beginning. However, it would be
much less efficient; that's why we won't step back to such an implementation
after already having the present one.
</p>

<h4>Pre-processing</h4>

<p>
The first action in every iteration of the outer loop is to do the first
processing step (s.a.) for the current element ("cur_el") -- in every iteration
exactly one element is pre-processed.
</p>

<p><pre>
  ++>NULL
  +
+---+
| ! |-. &lt;++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'  &lt;-- cur_el   &lt;== depth
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+
        | head |->| body |-.
        +------+  +------+ |
</pre></p>

<p>
First, we store any text from the current element node to the current open text
item by add_string().
</p>

<p><pre>
state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         &lt;-- first_item
| text |-->NULL  &lt;** last_item    &lt;== depth=1
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 
</pre></p>

<p>
The next thing is processing of line breaks and paragraph breaks, depending
what kind of element we have. More on this later, in <a
href="#textBlocks">Text Blocks</a> and <a
href="#blankLines">Blank Lines</a>.
</p>

<h4>Recursing</h4>

<p>
Then we recurse into the element (descend in the element tree).
</p>

<p><pre>
  ++>NULL
  +
+---+
| ! |-. &lt;++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'  &lt;-- cur_el
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+
        | head |->| body |-.     &lt;== depth
        +------+  +------+ |
</pre></p>

<p><pre>
state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         &lt;-- state[1].first_item   (depth 1)
| text |-->NULL  &lt;** state[1].last_item
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 
---
first_item -->NULL    &lt;== depth=2
last_item **>NULL
</pre></p>

<p>
This is done by push_state(). This function doubles the top of stack, and
returns a pointer to the newly created entry, which is used as the current
state. ("first_item" and "last_item" aren't copied, but set to NULL; "id_attr"
and "link_type" are set to -1.)
</p>

<p>
The stack stores all variables which are specific to every tree depth. Note
that the stack uses the depths from the element tree, not from the item tree.
The depths in the item tree are completely different, and change while
processing -- which is one of the most tricky parts about parse_struct().
</p>

<p>
Currently the data stored is:
</p>

<ul> <li>
 Visibilty of element's content ("visible")
</li> <li>
 Text attributes ("text_mode" and "high").
</li> <li>
 The nesting depth "list_depth" of item lists. (Determines the indent of list
 items.)
</li> <li>
 The two pointers "first_item" and "last_item", necessary to maintain the list
 of all items at a given depth.
</li> <li>
 The type of the link or form control created by the element in the
 "link_type" enum (a value of -1 indicates there is no link at all)
</li> <li>
 The URL of links or value of form elements in the "link_value" string
</li> <li>
 The "form_enabled" flag indicates whether a form element is to be submitted to the
 server
</li> <li>
 The name of a possible &lt;select> element, bequeathed to its &lt;option> elements
</li> <li>
 The kind of the &lt;select> element, also bequeathed
</li> <li>
 "link_start" stores the position where a link or inline anchor beginns inside
 the current string
</li> <li>
 "link_item" stores the text item in which a link/anchor beginns
</li> <li>
 For elements with an anchor, "id_attr" stores which attribute contains the
 anchor id (or name)
</li> </ul>

<p>
After descending, first some generic processing is performed.
</p>

<p>
This includes setting "cur_state->visible" depending on the parent element
type's "visible" property.
</p>

<p>
Also, "link_start" is set to the current string end, so any text generated
inside this element will become part of the link or anchor, if the element
creates one. If there is no string item open, 0 is stored, so the link will
begin at the start of the string if a new string beginns inside the element.
</p>

<p>
Aferwards, some element type specific handling is done. Mostly this is
outputing of special element indicators. (This could be made data-driven, and
probably it will do so soon.)
</p>

<p>
For some element types, also values of the current state are modified; this
would be the argument passing in a real recursive implementation.
</p>

<h4>Ascending</h4>

<p>
The last step of every outer loop iteration is returning from recursion
(ascending in the element tree). But in contrast to descending, ascending isn't
done once per outer loop iteration. Instead, there is an inner loop, that
ascends as often as needed to reach the level of the next element -- this can
be zero, once or several times.
</p>

<p>
We know how long we need to ascend by starting at the level of the current
element and looking for the parent of the new element; as soon as we find it,
we know we needn't ascend any more. "depth" is adjusted every time we ascend,
and all actions for leaving the element (unrecursing) are taken. ("depth" is
always one below the depth of "new_el".)
</p>

<p>
If the next element is a child of the current one, no ascending is necessary.
We have descended one step in the pre-processing step of the current iteration,
and this is ok; we keep it. ("cur_el" is alredy the parent of "list_next".)
</p>

<p><pre>
 ,-----------.
("header text")
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+
        | head |->| body |-. &lt;++++++++++++++   
        +------+  +------+ | &lt;+            +   
                           v  +            +   
                        +----++          +---+ 
            cur_el  --> | h1 |-. &lt;+   ,->| p |->
            new_el  xx> +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |      &lt;== depth
               list_next **> | ? |----'
                             +---+     
</pre></p>

<p>
If the next element is at the same level as the current one (single tags or
other elements with no sub-elements), we need to ascend exactly one time, to
get back to the level of the current element, after we have descended in the
pre-processing step.
</p>

<p><pre>
 ,-----------.
("header text")
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+ &lt;**
    --> | head |->| body |-. &lt;++++++++++++++
    xx> +------+  +------+ | &lt;+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. &lt;+   ,->| p |->      &lt;==
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                             | ? |----'
                             +---+     
</pre></p>

<p><pre>
 ,-----------.
("header text") &lt;xx
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+ &lt;**
    --> | head |->| body |-. &lt;++++++++++++++       &lt;==
        +------+  +------+ | &lt;+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. &lt;+   ,->| p |->
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                             | ? |----'
                             +---+     
</pre></p>

<p>
Of course it's a bit of overhead first to descend into an element, just to
ascend from it right after. But it saves a lot of code for special handling of
such childless elements. Many of the actions of both the first step and the
second step have to be done for them also -- putting them together and leaving
out the descending and ascending wouldn't save that much, while complicating
the code quite a lot. We may consider some way in the future if profiling shows
this would be rewarding.
</p>

<p>
If the next element is above the current one, we have to ascend more than once.
(Once to get back to the level of the current one, the others to ascend to the
new level.)
</p>

<p><pre>
 ,-----------.
("header text")     
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+    
        | head |->| body |-. &lt;++++++++++++++          
        +------+  +------+ | &lt;+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. &lt;+   ,->| p |-> &lt;**
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                         --> | ? |----'
                         xx> +---+     
 
                                            &lt;==
</pre></p>

<p><pre>
 ,-----------.
("header text")     
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+    
        | head |->| body |-. &lt;++++++++++++++          
        +------+  +------+ | &lt;+            +   
                           v  +            +   
                        +----++          +---+ 
                    xx> | h1 |-. &lt;+   ,->| p |-> &lt;**
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |        &lt;==
                         --> | ? |----'
                             +---+     
</pre></p>

<p><pre>
 ,-----------.
("header text")     
 `-+------+--'
   | html |-. &lt;++++++++
   +------+ | &lt;+      +
            v  +      +
        +------+  +------+ &lt;xx
        | head |->| body |-. &lt;++++++++++++++          
        +------+  +------+ | &lt;+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. &lt;+   ,->| p |-> &lt;**     &lt;==
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                         --> | ? |----'
                             +---+     
</pre></p>

<p>
In every ascending iteration, first we do some element type specific handling
again, mostly outputing element end indicators. Then we pop the previous state
from the stack.
</p>

<p><pre>
 ++>NULL
 +
 + .--------------------------------.
 + v                                |
+---+        &lt;xx                    |
| ! |-. &lt;++  &lt;**                    |
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |          &lt;==
 `-+------+--'                      |
   | html |-. &lt;+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  &lt;--
            `--------->| ? |--------'
                       +---+
</pre></p>

<p><pre>
state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---         ,-- first_item
+------+ &lt;--'     +------+   +------+ &lt;** last_item
| text |--[...]-->| text |-->| text |-->NULL              &lt;== depth=1
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'
</pre></p>

<p><pre>
 ++>NULL &lt;xx
 +
 + .--------------------------------.
 + v                                |
+---+                               |
| ! |-. &lt;++  &lt;**                    |          &lt;==
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |
 `-+------+--'                      |
   | html |-. &lt;+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  &lt;--
            `--------->| ? |--------'
                       +---+
</pre></p>

<p><pre>
first_item -->NULL        &lt;== depht=0
last_item **>NULL
---         ,-- state[1].first_item
+------+ &lt;--'     +------+   +------+ &lt;** state[1].last_item     (depth 1)
| text |--[...]-->| text |-->| text |-->NULL
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'
</pre></p>

<h4>Inserting into Item Tree</h4>

<p>
Afterwards, the probably most interesting part follows: The sub-items created
inside the element we are just leaving, are inserted into the item tree
properly.
</p>

<p>
If the element we are leaving enforces a box (looked up in "element_table[]"), a new
box item is created. (Box items are always created when leaving the element,
and thus *after* all items inside the box.)
</p>

<p><pre>
                                  -->    +-----+
                                  **> ,->| box |-->NULL     &lt;== depht=0
                                      |  +-----+==>NULL
                                      |      
                                      |      
                                      |      
                                      |      
                                      |
                                      |
+------+ &lt;--      +------+   +------+ | &lt;**           (depth 1)
| text |--[...]-->| text |-->| text |-'
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'
</pre></p>

<p>
The "parent" pointers of all immediate children are set to the new box item,
and "first_child" of the new item is set to the first of them.
</p>

<p><pre>
                                  -->    +-----+
                                  **> ,->| box |-->NULL     &lt;== depht=0
                                      |  +-----+==>NULL
                                      |  x  ^
  xxxxxxxx[...]xxxxxxxxxxxxxxxxxxxxxxxxxxx  +
  x                                   |     +
  x  +++++[...]++++++++++++++++++++++++++++++
  x  +                +          +    |
  v  +                +          +    |
+------+ &lt;--      +------+   +------+ | &lt;**           (depth 1)
| text |--[...]-->| text |-->| text |-'
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'
</pre></p>

<p>
If the element does not create a box, things are more tricky: We have to "lift"
all sub-elements to the new level. This is done by concatenating the list of
elements of the depth we are leaving to the list of elements of the depth we
are entering.
</p>

<p><pre>
state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         &lt;-- state[1].first_item   (depth 1)
| text |-->NULL  &lt;** state[1].last_item
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 
---
state[2]->first_item -->NULL   (depth 2)
state[2]->last_item **>NULL
---       ,-- state[3].first_item
+------+&lt;-'+-------+ &lt;** state[3].last_item    (depth 3)
| text |-->| blank |-->NULL           
+----*-+==>+-------+==>NULL  
  x  *         x      
  v  *         v      
NULL *       NULL     
     v     
 ,-------. 
("heading")
 `-------' 
---
+------+        &lt;-- first_item
| text |-->NULL &lt;** last_item      &lt;== depth=4
+----*-+==>NULL
  x  *       
  v  *       
NULL *       
     v       
 ,------------. 
("first...text")
 `------------' 
</pre></p>

<p><pre>
  +------+   &lt;+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
->| body |-. &lt;++++++++++++++                                              +
  +------+ | &lt;+            +                                              +
 xx^       v  +            +                                              +   
        +----++          +---+     &lt;++++++++++++++++++++                +---+ 
==>     | h1 |-. &lt;+   ,->| p |---. &lt;++                 +         **> ,->| p |->
depth=3 +----+ |  +   |  +---+   |   +                 +             |  +---+ 
               v  +   |          v   +                 +             |
          ,--------.  |  ,------------------.       ,----.           |
         (" heading") | (" first...newlines,")     (" and")          |
          `--+---+-'  |  `-----+----+-------'     +-+----+-+         |
             | ? |----'        | em |-. &lt;++    ,->| strong |-. &lt;++   |
             +---+             +----+ |   +    |  +--------+ |   +   |
                                      v   +    |             v   +   |
                           ,----------------.  |     ,------------.  |
                          (" emphasized text") |    (" strong text") |
                           `--------+---+---'  |     `-----+---+--'  |
                                    | ? |------'       --> | ? |-----'
                                    +---+                  +---+      
</pre></p>

<p><pre>
state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         &lt;-- state[1].first_item   (depth 1)
| text |-->NULL  &lt;** state[1].last_item
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 
---
state[2]->first_item -->NULL   (depth 2)
state[2]->last_item **>NULL
---       ,-- first_item
+------+&lt;-'+-------+   +------+ &lt;** last_item
| text |-->| blank |-->| text |-->NULL      &lt;== depth=3
+----*-+==>+-------+==>+----*-+==>NULL
  x  *         x         x  *       
  v  *         v         v  *       
NULL *       NULL      NULL *       
     v                      v       
 ,-------.          ,------------. 
("heading")        ("first...text")
 `-------'          `------------' 
</pre></p>

<p>
In the second case, we need an additional step if the element we are leaving
was a &lt;center> element (or some element with an align="center" attribute): In
this case all sub-elements have to be centered one by one. (Elements creating a
box are centered as a whole.)
</p>

<p>
Finally, we have to do some more line break/blank line handling.
</p>

</a>    <!--parseElementsProcessing-->

<a name="textBlocks">

<h3>Text Blocks</h3>

<p>
One single text block can be created by several elements. Every series of text
parts not interrupted by elements requirering line breaks (or blank lines)
around them is stored to a single text item; it can even contain newline
characters created by &lt;br> elements. This is done by calling add_string() every
time text data is encountered.
</p>

<p>
The color for the text added by some element is determined from the current
"state->text_mode" and "state->high". The normal color for the text mode is
looked up in "color_map[]", and than its bit 3 is negated if the text is
highlighted. This is more or less a hack; we'll have to replace this by some
serious attribute handling at some point...
</p>

<h4>add_string()</h4>

<p>
This function concatenates the new text to the "string" of "string_item", which
is a global variable pointing to the current open text item. If the new text
has other attributes than the last division of the "string" so far, a new
division is created for the new text; otherwise, the text is simply added to
the last division.
</p>

<p><pre>
text_item->string:
            div[0].end      div[1].end   div[2].end
                     v               v   v
text: "[...]newlines, emphasized text and"
       `~~~~~v~~~~~~'`~~~~~~~v~~~~~~'`~v'
                 div[1].color=MAGENTA  div[2].color=WHITE
    div[0].color=WHITE
</pre></p>

<p><pre>
+------------------------------------------------+
|+-----------+                                   |
||header text|                                   |
|+-----------+                                   |
|                                                |
|+-------+                                       |
||heading|                                       |
|+-------+                                       |
|                                                |
|+----------------------------------+            |
||[...]newlines, emphasized text and|            |
</pre></p>

<p><pre>
 ,------------------.       ,----.          
(" first...newlines,")     (" and")         
 `-----+----+-------'     +-+----+-+        
       | em |-. &lt;++    ,->| strong |-. &lt;++  
       +----+ |   +    |  +--------+ |   +  
              v   +    |             v   +  
   ,----------------.  |     ,------------. 
  (" emphasized text") |    (" strong text")
   `--------+---+---'  |     `-----+---+--' 
            | ? |------'       --> | ? |-->
            +---+                  +---+      
</pre></p>

<p><pre>
text_item->string:
            div[0].end      div[1].end   div[2].end  div[3].end
                     v               v   v           v
text: "[...]newlines, emphasized text and strong text"
       `~~~~~v~~~~~~'`~~~~~~~v~~~~~~'`~v'`~~~~~v~~~~'
                 div[1].color=MAGENTA          div[3].color=STRONG WHITE
      div[0].color=WHITE               div[2].color=WHITE
</pre></p>

<p><pre>
+------------------------------------------------+
|+-----------+                                   |
||header text|                                   |
|+-----------+                                   |
|                                                |
|+-------+                                       |
||heading|                                       |
|+-------+                                       |
|                                                |
|+----------------------------------------------+|
||[...]newlines, emphasized text and strong text||
</pre></p>

<p>
A new division can also be enforced, by an additional call of add_text() with
NULL as text before adding the next text part. (This is necessary to prevent
multiple consecutive links from being merged into one div, as link highlighting
is done div-wise.)
</p>

<p>
Appending to an existing text item is only possible if the last created item
was a text item, and there was no breaking element. (Neither the last one, nor
the new one, nor anyone in between.) Otherwise, a new text item has to be
created, and a new "String" structure for it. This is the only place where new
text items are created.
</p>

<p><pre>
|                                   |
|+---------------------------------+|
||starting with an evil center tag,||
|+---------------------------------+|
</pre></p>

<p><pre>
  +---+
->| p |-. &lt;++++++++++++++++++      
  +---+ | &lt;+                +      
        |  +                +      
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'  &lt;--
   | center |-. &lt;++    ,->| a |->
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       
</pre></p>

<p><pre>
|                                   |
|+---------------------------------+|
||starting with an evil center tag,||
|+---------------------------------+|
|+---------------------+            |
||this very...anchor em|            |
</pre></p>

<p>
If there is a space at the beginning of a string, it is discarded while
creating the string, for a text block always starts with a word.
</p>

</a>    <!--textBlocks-->

<h3>Line Breaks</h3>

<p>
As soon as an element forcing a line break is either entered or left (there are
no elements creating a break only before or only behind it), "string_item" is
set to NULL, indicating that no more text can be added to the last text item,
and new text has to create a new one.
</p>

<p><pre>
  +---+
->| p |-. &lt;++++++++++++++++++      
  +---+ | &lt;+                +      
        |  +                +      
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+ &lt;xx     `---+---+--'  &lt;**
   | center |-. &lt;++    ,->| a |->
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  | &lt;--            &lt;==
            | ? |------'
            +---+       
</pre></p>

<p><pre>
|                                   |
|+---------------------------------+|
||starting with an evil center tag,||
</pre></p>

<p><pre>
  +---+ &lt;xx
->| p |-. &lt;++++++++++++++++++      
  +---+ | &lt;+                +      
        |  +                +      
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'  &lt;**      &lt;==
   | center |-. &lt;++    ,->| a |->
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  | &lt;--
            | ? |------'
            +---+       
</pre></p>

<p><pre>
|                                   |
|+---------------------------------+|
||starting with an evil center tag,||
|+---------------------------------+|
</pre></p>

<a name="blankLines" id="blankLines">

<h3>Blank Lines</h3>

<p>
Probably the most tricky part is handling of blank lines. Similar to line
breaks, items needing blank lines have them before *and* after them. However,
when two such items meet, they have only *one* blank line between them.
Furthermore, a blank line is *never* inserted before the first or after the
last item inside a box. That's why blank lines cannot be simply inserted when
entering an element causing blank lines, or when leaving it. Instead, only
"requests" for blank lines are stored in "para_blank", and if certain
conditions are met, a blank line is inserted before the next item (inside
add_item()).
</p>

<p>
Actually, the blank line is inserted not in front of the new item, but after
the last item, at the same tree depth as that one. This is important, to ensure
that a blank line generated in front of a box is actually inserted *outside* of
the box, not inside it, as would be the case if it was inserted at the current
depth. The global "blank_depth" variable is responsible for this, and is set to
the current depth every time a blank line request is generated. The blank item
is then inserted directly to the "state" structure at "blank_depth" in
add_item(). This is surely very bad programming style ;-)
</p>

<p>
The blank line requests are managed by the global "para_blank" variable. A
value of 1 indicates that a blank line is needed, and will be generated by the
next add_item(). A value of 0 indicates that no blank line is needed in any
case; this situation can only occur when there are no items inside the current
box yet. A value of -1 indicates that a blank line *may* be necessary. That is
the case when we already have some items inside the current box, but the last
item does not need a blank line; a blank line needs to be inserted only when
the following item wants one.
</p>

<p>
When entering an element needing a blank line, a request is generated
("para_blank" set to 1 and "blank_depth" is stored) if "para_blank" was -1,
indicating that there are already items in the current box.
</p>

<p><pre>
      +
  +------+
->| body |-. &lt;++++++++++++++    
  +------+ | &lt;+            +    
           v  +            +    
        +----++          +---+  
   -->  | h1 |-. &lt;+   ,->| p |->
 cur_   +----+ |  +   |  +---+  
 tag           v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
             | ? |----'
             +---+     
</pre></p>

<p><pre>
+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
??? (para_blank=-1)
</pre></p>

<p><pre>
      +
  +------+
->| body |-. &lt;++++++++++++++    
  +------+ | &lt;+            +    
           v  +            +    
        +----++          +---+  
   -->  | h1 |-. &lt;+   ,->| p |->
   ==>  +----+ |  +   |  +---+  
 blank_        v  +   |
 depth    ,--------.  |
         (" heading") |
          `--+---+-'  |
       **>   | ? |----'
  list_next  +---+     
(new "cur_el")
</pre></p>

<p><pre>
+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
| (para_blank=1)   |
       ...
|+-------+         | (will be created later)
||heading|         |
|+-------+         |
</pre></p>

<p>
If "para_blank" is already 1, the request is left unchanged; a blank line will
be inserted already.
</p>

<p><pre>
|                  |
|+-------+         |
||heading|         |
|+-------+         |
| (para_blank=1)   |
</pre></p>

<p><pre>
   |
   v
+----+ &lt;==       +---+ &lt;--
| h1 |-. &lt;+   ,->| p |---. &lt;++
+----+ |  +   |  +---+   |   +
       v  +   |          v   +
  ,--------.  |  ,------------------. 
 (" heading") | (" first...newlines,")
  `--+---+-'  |  `-----+----+-------' 
     | ? |----'    **> | em |-.
     +---+             +----+ |
</pre></p>

<p><pre>
|                  |
|+-------+         |
||heading|         |
|+-------+         |
| (para_blank=1)   |
       ...
|+------------+    |
||first...text|    |
|+------------+    |
</pre></p>

<p>
If "para_blank" is 0 nothing is done, even if the element would normally need a
blank line: No blank line is ever inserted in front of the first item inside a
box.
</p>

<p><pre>
+-----------+
(para_blank=0)
</pre></p>

<p><pre>
+---+
| ! |-. &lt;+
+---+ |  +
      v  +
    +---++
--> | p |-. &lt;+
    +---+ |  +
          v  +
    ,---------.
   ("some text")
    `--+---+--'
   **> | ? |->
       +---+
</pre></p>

<p><pre>
+-----------+
     ...
|+---------+|
||some text||
|+---------+|
</pre></p>

<p>
When some item (text or box) is inserted by add_item(), "para_blank" is always
set to -1, as now there is at least one item in the current box.
</p>

<p><pre>
+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
| (para_blank=1)   |
</pre></p>

<p><pre>
      +
  +------+
->| body |-. &lt;++++++++++++++    
  +------+ | &lt;+            +    
           v  +            +    
        +----++          +---+  
        | h1 |-. &lt;+   ,->| p |->
        +----+ |  +   |  +---+  
               v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
         --> | ? |----'
             +---+     
</pre></p>

<p><pre>
+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
|                  |
|+-------+         |
||heading|         |
|+-------+         |
??? (para_blank=-1)
</pre></p>

<p>
When leaving an element needing a blank line, a request is generated also, and
will be handled in the next add_item(). Otherwise, the current state is kept.
</p>

<p><pre>
      +
  +------+
->| body |-. &lt;++++++++++++++    
  +------+ | &lt;+            +    
           v  +            +    
        +----++          +---+  
    xx> | h1 |-. &lt;+   ,->| p |-> &lt;**
new_el  +----+ |  +   |  +---+  
               v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
         --> | ? |----'
             +---+     
</pre></p>

<p><pre>
+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
|                  |
|+-------+         |
||heading|         |
|+-------+         |
??? (para_blank=-1)
</pre></p>

<p><pre>
      +
  +------+ &lt;xx
->| body |-. &lt;++++++++++++++    
  +------+ | &lt;+            +    
           v  +            +    
        +----++          +---+  
    ==> | h1 |-. &lt;+   ,->| p |-> &lt;**
        +----+ |  +   |  +---+  
               v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
         --> | ? |----'
             +---+     
</pre></p>

<p><pre>
+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
|                  |
|+-------+         |
||heading|         |
|+-------+         |
| (para_blank=1)   |
</pre></p>

<p>
When entering an element generating a box, "para_blank" is reset to 0.
</p>

<p><pre>
|                |
|+---------+     |
||some text|     |
|+---------+     |
??? (para_blank=-1)
</pre></p>

<p><pre>
  +--------+            +------+
->| center |-.       ,->| form |-.  &lt;--
  +--------+ |       |  +------+ |
             v       |           |
        ,---------.  |           |
       ("some text") |           v
        `--+---+--'  |         +---+
           | ? |-----'    **>  | p |->
           +---+               +---+
</pre></p>

<p><pre>
|                |
|+---------+     |
||some text|     |
|+---------+     |
|+--------------+|
(para_blank=0)
</pre></p>

<p>
However, it's left unchanged if there is already a request, indicating that a
blank line will be inserted *in front* of the box by the next add_item().
</p>

<p><pre>
|                |
|+---------+     |
||some text|     |
|+---------+     |
| (para_blank=1) |
</pre></p>

<p><pre>
  +---+            +------+
->| p |-. &lt;==   ,->| form |-.  &lt;--
  +---+ |       |  +------+ |
        v       |           |
   ,---------.  |           |
  ("some text") |           v
   `--+---+--'  |         +---+
      | ? |-----'    **>  | p |->
      +---+               +---+
</pre></p>

<p><pre>
|                |
|+---------+     |
||some text|     |
|+---------+     |
| (para_blank=1) |
|+--------------+|
</pre></p>

<p>
When leaving a box creating element, any request generated inside the box is
discarded. We never insert a blank line after the last item of a box.
</p>

<p><pre>
 ++>NULL
 + .--------------------------------.
 + v                                |
+---+        &lt;xx                    |
| ! |-. &lt;++  &lt;**                    |
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |
 `-+------+--'                      |
   | html |-. &lt;+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  &lt;--
            `--------->| ? |--------'
                       +---+
</pre></p>

<p><pre>
|+--------------+  |
||(a single tag)|  |
|+--------------+  |
| (para_blank=1)   |
</pre></p>

<p><pre>
 ++>NULL &lt;xx
 + .--------------------------------.
 + v                                |
+---+                               |
| ! |-. &lt;++  &lt;**                    |
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |
 `-+------+--'                      |
   | html |-. &lt;+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  &lt;--
            `--------->| ? |--------'
                       +---+
</pre></p>

<p><pre>
|+--------------+  |
||(a single tag)|  |
|+--------------+  |
+------------------+
??? (para_blank=-1)
</pre></p>

<p>
However, if the last request was generated *before* descending into the element
creating the box (only possible if the box is empty), it is kept, and the blank
is inserted when the box is added.
</p>

<p><pre>
|                |
|+---------+     |
||some text|     |
|+---------+     |
| (para_blank=1) |
|+--------------+|
</pre></p>

<p><pre>
  +---+            +------+ &lt;xx      +---+   
->| p |-. &lt;==   ,->| form |-.     ,->| p |->  &lt;**
  +---+ |       |  +------+ |     |  +---+
        v       |           |     |
   ,---------.  |           |     |
  ("some text") |           v     |
   `--+---+--'  |        +------+ |
      | ? |-----'   -->  | span |-'
      +---+              +------+
</pre></p>

<p><pre>
|                |
|+---------+     |
||some text|     |
|+---------+     |
|                |
|+--------------+|
|+--------------+|
</pre></p>

<h4>&lt;br> elements</h4>

<p>
&lt;br> elements used to break text blocks as described above. This however turned
out to be a bug: some elements (e.g. &lt;a>) can span over a &lt;br>, and these were
handled incorrectly with that approach.
</p>

<p>
Now simply a '\n' is added to the text block when &lt;br> is encountered, and then
correctly handled while <a href="#lineBreaking">Breaking String into
Lines</a>.
</p>

</a>    <!-- blankLines -->

<a name="links" id="links">

<h3>Links</h3>

<p>
When a link element is encountered (any &lt;a> element having a "href"),
"cur_state->link_type" is set (to FORM_NO), indicatating that the element
creates a link. The URL (from the "href" attribute) is saved in
"cur_state->link_value".
</p>

<p>
The link is then stored when leaving the link element. (After processing all
sub-elements.) The link start is set to "cur_state->link_start", which was the
string end position while entering the link element; the link end is set to the
current string end position. This way the link spans all text generated inside
the element.
</p>

<p>
There is some additional handlig necessary to workaround broken links, however.
(This used to be important to make forms work with SGML at all; now that full
SGML support is implemented, it probably only helps a few broken pages...)
</p>

<p>
When a link doesn't end in the same string as it started (checked by
"cur_state->link_item", which was the current string item when entering the
link element), it is stored in the starting string, not the current one.
However, if there was no active text item when the link started (indicating the
link starts at the beginning of a text block), we can't determine the starting
string. In this case, we store it in the current string -- normally, this is
the right thing to do; if the link spans multiple strings, on the other hand,
at least the last part ist stored this way.
</p>

<p>
Some magic is necessary to handle nested links: The link which is stored later
is the outer one, i.e. it starts *before* the previously stored inner link... To
handle this in a useful fashion, we also have to *store* it before the inner
link. Thus, instead of simply appending it at the end of the string's link list,
we have to shift all inner links one position behind, and put the new (outer)
link at the free position created in front of them.
</p>

<p>
Additionally, two hashes of the link URL and the text inside the link are
stored for each link; this is necessary to recognize the right link to
reactivate when a page is revisited but its content changed. See
<a href="hacking-page.html#reactivating">_Reactivating Link</a> in
hacking-page.* for details.
</p>

</a>    <!-- links -->

<h3>Forms</h3>

<p>
Form elements are handled very similar, and by the same code. The only
difference is that "link_type" is set to some form type instead of FORM_NO, and
the initial "value" of the element is stored in the structure tree so it will
be submitted to the server, and (for some form control types) displayed on the
page via <a href="hacking-links.html#setForm">set_form()</a> (see
hacking-links.*).
</p>

<p>
Also, "form_enabled" is set for all elements that will be submitted to the
server. It is always set for elements that are submitted unconditionally
(text/password/hidden input fields), and set to the initial state for elements
that are submitted depending on their state (radio buttons, checkboxes,
&lt;select> options). Submit buttons initially aren't enabled.
</p>

<p>
Some special handling is necessary for &lt;select> options: They do not have an
own "name" attribute to store in "Link->name" like other form elements; the
"name" for all options is given in the &lt;select> element instead. Thus it needs
to be passed to all the options by the state stack, in "select_name". The same
for the link type (FORM_OPTION or FORM_MULTIOPTION) of the option links: It
depends on the presence of the "multiple" attribute in the &lt;select> element,
and is passed in "select_type" for that reason.
</p>

<a name="virtual" id="virtual">

<h3>Virtual Items</h3>

<p>
There are two kinds of "virtual" Items: ITEM_BLOCK_ANCHOR and
ITEM_INLINE_ANCHOR. As the names suggest, both types are presently used for
anchors. The difference is that block anchors are created by block elements
with an "id" attribute, and span one or more block elements, while inline
anchors are created inside text blocks by the classical "a" element or by any
inline element with an "id", and span only a text part.
</p>

<p>
"virtual" means that these items do not affect layouting like other items; they
only create some additional structure.
</p>

<h4>Block Anchors</h4>

<p>
As block anchors span multiple other elements, they have to act as boxes,
containing a series of virtual children. In contrast to real box items, they do
not get their size assigned by the parent, and do not assign size to their
virtual children. The children get their size assigned by the real parent
directly, and the size of the virtual box is determined afterwards, from the
outer bounds of all virtual children.
</p>

<p>
In the present implementation, virtual box items aren't inserted normally as
parents of their virtual children into the item tree. They are simply inserted
as normal items after their virtual children, at the same tree depth. The
virtual box is created only by special pointers, handled only in the necessary
places; the normal tree traversal functions do not know about them. This is a
hack; the idea was to integrate them into the existing system changing as
little as possible. Most probably this will be replaced by a clean
implementation using real parent/children relations in the future.
</p>

<p><pre>
virtual tree:
                                                                  +-----+
                                                               ,->| box |
                                                               |  +-----+
                                                               |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +                                      +              +    |
+------+                           +--------------+   +------+ |
| text |-.                      ,->| block anchor |-->| text |-'
+------+=|======================|=>+--------------+==>+------+==>NULL
         |                      |    x     ^
         |    xxxxxxxxxxxxxxxxxxxxxxxx     +
         |    x  +++++++++++++++++++++++++++
         |    v  +         +    |
         |  +------+   +------+ |
         `->| text |-->| text |-'
            +------+   +------+==>NULL
</pre></p>

<p><pre>
real tree:
 
> virtual child                                              +-----+
                                                             ,->| box |
                  virtual box                                |  +-----+
           ,----------^----------.                           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +          +          +              +              +    |
+------+   +------+   +------+   +--------------+   +------+ |
| text |-->| text |-->| text |-->| block anchor |-->| text |-'
+------+==>+------+==>+------+==>+--------------+==>+------+==>NULL
             ^                          
                                       
             
</pre></p>

<h4>Inline Anchors</h4>

<p>
As inline anchors are contained within text elements, they have to act as
children of text items. Text item however can't have children in the present
implementation; thus, inline anchors presently are also only virtual children.
</p>

<p>
Similar to the virtual boxes created by block anchors, these virtual children
are stored after their virtual parent, at the same tree depth. This is a hack
just as the virtual boxes, and will be replaced also.
</p>

<p>
Additionally to the virtual parent (and the name of the anchor), inline anchors
need to store the position of their start and end inside the parent string.
</p>

<p><pre>
virtual tree:
 
%%%> anchor_start,anchor_end                                                +-----+
                                                                         .->| box |
                                                                         |  +-----+
                                                                         |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +                                                    +          +    |
+------+                                             +------+   +------+ |
| text |-.                                        .->| text |-->| text |-'
+------+=|========================================|=>+------+==>+------+==>NULL
         |                                        |    x ^ *
         |                                        |    x + *   ,--------------------.                           
         |    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx + **>("anchor1 and anchor2.")                          
         |    x     ++++++++++++++++++++++++++++++++++++++     `--------------------'                           
         |    x     +                   +         |             ^      ^    ^      ^
         |    x     +                   +         |             %      %    %      %
         |    x     +    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%    %      %
         |    x     +    %              +    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
         |    v     +    %              +    %    |
         |  +---------------+   +---------------+ |
         `->| inline anchor |-->| inline anchor |-'
            +---------------+==>+---------------+==>NULL
</pre></p>

<p><pre>
real tree:
 
> virtual parent                                                       +-----+
                                                                       .->| box |
                                                                       |  +-----+
                                                                       |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +          +              +                   +               +    |
+------+   +------+   +---------------+   +---------------+   +------+ |
| text |-->| text |-->| inline anchor |-->| inline anchor |-->| text |-'
+------+==>+------+==>+---------------+==>+---------------+==>+------+==>NULL
             *  ^             %                  %
%%%%%%%%%%%%%*%%%%%%%%%%%%%%%%                  %
%            *                                  %
%            *  %
%            v                                     %
%  ,--------------------.                          %
% ("anchor1 and anchor2.")                         %
%  `--------------------'                          %
%   ^      ^    ^      ^                           %
%   %      %    %      %                           %
%%%%%%%%%%%%    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
</pre></p>

<p>
Links are very similar to inline anchors, and probably will be stored the same
way as anchors in the future, once ITEM_INLINE is cleanly implemented. The
present different handling of links and anchors is by purpose -- to see which
method turns out to be better. (Presently, it looks much like the anchor-method
is better.)
</p>

</a>    <!-- virutal -->

<h3>Finishing</h3>

<p>
After all elements of the syntax tree are traversed, the structure tree is
finalized by setting "next" of the tree top (the global item) to point back to
itself, and "parent" point to the first item.
</p>

<p>
parse_struct() returns a "struct Item" pointer to the tree top. This is passed
as argument to the following passes by main().
</p>

<p>
Is is a bit strange that "parent" of the tree top points to the first item.
However, it is important to have some simple way of accessing the first item,
as some of the following passes start with the fist item; but only the pointer
to the tree top is passed on the following passes. Maybe we should use some
function that finds the first item by following "first_child" instead -- as
this is done only once in every processing pass, it wouldn't be too big an
unefficiency.
</p>

</a>    <!--parseStruct-->

<a name="freeItems" id="freeItems">

<h3>free_items()</h3>

<p>
This function is responsible for freeing the memory used by the item tree.
</p>

<p>
Just as <a href="#freeSyntax">free_syntax()</a>, the tree is
traversed by "list_next", and every node is freed, including all data belonging
to it.
</p>

<p>
For text items, the associated strings are freed also; for anchor items, the
anchor data is freed.
</p>

</a>    <!--freeItems-->

<a name="preRender" id="preRender">

<h2>7. pre-render.c</h2>

<p>
So far, the item tree only represents the structure of the page, i.e. the
dependencies of the items. The Items have no actual sizes nor positions yet.
</p>

<p><pre>
# ################################################################################################
 
# +-----------
# |header text
 
# ***
# *
 
# +-------
# |heading
 
# ***
# *
 
# +-----------------------------------------------------------------------------------------------
# |first paragraph of text; includes multiple spaces and newlines, emphasized text and strong text
 
# ***
# *
 
# +---------------------------------
# |starting with an evil center tag,
 
# +-----------------------------------------------------------------------------------------------
# |this very long second paragraph contains some special characters (including a simple space...):
# | &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  and hexal: ) but also an anchor
# | embedded inside a word (this anchor also is the only tag with parameters); and finally a blank
# | row                                                                                           
 
# +--------------
# |(a single tag)
</pre></p>

<p>
pre_render() assigns coordinates inside the layouted page to all items. It also
breaks the text blocks into lines.
</p>

<p>
This process is reversible (it doesn't alter any data stored before; it only
creates additional data), and thus resizing the output area of the viewer will
be possible without regenerating the item tree. (And without reloading the
file.)
</p>

<p>
The coordinates are stored to "x_start", "x_end", "y_start" and "y_end" of the
item structures. For every text item, the positions of all line breaks are
stored in "line_table[]". This is the only way line breaks are indicated; the
text string itself isn't altered. Thus the rendering function has to look at
this table while generating output. (<a href="#renderC">8. render.c</a>)
</p>

<p>
Besides of assigning the coordinates, pre_render() also generates the
"page_map[]". This is a structure telling which items occupy every line of the
output page. For every line, a table is generated, containing references to all
elements that show up in this line. It's described more thoroughly in
<a href="#createMap">create_map()</a>.
</p>

<p>
The processing is split into five smaller passes, executed successively by
pre_render(). It also could be done in one single, pseudo-recursive pass. Most
probably this would be more efficient; it would be harder to understand, too.
Maybe we will change this at some point...
</p>

<a name="calcWidth" id="calcWidth">

<h3>calc_width()</h3>

<p>
In the first pass, the minimal x-width of all items is calculated. This is done
by traversing the item tree by "list_next" (bottom to top). This ensures that
the sizes of all children are calculated before the parent. In every iteration
one item is processed. The actions taken depends on the type of the item.
</p>

<p>
For text items, the minimal width (stored in "x_end") is set to the width of
the longest word in the text block, as in other browsers. Note that we *can*
generate narrower text blocks; netrik can break words that do not fit on a
line, and probably also scrolling of items will be possible in the future.
(AFAIK no other browser does any of that, although it was recommended by the
W3C for years...)
</p>

<p><pre>
# ###############[...]
 
# +------+
# |header|
   ^     ^x_end=6
   0
 
# ***
# *
 
# +-------
# |heading
[...]
</pre></p>

<p>
For blank lines, the minimal width is set to 0 -- blank lines do not need any
width; anchor items as well.
</p>

<p><pre>
# ###############[...]
 
# +------+
# |header|
 
# **
# **
   ^x_end=0
 
# +-------
# |heading
[...]
</pre></p>

<p>
Finding the longest word is quite easy. The whole text block is processed char
by char in a loop. For each char, the current word length "len" is incremented.
When the word ends (space or string end encountered), "len" is reset to 0.
"longest" keeps the current maximum, and is stored to "x_end" after the whole
text block was processed.
</p>

<p>
For box items, the minimal width is the one of the widest sub-item.
</p>

<p><pre>
# ##############
 
# +------+     #
# |header|     #
 
# **           #
# **           #
 
# +-------+    #
# |heading|    #
 
# **           #
# **           #
 
# +----------+ #
# |emphasized| #
 
# **           #
# **           #
 
# +--------+   #
# |starting|   #
 
# +------------#
# |parameters);#
 
# +------+     #
# |single|     #
   ^           ^x_end
   0
</pre></p>

<p>
As all sub-items are processed before the parent, we already know the widths of
the sub-items when processing a box item. We simply go trough all immediate
children (start with "first_child" and go on by "next") and look for the
maximum.
</p>

</a>    <!--calcWidth-->

<a name="assignWidth" id="assignWidth">

<h3>assign_width()</h3>

<p>
The second pass assigns the x-coordinates (x_start and x_end) to all items.
(Presently, this is trivial: all items have the same coordinates as their
parent, which is the global item...) For text items, the positions of line
breaks are also calculated.
</p>

<h4>Traversing Item Tree Top to Bottom</h4>

<p>
In this pass, the tree is traversed top to bottom, as the coordinates of the
sub-items depend on the coordinates of the parent. This is a bit more
complicated than traversing bottom to top, as there is no equivalent of the
"list_next" pointer for this.
</p>

<p>
If the current item has children, we proceed with the first child. (Descend.)
</p>

<p><pre>
                                                         &lt;+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+&lt;==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
       cur_item xx>    +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
    new cur_item    |  +-----+=|===========|=>+-----+==>NULL
          |         |    x ^   |           |    x ^
  xxxxxxxx|xxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++|+++++++++++++++++   |    x  ++++++++++++
  v  +    |    +    |          |    v  +   |
+------+&lt;-'+------+ |          |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL
</pre></p>

<p>
If the current item has no children, we go to the "next" item.
</p>

<p><pre>
                                                         &lt;+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+&lt;==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ | &lt;--      |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x  ^xx     x                      x
  v          v                      v
  NULL       NULL                   NULL
</pre></p>

<p>
If there is no "next" item (we are already at the last item of this depth), we
have to ascend before we can go to the next item.
</p>

<p><pre>
                                                         &lt;+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+&lt;==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                --> ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ | &lt;xx      |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL
</pre></p>

<p><pre>
                                                         &lt;+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+&lt;==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+         -->    +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ | &lt;xx      |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL
</pre></p>

<p>
If we still do not have a "next" item after ascending, we ascend again.
</p>

<p><pre>
                                                         &lt;+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+&lt;==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+         -->    +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ |          |  +------+ |  &lt;xx
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL
</pre></p>

<p><pre>
                                                         &lt;+++
                                                            +
                                                         +-----+
                                                  --> ,->| box |-->NULL
                                                      |  +-----+&lt;==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ |          |  +------+ |  &lt;xx
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL
</pre></p>

<p>
After all items have been processed, we'll ascend until we get back to the top
item, which's "next" pointer points back to itself; Thus we stay at the tree
top after following "next", and the main loop is terminated.
</p>

<p><pre>
                                                         &lt;+++
                                                            +
                                                         +-----+
                                                  --> ,->| box |-->NULL
                                                      |  +-----+&lt;==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ |          |  +------+ |  &lt;xx
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL
</pre></p>

<h4>Assigning x-Coordinates</h4>

<p>
When processing an item, not the coordinates of "cur_item" are assigned, but
the coordinates of its *sub-items*. "cur_item" has had its coodinates assigned
while its parent was processed. This is an exact reversal of <a
href="#calcWidth">calc_width()</a>: Instead of "cur_item" getting
its size from its sub-items, the sub-items get their size from "cur_item".
</p>

<p>
The coordinates for the global item are assigned before calling assign_width().
x-start is set to 0, x-end is set to the output width passed from main(). (The
screen width when cfg.term_width is set; otherwise, either a constant value (in
--dump mode) or the maximum of screen width and page width.) Play around with
them if you like. (If set to a narrow box, you can see the word breaking work.)
</p>

<p><pre>
##################################################################################
 
#------+                                                                         #
#header|                                                                         #
 
#*                                                                               #
#*                                                                               #
[...]
#------------+                                                                   #
#parameters);|                                                                   #
 
#------+                                                                         #
#single|                                                                         #
 ^x_start=0                                                                      ^x_end=80
</pre></p>

<p>
For box (and form) items, the coordinates of all immediate children are simply
set to the ones of the box. (This will have to change at some point, but it's
ok for now...) It's done in a simple loop processing the children by
"first_child" and "next", just as in
<a href="#calcWidth">calc_width()</a>.
</p>

<p><pre>
##################################################################################
 
+--------------------------------------------------------------------------------+
|header text                                                                     |
 
**********************************************************************************
*                                                                                *
[...]
+--------------------------------------------------------------------------------+
|this very long second paragraph contains some special characters (including a   |
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  |
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the|
|only tag with parameters); and finally a blank row                              |
 
+--------------------------------------------------------------------------------+
|(a single tag)                                                                  |
</pre></p>

<p>
Blank items have no children. Nothing has to be done. The same for anchor items.
</p>

<p>
Text items have no children, too; but they need to be broken into lines. (This
could be done in a seperate pass...)
</p>

<a name="lineBreaking" id="lineBreaking">

<h4>Breaking String into Lines</h4>

<p>
The breaking into lines is a bit complicated, because netrik has the ability to
break words that do not fit on a single line. And: It does this smartly,
avoiding line breaks whenever possible.
</p>

<p>
The line breaking does not alter the string structure itself. The only output
is "line_table[]" (containing the positions of all line breaks inside the
string), and its length (The number of line breaks), which is stored as the
height of the text item in "y_end".
</p>

<p>
Before starting, some constants are calculated: The width of the text block is
the difference of x_end and x_start. The pointer to the start of the text
string is taken from the string structure and stored in "string_start".
</p>

<p>
The text block is processed word-wise. In each iteration of the outer loop one
word is processed. (There are some exceptions though for extremely long words;
this will be explained in a moment.)
</p>

<p>
First, "word_end" is retrieved. It points to the space, newline or string
terminator that ends the word to be processed. "next_word_start" is set to
"word_end+1", assuming that we will proceed with the next word (which starts
after the space); however, this may by altered later.
</p>

<p><pre>
                     line_table[0]
                     v
string: "I am a very simple and stupid example sentence."
         ^           ^      ^  ^^
         s           l      w  en
string_start  line_start        next_word_start
                               word_end
                            word_start
output:
 x_start     x_end
 |&lt;- width ->|
 v    12     v
+------------+
|I am a very |
|simple and stupid example sentence.
 ^      ^  ^^
 l      w  en
</pre></p>

<p>
Then we test if the whole word fits into the current open line (which's
starting position is stored in "line_start"). 
</p>

<p><pre>
                     v
string: "I am a very simple and stupid example sentence."
         ^           ^      ^  ^^^ 
         s           l      w  enb 
                                 line_start+width (line end/Break position)
                     |&lt;- width ->|
                         (12)
+------------+
|I am a very |
|simple and stupid example sentence.
 ^      ^  ^^^ 
 l      w  enb 
            e&lt;b -> no wrap
</pre></p>

<p>
If it does, we do nothing, and simply proceed with the next word. "word_start"
is set to "next_word_start" before the next iteration.
</p>

<p><pre>
                     v
string: "I am a very simple and stupid example sentence."
         ^           ^          ^
         s           l          n
                                ^
                                w
</pre></p>

<p>
Otherwise, we have to generate a line wrap, and put the word on a new line.
</p>

<p><pre>
                     v
string: "I am a very simple and stupid example sentence."
         ^           ^          ^^    ^^
         s           l          wb    en
+------------+
|I am a very |
|simple and stupid example sentence.
 ^          ^^    ^^
 l          wb    en
              e>=b -> wrap
</pre></p>

<p>
Normally, we simply wrap the line at the current word, by setting "line_start"
to "word_start", and adding a line break at this position to "line_table[]".
</p>

<p><pre>
                                line_table[1]
                     v          v
string: "I am a very simple and stupid example sentence."
         ^                      ^     ^^
         s                      w     en
                                ^
                                l
+------------+
|I am a very |
|simple and  |
|stupid example sentence.
 ^     ^^
 w     en
 ^
 l
</pre></p>

<p>
However, things are more complicated due to the ability of breaking too long
words. We have to decide whether we simply put the word on a new line as
described above, or if it's necessary/better to break it at the line end. And
this is the tricky part.
</p>

<p>
We can't just break all words, as it would cause many really unnecessary
breaks, which is really ugly and hard to read.
</p>

<p><pre>
+------------+
|Having aRea\|
|llyLongWord |
|followed by |
|anotherLong\|
|Word, I am  |
|quite inter\|  &lt;--
|esting.     |
+------------+
</pre></p>

<p>
Wrapping before every word that doesn't fit on the line end is also not
optimal:
</p>

<p><pre>
+------------+
|Having      |  &lt;--
|aReallyLong\|
|Word        |  &lt;--
|followed by |
|anotherLong\|
|Word, I am  |
|quite       |
|interesting.|
+------------+
</pre></p>

<p>
There is lots of space wasted, and it looks quite ugly.
</p>

<p>
A better solution is putting the beginning of a word on the current line always
as long as this doesn't introduce an additional (unnecessary) word break:
</p>

<p><pre>
+------------+
|Having aRea\|  &lt;--
|llyLongWord |
|followed by |
|anotherLong\|
|Word, I am  |
|quite       |  &lt;--
interesting. |
+------------+
</pre></p>

<p>
(The first arrow shows a case where the beginnig of the word was put on the
line end to better fill the space, the second one a case where this is not done
to avoid an unnecessary word break.)
</p>

<p>
The Problem here is to decide whether we can put the beginning of the word on
the line end without introducing an additional line break. For this, we
truncate the word, chopping off as many whole line widths ("width-1", because
of the break chars) from the end of the word as possible, and test if the
remaining part fits on the current line.
</p>

<p><pre>
                v      v       v        v
string: "I am a second example sentence with aVeryLongWord in me."
                                        ^    ^ ^    ^     ^^
                                        l    w t    b     en
                                               trunc_word_end=word_end-(width-1)
                                               |&lt;-      ->|
                                               width-1 (11)
+------------+
|I am a      |
|second      |
|example     |
|sentence    |
|with a\     |
 ^    ^ ^    ^
 l    w t    b
         t&lt;=b -> fill line
 
|VeryLongWord| (truncated part)
             ^
             e
</pre></p>

<p>
Or:
</p>

<p><pre>
         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                        ^    ^      ^ ^          ^^
                                        l    w      b t          en
+------------+
|I am a third|
|example     |
|sentence,   |
|with anEvenAB\
 ^    ^      ^ ^
 l    w      b t
             t>b -> put word on new line
 
|itLongerWord|
             ^
             e
</pre></p>

<p>
For a longer word, the truncation is done more than once:
</p>

<p><pre>
                      v       v         v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                        ^    ^   ^  ^       ^          ^          ^^
                                        l    w   T  b       t          t          en
                                                 (final)
+------------+
|I am another|
|example     |
|sentence,   |
|with aTe\   |
 ^    ^   ^  ^
 l    w   T  b  t&lt;=b -> fill line
 
|rriblyLongW\|
             ^
             t
|ordThatDoes\|
             ^
             t
|n'tSeemToEnd in me.
             ^^
             en
</pre></p>

<p>
or:
</p>

<p><pre>
                      v
string: "This example has again aTerriblyLongWordThatDoesn'tSeemToEnd in some other sentence."
                      ^         ^ ^ ^          ^          ^          ^^
                      l         w b T          t          t          en
+------------+
|This example|
|has again aTe\
 ^         ^ ^ ^
 l         w b T
              t>b -> don't fill
</pre></p>

<p>
This is done in a loop. We also could do it by a modulo division, but
evaluating a quite complicated expression is probably less efficient than a
simple loop, which is entered only in exceptional cases anyways. (Words wider
than a line are not very common, after all...) Also, it is easier to
understand, isn't it?...
</p>

<p>
Note that this only takes effect for words that actually need to be broken --
words shorter than the line width will never be truncated; the remaining part
will still not fit on the line, so the word is just put on the new line as
described above.
</p>

<p>
If we decide on the first way, we fill up the current line with the beginning
of the word. (The last char is kept empty for the word break indicator
displayed in the output.) The "line_start" is set accordingly, so that the rest
of the word will be put on the next line.
</p>

<p><pre>
                v      v       v        v          v
string: "I am a second example sentence with aVeryLongWord in me."
                                             ^     ^^     ^^
                                             w     lb     en
+------------+
|I am a      |
|second      |
|example     |
|sentence    |
|with aVeryL\|
|ongWord in me.
 ^      ^^
 l      en
</pre></p>

<p>
If the second way was chosen, things are quite easy: The line wrap is inserted
at the beginning of the word. Again, this is the same case as for short words
that aren't broken at all -- there is no extra handling for this.
</p>

<p><pre>
         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^                   ^^
                                             w                   en
                                             ^
                                             l
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^                   ^^
 l                   en
</pre></p>

<p>
Now we need to test if the remainder fits on the new line; if it does not, we
have to adjust "next_word_start" to make sure processing in the next loop
iteration will not continue with the next word, but with the part of the
current word that does not fit on the new line.
</p>

<p><pre>
         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^           ^       ^^
                                             l           b       en
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^           ^       ^^
 l           b       en
                e>=b -> more breaks necessary
</pre></p>

<p><pre>
         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^          ^^        
                                             l          nb        n
                                                                  (old value)
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^          ^^        
 l          nb        n
</pre></p>

<p>
This way we will scan for the word end again in the next iteration; but
starting with the part of the word that does not fit on the new line. (This is
unefficient of course, but this case is quite rare, and it's not worth adding
special code for handling this.)
</p>

<p><pre>
         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^          ^        ^^
                                             l          w        en
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^          ^^       ^^
 l          wb       en
</pre></p>

<p>
As the new line is already filled up with the previous word part, a line break
will always be inserted just in front of the remainder, and the remainder will
be put into another line.
</p>

<p><pre>
         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^          ^^       ^^
                                             l          wb       en
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^          ^^       ^^
 l          wb       en
                e>b -> wrap
</pre></p>

<p><pre>
         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                                        ^       ^^
                                                        wb       en
                                                        ^
                                                        l
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitL\|
|ongerWord in me."
 ^        ^^
 l        en
</pre></p>

<p>
Of course it's also possible that the remainder still doesn't fit on a line,
and has to be broken again.
</p>

<p><pre>
                      v       v         v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                   ^          ^^                  ^^
                                                   l          wb                  en
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWordThatDoesn'tSeemToEnd in me.
 ^          ^^                  ^^
 l          wb                  en
                   e>b -> wrap
</pre></p>

<p><pre>
                      v       v         v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                   ^          ^^       ^          ^^
                                                   l          wb       t          en
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWordThatDoes\|
 ^          ^^       ^
 l          wb       t
                 t>b -> don't fill
</pre></p>

<p><pre>
                      v       v         v          v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                              ^          ^^       ^
                                                              w          nb       e
                                                              ^
                                                              l
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWord\|
|ThatDoesn'tSeemToEnd in me.
 ^          ^^       ^
 l          nb       e
</pre></p>

<p>
This will be repeated, until the whole word is stored.
</p>

<p><pre>
                      v       v         v          v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                              ^          ^^       ^^
                                                              l          wb       en
                                                                            e>b -> wrap
</pre></p>

<p><pre>
                      v       v         v          v          v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                                         ^        ^^
                                                                         w        en
                                                                         ^
                                                                         l
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWord\|
|ThatDoesn't\|
|SeemToEnd in me.
 ^        ^^
 l        en
</pre></p>

<p>
Of course, a line break is always inserted in front of the word no matter if it
would fit on the old line, if the word follows a newline character.
</p>

<p>
Implementing this without considerable bloat and slowdown requires a little
trick: We test for the newline after normally scanning for the word end. Now if
there is a newline, we just set "word_end" to "word_start+width" -- we just
pretend that the current word is exactly as long as the line, so it will always
be put on a new line.
</p>

<p><pre>
                v      v       v        v      v      
string: "I am a stupid example sentence with a newline/in me."
         ^                                     ^       ^ ^^^
         s                                     l       w enb
+------------+
|I am a      |
[...]
|newline/in me.
 ^       ^ ^^^
 l       w enb
        ^newline in front of word
</pre></p>

<p><pre>
                v      v       v        v      v      
string: "I am a stupid example sentence with a newline/in me."
         ^                                     ^       ^  ^^        ^
         s                                     l       w  nb        e
+------------+
|I am a      |
[...]
|newline/in me.
 ^       ^  ^^        ^
 l       w  nb        e
                e>b -> wrap
</pre></p>

<p><pre>
+------------+
|I am a      |
[...]
|newline/    |
|in me.
 ^  ^        ^
 w  n        e
 ^
 l
</pre></p>

<p>
Note that "next_word_start" is *not* modified; this is important to ensure that
processing in the next iteration will continue with the following word
normally!
</p>

</a>    <!--lineBreaking-->
</a>    <!--assignWidth-->

<h3>calc_ywidth()</h3>

<p>
The third pass is simple again. calc_ywidth() calculates minimal heights
(y-widths) for all items.
</p>

<p>
Presently this is complete overkill; assign_ywidth() doesn't really need this.
However, it will be necessary as soon as interesting elements are
implemented (tables) -- which hopefully won't be too long now...
</p>

<p>
Like <a href="#calcWidth">calc_width()</a>, it traverses the tree
bottom to top, and for every item it stores the minimal height to "y_end".
</p>

<p>
Blank lines always have the height of 1. Anchor items have no height. (They are virtual...)
</p>

<p>
Text items have their height assigned already in
<a href="#assignWidth">assign_width()</a>, while <a
href="#lineBreaking">Breaking String into Lines</a>.
</p>

<p><pre>
##################################################################################
 
+--------------------------------------------------------------------------------+___
|header text                                                                     |__0
+--------------------------------------------------------------------------------+  1&lt;--y_end
 
**********************************************************************************___
*                                                                                *__0
**********************************************************************************  1&lt;--
[...]
+--------------------------------------------------------------------------------+___
|this very long second paragraph contains some special characters (including a   |  0
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  |  1
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the|  2
|only tag with parameters); and finally a blank row                              |__3
+--------------------------------------------------------------------------------+  4&lt;--
 
+--------------------------------------------------------------------------------+___
|(a single tag)                                                                  |__0
+--------------------------------------------------------------------------------+  1&lt;--
</pre></p>

<p>
The height of a box item is calculated by summing up the height of all its
sub-items. This is done by "first_child" and "next", just as seeking the widest
sub-item in <a href="#calcWidth">calc_width()</a>.
</p>

<p><pre>
##################################################################################
 
+--------------------------------------------------------------------------------+
|header text                                                                     |
+--------------------------------------------------------------------------------+ &lt;--  1
 
********************************************************************************** 
*                                                                                * 
********************************************************************************** &lt;-- +1
[...]                                                                                 [...]
+--------------------------------------------------------------------------------+ 
|this very long second paragraph contains some special characters (including a   | 
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  | 
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the| 
|only tag with parameters); and finally a blank row                              | 
+--------------------------------------------------------------------------------+ &lt;-- +4
 
+--------------------------------------------------------------------------------+ 
|(a single tag)                                                                  | 
+--------------------------------------------------------------------------------+ &lt;-- +1
 
################################################################################## &lt;## 13
                                                                                   item_tree->y_end
</pre></p>

<a name="assignYwidth" id="assignYwidth">

<h3>assign_ywidth()</h3>

<p>
The fourth pass is also fairly simple. It assigns y-coordinates to all items.
There is no initialisation necessary before calling it, as the y_end of the
global item is already set by calc_ywidth(), and y_start is always 0.
</p>

<p>
Like in <a href="#assignWidth">assign_width()</a>, the tree is
traversed top to bottom, and the sizes of all sub-items are assigned while
processing the parent.
</p>

<p>
Thus, text and blank items do not need any processing -- they do not have any
children.
</p>

<p>
For box items, the coordinates of all sub-items are assigned in a loop, like in
<a href="#assignWidth">assign_width()</a>. Every item is put
immediately after the previous one, i.e. it starts where the previous one ends.
We keep track of the current position by "y_pos". At the beginning it is
initialized to "y_start" of the box.
</p>

<p><pre>
##################################################################################
                                                                                   0 &lt;##           &lt;== y_pos
                                                                                     item_tree->y_start
</pre></p>

<p>
For every item, y_start is set to the current y_pos.
</p>

<p><pre>
                                                                                     y_end  y_start
                                                                                        |     |
+--###############################################################################____  v     v
|header text                                                                     |___0       &lt;--
*                                                                                *___1 &lt;--   &lt;**  
|heading                                                                         |___2 &lt;**   &lt;----
*                                                                                *___3 &lt;---- &lt;****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 &lt;**** &lt;--  
|and strong text                                                                 |___5       
*                                                                                *___6 &lt;--   &lt;**  
|starting with an evil center tag,                                               |___7 &lt;**   &lt;----
|this very long second paragraph contains some special characters (including a   |   8 &lt;---- &lt;--    &lt;==
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  |           cur_item->y_start=y_pos=8
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the|
|only tag with parameters); and finally a blank row                              |
+--------------------------------------------------------------------------------+ &lt;-- cur_item->y_end=4
 
+--------------------------------------------------------------------------------+ 
|(a single tag)                                                                  | 
+--------------------------------------------------------------------------------+ &lt;----  1
 
################################################################################## &lt;## 13
</pre></p>

<p>
"y_end" of an item is determined by adding the y-size (stored in "y_end" up to
now) to "y_start" of the item.
</p>

<p><pre>
+--###############################################################################____
|header text                                                                     |___0       &lt;--
*                                                                                *___1 &lt;--   &lt;**  
|heading                                                                         |___2 &lt;**   &lt;----
*                                                                                *___3 &lt;---- &lt;****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 &lt;**** &lt;--  
|and strong text                                                                 |___5       
*                                                                                *___6 &lt;--   &lt;**  
|starting with an evil center tag,                                               |___7 &lt;**   &lt;----
|this very long second paragraph contains some special characters (including a   |   8 &lt;---- &lt;--    &lt;==
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  |   9
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the|  10
|only tag with parameters); and finally a blank row                              |__11
+--------------------------------------------------------------------------------+  12 &lt;-- cur_item->y_end=cur_item->ystart+cur_item->y_end=12
 
+--------------------------------------------------------------------------------+
|(a single tag)                                                                  |
+--------------------------------------------------------------------------------+ &lt;----  1
 
################################################################################## &lt;## 13
</pre></p>

<p>
"y_pos" is adjusted to the end of the item.
</p>

<p><pre>
+--###############################################################################____
|header text                                                                     |___0       &lt;--
*                                                                                *___1 &lt;--   &lt;**  
|heading                                                                         |___2 &lt;**   &lt;----
*                                                                                *___3 &lt;---- &lt;****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 &lt;**** &lt;--  
|and strong text                                                                 |___5       
*                                                                                *___6 &lt;--   &lt;**  
|starting with an evil center tag,                                               |___7 &lt;**   &lt;----
|this very long second paragraph contains some special characters (including a   |   8 &lt;---- &lt;--
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  |   9
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the|  10
|only tag with parameters); and finally a blank row                              |__11
+--------------------------------------------------------------------------------+  12 &lt;--          &lt;==
 
+--------------------------------------------------------------------------------+
|(a single tag)                                                                  |
+--------------------------------------------------------------------------------+&lt;----  1
 
################################################################################## &lt;## 13
</pre></p>

<p>
In the next iteration, "y_pos" -- which now points to the end of the current
item -- is used as the beginning of the new item.
</p>

<p><pre>
+--###############################################################################____
|header text                                                                     |___0       &lt;--
*                                                                                *___1 &lt;--   &lt;**  
|heading                                                                         |___2 &lt;**   &lt;----
*                                                                                *___3 &lt;---- &lt;****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 &lt;**** &lt;--  
|and strong text                                                                 |___5       
*                                                                                *___6 &lt;--   &lt;**  
|starting with an evil center tag,                                               |___7 &lt;**   &lt;----
|this very long second paragraph contains some special characters (including a   |   8 &lt;---- &lt;--
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  |   9
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the|  10
|only tag with parameters); and finally a blank row                              |__11
|(a single tag)                                                                  |  12 &lt;--   &lt;----   &lt;==
+--------------------------------------------------------------------------------+&lt;----  1
 
################################################################################## &lt;## 13
</pre></p>

<a name="linkCoords" id="linkCoords">

<h4>Link Coordinates</h4>

<p>
Coordinates of links and anchors (both x and y!) are presently also assigned
here. Probably it would be a better idea to do that in extra pass... As soon as
the current link implementation is dropped and anchors are cleanly implemented,
the assignment can be cleanly and logically split between assign_xwidth() and
assign_ywidth(), just as for other item types.
</p>

<p>
Link coordinates are assigned by the parent text item. The text block is
scanned, one link after the other, for the lines containing the link start and
end. (The first line that ends after the link start, and the first line that
ends at/after the link end.) As an optimization, the search for the next link
is started in the line where the previous ended, not from beginning -- links
can't be nested.
</p>

<p>
After having the line, (and thus also the y coordinate), the x coordinate is
calculated by adding the link's relative position inside the line
(link[].start-line_start) to the x coordinate of the line start. (Which is
equal to the item start for normal text, but has to be calculated seperately
for every line in centered text items.)
</p>

<p>
Inline anchors are very similar; however, they get their coordinates while
processing the anchor item, not the text item containing them. (The text item
doesn't know anything about the anchors.) Anchors can be nested if created by
&lt;span> or so; thus the above optimization isn't possible. (It would be harder
anyhow, due to the anchors being processed every one on its own.)
</p>

<p>
Block anchors need another processing, of course: If they are empty, they keep
the coordinates assigned to them by the parent box; otherwise, all (immediate)
virtual children are scanned for the minimum/maximum for each of the four
coordinates; these are assigned as the coordinates of the anchor virtual box.
</p>

</a>    <!-- linkCoords -->

</a>    <!-- assignYwidth -->

<a name="createMap" id="createMap">

<h3>create_map()</h3>

<a name="pageMap" id="pageMap">

<p>
The last sub-pass generates the "page_map[]". (This could be also done inside
assign_ywidth()...) This map is necessary to quickly determine which elements
show up in the visible area of the page, when it is displayed in the viewer.
</p>

<p>
The page usage map stores references to all items that show up in any given
line of the output page. This is a very simple approach, but it should be
perfectly sufficient as long as netrik has no full graphic mode -- and this
will be probably for quite a while...
</p>

<p>
"page_map[]" is an array containing an "Item_list" structure for every line of
the output page. This structure is declared in items.h . It contains a count of
items in this line, and an array of pointers to the items.
</p>

</a>    <!--pageMap-->

<p>
The whole line map is allocated at the beginning. Afterwards, all items are
processed in a loop. For every item that is visible on the screen (presently
this are only text items), a reference to this item is stored to every line of
"page_map[]" between "y_start" and "y_end" of the item.
</p>

<p><pre>
                                                                                      line|page_map[line]
+--###############################################################################__  ----+--------------
|header text                                                                     |__    0 | t0
*                                                                                *__    1 | -
|heading                                                                         |__    2 | t1
*                                                                                *______3_|_-____
|first paragraph of text; includes multiple spaces and newlines, emphasized text |      4 | t2  &lt;-- cur_item->y_start
|and strong text                                                                 |______5_|_t2___
*                                                                                *__    6 | -   &lt;-- cur_item->y_end
|starting with an evil center tag,                                               |__    7 | -
|this very long second paragraph contains some special characters (including a   |      8 | -
|simple space...): &amp;; &lt;>"=/ plus a big gap###and two unicode escapes (decimal:  |      9 | -
|and hexal: ) but also an anchor embedded inside a word (this anchor also is the|     10 | -
|only tag with parameters); and finally a blank row                              |__    1 | -
|(a single tag)                                                                  |__   12 | -
###############################################################################--+  
</pre></p>

<p>
"page_map[]" is returned to pre_render(), and from there to main(), where it is
passed to <a href="#dump">dump()</a> or to
<a href="#render">render()</a> (via
<a href="hacking-pager.html#display">display()</a>, see
<a href="hacking-pager.html">hacking-pager.*</a>) along with "item_tree".
</p>

</a>    <!--createMap-->

<a name="freeMap" id="freeMap">

<h3>free_map()</h3>

<p>
This function frees the memory allocated for the page map.
</p>

<p>
First it goes through the table line by line, and frees the associated "list"
for each one. Afterwards, it frees the table itself.
</p>

</a>    <!--freeMap-->

</a>    <!--preRender-->

<a name="renderC" id="renderC">

<h2>8. render.c</h2>

<p>
With the item tree and the page usage map prepared in pre_render(), we can now
actually render the page. There are two different rendering functions:
</p>

<p>
dump() renders the whole page and dumps the output to the terminal. The output
is layouted correctly, using all the coordinates and text attributes.
</p>

<p>
render() renders only a specified area of the page, and outputs it to the
curses screen.
</p>

<a name="dump" id="dump">

<h3>dump()</h3>

<p>
The page is rendered line by line, using
"<a href="#pageMap">page_map[]</a>" (see
<a href="#createMap">create_map()</a> above) to determine which
items we need to render in every line. Of course this isn't really necessary,
as we do not ever have more than one item in a line presently. However, using
"page_map[]" here is *not* overkill, just for a change ;-) On the contrary,
this is a pragmatic approach. Dumping the page item by item without using
"page_map[]" would be more efficient; however, it would be also more
complicated than dumping line by line.
</p>

<p>
In each line, we process all items (from "page_map[line]") one after the other.
If there was actually more then one item (which is impossible presently...),
they would be printed one after the other -- with disasterous results... No
code is implemented for really handling this situation yet.
</p>

<p>
First action to do for each item is setting the cursor position to the
beginning of the text of this line. (Retrieved by
<a href="#linePos">line_pos()</a> and stored in "x_start".) This
we do by going forward as much character positions as necessary, by printing
that many space characters.
</p>

<p>
Now we can output the text itself. To know what is actually to be printed in
this line, the start and end positions of this line's text inside the string
(text block) are retrieved with <a href="#lineStartEnd">line_start() and
line_end()</a>. (And stored in "text_start" and "text_end".)
</p>

<p>
Having this, we print all attribute divisions in a loop. But first we have to
find the first one that shows up in the current line.
</p>

<p><pre>
                                               div[0].end     ...    div[3].end
                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
       |                  ^                    |                         ^
                          text_start                                   text_end
</pre></p>

<p>
In every iteration we print all the text between "div_start" end "div_end",
which normally point to the start and end of the current division. For the
first iteration, "div_start" is set to "text_start" -- we only want the part of
the div that acutally shows up in the line.
</p>

<p><pre>
                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
                          ^                    ^                         ^
                       div_start=text_start    div_end=div[0].end      text_end
 
output:
first paragraph of text; contains multiple
spaces and newlines,                               &lt;-- line
</pre></p>

<p>
The next div starts where the current one ends.
</p>

<p><pre>
                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
                          ^                    ^               ^         ^
                          text_start           div_start       div_end text_end
</pre></p>

<p>
The last division is truncated to "text_end" -- as with the first one, we only
want the part that shows up in the current line.
</p>

<p><pre>
                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
                          ^                                    ^         ^
                                                       div_start       div_end
                                                                      =text_end
 
output:
first paragraph of text; contains multiple
spaces and newlines, emphasized text and strong
</pre></p>

<p>
Before actually printing the text, it is copied into a temporary string, but
replacing all &amp;nbsp; characters by normal spaces. (There are various problems
resulting from putting real '\xa0' characters on the screen: For one, with
fonts where this char isn't really blank (always the case if the charset isn't
iso-8859-x) it's unusable. Also, copying via screen/GPM/X clipboard usually has
undesired results.)
</p>

<p>
Finally, we test if the line ends with a word break, and print the break
character if it does. We know it does when the character at the line end is a
word character (not a space, newline, or string end), because every word end is
followed by the space separating it from the next word; if there is no space at
the line end, we are inside a wrapped word.
</p>

<p><pre>
                    v                     v
text "Some sentence containing aVeryLongAndThusBrokenWord."
                                          ^
output:
Some sentence
containing aVeryLongAn\
...
dThusBrokenWord.
</pre></p>

</a>    <!--dump-->

<a name="render" id="render">

<h3>render()</h3>

<p>
render() works similar to <a href="#dump">dump()</a>. The main difference is how the output is
printed. However, there are a couple of differences in screen position handling
and other calculations also.
</p>

<p>
render() takes the starting positon of the rendered area inside the page, the
starting position on the screen, and the size of the rendered area as
arguments.
</p>

<p>
The area is processed line by line, and every line is processed item by item,
just like in dump().
</p>

<p>
"x_start" describes the starting column relative to the beginning of the
rendered area, not the screen. (dump() always dumps whole lines, and thus there
is no difference.) If it turns out that the line starts before the rendered
area, it has to be truncated. the ending position "x_end" is calculated in a
similar fashion.
</p>

<p>
Before the line is printed (div by div, as in dump()), the cursor is set to the
start position of the line. The column is the starting position relative to the
area ("x_start"), plus the starting position of the area on the screen;
likewise the row.
</p>

<p>
Before anything is printed, we test if some part of the line shows up inside
the rendered area at all. The word break indicator is also printed only if the
line ends inside the area.
</p>

<p>
If render() was called with the "overpaint" flag, the requested area is cleaned
before rendering anything, so any garbage will be removed. (Areas not
containing any text aren't affected otherwise.) This is done by overwriting the
desired part of each line with a string of spaces.
</p>

</a>    <!--render-->

<a name="dumpItems" id="dumpItems">

<h3>dump_items()</h3>

<p>
dump_items() dumps the item tree, including the text of text items. The text is
printed with correct attributes, but ignoring any coordinates and line breaks.
This function is for debugging purposes, and may be called anywhere inside or
after pre_render() (anywhere after parse_struct()).
</p>

<p>
The reason this function is in render.c is that it needs the same screen
handling functions as <a href="#dump">dump()</a>. Moreover, it
works in a very similar fashion.
</p>

<p>
The difference is that it does not dump line by line, but item by item
(traversing the tree top to bottom). After printing some information about the
item itself, it dumps the text division by division in the same way dump()
does, only it doesn't need to care about positions or line breaks; it dumps the
whole string at once.
</p>

</a>    <!--dumpItems-->

</a>    <!--renderC-->

<a name="itemsC" id="itemsC">

<h2>10. items.c</h2>

<p>
items.c contains a few simple helper functions intended to simplify retrieving
some common date from the structure (item) tree.
</p>

<p>
The advantage of using such helper functions, even if they are really simple,
is less code duplication -- which improves maintainability and probably makes
the code also easier to understand. (This is actually an approach toward
so-called object oriented programming...) On the other hand, these functions
are extremly inefficient, as they calculate intermediate values which could be
shared, and the calling overhead itself is fairly big for such simple
functions. (Effectively probably increasing code size.) Maybe we should try to
define them as macros, or just put them into an include file so they can be
inlined during optimized compilation. (Let the compiler decide...)
</p>

<a name="lineStartEnd" id="lineStartEnd">

<h3>line_start() and line_end()</h3>

<p>
line_start() and line_end() are used to find out at which position some
specific text line starts/ends inside the string of a (wrapped) text block.
</p>

<p>
The positions are read from the line_table[] (see
<a href="lineBreaking">Breaking String into Lines</a>). This is
normally trivial, but there are exceptions for the first and last lines.
</p>

<p>
line_end() has an additional quirk: If the line ends with a blank, the line end
position is decremented -- when a line wraps at a blank, this blank is always
discarded.
</p>

<p>
Note that the line number given to these functions is *not* the line number
relative to the start of the text block, but the page line.
</p>

</a>    <!-- lineStartEnd -->

<a name="linePos" id="linePos">

<h3>line_pos()</h3>

<p>
The horizontal page position of a single text line can be retrieved using
line_pos(). This is useful because of centered text items, where the line
starts do not equal the text block start, and differ from line to line.
</p>

<p>
The line number argument is in page coordinates just as in
<a href="#lineStartEnd">line_start() and line_end()</a> above.
</p>

</a>    <!-- linePos -->

</a>    <!-- itemsC -->

</body>
</html>