private static bool FutilityPruningCanBeApplied(int depth, int rootDepth, bool friendlyKingInCheck, bool pvNode, int alpha)
{
var maxDepth = SearchConstants.FutilityPruningMaxDepth + rootDepth / SearchConstants.FutilityPruningMaxDepthDivisor;
return(!pvNode && depth <= maxDepth && !friendlyKingInCheck && !IterativeDeepening.IsScoreNearCheckmate(alpha));
}
private static bool RazoringCanBeApplied(int depth, int rootDepth, bool friendlyKingInCheck, bool pvNode, int alpha)
{
return(!pvNode && depth >= SearchConstants.RazoringMinDepth && depth <= SearchConstants.RazoringMaxDepth && !friendlyKingInCheck && !IterativeDeepening.IsScoreNearCheckmate(alpha));
}
private static bool StaticNullMoveCanBeApplied(int depth, int rootDepth, bool friendlyKingInCheck, bool pvNode, int beta)
{
var maxDepth = SearchConstants.StaticNullMoveMaxDepth + rootDepth / SearchConstants.StaticNullMoveMaxDepthDivider;
return(!pvNode && depth <= maxDepth && !friendlyKingInCheck && !IterativeDeepening.IsScoreNearCheckmate(beta));
}
public static int FindBestMove(SearchContext context, int depth, int ply, int alpha, int beta, bool allowNullMove, bool friendlyKingInCheck, int extensionsCount)
{
if (context.Statistics.Nodes >= context.MaxNodesCount)
{
context.AbortSearch = true;
return(0);
}
if (context.AbortSearch)
{
return(0);
}
context.Statistics.Nodes++;
if (context.BoardState.Pieces[context.BoardState.ColorToMove][Piece.King] == 0)
{
context.Statistics.Leafs++;
return(SearchConstants.NoKingValue);
}
if (context.BoardState.IsKingChecked(ColorOperations.Invert(context.BoardState.ColorToMove)))
{
context.Statistics.Leafs++;
return(-SearchConstants.NoKingValue);
}
if (context.BoardState.IsThreefoldRepetition())
{
context.Statistics.Leafs++;
return(EvaluationConstants.ThreefoldRepetition);
}
if (context.BoardState.IsInsufficientMaterial())
{
var enemyColor = ColorOperations.Invert(context.BoardState.ColorToMove);
if (!friendlyKingInCheck && !context.BoardState.IsKingChecked(enemyColor))
{
context.Statistics.Leafs++;
return(EvaluationConstants.InsufficientMaterial);
}
}
if (context.BoardState.IsFiftyMoveRuleDraw())
{
context.Statistics.Leafs++;
return(EvaluationConstants.ThreefoldRepetition);
}
if (depth <= 0)
{
context.Statistics.Leafs++;
return(QuiescenceSearch.FindBestMove(context, depth, ply, alpha, beta));
}
var originalAlpha = alpha;
var pvNode = beta - alpha > 1;
var entry = TranspositionTable.Get(context.BoardState.Hash);
var hashMove = Move.Empty;
var bestMove = Move.Empty;
if (entry.Flags != TranspositionTableEntryFlags.Invalid && entry.IsKeyValid(context.BoardState.Hash))
{
#if DEBUG
context.Statistics.TTHits++;
#endif
if (entry.Flags != TranspositionTableEntryFlags.AlphaScore)
{
var isMoveLegal = context.BoardState.IsMoveLegal(entry.BestMove);
if (isMoveLegal)
{
hashMove = entry.BestMove;
bestMove = entry.BestMove;
#if DEBUG
context.Statistics.TTValidMoves++;
#endif
}
#if DEBUG
else
{
context.Statistics.TTInvalidMoves++;
}
#endif
}
if (entry.Depth >= depth)
{
entry.Score = (short)TranspositionTable.TTToRegularScore(entry.Score, ply);
switch (entry.Flags)
{
case TranspositionTableEntryFlags.AlphaScore:
{
if (entry.Score < beta)
{
beta = entry.Score;
}
break;
}
case TranspositionTableEntryFlags.ExactScore:
{
if (!pvNode || IterativeDeepening.IsScoreNearCheckmate(entry.Score))
{
return(entry.Score);
}
break;
}
case TranspositionTableEntryFlags.BetaScore:
{
if (entry.Score > alpha)
{
alpha = entry.Score;
}
break;
}
}
if (alpha >= beta)
{
context.Statistics.BetaCutoffs++;
return(entry.Score);
}
}
}
#if DEBUG
else
{
entry = TranspositionTableEntry.Empty;
context.Statistics.TTNonHits++;
}
#endif
if (RazoringCanBeApplied(depth, context.Statistics.Depth, friendlyKingInCheck, pvNode, alpha))
{
var fastEvaluation = Evaluation.FastEvaluate(context.BoardState, context.Statistics.EvaluationStatistics);
var margin = SearchConstants.RazoringMargin + (depth - SearchConstants.RazoringMinDepth) * SearchConstants.RazoringMarginMultiplier;
var futileAlpha = alpha - margin;
if (fastEvaluation < futileAlpha)
{
var result = QuiescenceSearch.FindBestMove(context, depth, ply, futileAlpha, futileAlpha + 1);
if (result <= futileAlpha)
{
#if DEBUG
context.Statistics.Razorings++;
#endif
return(futileAlpha);
}
#if DEBUG
else
{
context.Statistics.RazoringsRejected++;
}
#endif
}
}
if (StaticNullMoveCanBeApplied(depth, context.Statistics.Depth, friendlyKingInCheck, pvNode, beta))
{
var fastEvaluation = Evaluation.FastEvaluate(context.BoardState, context.Statistics.EvaluationStatistics);
var margin = SearchConstants.StaticNullMoveMargin + (depth - 1) * SearchConstants.StaticNullMoveMarginMultiplier;
var score = fastEvaluation - margin;
if (score >= beta)
{
#if DEBUG
context.Statistics.StaticNullMovePrunes++;
#endif
return(score);
}
}
if (NullMoveCanBeApplied(context.BoardState, depth, allowNullMove, pvNode, friendlyKingInCheck))
{
context.BoardState.MakeNullMove();
var score = -FindBestMove(context, depth - 1 - NullMoveGetReduction(depth), ply + 1, -beta, -beta + 1, false, false, extensionsCount);
context.BoardState.UndoNullMove();
if (score >= beta)
{
#if DEBUG
context.Statistics.NullMovePrunes++;
#endif
return(score);
}
}
if (IIDCanBeApplied(depth, entry.Flags, hashMove))
{
FindBestMove(context, depth - 1 - SearchConstants.IIDDepthReduction, ply, alpha, beta, allowNullMove, friendlyKingInCheck, extensionsCount);
var iidEntry = TranspositionTable.Get(context.BoardState.Hash);
if (iidEntry.IsKeyValid(context.BoardState.Hash))
{
hashMove = iidEntry.BestMove;
#if DEBUG
context.Statistics.IIDHits++;
#endif
}
}
var futilityPruningCanBeApplied = false;
var futilityPruningEvaluation = 0;
var futilityPruningMargin = 0;
if (FutilityPruningCanBeApplied(depth, context.Statistics.Depth, friendlyKingInCheck, pvNode, alpha))
{
futilityPruningCanBeApplied = true;
futilityPruningEvaluation = Evaluation.FastEvaluate(context.BoardState, context.Statistics.EvaluationStatistics);
futilityPruningMargin = SearchConstants.FutilityPruningMargin + (depth - 1) * SearchConstants.FutilityPruningMarginMultiplier;
}
Span <Move> moves = stackalloc Move[SearchConstants.MaxMovesCount];
Span <short> moveValues = stackalloc short[SearchConstants.MaxMovesCount];
var movesCount = 0;
var loudMovesGenerated = false;
var quietMovesGenerated = false;
var evasionMask = ulong.MaxValue;
if (friendlyKingInCheck && !context.BoardState.IsKingChecked(ColorOperations.Invert(context.BoardState.ColorToMove)))
{
var kingField = context.BoardState.Pieces[context.BoardState.ColorToMove][Piece.King];
var kingFieldIndex = BitOperations.BitScan(kingField);
evasionMask = KnightMovesGenerator.GetMoves(kingFieldIndex) |
QueenMovesGenerator.GetMoves(context.BoardState.OccupancySummary, kingFieldIndex);
}
if (hashMove == Move.Empty)
{
movesCount = context.BoardState.GetLoudMoves(moves, 0, evasionMask);
MoveOrdering.AssignLoudValues(context.BoardState, moves, moveValues, movesCount, depth, Move.Empty);
loudMovesGenerated = true;
#if DEBUG
context.Statistics.LoudMovesGenerated++;
#endif
if (movesCount == 0)
{
movesCount = context.BoardState.GetQuietMoves(moves, 0, evasionMask);
MoveOrdering.AssignQuietValues(context.BoardState, moves, moveValues, 0, movesCount, ply);
quietMovesGenerated = true;
#if DEBUG
context.Statistics.QuietMovesGenerated++;
#endif
}
}
else
{
moves[0] = hashMove;
moveValues[0] = MoveOrderingConstants.HashMove;
movesCount = 1;
}
var pvs = true;
var bestScore = 0;
var allMovesPruned = true;
for (var moveIndex = 0; moveIndex < movesCount; moveIndex++)
{
if (LMPCanBeApplied(context, depth, friendlyKingInCheck, quietMovesGenerated, moveIndex, movesCount, pvNode))
{
break;
}
MoveOrdering.SortNextBestMove(moves, moveValues, movesCount, moveIndex);
if (loudMovesGenerated && moves[moveIndex] == hashMove)
{
goto postLoopOperations;
}
if (loudMovesGenerated && !quietMovesGenerated && moveValues[moveIndex] < 100)
{
var loudMovesCount = movesCount;
movesCount = context.BoardState.GetQuietMoves(moves, movesCount, evasionMask);
MoveOrdering.AssignQuietValues(context.BoardState, moves, moveValues, loudMovesCount, movesCount, ply);
MoveOrdering.SortNextBestMove(moves, moveValues, movesCount, moveIndex);
quietMovesGenerated = true;
#if DEBUG
context.Statistics.QuietMovesGenerated++;
#endif
if (moves[moveIndex] == hashMove)
{
goto postLoopOperations;
}
}
if (context.MoveRestrictions != null && ply == 0)
{
if (!context.MoveRestrictions.Contains(moves[moveIndex]))
{
goto postLoopOperations;
}
}
context.BoardState.MakeMove(moves[moveIndex]);
var enemyKingInCheck = context.BoardState.IsKingChecked(context.BoardState.ColorToMove);
var extension = GetExtensions(depth, extensionsCount, enemyKingInCheck);
#if DEBUG
context.Statistics.Extensions += extension;
#endif
if (futilityPruningCanBeApplied && FutilityPruningCanBeAppliedForMove(context, moves[moveIndex], enemyKingInCheck, pvs))
{
var gain = FutilityPruningGetGain(context, moves[moveIndex]);
if (futilityPruningEvaluation + futilityPruningMargin + gain <= alpha)
{
#if DEBUG
context.Statistics.FutilityPrunes++;
#endif
context.BoardState.UndoMove(moves[moveIndex]);
goto postLoopOperations;
}
}
allMovesPruned = false;
if (pvs)
{
bestScore = -FindBestMove(context, depth - 1 + extension, ply + 1, -beta, -alpha, allowNullMove, enemyKingInCheck, extensionsCount + extension);
pvs = false;
}
else
{
var lmrReduction = 0;
if (LMRCanBeApplied(context, depth, friendlyKingInCheck, enemyKingInCheck, moveIndex, moves, moveValues))
{
lmrReduction = LMRGetReduction(pvNode, moveIndex);
}
var score = -FindBestMove(context, depth - lmrReduction - 1 + extension, ply + 1, -alpha - 1, -alpha, allowNullMove, enemyKingInCheck, extensionsCount + extension);
if (score > alpha)
{
if (pvNode)
{
score = -FindBestMove(context, depth - 1 + extension, ply + 1, -beta, -alpha, allowNullMove, enemyKingInCheck, extensionsCount + extension);
}
else
{
if (lmrReduction != 0)
{
score = -FindBestMove(context, depth - 1 + extension, ply + 1, -beta, -alpha, allowNullMove, enemyKingInCheck, extensionsCount + extension);
}
}
}
if (score > bestScore)
{
bestScore = score;
}
}
context.BoardState.UndoMove(moves[moveIndex]);
if (bestScore > alpha)
{
alpha = bestScore;
bestMove = moves[moveIndex];
if (alpha >= beta)
{
if (moves[moveIndex].IsQuiet())
{
KillerHeuristic.AddKillerMove(moves[moveIndex], context.BoardState.ColorToMove, ply);
HistoryHeuristic.AddHistoryMove(context.BoardState.ColorToMove, context.BoardState.PieceTable[moves[moveIndex].From], moves[moveIndex].To, depth);
}
#if DEBUG
if (moveIndex == 0)
{
context.Statistics.BetaCutoffsAtFirstMove++;
}
else
{
context.Statistics.BetaCutoffsNotAtFirstMove++;
}
#endif
context.Statistics.BetaCutoffs++;
break;
}
}
postLoopOperations:
if (!loudMovesGenerated)
{
movesCount = context.BoardState.GetLoudMoves(moves, 0, evasionMask);
MoveOrdering.AssignLoudValues(context.BoardState, moves, moveValues, movesCount, depth, hashMove);
moveIndex = -1;
loudMovesGenerated = true;
#if DEBUG
context.Statistics.LoudMovesGenerated++;
#endif
if (movesCount == 0)
{
movesCount = context.BoardState.GetQuietMoves(moves, 0, evasionMask);
MoveOrdering.AssignQuietValues(context.BoardState, moves, moveValues, 0, movesCount, ply);
quietMovesGenerated = true;
#if DEBUG
context.Statistics.QuietMovesGenerated++;
#endif
}
}
if (!quietMovesGenerated && moveIndex == movesCount - 1)
{
var loudMovesCount = movesCount;
movesCount = context.BoardState.GetQuietMoves(moves, movesCount, evasionMask);
MoveOrdering.AssignQuietValues(context.BoardState, moves, moveValues, loudMovesCount, movesCount, ply);
quietMovesGenerated = true;
#if DEBUG
context.Statistics.QuietMovesGenerated++;
#endif
}
}
// Don't save invalid scores to the transposition table
if (context.AbortSearch)
{
return(0);
}
if (allMovesPruned)
{
return(alpha);
}
// Don't add invalid move (done after checkmate) to prevent strange behaviors
if (bestScore == -SearchConstants.NoKingValue)
{
return(bestScore);
}
// Return draw score or checkmate score as leafs
if (bestScore == SearchConstants.NoKingValue)
{
if (friendlyKingInCheck)
{
return(-EvaluationConstants.Checkmate + ply);
}
return(0);
}
if (entry.Flags == TranspositionTableEntryFlags.Invalid || alpha != originalAlpha)
{
if (entry.Age != context.TranspositionTableEntryAge || entry.Depth <= depth)
{
var valueToSave = alpha;
var entryType = alpha <= originalAlpha ? TranspositionTableEntryFlags.AlphaScore :
alpha >= beta ? TranspositionTableEntryFlags.BetaScore :
TranspositionTableEntryFlags.ExactScore;
valueToSave = TranspositionTable.RegularToTTScore(alpha, ply);
TranspositionTable.Add(context.BoardState.Hash, new TranspositionTableEntry(
context.BoardState.Hash,
(short)valueToSave, bestMove,
(byte)depth, entryType,
(byte)context.TranspositionTableEntryAge)
);
#if DEBUG
if (entry.Flags != TranspositionTableEntryFlags.Invalid)
{
context.Statistics.TTReplacements++;
}
context.Statistics.TTAddedEntries++;
#endif
}
}
return(bestScore);
}