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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
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<TITLE>3.5 Implementing Text Indentation</TITLE>
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<A HREF="compile.html"><FONT COLOR="#FFFFFF">1 Compile</FONT></A><BR>
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<A HREF="implement.html"><FONT COLOR="#FFFFFF">3 Implement</FONT></A><BR>
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<P><FONT SIZE="5">3.5 Implementing Text Indentation</FONT><BR>
<P>At one point during the development of FunnelWeb, text
indentation was fully implemented. However, it was
subsequently removed because it was considered a dangerous
feature. This section records the way in which text
indentation was implemented so that if the feature ever has
to be put back, this technique can be used again.
<P>1. Create a new field in the <SAMP>sc_t</SAMP> record call
<SAMP>sc_postn</SAMP>.
<P>
<PRE>
char *sc_postn; /* Pointer in the range [sc_first,sc_last+1]. */
/* It is the minimum possible value of sc_postn for */
/* which EOL does not appear in *sc_postn..*sc_last. */
/* i.e. Points to the byte following the first EOL in */
/* the scrap or sc_first if EOL does not appear. */
</PRE>
<P>2. Modify the scanner so that it generates this field.
Sendtext should be modified so that
it accepts an argument for the <SAMP>p_postn</SAMP> value.
<P>
<PRE>
LOCAL void sendtext P_((p_ps_t,char *,char *,char *,bool));
LOCAL void sendtext(p_tkps,p_first,p_last,p_postn,is_white)
/* Appends a text token to the end of the token list. */
/* IN: p_ps is a pointer to a position structure giving the position of the */
/* first character of the token. */
/* IN: p_first and p_last point to the first and last byte of the text scrap. */
/* IN: p_postn has the same definition as sc_postn (see fwdata.h). */
/* IN: is_white should be set to TRUE iff scrap is entirely whitespace. */
p_ps_t p_tkps;
char *p_first;
char *p_last;
char *p_postn;
bool is_white;
{
tk_t token;
/* Empty text scraps should never be generated. */
assert(p_first<=p_last,"sendtext: Text scrap bounds are bad.");
/* If ch=EOL then we should be scanning more text, not shipping it! */
assert(ch!=EOL,"senttext: Shipping text while still more to scan.");
/* Check that p_postn is in range. See definition in fwdata.h. */
assert(p_first<=p_postn && p_postn<=p_last+1,
"sendtext: p_postn is out of range.");
/* Debug: Check the p_postn field using a brute force check. */
{
char *i,*j;
j=p_first;
for (i=p_first;i<=p_last;i++)
if (*i==EOL)
j=i+1;
assert(j==p_postn,"sendtext: sc_postn field is incorrect.");
}
/* Load the text token. */
token.tk_kind = TK_TEXT;
ASSIGN(token.tk_ps,*p_tkps);
token.tk_sc.sc_first = p_first;
token.tk_sc.sc_last = p_last;
token.tk_sc.sc_postn = p_postn;
token.tk_white = is_white;
token.tk_parno = 0;
ls_add(token_list,PV &token);
}
</PRE>
<P>Then all the calls to sendtext have to be changed:
<PRE>
/* @ instructs FunnelWeb to replace the special construct with the */
/* special character. Luckily one appears just before the @ !! */
/* Note: FALSE is OK because space is not a legal specialch. */
/* Note: Setting parameter p_postn to p_ch-1 is OK as EOL is not a */
/* legal specialch. */
sendtext(ps_spec,p_ch-1,p_ch-1,p_ch-1,FALSE);
break;
/* + instructs FunnelWeb to insert an EOL. We can't look to the end of */
/* the previous line to find an EOL as this might be the first line. */
/* Running ahead to the end of the line is expensive, and having the */
/* liner mini-package maintain a variable for it would be extra */
/* housekeeping. Instead of all this, we just point to a static. */
{CONST static char stateol = EOL;
sendtext(&ps_spec,&stateol,&stateol,(&stateol)+1,TRUE);}
break;
/* If we hit something that ends a text token */
/* then we can transmit a white text token. */
if (ch==specialch || ch==EOFCH)
{sendtext(&ps_start,p_first,p_ch-1,MAX(p_sol,p_first),TRUE); return;}
/* Otherwise we have some more (non-white) text to scan. */
/* We can then send a non-white text token. */
while (ch!=specialch && ch!=EOFCH)
NEXTCH;
sendtext(&ps_start,p_first,p_ch-1,MAX(p_sol,p_first),FALSE);
</PRE>
The dump code needs to be changed too!
<PRE>
wf_str(p_wf,"\"");
assert(token->tk_sc.sc_first !=NULL,"dm_tkls: NULL ptr1.");
assert(token->tk_sc.sc_last !=NULL,"dm_tkls: NULL ptr2.");
for (i=token->tk_sc.sc_first; i<=token->tk_sc.sc_last; i++)
{
if (i==token->tk_sc.sc_postn)
wf_str(p_wf,"<postn>");
if (*i=='\n')
wf_wl(p_wf,"");
else
dm_byte(p_wf,*((ubyte_ *) i));
}
if (i==token->tk_sc.sc_postn)
wf_str(p_wf,"<postn>");
wf_str(p_wf,"\"");
}
</PRE>
<P>3. Over in the Tangle module, create a massive array of
pointers to scraps to be used as a stack. Maintain pointers
into the stack called <SAMP>current</SAMP> and <I>base</I>
(similar to the blank indentation variables). Implement the
following:
<P>
<UL>
<LI> To write out a scrap, scan it byte by byte. Output
each byte. When you hit an EOL, pop the stack back to
<SAMP>base</SAMP>. Then write out an EOL followed by the stack
contents but writing each scrap only from <SAMP>postn</SAMP> to
end end of each scrap. When you have finished the new scrap,
push it on the stack.
<LI> When you hit a new macro to expand, save
<SAMP>base</SAMP>. Restore it later.
</UL>
<P>The <SAMP>postn</SAMP> field solves the big problem of how
to cope with something like this:
<P>
<PRE>
The rain in Spain
falls mainly @<on the plain@>
</PRE>
<P>The trouble is that we want to text indent the lines in
<SAMP>@<on the plain@></SAMP> with just "<TT>falls
mainly </TT>". However, this string is only part of a scrap. The
solution is to get the scanner to record, in the
<SAMP>postn</SAMP> field of each scrap, the position of the
first byte with a EOL-free run to the end of the scrap.
<P>This scheme is very efficient because all we are doing
is pushing and popping pointers to scraps on a stack array.
The main disadvantage is that the array must necessarily be
finite and would impose a limit on the depth of indentation
nesting.
<P>
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