cex: add support for state-item pair graph generation

* src/lssi.h, src/lssi.c, src/state-item.h, src/state-item.c: New.

src/lssi.h

@@ -0,0 +1,57 @@
+/* Lookahead sensative state item searches for counterexample generation
+
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of Bison, the GNU Compiler Compiler.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef LSSI_H
+# define LSSI_H
+
+# include "state-item.h"
+
+/*
+  All state-item graph nodes should also include a precise follow set (follow_L).
+  However, ignoring follow_L saves a lot of memory and is a pretty good approximation.
+  These functions exist to enforce restrictions caused by follow_L sets.
+ */
+
+/*
+ * find shortest lookahead-sensitive path of state-items to target such that
+ * next_sym is in the follow_L set of target in that position.
+*/
+gl_list_t shortest_path_from_start (state_item_number target,
+                                    symbol_number next_sym);
+
+/**
+ * Determine if the given terminal is in the given symbol set or can begin
+ * a nonterminal in the given symbol set.
+ */
+bool intersect_symbol (symbol_number sym, bitset syms);
+
+/**
+ * Determine if any symbol in ts is in syms
+ * or can begin with a nonterminal in syms.
+ */
+bool intersect (bitset ts, bitset syms);
+
+/**
+ * Compute a set of sequences of state-items that can make production steps
+ * to this state-item such that the resulting possible lookahead symbols are
+ * as given.
+ */
+gl_list_t lssi_reverse_production (const state_item *si, bitset lookahead);
+
+#endif /* LSSI_H */