private int PrincipalVariationSearch(Board board, Colour colour, byte depth, ushort ply, int alpha, int beta, uint[] parentPrincipalVariation)
{
++PositionCount;
if (depth == 0)
{
return(positionEvaluator.Evaluate(board) * (colour == Colour.White ? 1 : -1));
}
var transpositionKey = board.Key;
var existingTransposition = transpositionTable.Find(transpositionKey);
uint previousBestMove = 0;
if (existingTransposition.Key != 0)
{
if (existingTransposition.Depth > 1 && existingTransposition.Depth > depth)
{
return(existingTransposition.Evaluation);
}
if (existingTransposition.BestMove != 0)
{
previousBestMove = existingTransposition.BestMove;
}
}
var bestScore = int.MinValue;
var bestMove = 0u;
var principalVariation = new uint[64];
var moveCount = 1;
foreach (var move in GetNextMove(moveGenerator, ply, board, colour, previousBestMove))
{
board.MakeMove(move);
var evaluatedScore = 0;
var oppositeColour = colour.Opposite();
var nextDepth = (byte)(depth - 1);
var nextPly = (ushort)(ply + 1);
// https://www.chessprogramming.org/Principal_Variation_Search
if (moveCount == 1)
{
// Always do a full search on the first/PV move
evaluatedScore = -PrincipalVariationSearch(board, oppositeColour, nextDepth, nextPly, -beta, -alpha, principalVariation);
}
else
{
// Late Move Reduction http://mediocrechess.blogspot.com/2007/03/other-late-move-reduction-lmr.html
if (ply > 3 && moveCount > 3 && move.GetCapturePieceType() == PieceType.None && move.GetNumCheckers() == 0 && nextDepth > 0)
{
--nextDepth;
}
// Search with aspiration window
evaluatedScore = -PrincipalVariationSearch(board, oppositeColour, nextDepth, nextPly, -alpha - 1, -alpha, principalVariation);
if (alpha < evaluatedScore && evaluatedScore < beta)
{
// Re-search
evaluatedScore = -PrincipalVariationSearch(board, oppositeColour, nextDepth, nextPly, -beta, -evaluatedScore, principalVariation);
}
}
board.UnMakeMove(move);
if (alpha < evaluatedScore)
{
// alpha = Math.Max(bestScore, evaluatedScore); // alpha acts like max in MiniMax
alpha = evaluatedScore;
bestMove = move;
bestScore = evaluatedScore;
UpdatePrincipalVariation(principalVariation, parentPrincipalVariation, ply, move);
}
if (alpha > beta)
{
// Fail-hard beta-cutoff
break;
}
++moveCount;
if (ply == 1)
{
var info = new Info(PositionCount, stopWatch.ElapsedMilliseconds, depth, transpositionTable);
Info?.Invoke(this, new InfoEventArgs(info));
}
}
transpositionTable.Set(transpositionKey, depth, colour, bestScore, bestMove);
if (alpha == int.MinValue)
{
alpha = PieceValues.CheckmateValue + ply;
}
if (alpha == int.MaxValue)
{
alpha = -(PieceValues.CheckmateValue + ply);
}
return(alpha);
}
private int Evaluate() => positionEvaluator.Evaluate(board);