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Reduce more if multiple moves exceed alpha

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Idea of this patch is the following: in case we already have four moves that
exceeded alpha in the current node, the probability of finding fifth should
be reasonably low. Note that four is completely arbitrary - there could and
probably should be some tweaks, both in tweaking best move count threshold
for more reductions and tweaking how they work - for example making more
reductions with best move count linearly.

passed STC:
https://tests.stockfishchess.org/tests/view/615f614783dd501a05b0aee2
LLR: 2.94 (-2.94,2.94) <-0.50,2.50>
Total: 141816 W: 36056 L: 35686 D: 70074
Ptnml(0-2): 499, 15131, 39273, 15511, 494

passed LTC:
https://tests.stockfishchess.org/tests/view/615fdff683dd501a05b0af35
LLR: 2.94 (-2.94,2.94) <0.50,3.50>
Total: 68536 W: 17221 L: 16891 D: 34424
Ptnml(0-2): 38, 6573, 20725, 6885, 47

closes https://github.com/official-stockfish/Stockfish/pull/3736

Bench: 6131513
This commit is contained in:
Michael Chaly 2021-10-09 02:15:43 +03:00 • committed by Stéphane Nicolet
parent f21a66f70d
commit c8459b18ba
1 changed files with 8 additions and 4 deletions
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12
src/search.cpp
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@ -591,13 +591,13 @@ namespace {
bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
ttCapture, singularQuietLMR, noLMRExtension;
Piece movedPiece;
int moveCount, captureCount, quietCount;
int moveCount, captureCount, quietCount, bestMoveCount;
// Step 1. Initialize node
ss->inCheck = pos.checkers();
priorCapture = pos.captured_piece();
Color us = pos.side_to_move();
moveCount = captureCount = quietCount = ss->moveCount = 0;
moveCount = bestMoveCount = captureCount = quietCount = ss->moveCount = 0;
bestValue = -VALUE_INFINITE;
maxValue = VALUE_INFINITE;
@ -1186,8 +1186,9 @@ moves_loop: // When in check, search starts here
{
Depth r = reduction(improving, depth, moveCount, rangeReduction > 2);
// Decrease reduction if on the PV (~1 Elo)
if (PvNode)
// Decrease reduction if on the PV (~2 Elo)
if ( PvNode
&& bestMoveCount <= 3)
r--;
// Decrease reduction if the ttHit running average is large (~0 Elo)
@ -1340,7 +1341,10 @@ moves_loop: // When in check, search starts here
update_pv(ss->pv, move, (ss+1)->pv);
if (PvNode && value < beta) // Update alpha! Always alpha < beta
{
alpha = value;
bestMoveCount++;
}
else
{
assert(value >= beta); // Fail high