private async Task <EvaluatedMoveCollection> EvaluateMovesToDepthAsync(GameBoard gameBoard, int depth, IEnumerable <EvaluatedMove> movesToEvaluate, CancellationToken token)
{
double alpha = double.NegativeInfinity;
double beta = double.PositiveInfinity;
int color = gameBoard.CurrentTurnColor == PlayerColor.White ? 1 : -1;
double alphaOriginal = alpha;
double?bestValue = null;
EvaluatedMoveCollection evaluatedMoves = new EvaluatedMoveCollection();
bool firstMove = true;
foreach (EvaluatedMove moveToEvaluate in movesToEvaluate)
{
bool updateAlpha = false;
if (token.IsCancellationRequested)
{
// Cancel
return(new EvaluatedMoveCollection(movesToEvaluate, false));
}
gameBoard.TrustedPlay(moveToEvaluate.Move);
double?value = null;
if (firstMove)
{
// Full window search
value = -1 * await PrincipalVariationSearchAsync(gameBoard, depth - 1, -beta, -alpha, -color, OrderType.Default, token);
updateAlpha = true;
firstMove = false;
}
else
{
// Null window search
value = -1 * await PrincipalVariationSearchAsync(gameBoard, depth - 1, -alpha - double.Epsilon, -alpha, -color, OrderType.Default, token);
if (value.HasValue && value > alpha && value < beta)
{
// Research with full window
value = -1 * await PrincipalVariationSearchAsync(gameBoard, depth - 1, -beta, -alpha, -color, OrderType.Default, token);
updateAlpha = true;
}
}
gameBoard.UndoLastMove();
if (!value.HasValue)
{
// Cancel occurred during evaluation
return(new EvaluatedMoveCollection(movesToEvaluate, false));
}
EvaluatedMove evaluatedMove = new EvaluatedMove(moveToEvaluate.Move, value.Value, depth);
evaluatedMoves.Add(evaluatedMove);
if (updateAlpha)
{
alpha = Math.Max(alpha, value.Value);
}
if (!bestValue.HasValue || value >= bestValue)
{
bestValue = value;
}
if (bestValue >= beta)
{
// A winning move has been found, since beta is always infinity in this function
break;
}
}
ulong key = gameBoard.ZobristKey;
TranspositionTableEntry tEntry = new TranspositionTableEntry();
if (bestValue <= alphaOriginal)
{
// Losing move since alphaOriginal os negative infinity in this function
tEntry.Type = TranspositionTableEntryType.UpperBound;
}
else
{
// Move is a lower bound winning move if bestValue >= beta (always infinity in this function), otherwise it's exact
tEntry.Type = bestValue >= beta ? TranspositionTableEntryType.LowerBound : TranspositionTableEntryType.Exact;
tEntry.BestMove = evaluatedMoves.BestMove.Move;
}
tEntry.Value = bestValue.Value;
tEntry.Depth = depth;
TranspositionTable.Store(key, tEntry);
return(evaluatedMoves);
}
private async Task <EvaluatedMoveCollection> EvaluateMovesAsync(GameBoard gameBoard, BestMoveParams bestMoveParams, CancellationToken token)
{
EvaluatedMoveCollection movesToEvaluate = new EvaluatedMoveCollection();
EvaluatedMove bestMove = null;
// Try to get cached best move if available
ulong key = gameBoard.ZobristKey;
if (TranspositionTable.TryLookup(key, out TranspositionTableEntry tEntry) && null != tEntry.BestMove)
{
bestMove = new EvaluatedMove(tEntry.BestMove, tEntry.Value, tEntry.Depth);
OnBestMoveFound(bestMoveParams, bestMove);
}
if (null != bestMove && double.IsPositiveInfinity(bestMove.ScoreAfterMove))
{
// Winning move, don't search
movesToEvaluate.Add(bestMove);
return(movesToEvaluate);
}
List <EvaluatedMove> validMoves = GetPreSortedValidMoves(gameBoard, bestMove);
movesToEvaluate.Add(validMoves, false);
if (movesToEvaluate.Count <= 1 || bestMoveParams.MaxSearchDepth == 0)
{
// No need to search
return(movesToEvaluate);
}
// Iterative search
int depth = 1 + Math.Max(0, movesToEvaluate.BestMove.Depth);
while (depth <= bestMoveParams.MaxSearchDepth)
{
// Start LazySMP helper threads
CancellationTokenSource helperCTS = new CancellationTokenSource();
Task[] helperThreads = StartHelperThreads(gameBoard, depth, bestMoveParams.MaxHelperThreads, helperCTS);
// "Re-sort" moves to evaluate based on the next iteration
movesToEvaluate = await EvaluateMovesToDepthAsync(gameBoard, depth, movesToEvaluate, token);
// End LazySMP helper threads
EndHelperThreads(helperThreads, helperCTS);
// Fire BestMoveFound for current depth
OnBestMoveFound(bestMoveParams, movesToEvaluate.BestMove);
if (double.IsInfinity(movesToEvaluate.BestMove.ScoreAfterMove))
{
// The best move ends the game, stop searching
break;
}
// Prune game-losing moves if possible
movesToEvaluate.PruneGameLosingMoves();
if (movesToEvaluate.Count <= 1)
{
// Only one move, no reason to keep looking
break;
}
if (token.IsCancellationRequested)
{
// Cancelled, stop searching
break;
}
depth = 1 + Math.Max(depth, movesToEvaluate.BestMove.Depth);
}
return(movesToEvaluate);
}