private Option <int> CalculateEvaluationRecursively(
Position.Position position,
int currentDepth)
{
var bestMove = Move.Move.NullMove;
var bestMoveEvaluation = MinEvaluationValue;
bool hasLegalMove = false;
var playerEvaluationMultiplier = GetPlayerEvaluationMultiplier(position.PlayerToMove);
var playerToMove = position.PlayerToMove;
var pseudoLegalMoves = MoveGenerator.GeneratePseudoLegalMoves(position);
foreach (var move in pseudoLegalMoves)
{
position.PlayMove(move);
if (!IsPlayedMoveLegal(position))
{
position.UndoMove();
continue;
}
hasLegalMove = true;
int movePriority = CalculateMovePriority(move, currentDepth);
if (movePriority < MinPriorityToConsiderMove)
{
position.UndoMove();
continue;
}
evaluator.OnMovePlayed(move, playerToMove);
var evaluation = -CalculateEvaluationRecursively(
position,
currentDepth + 1)
.ValueOr(() => - playerEvaluationMultiplier * evaluator.GetCurrentEvaluation());
if (evaluation >= bestMoveEvaluation)
{
bestMove = move;
bestMoveEvaluation = evaluation;
}
position.UndoMove();
evaluator.OnUndoMove();
}
if (!hasLegalMove)
{
return(Option.Some(
GetEvaluationForPositionWithoutLegalMoves(
position,
playerEvaluationMultiplier)));
}
return(bestMove.IsNullMove ? Option.None <int>() : Option.Some(bestMoveEvaluation));
}