/*-------------------------------------------------------------------------
 *
 * parser.c
 *		Main entry point/driver for PostgreSQL grammar
 *
 * This should match src/backend/parser/parser.c, except that we do not
 * need to bother with re-entrant interfaces.
 *
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  src/interfaces/ecpg/preproc/parser.c
 *
 *-------------------------------------------------------------------------
 */

#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 */
static char *lookahead_yytext;	/* start current token */
static char *lookahead_end;		/* end of current token */
static char lookahead_hold_char;	/* to be put back at *lookahead_end */


/*
 * Intermediate filter between parser and base lexer (base_yylex in scan.l).
 *
 * This 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 replacing 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 that 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;
	int			cur_token_length;
	YYSTYPE		cur_yylval;
	YYLTYPE		cur_yylloc;
	char	   *cur_yytext;

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

	/*
	 * If this token isn't one that requires lookahead, just return it.  If it
	 * does, determine the token length.  (We could get that via strlen(), but
	 * since we have such a small set of possibilities, hardwiring seems
	 * feasible and more efficient.)
	 */
	switch (cur_token)
	{
		case NOT:
			cur_token_length = 3;
			break;
		case NULLS_P:
			cur_token_length = 5;
			break;
		case WITH:
			cur_token_length = 4;
			break;
		default:
			return cur_token;
	}

	/*
	 * Identify end+1 of current token.  base_yylex() has temporarily stored a
	 * '\0' here, and will undo that when we call it again.  We need to redo
	 * it to fully revert the lookahead call for error reporting purposes.
	 */
	lookahead_end = base_yytext + cur_token_length;
	Assert(*lookahead_end == '\0');

	/* Save and restore lexer output variables around the call */
	cur_yylval = base_yylval;
	cur_yylloc = base_yylloc;
	cur_yytext = base_yytext;

	/* Get next token, saving outputs into lookahead variables */
	next_token = base_yylex();

	lookahead_token = next_token;
	lookahead_yylval = base_yylval;
	lookahead_yylloc = base_yylloc;
	lookahead_yytext = base_yytext;

	base_yylval = cur_yylval;
	base_yylloc = cur_yylloc;
	base_yytext = cur_yytext;

	/* Now revert the un-truncation of the current token */
	lookahead_hold_char = *lookahead_end;
	*lookahead_end = '\0';

	have_lookahead = true;

	/* Replace cur_token if needed, based on lookahead */
	switch (cur_token)
	{
		case NOT:
			/* Replace NOT by NOT_LA if it's followed by BETWEEN, IN, etc */
			switch (next_token)
			{
				case BETWEEN:
				case IN_P:
				case LIKE:
				case ILIKE:
				case SIMILAR:
					cur_token = NOT_LA;
					break;
			}
			break;

		case NULLS_P:
			/* Replace NULLS_P by NULLS_LA if it's followed by FIRST or LAST */
			switch (next_token)
			{
				case FIRST_P:
				case LAST_P:
					cur_token = NULLS_LA;
					break;
			}
			break;

		case WITH:
			/* Replace WITH by WITH_LA if it's followed by TIME or ORDINALITY */
			switch (next_token)
			{
				case TIME:
				case ORDINALITY:
					cur_token = WITH_LA;
					break;
			}
			break;
	}

	return cur_token;
}
