Reduce futility_margin if opponents last move was bad

This reduces the futiltiy_margin if our opponents last move was bad by
around ~1/3 when not improving and ~1/2.7 when improving, the idea being
to retroactively futility prune moves that were played, but turned out
to be bad.  A bad move is being defined as their staticEval before their
move being lower as our staticEval now is. If the depth is 2 and we are
improving the opponent worsening flag is not set, in order to not risk
having a too low futility_margin, due to the fact that when these
conditions are met the futility_margin already drops quite low.

src/search.cpp

@@ -50,9 +50,12 @@ using namespace Search;
 namespace {
 
 // Futility margin
-Value futility_margin(Depth d, bool noTtCutNode, bool improving) {
-    Value futilityMult = 138 - 28 * noTtCutNode;
-    return (futilityMult * d - 3 * futilityMult / 2 * improving);
+Value futility_margin(Depth d, bool noTtCutNode, bool improving, bool oppWorsening) {
+    Value futilityMult       = 138 - 28 * noTtCutNode;
+    Value improvingDeduction = 3 * improving * futilityMult / 2;
+    Value worseningDeduction = (331 + 45 * improving) * oppWorsening * futilityMult / 1024;
+
+    return futilityMult * d - improvingDeduction - worseningDeduction;
 }
 
 constexpr int futility_move_count(bool improving, Depth depth) {
@@ -472,7 +475,7 @@ Value Search::Worker::search(
     Move     ttMove, move, excludedMove, bestMove;
     Depth    extension, newDepth;
     Value    bestValue, value, ttValue, eval, maxValue, probCutBeta;
-    bool     givesCheck, improving, priorCapture;
+    bool     givesCheck, improving, priorCapture, opponentWorsening;
     bool     capture, moveCountPruning, ttCapture;
     Piece    movedPiece;
     int      moveCount, captureCount, quietCount;
@@ -644,6 +647,8 @@ Value Search::Worker::search(
                 ? ss->staticEval > (ss - 2)->staticEval
                 : (ss - 4)->staticEval != VALUE_NONE && ss->staticEval > (ss - 4)->staticEval;
 
+    opponentWorsening = ss->staticEval + (ss - 1)->staticEval > 2 && (depth != 2 || !improving);
+
     // Step 6. Razoring (~1 Elo)
     // If eval is really low check with qsearch if it can exceed alpha, if it can't,
     // return a fail low.
@@ -657,7 +662,7 @@ Value Search::Worker::search(
     // Step 7. Futility pruning: child node (~40 Elo)
     // The depth condition is important for mate finding.
     if (!ss->ttPv && depth < 8
-        && eval - futility_margin(depth, cutNode && !ss->ttHit, improving)
+        && eval - futility_margin(depth, cutNode && !ss->ttHit, improving, opponentWorsening)
                - (ss - 1)->statScore / 253
              >= beta
         && eval >= beta && eval < 19543  // smaller than wins.