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Simplify reductions on singular extension

Browse source 
Current master employs a scheme to adjust reductions on singular
nodes that is somewhat controversial, see
https://github.com/official-stockfish/Stockfish/pull/2167

This patch removes this use of a search result outside of [a,b],
by observing that the main effect of this code is to adjust the
reduction by an average of ~2 (1.7) rather than 1.

Claims the first blue at STC and LTC:

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 30142 W: 6547 L: 6442 D: 17153
http://tests.stockfishchess.org/tests/view/5daf16c40ebc5902c06da566

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 45715 W: 7380 L: 7298 D: 31037
http://tests.stockfishchess.org/tests/view/5daf2f3c0ebc5902c06da6c7

Closes https://github.com/official-stockfish/Stockfish/pull/2367

Bench: 5115841
This commit is contained in:
Joost VandeVondele 2019-10-21 22:21:50 +02:00 • committed by Stéphane Nicolet
parent 215cd19108
commit 7e89a71624
1 changed files with 7 additions and 9 deletions
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16
src/search.cpp
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@ -595,16 +595,16 @@ namespace {
Depth extension, newDepth;
Value bestValue, value, ttValue, eval, maxValue;
bool ttHit, ttPv, inCheck, givesCheck, improving, doLMR, priorCapture;
bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture, singularLMR;
Piece movedPiece;
int moveCount, captureCount, quietCount, singularLMR;
int moveCount, captureCount, quietCount;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
inCheck = pos.checkers();
priorCapture = pos.captured_piece();
Color us = pos.side_to_move();
moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
moveCount = captureCount = quietCount = ss->moveCount = 0;
bestValue = -VALUE_INFINITE;
maxValue = VALUE_INFINITE;
@ -917,7 +917,7 @@ moves_loop: // When in check, search starts from here
ss->killers);
value = bestValue;
moveCountPruning = false;
singularLMR = moveCountPruning = false;
ttCapture = ttMove && pos.capture_or_promotion(ttMove);
// Mark this node as being searched
@ -980,10 +980,7 @@ moves_loop: // When in check, search starts from here
if (value < singularBeta)
{
extension = 1;
singularLMR++;
if (value < singularBeta - std::min(4 * depth, 36))
singularLMR++;
singularLMR = true;
}
// Multi-cut pruning
@ -1106,7 +1103,8 @@ moves_loop: // When in check, search starts from here
r--;
// Decrease reduction if ttMove has been singularly extended
r -= singularLMR;
if (singularLMR)
r -= 2;
if (!captureOrPromotion)
{