From b34a690cd4aa6d828ae0f47b427167f4e6392db7 Mon Sep 17 00:00:00 2001
From: rn5f107s2 <clemens.lerchl@gmail.com>
Date: Thu, 30 May 2024 21:18:42 +0200
Subject: [PATCH] MCP more after a bad singular search

The idea is, that if we have the information that the singular search failed low and therefore produced an upperbound score, we can use the score from singularsearch as approximate upperbound as to what bestValue our non ttMoves will produce. If this value is well below alpha, we assume that all non-ttMoves will score below alpha and therfore can skip more moves.
This patch also sets up variables for future patches wanting to use teh singular search result outside of singular extensions, in singularBound and singularValue, meaning further patches using this search result to affect various pruning techniques can be tried.

Passed STC:
https://tests.stockfishchess.org/tests/view/6658d13e6b0e318cefa90120
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 85632 W: 22112 L: 21725 D: 41795
Ptnml(0-2): 243, 10010, 21947, 10349, 267

Passed LTC:
https://tests.stockfishchess.org/tests/view/6658dd356b0e318cefa9016a
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 243978 W: 62014 L: 61272 D: 120692
Ptnml(0-2): 128, 26598, 67791, 27348, 124

closes https://github.com/official-stockfish/Stockfish/pull/5325

bench 1397172
---
 src/search.cpp | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index 4086d50f..f738530a 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -550,11 +550,12 @@ Value Search::Worker::search(
     Key      posKey;
     Move     ttMove, move, excludedMove, bestMove;
     Depth    extension, newDepth;
-    Value    bestValue, value, ttValue, eval, maxValue, probCutBeta;
+    Value    bestValue, value, ttValue, eval, maxValue, probCutBeta, singularValue;
     bool     givesCheck, improving, priorCapture, opponentWorsening;
     bool     capture, moveCountPruning, ttCapture;
     Piece    movedPiece;
     int      moveCount, captureCount, quietCount;
+    Bound    singularBound;
 
     // Step 1. Initialize node
     Worker* thisThread = this;
@@ -923,6 +924,8 @@ moves_loop:  // When in check, search starts here
 
     value            = bestValue;
     moveCountPruning = false;
+    singularValue    = VALUE_INFINITE;
+    singularBound    = BOUND_NONE;
 
     // Step 13. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
@@ -972,7 +975,9 @@ moves_loop:  // When in check, search starts here
         if (!rootNode && pos.non_pawn_material(us) && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
         {
             // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~8 Elo)
-            moveCountPruning = moveCount >= futility_move_count(improving, depth);
+            moveCountPruning =
+              moveCount >= futility_move_count(improving, depth)
+                             - (singularBound == BOUND_UPPER && singularValue < alpha - 50);
 
             // Reduced depth of the next LMR search
             int lmrDepth = newDepth - r;
@@ -1058,8 +1063,9 @@ moves_loop:  // When in check, search starts here
                 Depth singularDepth = newDepth / 2;
 
                 ss->excludedMove = move;
-                value =
+                value            = singularValue =
                   search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
+                singularBound    = singularValue >= singularBeta ? BOUND_LOWER : BOUND_UPPER;
                 ss->excludedMove = Move::none();
 
                 if (value < singularBeta)