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Use extension concept for check-extensions and IIR
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@eduherminio
eduherminio committed Jan 9, 2025
1 parent f3b7787 commit fca774d
Showing 1 changed file with 20 additions and 12 deletions.
32 changes: 20 additions & 12 deletions src/Lynx/Search/NegaMax.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,8 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool cutnode, Cance

bool isRoot = ply == 0;
bool pvNode = beta - alpha > 1;
int extension = 0;

ShortMove ttBestMove = default;
NodeType ttElementType = default;
int ttScore = default;
Expand All @@ -63,7 +65,7 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool cutnode, Cance
else if (depth <= Configuration.EngineSettings.TTHit_NoCutoffExtension_MaxDepth)
{
// Extension idea from Stormphrax
++depth;
++extension;
}
}

Expand All @@ -74,7 +76,7 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool cutnode, Cance
// which we'll be able to use later for the full depth search
if (ttElementType == default && depth >= Configuration.EngineSettings.IIR_MinDepth)
{
--depth;
--extension;
}
}

Expand All @@ -94,10 +96,10 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool cutnode, Cance

if (isInCheck)
{
++depth;
++extension;
staticEval = position.StaticEvaluation(Game.HalfMovesWithoutCaptureOrPawnMove).Score;
}
else if (depth <= 0)
else if (depth + extension <= 0)
{
if (MoveGenerator.CanGenerateAtLeastAValidMove(position))
{
Expand Down Expand Up @@ -170,7 +172,7 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool cutnode, Cance

if (score < beta) // Static evaluation + bonus indicates fail-low node
{
if (depth == 1)
if (depth + extension == 1) // TODO extension
{
var qSearchScore = QuiescenceSearch(ply, alpha, beta, cancellationToken);

Expand Down Expand Up @@ -216,7 +218,7 @@ private int NegaMax(int depth, int ply, int alpha, int beta, bool cutnode, Cance
// 3 + (depth / 3) + Math.Min((staticEval - beta) / 200, 3));

var gameState = position.MakeNullMove();
var nmpScore = -NegaMax(depth - 1 - nmpReduction, ply + 1, -beta, -beta + 1, !cutnode, cancellationToken, parentWasNullMove: true);
var nmpScore = -NegaMax(depth + extension - 1 - nmpReduction, ply + 1, -beta, -beta + 1, !cutnode, cancellationToken, parentWasNullMove: true);
position.UnMakeNullMove(gameState);

if (nmpScore >= beta)
Expand Down Expand Up @@ -310,7 +312,7 @@ void RevertMove()
_tt.PrefetchTTEntry(position);
bool isCutNode = !pvNode && !cutnode; // Linter 'simplification' of pvNode ? false : !cutnode
#pragma warning disable S2234 // Arguments should be passed in the same order as the method parameters
score = -NegaMax(depth - 1, ply + 1, -beta, -alpha, isCutNode, cancellationToken);
score = -NegaMax(depth + extension - 1, ply + 1, -beta, -alpha, isCutNode, cancellationToken);
#pragma warning restore S2234 // Arguments should be passed in the same order as the method parameters
}
else
Expand Down Expand Up @@ -360,6 +362,9 @@ void RevertMove()

// 🔍 Late Move Reduction (LMR) - search with reduced depth
// Impl. based on Ciekce (Stormphrax) and Martin (Motor) advice, and Stormphrax & Akimbo implementations
var newDepth = depth - 1 + extension;
int reducedDepth = newDepth;

if (isNotGettingCheckmated)
{
if (!isCapture
Expand Down Expand Up @@ -396,7 +401,7 @@ void RevertMove()

// Don't allow LMR to drop into qsearch or increase the depth
// depth - 1 - depth +2 = 1, min depth we want
reduction = Math.Clamp(reduction, 0, depth - 2);
reduction = Math.Clamp(reduction, 0, newDepth - 1);
}

// 🔍 Static Exchange Evaluation (SEE) reduction
Expand All @@ -406,12 +411,15 @@ void RevertMove()
&& moveScores[moveIndex] >= EvaluationConstants.BadCaptureMoveBaseScoreValue)
{
reduction += Configuration.EngineSettings.SEE_BadCaptureReduction;
reduction = Math.Clamp(reduction, 0, depth - 1);
reduction = Math.Clamp(reduction, 0, newDepth);
}

// Don't allow LMR to drop into qsearch or increase the depth
reducedDepth -= reduction;
}

// Search with reduced depth and zero window
score = -NegaMax(depth - 1 - reduction, ply + 1, -alpha - 1, -alpha, cutnode: true, cancellationToken);
score = -NegaMax(reducedDepth, ply + 1, -alpha - 1, -alpha, cutnode: true, cancellationToken);

// 🔍 Principal Variation Search (PVS)
if (score > alpha && reduction > 0)
Expand All @@ -421,14 +429,14 @@ void RevertMove()
// https://web.archive.org/web/20071030220825/http://www.brucemo.com/compchess/programming/pvs.htm

// Search with full depth but narrowed score bandwidth
score = -NegaMax(depth - 1, ply + 1, -alpha - 1, -alpha, !cutnode, cancellationToken);
score = -NegaMax(newDepth, ply + 1, -alpha - 1, -alpha, !cutnode, cancellationToken);
}

if (score > alpha && score < beta)
{
// PVS Hypothesis invalidated -> search with full depth and full score bandwidth
#pragma warning disable S2234 // Arguments should be passed in the same order as the method parameters
score = -NegaMax(depth - 1, ply + 1, -beta, -alpha, cutnode: false, cancellationToken);
score = -NegaMax(newDepth, ply + 1, -beta, -alpha, cutnode: false, cancellationToken);
#pragma warning restore S2234 // Arguments should be passed in the same order as the method parameters
}
}
Expand Down

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