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🔍 Don't always stop search when a mate is found #827

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merged 2 commits into from
Jun 26, 2024
Merged

🔍 Don't always stop search when a mate is found #827

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4db503a
Don't stop search on every mate found
eduherminio Jun 25, 2024
f1ed209
Merge branch 'main' into dont-always-stop-search-on-mate
eduherminio Jun 26, 2024
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7 changes: 7 additions & 0 deletions src/Lynx/Constants.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -358,6 +358,13 @@ public static class Constants
/// </summary>
public const int ArrayDepthMargin = 16;

/// <summary>
/// Given mate in <see cref="MateDistanceMarginToStopSearching"/> (so this is full moves)
/// </summary>
public const int MateDistanceMarginToStopSearching = 2;

public const int MaxMateDistanceToStopSearching = 100 - (2 * MateDistanceMarginToStopSearching);

/// <summary>
/// From https://lichess.org/RViT3UWL
/// Failes input parsing fail if default input buffer size is used (see Lynx.Cli.Listener
Expand Down
39 changes: 24 additions & 15 deletions src/Lynx/Search/IDDFS.cs
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Expand Up @@ -92,6 +92,8 @@ public SearchResult IDDFS(int maxDepth, int softLimitTimeBound)
_engineWriter.TryWrite(InfoCommand.SearchResultInfo(lastSearchResult));
}

int mate = 0;

do
{
_absoluteSearchCancellationTokenSource.Token.ThrowIfCancellationRequested();
Expand Down Expand Up @@ -142,11 +144,14 @@ public SearchResult IDDFS(int maxDepth, int softLimitTimeBound)

var bestEvaluationAbs = Math.Abs(bestEvaluation);
isMateDetected = bestEvaluationAbs > EvaluationConstants.PositiveCheckmateDetectionLimit;
mate = isMateDetected
? Utils.CalculateMateInX(bestEvaluation, bestEvaluationAbs)
: 0;

lastSearchResult = UpdateLastSearchResult(lastSearchResult, bestEvaluation, alpha, beta, depth, isMateDetected, bestEvaluationAbs);
lastSearchResult = UpdateLastSearchResult(lastSearchResult, bestEvaluation, alpha, beta, depth, mate, bestEvaluationAbs);

_engineWriter.TryWrite(InfoCommand.SearchResultInfo(lastSearchResult));
} while (StopSearchCondition(++depth, maxDepth, isMateDetected, softLimitTimeBound));
} while (StopSearchCondition(++depth, maxDepth, mate, softLimitTimeBound));
}
catch (OperationCanceledException)
{
Expand All @@ -169,23 +174,33 @@ public SearchResult IDDFS(int maxDepth, int softLimitTimeBound)

var finalSearchResult = GenerateFinalSearchResult(lastSearchResult, bestEvaluation, alpha, beta, depth, firstLegalMove, isCancelled);

if (isMateDetected && finalSearchResult.Mate + Game.HalfMovesWithoutCaptureOrPawnMove < 96)
if (Configuration.EngineSettings.UseOnlineTablebaseInRootPositions
&& isMateDetected
&& (finalSearchResult.Mate * 2) + Game.HalfMovesWithoutCaptureOrPawnMove < Constants.MaxMateDistanceToStopSearching)
{
_logger.Info("Engine search found a short enough mate, cancelling online tb probing if still active");
_searchCancellationTokenSource.Cancel();
_logger.Info("Engine search found a short enough mate, cancelling online tb probing if still active");
}

_engineWriter.TryWrite(InfoCommand.SearchResultInfo(finalSearchResult));

return finalSearchResult;
}

private bool StopSearchCondition(int depth, int maxDepth, bool isMateDetected, int softLimitTimeBound)
private bool StopSearchCondition(int depth, int maxDepth, int mate, int softLimitTimeBound)
{
if (isMateDetected)
if (mate != 0)
{
_logger.Info("Stopping at depth {0}: mate detected", depth - 1);
return false;
var winningMateThreshold = (100 - Game.HalfMovesWithoutCaptureOrPawnMove) / 2;
_logger.Info("Depth {0}: mate in {1} detected ({2} moves until draw by repetition)", depth - 1, mate, winningMateThreshold);

if (mate < 0 || mate + Constants.MateDistanceMarginToStopSearching < winningMateThreshold)
{
_logger.Info("Stopping search: mate is short enough");
return false;
}

_logger.Info("Search continues, hoping to find a faster mate");
}

if (depth >= Configuration.EngineSettings.MaxDepth)
Expand Down Expand Up @@ -277,17 +292,11 @@ private bool OnlyOneLegalMove(ref Move firstLegalMove, [NotNullWhen(true)] out S

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private SearchResult UpdateLastSearchResult(SearchResult? lastSearchResult,
int bestEvaluation, int alpha, int beta, int depth, bool isMateDetected, int bestEvaluationAbs)
int bestEvaluation, int alpha, int beta, int depth, int mate, int bestEvaluationAbs)
{
var pvMoves = _pVTable.TakeWhile(m => m != default).ToList();
var maxDepthReached = _maxDepthReached.LastOrDefault(item => item != default);

int mate = default;
if (isMateDetected)
{
mate = Utils.CalculateMateInX(bestEvaluation, bestEvaluationAbs);
}

var elapsedTime = _stopWatch.ElapsedMilliseconds;

_previousSearchResult = lastSearchResult;
Expand Down
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