/// <summary>
/// Gets the list pf PV nodes for the specified bitboard and color. Nodes count is limited by
/// <see cref="AIConstants.MaxDepth"/> value to avoid infinite repetitions.
/// </summary>
/// <param name="bitboard">The initial bitboard.</param>
/// <param name="color">The initial color.</param>
/// <returns>The list of PV nodes.</returns>
private PVNodesList GetPVNodes(Bitboard bitboard, Color color)
{
var pvNodes = new PVNodesList();
var boardHash = bitboard.GetHashForColor(color);
while (_transpositionTable.Exists(boardHash) && pvNodes.Count < AIConstants.MaxDepth)
{
var pvNode = _transpositionTable.Get(boardHash);
if (pvNode.BestMove == null)
{
break;
}
pvNodes.Add(pvNode.BestMove);
bitboard = bitboard.Move(pvNode.BestMove);
color = ColorOperations.Invert(color);
boardHash = bitboard.GetHashForColor(color);
}
return(pvNodes);
}
/// <summary>
/// Temporary method to calculating best move.
/// </summary>
/// <param name="color">The player color.</param>
/// <param name="bitboard">The bitboard.</param>
/// <param name="depth">The current depth.</param>
/// <param name="bestMove">The best possible move from nested nodes.</param>
/// <param name="stats">The AI stats.</param>
/// <returns>The evaluation score of best move.</returns>
public int Do(Color color, Bitboard bitboard, int depth, int alpha, int beta, AIStats stats)
{
var bestValue = AIConstants.InitialAlphaValue;
var enemyColor = ColorOperations.Invert(color);
var boardHash = bitboard.GetHashForColor(color);
var originalAlpha = alpha;
stats.TotalNodes++;
if (bitboard.IsThreefoldRepetition())
{
stats.EndNodes++;
return(0);
}
if (_transpositionTable.Exists(boardHash))
{
var transpositionNode = _transpositionTable.Get(boardHash);
if (transpositionNode.Depth >= depth)
{
stats.TranspositionTableHits++;
switch (transpositionNode.Type)
{
case ScoreType.Exact:
{
return(transpositionNode.Score);
}
case ScoreType.LowerBound:
{
alpha = Math.Max(alpha, transpositionNode.Score);
break;
}
case ScoreType.UpperBound:
{
beta = Math.Min(beta, transpositionNode.Score);
break;
}
}
if (alpha >= beta)
{
return(transpositionNode.Score);
}
}
}
if (depth <= 0)
{
stats.EndNodes++;
return(_quiescenceSearch.Do(color, bitboard, alpha, beta, stats));
}
var whiteGeneratorMode = GetGeneratorMode(color, Color.White);
var blackGeneratorMode = GetGeneratorMode(color, Color.Black);
bitboard.Calculate(whiteGeneratorMode, blackGeneratorMode, false);
if (bitboard.IsCheck(enemyColor))
{
stats.EndNodes++;
return(AIConstants.MateValue + depth);
}
Move bestMove = null;
var availableMoves = SortMoves(color, bitboard, bitboard.Moves);
var firstMove = true;
foreach (var move in availableMoves)
{
var bitboardAfterMove = bitboard.Move(move);
var nodeValue = 0;
if (firstMove)
{
nodeValue = -Do(enemyColor, bitboardAfterMove, depth - 1, -beta, -alpha, stats);
firstMove = false;
}
else
{
nodeValue = -Do(enemyColor, bitboardAfterMove, depth - 1, -alpha - 1, -alpha, stats);
if (nodeValue > alpha && nodeValue < beta)
{
bitboardAfterMove = bitboard.Move(move);
nodeValue = -Do(enemyColor, bitboardAfterMove, depth - 1, -beta, -alpha, stats);
}
}
if (nodeValue > bestValue)
{
bestValue = nodeValue;
bestMove = move;
}
alpha = Math.Max(nodeValue, alpha);
if (alpha >= beta)
{
stats.AlphaBetaCutoffs++;
break;
}
}
if (bestValue == -(AIConstants.MateValue + depth - 1) && !bitboard.IsCheck(color))
{
stats.EndNodes++;
return(0);
}
var updateTranspositionNode = new TranspositionNode();
updateTranspositionNode.Score = bestValue;
updateTranspositionNode.Depth = depth;
updateTranspositionNode.BestMove = bestMove;
if (bestValue <= originalAlpha)
{
updateTranspositionNode.Type = ScoreType.UpperBound;
}
else if (bestValue >= beta)
{
updateTranspositionNode.Type = ScoreType.LowerBound;
}
else
{
updateTranspositionNode.Type = ScoreType.Exact;
}
_transpositionTable.AddOrUpdate(boardHash, updateTranspositionNode);
return(bestValue);
}
/// <summary>
/// Regular search, the core of AI algorithms.
/// </summary>
/// <param name="color">The player color.</param>
/// <param name="bitboard">The bitboard.</param>
/// <param name="depth">The current depth.</param>
/// <param name="alpha">The alpha value.</param>
/// <param name="beta">The beta value.</param>
/// <param name="deadline">The deadline (time after which search is immediately terminated).</param>
/// <param name="helper">The flag indicating whether the search is an helper or not.</param>
/// <param name="stats">The AI stats.</param>
/// <returns>The evaluation score of best move.</returns>
public int Do(Color color, Bitboard bitboard, int depth, int alpha, int beta, long deadline, bool helper, AIStats stats)
{
var root = stats.TotalNodes == 0;
var bestValue = AIConstants.InitialAlphaValue;
var enemyColor = ColorOperations.Invert(color);
var boardHash = bitboard.GetHashForColor(color);
var originalAlpha = alpha;
stats.TotalNodes++;
if (bitboard.IsThreefoldRepetition())
{
stats.EndNodes++;
return(0);
}
#if TRANSPOSITION_TABLE
if (_transpositionTable.Exists(boardHash))
{
var transpositionNode = _transpositionTable.Get(boardHash);
if (transpositionNode.Depth >= depth)
{
stats.TranspositionTableHits++;
switch (transpositionNode.Type)
{
case ScoreType.Exact:
{
return(transpositionNode.Score);
}
case ScoreType.LowerBound:
{
alpha = Math.Max(alpha, transpositionNode.Score);
break;
}
case ScoreType.UpperBound:
{
beta = Math.Min(beta, transpositionNode.Score);
break;
}
}
if (alpha >= beta)
{
return(transpositionNode.Score);
}
}
}
#endif
if (depth <= 0)
{
stats.EndNodes++;
#if QUIESCENCE_SEARCH
return(_quiescenceSearch.Do(color, bitboard, alpha, beta, stats));
#else
bitboard.Calculate(GeneratorMode.CalculateAttacks, false);
return(bitboard.GetEvaluation());
#endif
}
var whiteGeneratorMode = GetGeneratorMode(color, Color.White);
var blackGeneratorMode = GetGeneratorMode(color, Color.Black);
bitboard.Calculate(whiteGeneratorMode, blackGeneratorMode, false);
if (bitboard.IsCheck(enemyColor))
{
stats.EndNodes++;
return(AIConstants.MateValue + depth);
}
Move bestMove = null;
var availableMoves = SortMoves(color, depth, bitboard, bitboard.Moves, helper);
var firstMove = true;
foreach (var move in availableMoves)
{
if (DateTime.Now.Ticks >= deadline)
{
break;
}
if (root)
{
if (_patternsDetector.IsPattern(bitboard, move))
{
continue;
}
}
var bitboardAfterMove = bitboard.Move(move);
var nodeValue = 0;
if (firstMove)
{
nodeValue = -Do(enemyColor, bitboardAfterMove, depth - 1, -beta, -alpha, deadline, helper, stats);
#if NEGASCOUT
firstMove = false;
#endif
}
else
{
nodeValue = -Do(enemyColor, bitboardAfterMove, depth - 1, -alpha - 1, -alpha, deadline, helper, stats);
if (nodeValue > alpha && nodeValue < beta)
{
bitboardAfterMove = bitboard.Move(move);
nodeValue = -Do(enemyColor, bitboardAfterMove, depth - 1, -beta, -alpha, deadline, helper, stats);
}
}
if (nodeValue > bestValue)
{
bestValue = nodeValue;
bestMove = move;
}
alpha = Math.Max(nodeValue, alpha);
if (alpha >= beta)
{
if (move is QuietMove)
{
_historyTable.AddKiller(color, depth, bestMove);
_killerTable.AddKiller(depth, move);
}
#if ALPHABETA_PRUNNING
stats.AlphaBetaCutoffs++;
break;
#endif
}
}
if (bestValue == -(AIConstants.MateValue + depth - 1) && !bitboard.IsCheck(color))
{
stats.EndNodes++;
return(0);
}
var updateTranspositionNode = new TranspositionNode
{
Score = bestValue,
Depth = depth,
BestMove = bestMove,
Type = GetTranspositionNodeType(originalAlpha, beta, bestValue)
};
_transpositionTable.AddOrUpdate(boardHash, updateTranspositionNode);
return(bestValue);
}