public Position MakeAMove(IBoard board)
{
this.leafCount = 0;
IBoard boardCopy = this.boardFactory.DeepCloneBoard(board);
AiSearchResult result = Search(isMaxSearch: true,
board: boardCopy,
curPlayer: player,
depth: 0,
minPossibleScore: double.NegativeInfinity,
maxPossibleScore: double.PositiveInfinity);
Position bestMove = result.Moves.Peek().Move;
Debug.WriteLine($"{player} best move {bestMove}, score {result.Score},"
+ $" leaf count {this.leafCount}, moves {string.Join(",", result.Moves)}.");
return(bestMove);
}
private AiSearchResult Search(bool isMaxSearch,
IBoard board,
PieceType curPlayer,
int depth,
double minPossibleScore,
double maxPossibleScore)
{
if (depth >= maxDepth || board.IsFull() || HasWinner(board))
{
this.leafCount++;
// Return the score of current board.
double score = scorer.GetScore(board, player);
return(new AiSearchResult(score));
}
PieceType otherPlayer = curPlayer.GetOther();
double worseScore = isMaxSearch ? minPossibleScore : maxPossibleScore;
AiSearchResult bestMove = new AiSearchResult(worseScore);
foreach (Position move in this.moveEnumerator.GetMoves(board, curPlayer))
{
// Make a move.
board.Set(move, curPlayer);
// Search deeper moves.
AiSearchResult result = Search(!isMaxSearch, board, otherPlayer, depth + 1, minPossibleScore, maxPossibleScore);
// Undo the move.
board.Set(move, PieceType.Empty);
double score = result.Score;
if (isMaxSearch)
{
if (score > bestMove.Score)
{
result.Moves.Push(new PlayerAndMove(curPlayer, move));
bestMove = result;
if (score >= maxPossibleScore)
{
// Result is better than max possible score, so this whole max search branch is meaningless.
// Just stop searching and return meaningless result.
break;
}
minPossibleScore = score;
}
}
else
{
if (score < bestMove.Score)
{
result.Moves.Push(new PlayerAndMove(curPlayer, move));
bestMove = result;
if (score <= minPossibleScore)
{
// Result is worse than min possible score, so this whole min search branch is meaningless.
// Just stop searching and return meaningless result.
break;
}
maxPossibleScore = score;
}
}
}
return(bestMove);
}
