Remove history counters

Instead decrement history value on failure.

After 999 games at 1+0

Mod vs Orig  +236 =558 -204 51.60% +11 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/history.cpp

@@ -42,8 +42,6 @@ History::History() { clear(); }
 
 void History::clear() {
   memset(history, 0, 2 * 8 * 64 * sizeof(int));
-  memset(successCount, 0, 2 * 8 * 64 * sizeof(int));
-  memset(failureCount, 0, 2 * 8 * 64 * sizeof(int));
 }
 
 
@@ -58,7 +56,6 @@ void History::success(Piece p, Square to, Depth d) {
   assert(square_is_ok(to));
 
   history[p][to] += int(d) * int(d);
-  successCount[p][to]++;
 
   // Prevent history overflow
   if (history[p][to] >= HistoryMax)
@@ -72,12 +69,14 @@ void History::success(Piece p, Square to, Depth d) {
 /// called for each non-capturing move which failed to produce a beta cutoff
 /// at a node where a beta cutoff was finally found.
 
-void History::failure(Piece p, Square to) {
+void History::failure(Piece p, Square to, Depth d) {
 
   assert(piece_is_ok(p));
   assert(square_is_ok(to));
 
-  failureCount[p][to]++;
+  history[p][to] -= int(d) * int(d);
+  if (history[p][to] < 0)
+      history[p][to] = 0;
 }
 
 
@@ -91,15 +90,3 @@ int History::move_ordering_score(Piece p, Square to) const {
 
   return history[p][to];
 }
-
-
-/// History::ok_to_prune() decides whether a move has been sufficiently
-/// unsuccessful that it makes sense to prune it entirely.
-
-bool History::ok_to_prune(Piece p, Square to, Depth d) const {
-
-  assert(piece_is_ok(p));
-  assert(square_is_ok(to));
-
-  return (int(d) * successCount[p][to] < failureCount[p][to]);
-}

src/history.h

@@ -47,14 +47,11 @@ public:
   History();
   void clear();
   void success(Piece p, Square to, Depth d);
-  void failure(Piece p, Square to);
+  void failure(Piece p, Square to, Depth d);
   int move_ordering_score(Piece p, Square to) const;
-  bool ok_to_prune(Piece p, Square to, Depth d) const;
 
 private:
   int history[16][64];  // [piece][square]
-  int successCount[16][64];
-  int failureCount[16][64];
 };
 
 

src/search.cpp

@@ -288,7 +288,7 @@ namespace {
   bool move_is_killer(Move m, const SearchStack& ss);
   Depth extension(const Position& pos, Move m, bool pvNode, bool capture, bool check, bool singleReply, bool mateThreat, bool* dangerous);
   bool ok_to_do_nullmove(const Position& pos);
-  bool ok_to_prune(const Position& pos, Move m, Move threat, Depth d);
+  bool ok_to_prune(const Position& pos, Move m, Move threat);
   bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
   void update_history(const Position& pos, Move m, Depth depth, Move movesSearched[], int moveCount);
   void update_killers(Move m, SearchStack& ss);
@@ -1510,9 +1510,9 @@ namespace {
           && !captureOrPromotion
           &&  move != ttMove)
       {
-          // History pruning. See ok_to_prune() definition
+          // Move count based pruning
           if (   moveCount >= FutilityMoveCountMargin
-              && ok_to_prune(pos, move, ss[ply].threatMove, depth)
+              && ok_to_prune(pos, move, ss[ply].threatMove)
               && bestValue > value_mated_in(PLY_MAX))
               continue;
 
@@ -1841,9 +1841,9 @@ namespace {
           && !dangerous
           && !captureOrPromotion)
       {
-          // History pruning. See ok_to_prune() definition
+          // Move count based pruning
           if (   moveCount >= 2 + int(sp->depth)
-              && ok_to_prune(pos, move, ss[sp->ply].threatMove, sp->depth)
+              && ok_to_prune(pos, move, ss[sp->ply].threatMove)
               && sp->bestValue > value_mated_in(PLY_MAX))
               continue;
 
@@ -2464,7 +2464,7 @@ namespace {
   // non-tactical moves late in the move list close to the leaves are
   // candidates for pruning.
 
-  bool ok_to_prune(const Position& pos, Move m, Move threat, Depth d) {
+  bool ok_to_prune(const Position& pos, Move m, Move threat) {
 
     assert(move_is_ok(m));
     assert(threat == MOVE_NONE || move_is_ok(threat));
@@ -2498,11 +2498,7 @@ namespace {
         && pos.move_attacks_square(m, tto))
         return false;
 
-    // Case 4: Don't prune moves with good history
+    // Case 4: If the moving piece in the threatened move is a slider, don't
-    if (!H.ok_to_prune(pos.piece_on(mfrom), mto, d))
-        return false;
-
-    // Case 5: If the moving piece in the threatened move is a slider, don't
     // prune safe moves which block its ray.
     if (  !PruneBlockingMoves
         && threat != MOVE_NONE
@@ -2543,7 +2539,7 @@ namespace {
     {
         assert(m != movesSearched[i]);
         if (!pos.move_is_capture_or_promotion(movesSearched[i]))
-            H.failure(pos.piece_on(move_from(movesSearched[i])), move_to(movesSearched[i]));
+            H.failure(pos.piece_on(move_from(movesSearched[i])), move_to(movesSearched[i]), depth);
     }
   }