src/nullable.c

   1 /* Part of the bison parser generator,
   2    Copyright 1984, 1989, 2000 Free Software Foundation, Inc.
   3
   4    This file is part of Bison, the GNU Compiler Compiler.
   5
   6    Bison is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 2, or (at your option)
   9    any later version.
  10
  11    Bison is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with Bison; see the file COPYING.  If not, write to
  18    the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  19    Boston, MA 02111-1307, USA.  */
  20
  21
  22 /* Set up NULLABLE, a vector saying which nonterminals can expand into
  23    the null string.  NULLABLE[I - NTOKENS] is nonzero if symbol I can
  24    do so.  */
  25
  26 #include "system.h"
  27 #include "types.h"
  28 #include "gram.h"
  29 #include "nullable.h"
  30
  31 char *nullable = NULL;
  32
  33 void
  34 set_nullable (void)
  35 {
  36   short *r;
  37   short *s1;
  38   short *s2;
  39   int ruleno;
  40   int symbol;
  41   shorts *p;
  42
  43   short *squeue;
  44   short *rcount;
  45   shorts **rsets;
  46   shorts *relts;
  47   char any_tokens;
  48   short *r1;
  49
  50 #ifdef  TRACE
  51   fprintf (stderr, _("Entering set_nullable"));
  52 #endif
  53
  54   nullable = XCALLOC (char, nvars) - ntokens;
  55
  56   squeue = XCALLOC (short, nvars);
  57   s1 = s2 = squeue;
  58
  59   rcount = XCALLOC (short, nrules + 1);
  60   rsets = XCALLOC (shorts *, nvars) - ntokens;
  61   /* This is said to be more elements than we actually use.
  62      Supposedly nitems - nrules is enough.
  63      But why take the risk?  */
  64   relts = XCALLOC (shorts, nitems + nvars + 1);
  65   p = relts;
  66
  67   r = ritem;
  68   while (*r)
  69     {
  70       if (*r < 0)
  71         {
  72           symbol = rlhs[-(*r++)];
  73           if (symbol >= 0 && !nullable[symbol])
  74             {
  75               nullable[symbol] = 1;
  76               *s2++ = symbol;
  77             }
  78         }
  79       else
  80         {
  81           r1 = r;
  82           any_tokens = 0;
  83           for (symbol = *r++; symbol > 0; symbol = *r++)
  84             {
  85               if (ISTOKEN (symbol))
  86                 any_tokens = 1;
  87             }
  88
  89           if (!any_tokens)
  90             {
  91               ruleno = -symbol;
  92               r = r1;
  93               for (symbol = *r++; symbol > 0; symbol = *r++)
  94                 {
  95                   rcount[ruleno]++;
  96                   p->next = rsets[symbol];
  97                   p->value = ruleno;
  98                   rsets[symbol] = p;
  99                   p++;
 100                 }
 101             }
 102         }
 103     }
 104
 105   while (s1 < s2)
 106     {
 107       p = rsets[*s1++];
 108       while (p)
 109         {
 110           ruleno = p->value;
 111           p = p->next;
 112           if (--rcount[ruleno] == 0)
 113             {
 114               symbol = rlhs[ruleno];
 115               if (symbol >= 0 && !nullable[symbol])
 116                 {
 117                   nullable[symbol] = 1;
 118                   *s2++ = symbol;
 119                 }
 120             }
 121         }
 122     }
 123
 124   XFREE (squeue);
 125   XFREE (rcount);
 126   XFREE (rsets + ntokens);
 127   XFREE (relts);
 128 }
 129
 130
 131 void
 132 free_nullable (void)
 133 {
 134   XFREE (nullable + ntokens);
 135 }