parser.c

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/*-------------------------------------------------------------------------
 *
 * 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) 1996-2019, 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 "preproc_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;
}