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Introduce Triple Extensions #5027

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Introduce Triple Extensions #5027

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c5ef342
Introduce Triple Extensions
Viren6 Feb 1, 2024
7bd9dd7
Fix double extension cap
Viren6 Feb 2, 2024
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15 changes: 5 additions & 10 deletions src/search.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -529,7 +529,7 @@ Value Search::Worker::search(
Move ttMove, move, excludedMove, bestMove;
Depth extension, newDepth;
Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
bool givesCheck, improving, priorCapture, singularQuietLMR;
bool givesCheck, improving, priorCapture;
bool capture, moveCountPruning, ttCapture;
Piece movedPiece;
int moveCount, captureCount, quietCount;
Expand Down Expand Up @@ -907,7 +907,7 @@ Value Search::Worker::search(
contHist, &thisThread->pawnHistory, countermove, ss->killers);

value = bestValue;
moveCountPruning = singularQuietLMR = false;
moveCountPruning = false;

// Step 13. Loop through all pseudo-legal moves until no moves remain
// or a beta cutoff occurs.
Expand Down Expand Up @@ -1042,12 +1042,11 @@ Value Search::Worker::search(
if (value < singularBeta)
{
extension = 1;
singularQuietLMR = !ttCapture;

// Avoid search explosion by limiting the number of double extensions
if (!PvNode && value < singularBeta - 2 && ss->doubleExtensions <= 12)
if (!PvNode && value < singularBeta - 2 && ss->doubleExtensions <= 15)
{
extension = 2;
extension = 2 + (value < singularBeta - 200 && !ttCapture);
depth += depth < 15;
}
}
Expand Down Expand Up @@ -1098,7 +1097,7 @@ Value Search::Worker::search(

// Add extension to new depth
newDepth += extension;
ss->doubleExtensions = (ss - 1)->doubleExtensions + (extension == 2);
ss->doubleExtensions = (ss - 1)->doubleExtensions + (extension >= 2);

// Speculative prefetch as early as possible
prefetch(tt.first_entry(pos.key_after(move)));
Expand Down Expand Up @@ -1132,10 +1131,6 @@ Value Search::Worker::search(
if (PvNode && tte->bound() != BOUND_UPPER)
r--;

// Decrease reduction if a quiet ttMove has been singularly extended (~1 Elo)
if (singularQuietLMR)
r--;

// Increase reduction on repetition (~1 Elo)
if (move == (ss - 4)->currentMove && pos.has_repeated())
r += 2;
Expand Down
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