/*-------------------------------------------------------------------------
 *
 * parser.c
 *		Main entry point/driver for PostgreSQL grammar
 *
 * Note that the grammar is not allowed to perform any table access
 * (since we need to be able to do basic parsing even while inside an
 * aborted transaction).  Therefore, the data structures returned by
 * the grammar are "raw" parsetrees that still need to be analyzed by
 * analyze.c and related files.
 *
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/interfaces/ecpg/preproc/parser.c,v 1.5 2009/01/01 17:24:02 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres_fe.h"

#include "extern.h"
#include "preproc.h"


static bool have_lookahead;		/* is lookahead info valid? */
static int	lookahead_token;	/* one-token lookahead */
static YYSTYPE lookahead_yylval;	/* yylval for lookahead token */
static YYLTYPE lookahead_yylloc;	/* yylloc for lookahead token */


/*
 * Intermediate filter between parser and base lexer (base_yylex in scan.l).
 *
 * The filter is needed because in some cases the standard SQL grammar
 * requires more than one token lookahead.  We reduce these cases to one-token
 * lookahead by combining tokens here, in order to keep the grammar LALR(1).
 *
 * Using a filter is simpler than trying to recognize multiword tokens
 * directly in scan.l, because we'd have to allow for comments between the
 * words.  Furthermore it's not clear how to do it without re-introducing
 * scanner backtrack, which would cost more performance than this filter
 * layer does.
 */
int
filtered_base_yylex(void)
{
	int			cur_token;
	int			next_token;
	YYSTYPE		cur_yylval;
	YYLTYPE		cur_yylloc;

	/* Get next token --- we might already have it */
	if (have_lookahead)
	{
		cur_token = lookahead_token;
		base_yylval = lookahead_yylval;
		base_yylloc = lookahead_yylloc;
		have_lookahead = false;
	}
	else
		cur_token = base_yylex();

	/* Do we need to look ahead for a possible multiword token? */
	switch (cur_token)
	{
		case NULLS_P:

			/*
			 * NULLS FIRST and NULLS LAST must be reduced to one token
			 */
			cur_yylval = base_yylval;
			cur_yylloc = base_yylloc;
			next_token = base_yylex();
			switch (next_token)
			{
				case FIRST_P:
					cur_token = NULLS_FIRST;
					break;
				case LAST_P:
					cur_token = NULLS_LAST;
					break;
				default:
					/* save the lookahead token for next time */
					lookahead_token = next_token;
					lookahead_yylval = base_yylval;
					lookahead_yylloc = base_yylloc;
					have_lookahead = true;
					/* and back up the output info to cur_token */
					base_yylval = cur_yylval;
					base_yylloc = cur_yylloc;
					break;
			}
			break;

		case WITH:

			/*
			 * WITH TIME must be reduced to one token
			 */
			cur_yylval = base_yylval;
			cur_yylloc = base_yylloc;
			next_token = base_yylex();
			switch (next_token)
			{
				case TIME:
					cur_token = WITH_TIME;
					break;
				default:
					/* save the lookahead token for next time */
					lookahead_token = next_token;
					lookahead_yylval = base_yylval;
					lookahead_yylloc = base_yylloc;
					have_lookahead = true;
					/* and back up the output info to cur_token */
					base_yylval = cur_yylval;
					base_yylloc = cur_yylloc;
					break;
			}
			break;

		default:
			break;
	}

	return cur_token;
}
