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82 lines
2.0 KiB
82 lines
2.0 KiB
#include <stdlib.h>
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#include "y1.h"
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/*
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* 12-Apr-83 (RBD) Add symbolic exit status
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*/
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void cempty(void)
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{
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/* mark nonterminals which derive the empty string */
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/* also, look for nonterminals which don't derive any token strings */
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# define EMPTY 1
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# define WHOKNOWS 0
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# define OK 1
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register i, *p;
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/* first, use the array pempty to detect productions that can never be reduced */
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/* set pempty to WHONOWS */
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aryfil( pempty, nnonter+1, WHOKNOWS );
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/* now, look at productions, marking nonterminals which derive something */
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more:
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PLOOP(0,i)
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{
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if( pempty[ *prdptr[i] - NTBASE ] ) continue;
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for( p=prdptr[i]+1; *p>=0; ++p )
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{
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if( *p>=NTBASE && pempty[ *p-NTBASE ] == WHOKNOWS ) break;
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}
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if( *p < 0 )
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{
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/* production can be derived */
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pempty[ *prdptr[i]-NTBASE ] = OK;
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goto more;
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}
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}
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/* now, look at the nonterminals, to see if they are all OK */
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NTLOOP(i)
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{
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/* the added production rises or falls as the start symbol ... */
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if( i == 0 ) continue;
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if( pempty[ i ] != OK )
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{
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fatfl = 0;
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error( "nonterminal %s never derives any token string", nontrst[i].name );
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}
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}
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if( nerrors )
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{
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summary();
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exit(EX_ERR);
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}
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/* now, compute the pempty array, to see which nonterminals derive the empty string */
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/* set pempty to WHOKNOWS */
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aryfil( pempty, nnonter+1, WHOKNOWS );
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/* loop as long as we keep finding empty nonterminals */
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again:
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PLOOP(1,i)
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{
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if( pempty[ *prdptr[i]-NTBASE ]==WHOKNOWS )
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{
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/* not known to be empty */
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for( p=prdptr[i]+1; *p>=NTBASE && pempty[*p-NTBASE]==EMPTY ; ++p ) ;
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if( *p < 0 )
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{
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/* we have a nontrivially empty nonterminal */
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pempty[*prdptr[i]-NTBASE] = EMPTY;
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goto again; /* got one ... try for another */
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}
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}
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}
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}
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