Combination of two ideas:

VoyagerOne · glinscott · commit 8fd34d776307 · 2015-10-12T14:00:54.000-07:00
Apply bonus for the prior CMH that caused a fail low.

Balance Stats: CMH and History bonuses are updated differently.
This eliminates the "fudge" factor weight when scoring moves. Also
eliminated discontinuity in the gravity history stat formula. (i.e. stat
scores will no longer inverse when depth exceeds 22)

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 21802 W: 4107 L: 3887 D: 13808

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 46036 W: 7046 L: 6756 D: 32234

Bench: 7677367
diff --git a/src/movepick.cpp b/src/movepick.cpp
@@ -146,7 +146,7 @@ void MovePicker::score<QUIETS>() {
 
   for (auto& m : *this)
       m.value =  history[pos.moved_piece(m)][to_sq(m)]
-               + cmh[pos.moved_piece(m)][to_sq(m)] * 3;
+               + cmh[pos.moved_piece(m)][to_sq(m)];
 }
 
 template<>
diff --git a/src/movepick.h b/src/movepick.h
@@ -39,7 +39,7 @@
 template<typename T>
 struct Stats {
 
-  static const Value Max = Value(1 << 28);
+  static const Value Max = Value(1<<28);
 
   const T* operator[](Piece pc) const { return table[pc]; }
   T* operator[](Piece pc) { return table[pc]; }
@@ -51,10 +51,20 @@ struct Stats {
         table[pc][to] = m;
   }
 
-  void update(Piece pc, Square to, Value v) {
+  void updateH(Piece pc, Square to, Value v) {
 
-    table[pc][to] -= table[pc][to] * std::min(abs(v), 512) / 512;
-    table[pc][to] += v * 64;
+    if (abs(int(v)) >= 324) 
+        return;
+    table[pc][to] -= table[pc][to] * abs(int(v)) / 324;
+    table[pc][to] += int(v) * 32;
+  }
+
+  void updateCMH(Piece pc, Square to, Value v) {
+
+    if (abs(int(v)) >= 324) 
+        return;
+    table[pc][to] -= table[pc][to] * abs(int(v)) / 512;
+    table[pc][to] += int(v) * 64;
   }
 
 private:
diff --git a/src/search.cpp b/src/search.cpp
@@ -1132,6 +1132,19 @@ namespace {
     else if (bestMove && !pos.capture_or_promotion(bestMove))
         update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
 
+    // Bonus for prior countermove that caused the fail low
+    else if (!bestMove)
+    {
+        if (is_ok((ss - 2)->currentMove) && is_ok((ss - 1)->currentMove) && !pos.captured_piece_type() && !inCheck && depth>=3*ONE_PLY)
+        {
+            Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
+            Square prevSq = to_sq((ss - 1)->currentMove);
+            Square prevPrevSq = to_sq((ss - 2)->currentMove);
+            HistoryStats& flMoveCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
+            flMoveCmh.updateCMH(pos.piece_on(prevSq), prevSq, bonus);
+        }
+    }
+
     tte->save(posKey, value_to_tt(bestValue, ss->ply),
               bestValue >= beta ? BOUND_LOWER :
               PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
@@ -1405,29 +1418,29 @@ namespace {
     Square prevSq = to_sq((ss-1)->currentMove);
     HistoryStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
 
-    History.update(pos.moved_piece(move), to_sq(move), bonus);
+    History.updateH(pos.moved_piece(move), to_sq(move), bonus);
 
     if (is_ok((ss-1)->currentMove))
     {
         Countermoves.update(pos.piece_on(prevSq), prevSq, move);
-        cmh.update(pos.moved_piece(move), to_sq(move), bonus);
+        cmh.updateCMH(pos.moved_piece(move), to_sq(move), bonus);
     }
 
     // Decrease all the other played quiet moves
     for (int i = 0; i < quietsCnt; ++i)
     {
-        History.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
+        History.updateH(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
 
         if (is_ok((ss-1)->currentMove))
-            cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
+            cmh.updateCMH(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
     }
 
     // Extra penalty for PV move in previous ply when it gets refuted
     if (is_ok((ss-2)->currentMove) && (ss-1)->moveCount == 1 && !pos.captured_piece_type())
     {
         Square prevPrevSq = to_sq((ss-2)->currentMove);
         HistoryStats& ttMoveCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
-        ttMoveCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * depth / ONE_PLY - 1);
+        ttMoveCmh.updateCMH(pos.piece_on(prevSq), prevSq, -bonus - 2 * depth / ONE_PLY - 1);
     }
   }
 

Original file line number	Diff line number	Diff line change
`@@ -146,7 +146,7 @@ void MovePicker::score<QUIETS>() {`
`146`	`146`
`147`	`147`	`for (auto& m : *this)`
`148`	`148`	`m.value = history[pos.moved_piece(m)][to_sq(m)]`
`149`		`- + cmh[pos.moved_piece(m)][to_sq(m)] * 3;`
	`149`	`+ + cmh[pos.moved_piece(m)][to_sq(m)];`
`150`	`150`	`}`
`151`	`151`
`152`	`152`	`template<>`