File: equivalents.w

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% equivalents.w
%
% Copyright 2009-2010 Taco Hoekwater <taco@@luatex.org>
%
% This file is part of LuaTeX.
%
% LuaTeX is free software; you can redistribute it and/or modify it under
% the terms of the GNU General Public License as published by the Free
% Software Foundation; either version 2 of the License, or (at your
% option) any later version.
%
% LuaTeX is distributed in the hope that it will be useful, but WITHOUT
% ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
% FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
% License for more details.
%
% You should have received a copy of the GNU General Public License along
% with LuaTeX; if not, see <http://www.gnu.org/licenses/>.

@ @c

#include "ptexlib.h"

halfword last_cs_name = null_cs;

/* |eqtb[p]| has just been restored or retained */

static void diagnostic_trace(halfword p, const char *s)
{
    begin_diagnostic();
    print_char('{');
    tprint(s);
    print_char(' ');
    show_eqtb(p);
    print_char('}');
    end_diagnostic(false);
}

@ @c
void show_eqtb_meaning(halfword n);     /* forward */

@ Now that we have studied the data structures for \TeX's semantic routines,
we ought to consider the data structures used by its syntactic routines. In
other words, our next concern will be
the tables that \TeX\ looks at when it is scanning
what the user has written.

The biggest and most important such table is called |eqtb|. It holds the
current ``equivalents'' of things; i.e., it explains what things mean
or what their current values are, for all quantities that are subject to
the nesting structure provided by \TeX's grouping mechanism. There are six
parts to |eqtb|:

\yskip\hang 1) |eqtb[null_cs]| holds the current equivalent of the
zero-length control sequence.

\yskip\hang 2) |eqtb[hash_base..(glue_base-1)]| holds the current
equivalents of single- and multiletter control sequences.

\yskip\hang 3) |eqtb[glue_base..(local_base-1)]| holds the current
equivalents of glue parameters like the current baselineskip.

\yskip\hang 4) |eqtb[local_base..(int_base-1)]| holds the current
equivalents of local halfword quantities like the current box registers,
the current ``catcodes,'' the current font, and a pointer to the current
paragraph shape.

\yskip\hang 5) |eqtb[int_base..(dimen_base-1)]| holds the current
equivalents of fullword integer parameters like the current hyphenation
penalty.

\yskip\hang 6) |eqtb[dimen_base..eqtb_size]| holds the current equivalents
of fullword dimension parameters like the current hsize or amount of
hanging indentation.

\yskip\noindent Note that, for example, the current amount of
baselineskip glue is determined by the setting of a particular location
in region~3 of |eqtb|, while the current meaning of the control sequence
`\.{\\baselineskip}' (which might have been changed by \.{\\def} or
\.{\\let}) appears in region~2.

@ The last two regions of |eqtb| have fullword values instead of the
three fields |eq_level|, |eq_type|, and |equiv|. An |eq_type| is unnecessary,
but \TeX\ needs to store the |eq_level| information in another array
called |xeq_level|.

@c
memory_word *eqtb;
halfword eqtb_top;              /* maximum of the |eqtb| */
quarterword xeq_level[(eqtb_size + 1)];

@ @c
void initialize_equivalents(void)
{
    int k;
    for (k = int_base; k <= eqtb_size; k++)
        xeq_level[k] = level_one;
}

@ The nested structure provided by `$\.{\char'173}\ldots\.{\char'175}$' groups
in \TeX\ means that |eqtb| entries valid in outer groups should be saved
and restored later if they are overridden inside the braces. When a new |eqtb|
value is being assigned, the program therefore checks to see if the previous
entry belongs to an outer level. In such a case, the old value is placed
on the |save_stack| just before the new value enters |eqtb|. At the
end of a grouping level, i.e., when the right brace is sensed, the
|save_stack| is used to restore the outer values, and the inner ones are
destroyed.

Entries on the |save_stack| are of type |save_record|. The top item on
this stack is |save_stack[p]|, where |p=save_ptr-1|; it contains three
fields called |save_type|, |save_level|, and |save_value|, and it is
interpreted in one of four ways:

\yskip\hang 1) If |save_type(p)=restore_old_value|, then
|save_value(p)| is a location in |eqtb| whose current value should
be destroyed at the end of the current group and replaced by |save_word(p-1)|
(|save_type(p-1)==saved_eqtb|).
Furthermore if |save_value(p)>=int_base|, then |save_level(p)| should
replace the corresponding entry in |xeq_level| (if |save_value(p)<int_base|,
then the level is part of |save_word(p-1)|).

\yskip\hang 2) If |save_type(p)=restore_zero|, then |save_value(p)|
is a location in |eqtb| whose current value should be destroyed at the end
of the current group, when it should be
replaced by the current value of |eqtb[undefined_control_sequence]|.

\yskip\hang 3) If |save_type(p)=insert_token|, then |save_value(p)|
is a token that should be inserted into \TeX's input when the current
group ends.

\yskip\hang 4) If |save_type(p)=level_boundary|, then |save_level(p)|
is a code explaining what kind of group we were previously in, and
|save_value(p)| points to the level boundary word at the bottom of
the entries for that group. Furthermore, |save_value(p-1)| contains the
source line number at which the current level of grouping was entered,
this field has itself a type: |save_type(p-1)==saved_line|.

Besides this `official' use, various subroutines push temporary
variables on the save stack when it is handy to do so. These all have
an explicit |save_type|, and they are:

|saved_adjust| signifies an adjustment is beging scanned,
|saved_insert| an insertion is being scanned,
|saved_disc| the \.{\\discretionary} sublist we are working on right now,
|saved_boxtype| whether a \.{\\localbox} is \.{\\left} or \.{\\right},
|saved_textdir| a text direction to be restored,
|saved_eqno| diffentiates between \.{\\eqno} and \.{\\leqno},
|saved_choices| the \.{\\mathchoices} sublist we are working on right now,
|saved_math| and interrupted math list,
|saved_boxcontext| the box context value,
|saved_boxspec| the box \.{to} or \.{spread} specification,
|saved_boxdir| the box \.{dir} specification,
|saved_boxattr| the box \.{attr} specification,
|saved_boxpack| the box \.{pack} specification.

@ The global variable |cur_group| keeps track of what sort of group we are
currently in. Another global variable, |cur_boundary|, points to the
topmost |level_boundary| word.  And |cur_level| is the current depth of
nesting. The routines are designed to preserve the condition that no entry
in the |save_stack| or in |eqtb| ever has a level greater than |cur_level|.

@c
save_record *save_stack;
int save_ptr;                        /* first unused entry on |save_stack| */
int max_save_stack;                  /* maximum usage of save stack */
quarterword cur_level = level_one;   /* current nesting level for groups */
group_code cur_group = bottom_level; /* current group type */
int cur_boundary;                    /* where the current level begins */

@ At this time it might be a good idea for the reader to review the introduction
to |eqtb| that was given above just before the long lists of parameter names.
Recall that the ``outer level'' of the program is |level_one|, since
undefined control sequences are assumed to be ``defined'' at |level_zero|.



@ The following macro is used to test if there is room for up to eight more
entries on |save_stack|. By making a conservative test like this, we can
get by with testing for overflow in only a few places.

@c
#define check_full_save_stack() do { \
    if (save_ptr>max_save_stack) { \
        max_save_stack=save_ptr; \
        if (max_save_stack>save_size-8) \
            overflow("save size",(unsigned)save_size); \
    } \
} while (0)

@ Procedure |new_save_level| is called when a group begins. The
argument is a group identification code like `|hbox_group|'. After
calling this routine, it is safe to put six more entries on |save_stack|.

In some cases integer-valued items are placed onto the
|save_stack| just below a |level_boundary| word, because this is a
convenient place to keep information that is supposed to ``pop up'' just
when the group has finished.
For example, when `\.{\\hbox to 100pt}' is being treated, the 100pt
dimension is stored on |save_stack| just before |new_save_level| is
called.

@c
void new_save_level(group_code c)
{   /* begin a new level of grouping */
    check_full_save_stack();
    set_saved_record(0, saved_line, 0, line);
    incr(save_ptr);
    save_type(save_ptr) = level_boundary;
    save_level(save_ptr) = cur_group;
    save_value(save_ptr) = cur_boundary;
    if (cur_level == max_quarterword)
        overflow("grouping levels", max_quarterword - min_quarterword);
    /* quit if |(cur_level+1)| is too big to be stored in |eqtb| */
    cur_boundary = save_ptr;
    cur_group = c;
    if (tracing_groups_par > 0)
        group_trace(false);
    incr(cur_level);
    incr(save_ptr);
}

@ @c
static const char *save_stack_type(int v)
{
    const char *s = "";
    switch (save_type(v)) {
        case restore_old_value: s = "restore_old_value"; break;
        case restore_zero:      s = "restore_zero";      break;
        case insert_token:      s = "insert_token";      break;
        case level_boundary:    s = "level_boundary";    break;
        case saved_line:        s = "saved_line";        break;
        case saved_adjust:      s = "saved_adjust";      break;
        case saved_insert:      s = "saved_insert";      break;
        case saved_disc:        s = "saved_disc";        break;
        case saved_boxtype:     s = "saved_boxtype";     break;
        case saved_textdir:     s = "saved_textdir";     break;
        case saved_eqno:        s = "saved_eqno";        break;
        case saved_choices:     s = "saved_choices";     break;
        case saved_math:        s = "saved_math";        break;
        case saved_boxcontext:  s = "saved_boxcontext";  break;
        case saved_boxspec:     s = "saved_boxspec";     break;
        case saved_boxdir:      s = "saved_boxdir";      break;
        case saved_boxattr:     s = "saved_boxattr";     break;
        case saved_boxpack:     s = "saved_boxpack";     break;
        case saved_eqtb:        s = "saved_eqtb";        break;
        default: break;
    }
    return s;
}

@ @c
void print_save_stack(void)
{
    int i;
    begin_diagnostic();
    selector = term_and_log;
    print_ln();
    for (i = (save_ptr - 1); i >= 0; i--) {
        tprint("save_stack[");
        if (i < 100)
            print_char(' ');
        if (i < 10)
            print_char(' ');
        print_int(i);
        tprint("]: ");
        tprint(save_stack_type(i));
        switch (save_type(i)) {
            case restore_old_value:
                tprint(", ");
                show_eqtb_meaning(save_value(i));
                tprint("=");
                if (save_value(i) >= int_base) {
                    print_int(save_word(i - 1).cint);
                } else {
                    print_int(eq_type_field(save_word(i - 1)));
                    print_char(',');        /* |print_int(eq_level_field(save_word(i-1)));| */
                    print_int(equiv_field(save_word(i - 1)));
                }
                i--;
                break;
            case restore_zero:
                tprint(", ");
                show_eqtb_meaning(save_value(i));
                break;
            case insert_token:
                tprint(", ");
                {
                    halfword p = get_avail();
                    set_token_info(p, save_value(i));
                    show_token_list(p, null, 1);
                    free_avail(p);
                }
                break;
            case level_boundary:
                tprint(", old group=");
                print_int(save_level(i));
                tprint(", boundary = ");
                print_int(save_value(i));
                tprint(", line = ");
                print_int(save_value(i - 1));
                i--;
                break;
            case saved_adjust:
                tprint(", ");
                print_int(save_level(i));   /* vadjust vs vadjust pre */
                break;
            case saved_insert:
                tprint(", ");
                print_int(save_value(i));   /* insert number */
                break;
            case saved_boxtype:    /* \.{\\localleftbox} vs \.{\\localrightbox} */
                tprint(", ");
                print_int(save_value(i));
                break;
            case saved_eqno:       /* \.{\\eqno} vs \.{\\leqno} */
                tprint(", ");
                print_int(save_value(i));
                break;
            case saved_disc:
            case saved_choices:
                tprint(", ");
                print_int(save_value(i));
                break;
            case saved_math:
                tprint(", listptr=");
                print_int(save_value(i));
                break;
            case saved_boxcontext:
                tprint(", ");
                print_int(save_value(i));
                break;
            case saved_boxspec:
                tprint(", spec=");
                print_int(save_level(i));
                tprint(", dimen=");
                print_int(save_value(i));
                break;
            case saved_textdir:
            case saved_boxdir:
                tprint(", ");
                print_dir(dir_dir(save_value(i)));
                break;
            case saved_boxattr:
            case saved_boxpack:
                tprint(", ");
                print_int(save_value(i));
                break;
            case saved_line:
            case saved_eqtb:
                break;
            default:
                break;
        }
        print_ln();
    }
    end_diagnostic(true);
}

@ The \.{\\showgroups} command displays all currently active grouping
  levels.

@ The modifications of \TeX\ required for the display produced by the
  |show_save_groups| procedure were first discussed by Donald~E. Knuth in
  {\sl TUGboat\/} {\bf 11}, 165--170 and 499--511, 1990.
  @^Knuth, Donald Ervin@>

  In order to understand a group type we also have to know its mode.
  Since unrestricted horizontal modes are not associated with grouping,
  they are skipped when traversing the semantic nest.

@c
void show_save_groups(void)
{
    int p = nest_ptr;           /* index into |nest| */
    int m;                      /* mode */
    save_pointer v = save_ptr;  /* saved value of |save_ptr| */
    quarterword l = cur_level;  /* saved value of |cur_level| */
    group_code c = cur_group;   /* saved value of |cur_group| */
    int a = 1;                  /* to keep track of alignments */
    int i;
    quarterword j;
    const char *s = NULL;
    save_ptr = cur_boundary;
    decr(cur_level);
    tprint_nl("");
    print_ln();
    while (1) {
        tprint_nl("### ");
        print_group(true);
        if (cur_group == bottom_level)
            goto DONE;
        do {
            m = nest[p].mode_field;
            if (p > 0)
                decr(p);
            else
                m = vmode;
        } while (m == hmode);
        tprint(" (");
        switch (cur_group) {
            case simple_group:
                incr(p);
                goto FOUND2;
                break;
            case hbox_group:
            case adjusted_hbox_group:
                s = "hbox";
                break;
            case vbox_group:
                s = "vbox";
                break;
            case vtop_group:
                s = "vtop";
                break;
            case align_group:
                if (a == 0) {
                    if (m == -vmode)
                        s = "halign";
                    else
                        s = "valign";
                    a = 1;
                    goto FOUND1;
                } else {
                    if (a == 1)
                        tprint("align entry");
                    else
                        tprint_esc("cr");
                    if (p >= a)
                        p = p - a;
                    a = 0;
                    goto FOUND;
                }
                break;
            case no_align_group:
                incr(p);
                a = -1;
                tprint_esc("noalign");
                goto FOUND2;
                break;
            case output_group:
                tprint_esc("output");
                goto FOUND;
                break;
            case math_group:
                goto FOUND2;
                break;
            case disc_group:
                tprint_esc("discretionary");
                for (i = 1; i < 3; i++)
                    if (i <= saved_value(-2))
                        tprint("{}");
                goto FOUND2;
                break;
            case math_choice_group:
                tprint_esc("mathchoice");
                for (i = 1; i < 4; i++)
                    if (i <= saved_value(-3))       /* different offset because |-2==saved_textdir| */
                        tprint("{}");
                goto FOUND2;
                break;
            case insert_group:
                if (saved_type(-1) == saved_adjust) {
                    tprint_esc("vadjust");
                    if (saved_level(-1) != 0)
                        tprint(" pre");
                } else {
                    tprint_esc("insert");
                    print_int(saved_value(-1));
                }
                goto FOUND2;
                break;
            case vcenter_group:
                s = "vcenter";
                goto FOUND1;
                break;
            case semi_simple_group:
                incr(p);
                tprint_esc("begingroup");
                goto FOUND;
                break;
            case math_shift_group:
                if (m == mmode) {
                    print_char('$');
                } else if (nest[p].mode_field == mmode) {
                    print_cmd_chr(eq_no_cmd, saved_value(-2));
                    goto FOUND;
                }
                print_char('$');
                goto FOUND;
                break;
            case math_left_group:
                if (subtype(nest[p + 1].eTeX_aux_field) == left_noad_side)
                    tprint_esc("left");
                else
                    tprint_esc("middle");
                goto FOUND;
                break;
            default:
                confusion("showgroups");
                break;
        }
        /* Show the box context */
        i = saved_value(-5);
        if (i != 0) {
            if (i < box_flag) {
                if (abs(nest[p].mode_field) == vmode)
                    j = hmove_cmd;
                else
                    j = vmove_cmd;
                if (i > 0)
                    print_cmd_chr(j, 0);
                else
                    print_cmd_chr(j, 1);
                print_scaled(abs(i));
                tprint("pt");
            } else if (i < ship_out_flag) {
                if (i >= global_box_flag) {
                    tprint_esc("global");
                    i = i - (global_box_flag - box_flag);
                }
                tprint_esc("setbox");
                print_int(i - box_flag);
                print_char('=');
            } else {
                print_cmd_chr(leader_ship_cmd, i - (leader_flag - a_leaders));
            }
        }
      FOUND1:
        tprint_esc(s);
        /* Show the box packaging info */
        {
            /* offsets may vary */
            int ii = -1;
            while (saved_type(ii) != saved_boxspec)
                ii--;
            if (saved_value(ii) != 0) {
                print_char(' ');
                if (saved_level(ii) == exactly)
                    tprint("to");
                else
                    tprint("spread");
                print_scaled(saved_value(ii));
                tprint("pt");
            }
        }
      FOUND2:
        print_char('{');
      FOUND:
        print_char(')');
        decr(cur_level);
        cur_group = save_level(save_ptr);
        save_ptr = save_value(save_ptr);
    }
  DONE:
    save_ptr = v;
    cur_level = l;
    cur_group = c;
}

@ Just before an entry of |eqtb| is changed, the following procedure should
be called to update the other data structures properly. It is important
to keep in mind that reference counts in |mem| include references from
within |save_stack|, so these counts must be handled carefully.
@^reference counts@>

@c
/* we don't need to destroy when an assignment has the same node */

void eq_destroy(memory_word w)
{                               /* gets ready to forget |w| */
    halfword q;                 /* |equiv| field of |w| */
    switch (eq_type_field(w)) {
        case call_cmd:
        case long_call_cmd:
        case outer_call_cmd:
        case long_outer_call_cmd:
            delete_token_ref(equiv_field(w));
            break;
        case glue_ref_cmd:
            flush_node(equiv_field(w));
            break;
        case shape_ref_cmd:
            q = equiv_field(w);     /* we need to free a \.{\\parshape} block */
            if (q != null)
                flush_node(q);
            break;                  /* such a block is |2n+1| words long, where |n=vinfo(q)| */
        case box_ref_cmd:
            flush_node_list(equiv_field(w));
            break;
        default:
            break;
    }
}

@ To save a value of |eqtb[p]| that was established at level |l|, we
can use the following subroutine.

@c
void eq_save(halfword p, quarterword l)
{                               /* saves |eqtb[p]| */
    check_full_save_stack();
    if (l == level_zero) {
        save_type(save_ptr) = restore_zero;
    } else {
        save_word(save_ptr) = eqtb[p];
        save_type(save_ptr) = saved_eqtb;
        incr(save_ptr);
        save_type(save_ptr) = restore_old_value;
    }
    save_level(save_ptr) = l;
    save_value(save_ptr) = p;
    incr(save_ptr);
}

@ The procedure |eq_define| defines an |eqtb| entry having specified
|eq_type| and |equiv| fields, and saves the former value if appropriate.
This procedure is used only for entries in the first four regions of |eqtb|,
i.e., only for entries that have |eq_type| and |equiv| fields.
After calling this routine, it is safe to put four more entries on
|save_stack|, provided that there was room for four more entries before
the call, since |eq_save| makes the necessary test.

@ new data for |eqtb|
@c
void eq_define(halfword p, quarterword t, halfword e)
{
    boolean trace = tracing_assigns_par > 0;
    if ((eq_type(p) == t) && (equiv(p) == e)) {
        if (trace)
            diagnostic_trace(p, "reassigning");
        eq_destroy(eqtb[p]);
        return;
    }
    if (trace)
        diagnostic_trace(p, "changing");
    if (eq_level(p) == cur_level)
        eq_destroy(eqtb[p]);
    else if (cur_level > level_one)
        eq_save(p, eq_level(p));
    set_eq_level(p, cur_level);
    set_eq_type(p, t);
    set_equiv(p, e);
    if (trace)
        diagnostic_trace(p, "into");
}

@ The counterpart of |eq_define| for the remaining (fullword) positions in
|eqtb| is called |eq_word_define|. Since |xeq_level[p]>=level_one| for all
|p|, a `|restore_zero|' will never be used in this case.

@c
void eq_word_define(halfword p, int w)
{
    boolean trace = tracing_assigns_par > 0;
    if (eqtb[p].cint == w) {
        if (trace)
            diagnostic_trace(p, "reassigning");
        return;
    }
    if (trace)
        diagnostic_trace(p, "changing");
    if (xeq_level[p] != cur_level) {
        eq_save(p, xeq_level[p]);
        xeq_level[p] = cur_level;
    }
    eqtb[p].cint = w;
    if (trace)
        diagnostic_trace(p, "into");
}


@ The |eq_define| and |eq_word_define| routines take care of local definitions.
@^global definitions@>
Global definitions are done in almost the same way, but there is no need
to save old values, and the new value is associated with |level_one|.

@c
void geq_define(halfword p, quarterword t, halfword e)
{                               /* global |eq_define| */
    boolean trace = tracing_assigns_par > 0;
    if (trace)
        diagnostic_trace(p, "globally changing");
    eq_destroy(eqtb[p]);
    set_eq_level(p, level_one);
    set_eq_type(p, t);
    set_equiv(p, e);
    if (trace)
        diagnostic_trace(p, "into");
}

void geq_word_define(halfword p, int w)
{                               /* global |eq_word_define| */
    boolean trace = tracing_assigns_par > 0;
    if (trace)
        diagnostic_trace(p, "globally changing");
    eqtb[p].cint = w;
    xeq_level[p] = level_one;
    if (trace)
        diagnostic_trace(p, "into");
}

@ Subroutine |save_for_after| puts a token on the stack for save-keeping.

@c
void save_for_after(halfword t)
{
    if (cur_level > level_one) {
        check_full_save_stack();
        save_type(save_ptr) = insert_token;
        save_level(save_ptr) = level_zero;
        save_value(save_ptr) = t;
        incr(save_ptr);
    }
}

@ The |unsave| routine goes the other way, taking items off of |save_stack|.
This routine takes care of restoration when a level ends; everything
belonging to the topmost group is cleared off of the save stack.

@c
void unsave(void)
{                               /* pops the top level off the save stack */
    halfword p;                 /* position to be restored */
    quarterword l = level_one;  /* saved level, if in fullword regions of |eqtb| */
    boolean a = false;          /* have we already processed an \.{\\aftergroup} ? */
    unsave_math_codes(cur_level);
    unsave_cat_codes(cat_code_table_par, cur_level);
    unsave_text_codes(cur_level);
    unsave_math_data(cur_level);
    if (cur_level > level_one) {
        boolean trace = tracing_restores_par > 0;
        decr(cur_level);
        /* Clear off top level from |save_stack| */
        while (true) {
            decr(save_ptr);
            if (save_type(save_ptr) == level_boundary)
                break;
            p = save_value(save_ptr);
            if (save_type(save_ptr) == insert_token) {
                reinsert_token(a, p);
                a = true; /* always ... always etex now */
            } else {
                if (save_type(save_ptr) == restore_old_value) {
                    l = save_level(save_ptr);
                    decr(save_ptr);
                } else {
                    save_word(save_ptr) = eqtb[undefined_control_sequence];
                }
                /* Store |save_stack[save_ptr]| in |eqtb[p]|, unless
                   |eqtb[p]| holds a global value */
                /* A global definition, which sets the level to |level_one|,
                   will not be undone by |unsave|. If at least one global definition of
                   |eqtb[p]| has been carried out within the group that just ended, the
                   last such definition will therefore survive.
                 */
                if (p < int_base || p > eqtb_size) {
                    if (eq_level(p) == level_one) {
                        eq_destroy(save_word(save_ptr));        /* destroy the saved value */
                        if (trace)
                            diagnostic_trace(p, "retaining");
                    } else {
                        eq_destroy(eqtb[p]);    /* destroy the current value */
                        eqtb[p] = save_word(save_ptr);  /* restore the saved value */
                        if (trace)
                            diagnostic_trace(p, "restoring");
                    }
                } else if (xeq_level[p] != level_one) {
                    eqtb[p] = save_word(save_ptr);
                    xeq_level[p] = l;
                    if (trace)
                        diagnostic_trace(p, "restoring");
                } else {
                    if (trace)
                        diagnostic_trace(p, "retaining");
                }
            }
        }
        if (tracing_groups_par > 0)
            group_trace(true);
        if (grp_stack[in_open] == cur_boundary)
            group_warning();    /* groups possibly not properly nested with files */
        cur_group = save_level(save_ptr);
        cur_boundary = save_value(save_ptr);
        decr(save_ptr);
    } else {
        confusion("curlevel");  /* |unsave| is not used when |cur_group=bottom_level| */
    }
    attr_list_cache = cache_disabled;
}

@ Most of the parameters kept in |eqtb| can be changed freely, but there's
an exception:  The magnification should not be used with two different
values during any \TeX\ job, since a single magnification is applied to an
entire run. The global variable |mag_set| is set to the current magnification
whenever it becomes necessary to ``freeze'' it at a particular value.

@c
int mag_set = 0; /* if nonzero, this magnification should be used henceforth */

@ The |prepare_mag| subroutine is called whenever \TeX\ wants to use |mag|
for magnification.

@c
void prepare_mag(void)
{
    if ((mag_set > 0) && (mag_par != mag_set)) {
        print_err("Incompatible magnification (");
        print_int(mag_par);
        tprint(");");
        tprint_nl(" the previous value will be retained");
        help2("I can handle only one magnification ratio per job. So I've",
              "reverted to the magnification you used earlier on this run.");
        int_error(mag_set);
        geq_word_define(int_base + mag_code, mag_set);  /* |mag:=mag_set| */
    }
    if ((mag_par <= 0) || (mag_par > 32768)) {
        print_err("Illegal magnification has been changed to 1000");
        help1("The magnification ratio must be between 1 and 32768.");
        int_error(mag_par);
        geq_word_define(int_base + mag_code, 1000);
    }
    if ((mag_set == 0) && (mag_par != mag_set)) {
        if (mag_par != 1000)
            one_true_inch = xn_over_d(one_hundred_inch, 10, mag_par);
        else
            one_true_inch = one_inch;
    }
    mag_set = mag_par;
}

@ Let's pause a moment now and try to look at the Big Picture.
The \TeX\ program consists of three main parts: syntactic routines,
semantic routines, and output routines. The chief purpose of the
syntactic routines is to deliver the user's input to the semantic routines,
one token at a time. The semantic routines act as an interpreter
responding to these tokens, which may be regarded as commands. And the
output routines are periodically called on to convert box-and-glue
lists into a compact set of instructions that will be sent
to a typesetter. We have discussed the basic data structures and utility
routines of \TeX, so we are good and ready to plunge into the real activity by
considering the syntactic routines.

Our current goal is to come to grips with the |get_next| procedure,
which is the keystone of \TeX's input mechanism. Each call of |get_next|
sets the value of three variables |cur_cmd|, |cur_chr|, and |cur_cs|,
representing the next input token.
$$\vbox{\halign{#\hfil\cr
  \hbox{|cur_cmd| denotes a command code from the long list of codes
   given above;}\cr
  \hbox{|cur_chr| denotes a character code or other modifier of the command
   code;}\cr
  \hbox{|cur_cs| is the |eqtb| location of the current control sequence,}\cr
  \hbox{\qquad if the current token was a control sequence,
   otherwise it's zero.}\cr}}$$
Underlying this external behavior of |get_next| is all the machinery
necessary to convert from character files to tokens. At a given time we
may be only partially finished with the reading of several files (for
which \.{\\input} was specified), and partially finished with the expansion
of some user-defined macros and/or some macro parameters, and partially
finished with the generation of some text in a template for \.{\\halign},
and so on. When reading a character file, special characters must be
classified as math delimiters, etc.; comments and extra blank spaces must
be removed, paragraphs must be recognized, and control sequences must be
found in the hash table. Furthermore there are occasions in which the
scanning routines have looked ahead for a word like `\.{plus}' but only
part of that word was found, hence a few characters must be put back
into the input and scanned again.

To handle these situations, which might all be present simultaneously,
\TeX\ uses various stacks that hold information about the incomplete
activities, and there is a finite state control for each level of the
input mechanism. These stacks record the current state of an implicitly
recursive process, but the |get_next| procedure is not recursive.
Therefore it will not be difficult to translate these algorithms into
low-level languages that do not support recursion.

@c
int cur_cmd;                    /* current command set by |get_next| */
halfword cur_chr;               /* operand of current command */
halfword cur_cs;                /* control sequence found here, zero if none found */
halfword cur_tok;               /* packed representative of |cur_cmd| and |cur_chr| */

@ Here is a procedure that displays the current command.

@c
void show_cur_cmd_chr(void)
{
    int n;                      /* level of \.{\\if...\\fi} nesting */
    int l;                      /* line where \.{\\if} started */
    halfword p;
    begin_diagnostic();
    tprint_nl("{");
    if (mode_par != shown_mode) {
        print_mode(mode_par);
        tprint(": ");
        shown_mode = mode_par;
    }
    print_cmd_chr((quarterword) cur_cmd, cur_chr);
    if (tracing_ifs_par > 0) {
        if (cur_cmd >= if_test_cmd) {
            if (cur_cmd <= fi_or_else_cmd) {
                tprint(": ");
                if (cur_cmd == fi_or_else_cmd) {
                    print_cmd_chr(if_test_cmd, cur_if);
                    print_char(' ');
                    n = 0;
                    l = if_line;
                } else {
                    n = 1;
                    l = line;
                }
                p = cond_ptr;
                while (p != null) {
                    incr(n);
                    p = vlink(p);
                }
                tprint("(level ");
                print_int(n);
                print_char(')');
                print_if_line(l);
            }
        }
    }
    print_char('}');
    end_diagnostic(false);
}

@ Here is a procedure that displays the contents of |eqtb[n]| symbolically.

@c
void show_eqtb(halfword n)
{
    if (n < null_cs) {
        /* this can't happen */
        print_char('?');
    } else if ((n < glue_base) || ((n > eqtb_size) && (n <= eqtb_top))) {
        /*
            Show equivalent |n|, in region 1 or 2

            Here is a routine that displays the current meaning of an |eqtb| entry
            in region 1 or~2. (Similar routines for the other regions will appear
            below.)
        */

        sprint_cs(n);
        print_char('=');
        print_cmd_chr(eq_type(n), equiv(n));
        if (eq_type(n) >= call_cmd) {
            print_char(':');
            show_token_list(token_link(equiv(n)), null, 32);
        }
    } else if (n < local_base) {
        /*
            Show equivalent |n|, in region 3

            All glue parameters and registers are initially `\.{0pt plus0pt minus0pt}'.
        */
        if (n < skip_base) {
            if (n < glue_base + thin_mu_skip_code)
                print_cmd_chr(assign_glue_cmd, n);
            else
                print_cmd_chr(assign_mu_glue_cmd, n);
            print_char('=');
            if (n < glue_base + thin_mu_skip_code)
                print_spec(equiv(n), "pt");
            else
                print_spec(equiv(n), "mu");
        } else if (n < mu_skip_base) {
            tprint_esc("skip");
            print_int(n - skip_base);
            print_char('=');
            print_spec(equiv(n), "pt");
        } else {
            tprint_esc("muskip");
            print_int(n - mu_skip_base);
            print_char('=');
            print_spec(equiv(n), "mu");
        }

    } else if (n < int_base) {
        /*
            Show equivalent |n|, in region 4

            We initialize most things to null or undefined values. An undefined font
            is represented by the internal code |font_base|.

            However, the character code tables are given initial values based on the
            conventional interpretation of ASCII code. These initial values should
            not be changed when \TeX\ is adapted for use with non-English languages;
            all changes to the initialization conventions should be made in format
            packages, not in \TeX\ itself, so that global interchange of formats is
            possible.
        */
        if ((n == par_shape_loc) || ((n >= etex_pen_base) && (n < etex_pens))) {
            if (n == par_shape_loc)
                print_cmd_chr(set_tex_shape_cmd, n);
            else
                print_cmd_chr(set_etex_shape_cmd, n);
            print_char('=');
            if (equiv(n) == null) {
                print_char('0');
            } else if (n > par_shape_loc) {
                print_int(penalty(equiv(n)));
                print_char(' ');
                print_int(penalty(equiv(n) + 1));
                if (penalty(equiv(n)) > 1)
                    tprint_esc("ETC.");
            } else {
                print_int(vinfo(par_shape_par_ptr + 1));
            }
        } else if (n < toks_base) {
            print_cmd_chr(assign_toks_cmd, n);
            print_char('=');
            if (equiv(n) != null)
                show_token_list(token_link(equiv(n)), null, 32);
        } else if (n < box_base) {
            tprint_esc("toks");
            print_int(n - toks_base);
            print_char('=');
            if (equiv(n) != null)
                show_token_list(token_link(equiv(n)), null, 32);
        } else if (n < cur_font_loc) {
            tprint_esc("box");
            print_int(n - box_base);
            print_char('=');
            if (equiv(n) == null) {
                tprint("void");
            } else {
                depth_threshold = 0;
                breadth_max = 1;
                show_node_list(equiv(n));
            }
        } else if (n == cur_font_loc) {
            /* Show the font identifier in |eqtb[n]| */
            tprint("current font");
            print_char('=');
            print_esc(hash[font_id_base + equiv(n)].rh);        /* that's |font_id_text(equiv(n))| */
        }
    } else if (n < dimen_base) {
        /* Show equivalent |n|, in region 5 */
        if (n < dir_base) {
            print_cmd_chr(assign_int_cmd, n);
            print_char('=');
            print_int(eqtb[n].cint);
        } else if (n < count_base) {
            print_cmd_chr(assign_dir_cmd, n);
            print_char(' ');
            print_dir(eqtb[n].cint);
        } else if (n < attribute_base) {
            tprint_esc("count");
            print_int(n - count_base);
            print_char('=');
            print_int(eqtb[n].cint);
        } else if (n < del_code_base) {
            tprint_esc("attribute");
            print_int(n - attribute_base);
            print_char('=');
            print_int(eqtb[n].cint);
        }
    } else if (n <= eqtb_size) {
        /* Show equivalent |n|, in region 6 */
        if (n < scaled_base) {
            print_cmd_chr(assign_dimen_cmd, n);
        } else {
            tprint_esc("dimen");
            print_int(n - scaled_base);
        }
        print_char('=');
        print_scaled(eqtb[n].cint);
        tprint("pt");
    } else {
        /* this can't happen either */
        print_char('?');
    }
}

@ @c
void show_eqtb_meaning(halfword n)
{
    if (n < null_cs) {
        /* this can't happen */
        print_char('?');
    } else if ((n < glue_base) || ((n > eqtb_size) && (n <= eqtb_top))) {
        /*
            Show equivalent |n|, in region 1 or 2

            Here is a routine that displays the current meaning of an |eqtb| entry
            in region 1 or~2. (Similar routines for the other regions will appear
            below.)
        */
        sprint_cs(n);
    } else if (n < local_base) {
        /*
            Show equivalent |n|, in region 3

            All glue parameters and registers are initially `\.{0pt plus0pt minus0pt}'.
        */
        if (n < skip_base) {
            if (n < glue_base + thin_mu_skip_code)
                print_cmd_chr(assign_glue_cmd, n);
            else
                print_cmd_chr(assign_mu_glue_cmd, n);
        } else if (n < mu_skip_base) {
            tprint_esc("skip");
            print_int(n - skip_base);
        } else {
            tprint_esc("muskip");
            print_int(n - mu_skip_base);
        }

    } else if (n < int_base) {
        /*
            Show equivalent |n|, in region 4

            We initialize most things to null or undefined values. An undefined font
            is represented by the internal code |font_base|.

            However, the character code tables are given initial values based on the
            conventional interpretation of ASCII code. These initial values should
            not be changed when \TeX\ is adapted for use with non-English languages;
            all changes to the initialization conventions should be made in format
            packages, not in \TeX\ itself, so that global interchange of formats is
            possible.
        */
        if ((n == par_shape_loc) || ((n >= etex_pen_base) && (n < etex_pens))) {
            if (n == par_shape_loc)
                print_cmd_chr(set_tex_shape_cmd, n);
            else
                print_cmd_chr(set_etex_shape_cmd, n);
        } else if (n < toks_base) {
            print_cmd_chr(assign_toks_cmd, n);
        } else if (n < box_base) {
            tprint_esc("toks");
            print_int(n - toks_base);
        } else if (n < cur_font_loc) {
            tprint_esc("box");
            print_int(n - box_base);
        } else if (n == cur_font_loc) {
            /* Show the font identifier in |eqtb[n]| */
            tprint("current font");
        }
    } else if (n < dimen_base) {
        /* Show equivalent |n|, in region 5 */
        if (n < dir_base) {
            print_cmd_chr(assign_int_cmd, n);
        } else if (n < count_base) {
            print_cmd_chr(assign_dir_cmd, n);
        } else if (n < attribute_base) {
            tprint_esc("count");
            print_int(n - count_base);
        } else if (n < del_code_base) {
            tprint_esc("attribute");
            print_int(n - attribute_base);
        }
    } else if (n <= eqtb_size) {
        /* Show equivalent |n|, in region 6 */
        if (n < scaled_base) {
            print_cmd_chr(assign_dimen_cmd, n);
        } else {
            tprint_esc("dimen");
            print_int(n - scaled_base);
        }
    } else {
        /* this can't happen either */
        print_char('?');
    }
}