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LAC = lookahead correction. See discussion at <http://lists.gnu.org/archive/html/bison-patches/2009-09/msg00034.html>. However, one point there must be corrected: because of %nonassoc, LAC is *not* always redundant for lr.type=canonical-lr. * data/yacc.c: Accept values of "none" (default) or "full" for parse.lac. Accept %define parse.lac.es-capacity to specify capacity of LAC's temporary exploratory stack. It defaults to 20 and, for now, will not grow dynamically. (b4_lac_flag, b4_lac_if): New m4 macros. Evaluate as true for parse.lac!=none. (YYBACKUP): Invoke YY_LAC_DISCARD. (YY_LAC_ESTABLISH, YY_LAC_DISCARD): New cpp macros that invoke yy_lac and track when it needs to be invoked (yy_lac): New function that, given the current stack, determines whether a token can eventually be shifted. Return status mimics yyparse return status. (yysyntax_error): Change yystate argument to yyssp so stack top can be passed to yy_lac. If LAC is requested, build expected token list by invoking yy_lac for every token instead of just checking the current state for lookaheads. Return 2 if yy_lac exhausts memory. (yyparse, yypush_parse): Use local variable yy_lac_established and cpp macros YY_LAC_ESTABLISH and YY_LAC_DISCARD to implement LAC. Update yysyntax_error invocation. Add yyexhaustedlab code if LAC is requested. * tests/conflicts.at (%nonassoc and eof): Extend to check the effect of each of -Dlr.type=canonical-lr and -Dparse.lac=full. (parse.error=verbose and consistent errors): Likewise. (LAC: %nonassoc requires splitting canonical LR states): New test group demonstrating how LAC can fix canonical LR. * tests/input.at (LAC: Errors for %define): New test group. * tests/regression.at (LAC: Exploratory stack): New test group. (LAC: Memory exhaustion): New test group.
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