feat(sat): phase saving. remember polarity of decisions

src/sat/solver.ml

@@ -1600,11 +1600,14 @@ let pick_branch_lit ~full self : bool =
     | [] ->
       (match H.remove_min self.order with
       | v ->
-        pick_with_given_atom
+        let pol = Var.last_pol self.store v in
-          (if Var.default_pol self.store v then
+        let atom =
+          if pol then
             Atom.pa v
           else
-            Atom.na v)
+            Atom.na v
+        in
+        pick_with_given_atom atom
       | exception Not_found -> false)
   (* pick a decision, trying [atom] first if it's not assigned yet. *)
   and pick_with_given_atom (atom : atom) : bool =
@@ -1615,9 +1618,12 @@ let pick_branch_lit ~full self : bool =
         || Atom.is_true self.store (Atom.na v));
       pick_lit ()
     ) else (
+      (* [atom] is not assigned, we can decide it *)
       new_decision_level self;
       let current_level = decision_level self in
       enqueue_bool self atom ~level:current_level Decision;
+      (* remember polarity *)
+      Var.set_last_pol self.store v (Atom.sign atom);
       Stat.incr self.n_decisions;
       Event.emit self.on_decision (Atom.lit self.store atom);
       true

src/sat/store.ml

@@ -23,6 +23,7 @@ type t = {
   v_reason: var_reason option Vec.t; (* reason for assignment *)
   v_seen: Bitvec.t; (* generic temporary marker *)
   v_default_polarity: Bitvec.t; (* default polarity in decisions *)
+  v_last_polarity: Bitvec.t; (* last polarity when deciding this *)
   mutable v_count: int;
   (* atoms *)
   a_is_true: Bitvec.t;
@@ -55,6 +56,7 @@ let create ?(size = `Big) ~stat () : t =
     v_reason = Vec.create ();
     v_seen = Bitvec.create ();
     v_default_polarity = Bitvec.create ();
+    v_last_polarity = Bitvec.create ();
     v_count = 0;
     a_is_true = Bitvec.create ();
     a_form = Vec.create ();
@@ -95,8 +97,16 @@ module Var = struct
   let[@inline] unmark self v = Bitvec.set self.v_seen (v : var :> int) false
   let[@inline] marked self v = Bitvec.get self.v_seen (v : var :> int)
 
+  let[@inline] set_last_pol self v b =
+    Bitvec.set self.v_last_polarity (v : var :> int) b
+
+  let[@inline] last_pol self v =
+    Bitvec.get self.v_last_polarity (v : var :> int)
+
   let[@inline] set_default_pol self v b =
-    Bitvec.set self.v_default_polarity (v : var :> int) b
+    Bitvec.set self.v_default_polarity (v : var :> int) b;
+    (* also update last polarity *)
+    set_last_pol self v b
 
   let[@inline] default_pol self v =
     Bitvec.get self.v_default_polarity (v : var :> int)
@@ -340,6 +350,7 @@ let alloc_var_uncached_ ?default_pol:(pol = true) self (form : Lit.t) : var =
     v_reason;
     v_seen;
     v_default_polarity;
+    v_last_polarity;
     stat_n_atoms;
     a_is_true;
     a_seen;
@@ -365,6 +376,8 @@ let alloc_var_uncached_ ?default_pol:(pol = true) self (form : Lit.t) : var =
   Bitvec.ensure_size v_seen v_idx;
   Bitvec.ensure_size v_default_polarity v_idx;
   Bitvec.set v_default_polarity v_idx pol;
+  Bitvec.ensure_size v_last_polarity v_idx;
+  Bitvec.set v_last_polarity v_idx pol;
 
   assert (Vec.size a_form = 2 * (v : var :> int));
   Bitvec.ensure_size a_is_true (2 * (v : var :> int));

src/sat/store.mli

@@ -37,6 +37,8 @@ module Var : sig
   val marked : store -> var -> bool
   val set_default_pol : store -> var -> bool -> unit
   val default_pol : store -> var -> bool
+  val set_last_pol : store -> var -> bool -> unit
+  val last_pol : store -> var -> bool
   val heap_idx : store -> var -> int
   val set_heap_idx : store -> var -> int -> unit
 end