/// <summary>
/// Min max searching algorithm with defined cutoff depth.
/// </summary>
/// <returns>The column that will be moved played in.</returns>
private int MinimaxCutoffSearch(BitBoard board)
{
int maxDepth = 7;
var openMoveValue = 0.0m;
var movedColumn = -1;
var alpha = decimal.MinValue;
var beta = decimal.MaxValue;
var nodeCounter = new NodeCounter();
var stopWatch = new Stopwatch();
stopWatch.Start();
if (AiColor == DiscColor.Red)
{
(openMoveValue, movedColumn) = MinValue(board, maxDepth, alpha, beta, nodeCounter, AiColor);
}
else
{
(openMoveValue, movedColumn) = MaxValue(board, maxDepth, alpha, beta, nodeCounter, AiColor);
}
stopWatch.Stop();
var elapsed = stopWatch.Elapsed;
var elapsedTime = String.Format("Searched for {0:00}.{1:0000} seconds",
elapsed.Seconds, elapsed.Milliseconds);
Console.WriteLine(elapsedTime);
Console.WriteLine($"Column {movedColumn} was chosen.");
Console.WriteLine($"{nodeCounter.TotalNodes} were explored.");
return(movedColumn);
}
public void UpdateBoard(BitBoard board)
{
GameBoard = board;
}
private (decimal BoardScore, int Column) MinValue(
BitBoard board,
int depth,
decimal alpha,
decimal beta,
NodeCounter nodeCounter,
DiscColor movingColor)
{
nodeCounter.Increment();
var openColumns = GetOpenColumns(in board);
// TODO I think these columns are safe but not certain
// I didn't see any problems with this during some test games, may require more vetting though
// Drawn game
if (openColumns.Count == 0)
{
return(0.0m, -1);
}
if (depth == 0 ||
CheckVictory(in board) != DiscColor.None)
{
return(EvaluateBoardState(in board, movingColor), -1);
}
// Win and return immediately if possible
var winningMove = FindKillerMove(in board, movingColor);
if (winningMove.HasWinner &&
winningMove.Winner == movingColor)
{
var winningBoard = BitBoardMove(in board, winningMove.Column, movingColor);
var winningScore = EvaluateBoardState(in winningBoard, movingColor);
return(winningScore, winningMove.Column);
}
// Stop the opponent from winning if possible
var oppWinningMove = FindKillerMove(in board, OpponentColor);
if (oppWinningMove.HasWinner)
{
var stopWinningBoard = BitBoardMove(in board, oppWinningMove.Column, AiColor);
var stopWinningScore = EvaluateBoardState(in stopWinningBoard, movingColor);
return(stopWinningScore, oppWinningMove.Column);
}
decimal minimumMoveValue = decimal.MaxValue;
int movedColumn = -1;
foreach (int openMove in openColumns)
{
var newState = BitBoardMove(in board, openMove, movingColor);
var childMaxValue = MaxValue(newState, depth - 1, alpha, beta, nodeCounter, ChangeTurnColor(movingColor)).BoardScore;
if (childMaxValue < minimumMoveValue)
{
minimumMoveValue = childMaxValue;
movedColumn = openMove;
}
if (minimumMoveValue <= alpha)
{
return(minimumMoveValue, openMove);
}
beta = Math.Min(beta, minimumMoveValue);
}
return(minimumMoveValue, movedColumn);
}
