src/nullable.c

   1 /* Part of the bison parser generator,
   2    Copyright (C) 1984, 1989 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, 675 Mass Ave, Cambridge, MA 02139, USA.  */
  19
  20
  21 /* set up nullable, a vector saying which nonterminals can expand into the null string.
  22    nullable[i - ntokens] is nonzero if symbol i can do so.  */
  23
  24 #include <stdio.h>
  25 #include "system.h"
  26 #include "types.h"
  27 #include "gram.h"
  28 #include "new.h"
  29
  30
  31 char *nullable;
  32
  33
  34 void
  35 set_nullable()
  36 {
  37   register short *r;
  38   register short *s1;
  39   register short *s2;
  40   register int ruleno;
  41   register int symbol;
  42   register shorts *p;
  43
  44   short *squeue;
  45   short *rcount;
  46   shorts **rsets;
  47   shorts *relts;
  48   char any_tokens;
  49   short *r1;
  50
  51 #ifdef  TRACE
  52   fprintf(stderr, "Entering set_nullable");
  53 #endif
  54
  55   nullable = NEW2(nvars, char) - ntokens;
  56
  57   squeue = NEW2(nvars, short);
  58   s1 = s2 = squeue;
  59
  60   rcount = NEW2(nrules + 1, short);
  61   rsets = NEW2(nvars, shorts *) - ntokens;
  62   /* This is said to be more elements than we actually use.
  63      Supposedly nitems - nrules is enough.
  64      But why take the risk?  */
  65   relts = NEW2(nitems + nvars + 1, shorts);
  66   p = relts;
  67
  68   r = ritem;
  69   while (*r)
  70     {
  71       if (*r < 0)
  72         {
  73           symbol = rlhs[-(*r++)];
  74           if (symbol >= 0 && !nullable[symbol])
  75             {
  76               nullable[symbol] = 1;
  77               *s2++ = symbol;
  78             }
  79         }
  80       else
  81         {
  82           r1 = r;
  83           any_tokens = 0;
  84           for (symbol = *r++; symbol > 0; symbol = *r++)
  85             {
  86               if (ISTOKEN(symbol))
  87                 any_tokens = 1;
  88             }
  89
  90           if (!any_tokens)
  91             {
  92               ruleno = -symbol;
  93               r = r1;
  94               for (symbol = *r++; symbol > 0; symbol = *r++)
  95                 {
  96                   rcount[ruleno]++;
  97                   p->next = rsets[symbol];
  98                   p->value = ruleno;
  99                   rsets[symbol] = p;
 100                   p++;
 101                 }
 102             }
 103         }
 104     }
 105
 106   while (s1 < s2)
 107     {
 108       p = rsets[*s1++];
 109       while (p)
 110         {
 111           ruleno = p->value;
 112           p = p->next;
 113           if (--rcount[ruleno] == 0)
 114             {
 115               symbol = rlhs[ruleno];
 116               if (symbol >= 0 && !nullable[symbol])
 117                 {
 118                   nullable[symbol] = 1;
 119                   *s2++ = symbol;
 120                 }
 121             }
 122         }
 123     }
 124
 125   FREE(squeue);
 126   FREE(rcount);
 127   FREE(rsets + ntokens);
 128   FREE(relts);
 129 }
 130
 131
 132 void
 133 free_nullable()
 134 {
 135   FREE(nullable + ntokens);
 136 }