adding BDD-based variable elimination routine

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/sat/sat_elim_vars.h

@@ -0,0 +1,61 @@
+/*++
+Copyright (c) 2017 Microsoft Corporation
+
+Module Name:
+
+    sat_elim_vars.h
+
+Abstract:
+
+    Helper class for eliminating variables
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2017-10-14
+
+Revision History:
+
+--*/
+#ifndef SAT_ELIM_VARS_H_
+#define SAT_ELIM_VARS_H_
+
+#include "sat/sat_types.h"
+#include "sat/sat_bdd.h"
+
+namespace sat {
+    class solver;
+    class simplifier;
+
+    class elim_vars {
+        friend class simplifier;
+
+        simplifier& simp;
+        solver&     s;
+        bdd_manager m;
+
+        svector<bool_var> m_vars;
+        unsigned_vector   m_mark;
+        unsigned          m_mark_lim;
+        unsigned_vector   m_var2index;
+
+        unsigned m_max_literals;
+
+        unsigned num_vars() const { return m_vars.size(); }
+        void reset_mark();
+        void mark_var(bool_var v);
+        bool mark_literals(clause_use_list & occs);
+        bool mark_literals(literal lit);
+        bdd make_clauses(clause_use_list & occs);
+        bdd make_clauses(literal lit);
+        bdd mk_literal(literal l);
+        void get_clauses(bdd const& b, literal_vector& lits, clause_vector& clauses, literal_vector& units);
+        void add_clauses(bdd const& b, literal_vector& lits);
+
+    public:
+        elim_vars(simplifier& s);
+        bool operator()(bool_var v);
+    };
+
+};
+
+#endif