src/nullable.c

   1 /* Part of the bison parser generator,
   2    Copyright 1984, 1989, 2000, 2001 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 "getargs.h"
  28 #include "reader.h"
  29 #include "types.h"
  30 #include "gram.h"
  31 #include "nullable.h"
  32
  33 char *nullable = NULL;
  34
  35 static void
  36 nullable_print (FILE *out)
  37 {
  38   int i;
  39   fputs ("NULLABLE\n", out);
  40   for (i = ntokens; i < nsyms; i++)
  41     fprintf (out, "\t%s: %s\n", tags[i], nullable[i] ? "yes" : "no");
  42   fputs ("\n\n", out);
  43 }
  44
  45 void
  46 set_nullable (void)
  47 {
  48   short *r;
  49   short *s1;
  50   short *s2;
  51   shorts *p;
  52
  53   short *squeue;
  54   short *rcount;
  55   shorts **rsets;
  56   shorts *relts;
  57
  58   if (trace_flag)
  59     fprintf (stderr, "Entering set_nullable\n");
  60
  61   nullable = XCALLOC (char, nvars) - ntokens;
  62
  63   squeue = XCALLOC (short, nvars);
  64   s1 = s2 = squeue;
  65
  66   rcount = XCALLOC (short, nrules + 1);
  67   rsets = XCALLOC (shorts *, nvars) - ntokens;
  68   /* This is said to be more elements than we actually use.
  69      Supposedly nitems - nrules is enough.
  70      But why take the risk?  */
  71   relts = XCALLOC (shorts, nitems + nvars + 1);
  72   p = relts;
  73
  74   r = ritem;
  75   while (*r)
  76     {
  77       if (*r < 0)
  78         {
  79           int symbol = rule_table[-(*r++)].lhs;
  80           if (symbol >= 0 && !nullable[symbol])
  81             {
  82               nullable[symbol] = 1;
  83               *s2++ = symbol;
  84             }
  85         }
  86       else
  87         {
  88           int any_tokens = 0;
  89           int symbol;
  90           short *r1 = r;
  91           for (symbol = *r++; symbol > 0; symbol = *r++)
  92             if (ISTOKEN (symbol))
  93               any_tokens = 1;
  94
  95           if (!any_tokens)
  96             {
  97               int ruleno = -symbol;
  98               r = r1;
  99               for (symbol = *r++; symbol > 0; symbol = *r++)
 100                 {
 101                   rcount[ruleno]++;
 102                   p->next = rsets[symbol];
 103                   p->value = ruleno;
 104                   rsets[symbol] = p;
 105                   p++;
 106                 }
 107             }
 108         }
 109     }
 110
 111   while (s1 < s2)
 112     {
 113       p = rsets[*s1++];
 114       while (p)
 115         {
 116           int ruleno = p->value;
 117           p = p->next;
 118           if (--rcount[ruleno] == 0)
 119             {
 120               int symbol = rule_table[ruleno].lhs;
 121               if (symbol >= 0 && !nullable[symbol])
 122                 {
 123                   nullable[symbol] = 1;
 124                   *s2++ = symbol;
 125                 }
 126             }
 127         }
 128     }
 129
 130   XFREE (squeue);
 131   XFREE (rcount);
 132   XFREE (rsets + ntokens);
 133   XFREE (relts);
 134
 135   if (trace_flag)
 136     nullable_print (stderr);
 137 }
 138
 139
 140 void
 141 free_nullable (void)
 142 {
 143   XFREE (nullable + ntokens);
 144 }