/// <summary>
/// Initial minimax starting method. This method kicks off the algoritm and finds the best move for the current player.
/// If two or more moves have the same value, the best move is choosen randomly from the moves
/// </summary>
/// <param name="board">The board.</param>
/// <returns>Best move for the current player</returns>
public static CheckersMove MinimaxStart(CheckerBoard board)
{
int alpha = int.MinValue;
int beta = int.MaxValue;
thinking = true;
List <CheckersMove> possibleMoves = board.GetMovesForPlayer();
List <int> values = new List <int>();
Logger.Info(string.Format("Max is {0}", board.CurrentPlayerTurn));
if (possibleMoves.IsNullOrEmpty())
{
return(null);
}
foreach (CheckersMove move in possibleMoves)
{
CheckersMove moveToMake = move;
CheckerBoard boardToMakeMoveOn = board;
do
{
Logger.Debug("Board Before");
Logger.Debug(boardToMakeMoveOn.ToString());
boardToMakeMoveOn = (CheckerBoard)boardToMakeMoveOn.GetMinimaxClone();
boardToMakeMoveOn.MakeMoveOnBoard((CheckersMove)moveToMake.GetMinimaxClone());
moveToMake = moveToMake.NextMove;
Logger.Debug("Board After");
Logger.Debug(boardToMakeMoveOn.ToString());
}while (moveToMake != null);
values.Add(Minimax(boardToMakeMoveOn, Settings.AIDepth - 1, alpha, beta, false, board.CurrentPlayerTurn));
}
int maxHeuristics = int.MinValue;
foreach (int value in values)
{
if (value >= maxHeuristics)
{
maxHeuristics = value;
}
}
//filter the list of moves based on max value
List <CheckersMove> bestMoves = new List <CheckersMove>();
for (int i = 0; i < values.Count; i++)
{
if (values[i] == maxHeuristics)
{
bestMoves.Add(possibleMoves[i]);
}
}
counter = 0;
thinking = false;
Logger.Info("Node Values: " + string.Join(",", values.Select(x => x.ToString()).ToArray()));
return(bestMoves[Rng.Next(bestMoves.Count)]);
}
/// <summary>
/// Runs the minimax algoritm with alpha beta pruning
/// </summary>
/// <param name="board">The board.</param>
/// <param name="depth">The depth.</param>
/// <param name="alpha">The alpha.</param>
/// <param name="beta">The beta.</param>
/// <param name="isMax">if set to <c>true</c> [is maximum].</param>
/// <param name="rootPlayer">The root player.</param>
/// <returns>Minimax value for this board state</returns>
private static int Minimax(CheckerBoard board, int depth, int alpha, int beta, bool isMax, PlayerColor rootPlayer)
{
List <CheckersMove> possibleMoves = board.GetMovesForPlayer();
if (depth == 0 || possibleMoves.Count == 0)
{
return(Score(board, rootPlayer));
}
int value = 0;
if (isMax)
{
value = int.MinValue;
foreach (CheckersMove move in possibleMoves)
{
CheckersMove moveToMake = move;
CheckerBoard boardToMakeMoveOn = board;
do
{
boardToMakeMoveOn = (CheckerBoard)boardToMakeMoveOn.GetMinimaxClone();
boardToMakeMoveOn.MakeMoveOnBoard((CheckersMove)moveToMake.GetMinimaxClone());
moveToMake = moveToMake.NextMove;
}while (moveToMake != null);
int result = Minimax(boardToMakeMoveOn, depth - 1, alpha, beta, false, rootPlayer);
value = Math.Max(result, value);
alpha = Math.Max(alpha, value);
if (alpha >= beta)
{
Logger.Debug("Branch was pruned");
break;
}
}
}
else
{
value = int.MaxValue;
foreach (CheckersMove move in possibleMoves)
{
CheckersMove moveToMake = move;
CheckerBoard boardToMakeMoveOn = board;
do
{
boardToMakeMoveOn = (CheckerBoard)boardToMakeMoveOn.GetMinimaxClone();
boardToMakeMoveOn.MakeMoveOnBoard((CheckersMove)moveToMake.GetMinimaxClone());
moveToMake = moveToMake.NextMove;
}while (moveToMake != null);
int result = Minimax(boardToMakeMoveOn, depth - 1, alpha, beta, true, rootPlayer);
value = Math.Min(result, value);
beta = Math.Min(alpha, value);
if (alpha >= beta)
{
Logger.Debug("Branch was pruned");
break;
}
}
}
return(value);
}