improved.c

compiler

/*@A (C) 1992 Allen I. Holub                                                */
/* Revised parser  */

#include <stdio.h>
#include "lex.h"

void    factor      ( void );
void    term        ( void );
void    expression  ( void );

statements()
{
    /*  statements -> expression SEMI |  expression SEMI statements */

    while( !match(EOI) )
    {
        expression();

        if( match( SEMI ) )
            advance();
        else
            fprintf( stderr, "%d: Inserting missing semicolon\n", yylineno );
    }
}

void    expression()
{
    /* expression  -> term expression'
     * expression' -> PLUS term expression' |  epsilon
     */

    if( !legal_lookahead( NUM_OR_ID, LP, 0 ) )
	return;

    term();
    while( match( PLUS ) )
    {
        advance();
        term();
    }
}

void    term()
{
    if( !legal_lookahead( NUM_OR_ID, LP, 0 ) )
	return;

    factor();
    while( match( TIMES ) )
    {
        advance();
        factor();
    }
}

void    factor()
{
    if( !legal_lookahead( NUM_OR_ID, LP, 0 ) )
	return;

    if( match(NUM_OR_ID) )
        advance();

    else if( match(LP) )
    {
        advance();
        expression();
        if( match(RP) )
            advance();
        else
            fprintf( stderr, "%d: Mismatched parenthesis\n", yylineno );
    }
    else
	fprintf( stderr, "%d: Number or identifier expected\n", yylineno );
}
#include <stdarg.h>

#define MAXFIRST 16
#define SYNCH	 SEMI

int	legal_lookahead(  first_arg )
int	first_arg;
{
    /* Simple error detection and recovery. Arguments are a 0-terminated list of
     * those tokens that can legitimately come next in the input. If the list is
     * empty, the end of file must come next. Print an error message if
     * necessary. Error recovery is performed by discarding all input symbols
     * until one that's in the input list is found
     *
     * Return true if there's no error or if we recovered from the error,
     * false if we can't recover.
     */

    va_list  	args;
    int		tok;
    int		lookaheads[MAXFIRST], *p = lookaheads, *current;
    int		error_printed = 0;
    int		rval	      = 0;

    va_start( args, first_arg );

    if( !first_arg )
    {
	if( match(EOI) )
	    rval = 1;
    }
    else
    {
	*p++ = first_arg;
	while( (tok = va_arg(args, int)) && p < &lookaheads[MAXFIRST] )
	    *p++ = tok;

	while( !match( SYNCH ) )
	{
	    for( current = lookaheads; current < p ; ++current )
		if( match( *current ) )
		{
		    rval = 1;
		    goto exit;
		}

	    if( !error_printed )
	    {
		fprintf( stderr, "Line %d: Syntax error\n", yylineno );
		error_printed = 1;
	    }

	    advance();
	}
    }

exit:
    va_end( args )
    return rval;
}