/// <summary>
/// Should only be called through the public Search method.
/// </summary>
/// <param name="state">The game state to consider.</param>
/// <param name="isLightPlayer">The player to move.</param>
/// <param name="alpha">The alpha pruning value.</param>
/// <param name="beta">The beta pruning value.</param>
/// <param name="depth">The current search depth.</param>
/// <returns>A MinimaxMove that represents the best move found.</returns>
/// <remarks>
/// The initial alpha value should be Int32.MinValue, the initial beta value
/// should be Int32.MaxValue, and the initial depth value should be 0.
///
/// The search will terminate ASAP if the m_ct cancellation token is signaled.
///
/// This method is thread-safe.
/// </remarks>
private MinimaxMove InternalSearch(MinimaxSpot[,] state, bool isLightPlayer, int alpha, int beta, int depth)
{
// Stop the search if...
if (TerminalTest(state) || depth >= _maxDepth || _cancellationToken.IsCancellationRequested)
{
_movesConsidered++;
return(new MinimaxMove(EvaluateHeuristic(state)));
}
// Initialize the best move for this recursive call.
var bestMove = new MinimaxMove(isLightPlayer ? int.MinValue : int.MaxValue);
// Get the valid moves for this recursive call.
var validMoves = GetValidMoves(state, isLightPlayer);
// If there are valid moves, recurse on each.
var consideredLocalMoves = false;
foreach (var move in validMoves)
{
consideredLocalMoves = true;
var currentMove = move;
currentMove.Value = InternalSearch(GetInsight(state, currentMove, isLightPlayer), !isLightPlayer, alpha, beta, depth + 1).Value;
if (isLightPlayer)
{
if (currentMove.Value > bestMove.Value)
{
bestMove = currentMove;
}
if (bestMove.Value >= beta)
{
break;
}
alpha = Math.Max(alpha, bestMove.Value.Value);
}
else
{
if (currentMove.Value < bestMove.Value)
{
bestMove = currentMove;
}
if (bestMove.Value <= alpha)
{
break;
}
beta = Math.Min(beta, bestMove.Value.Value);
}
}
// If there were no valid moves, still calculate the value.
if (!consideredLocalMoves)
{
bestMove.Value = InternalSearch(state, !isLightPlayer, alpha, beta, depth + 1).Value;
}
return(bestMove);
}
protected override MinimaxSpot[,] GetInsight(MinimaxSpot[,] state, MinimaxMove move, bool isLightPlayer)
{
var insightState = new MinimaxSpot[_numRows, _numCols];
for (int i = 0; i < _numRows; i++)
{
for (int j = 0; j < _numCols; j++)
{
insightState[i, j] = state[i, j];
}
}
insightState[move.Row, move.Col] = isLightPlayer ? MinimaxSpot.Light : MinimaxSpot.Dark;
return(insightState);
}
/// <summary>
/// Returns the best move resulting from a Minimax, alpha-beta pruning search on the given state.
/// </summary>
/// <param name="state">The state to consider.</param>
/// <param name="isLightPlayer">The player to move.</param>
/// <param name="inParallel">A boolean indicating whether to use the parallel algorithm.</param>
/// <returns>A MinimaxMove that represents the best move found.</returns>
/// <remarks>
/// This method will only return a MinimaxMove(-1...) if there are no valid moves.
/// </remarks>
public MinimaxMove Search(MinimaxSpot[,] state, bool isLightPlayer, bool inParallel)
{
// Initialize a bunch of state.
_maxDepth = MaxDepth == -1 ? int.MaxValue : MaxDepth;
_degOfParallelism = DegreeOfParallelism;
_timeLimit = TimeLimit;
_taskCount = 0;
_movesConsidered = 0;
var curCts = _cancellationTokenSource = new CancellationTokenSource();
_cancellationToken = _cancellationTokenSource.Token;
var aiMove = new MinimaxMove(-1, -1, null);
// Start the timeout timer. Done using a dedicated thread to minimize delay
// in cancellation due to lack of threads in the pool to run the callback.
var timeoutTask = Task.Factory.StartNew(() =>
{
Thread.Sleep(_timeLimit);
curCts.Cancel();
}, TaskCreationOptions.LongRunning);
// Do the search
aiMove = inParallel
? InternalSearchTPL(state, isLightPlayer, int.MinValue, int.MaxValue, 0, CancellationToken.None)
: InternalSearch(state, isLightPlayer, int.MinValue, int.MaxValue, 0);
// Make sure that MinimaxMove(-1...) is only returned if there are no valid moves, because
// InternalSearch* may return MinimaxMove(-1...) if none of the valid moves beats Int32.Min/Max.
if (aiMove.Row == -1)
{
foreach (var move in GetValidMoves(state, isLightPlayer))
{
aiMove = move;
aiMove.Value = isLightPlayer ? int.MinValue : int.MaxValue;
break;
}
}
return(aiMove);
}
/// <summary> /// Returns the game state that results when the given player plays the given move on the given /// state. If the move is invalid, the new state should be the same as the old state. /// Must be thread-safe. /// </summary> /// <param name="state">The state to play a move on.</param> /// <param name="move">The move to play.</param> /// <param name="isLightPlayer">The player to play the move.</param> /// <returns>A MinimaxSpot matrix that represents the insight state.</returns> protected abstract MinimaxSpot[,] GetInsight(MinimaxSpot[,] state, MinimaxMove move, bool isLightPlayer);
/// <summary>
/// Should only be called through the public Search method.
/// </summary>
/// <param name="state">The game state to consider.</param>
/// <param name="isLightPlayer">The player to move.</param>
/// <param name="alpha">The alpha pruning value.</param>
/// <param name="beta">The beta pruning value.</param>
/// <param name="depth">The current search depth.</param>
/// <param name="token">The pruning token.</param>
/// <returns>A MinimaxMove that represents the best move found.</returns>
/// <remarks>
/// The initial alpha value should be Int32.MinValue, the initial beta value
/// should be Int32.MaxValue, the initial depth value should be 0, and the
/// initial token should be a non-settable token.
///
/// The search will terminate ASAP if the m_ct cancellation token is signaled.
///
/// This method is thread-safe.
/// </remarks>
private MinimaxMove InternalSearchTPL(MinimaxSpot[,] state, bool isLightPlayer, int alpha, int beta, int depth, CancellationToken token)
{
// Stop the search if...
if (TerminalTest(state) || depth >= _maxDepth || _cancellationToken.IsCancellationRequested)
{
_movesConsidered++; // NOTE: this is racy and may be lower than the actual count, but it only needs to be an appx
return(new MinimaxMove(EvaluateHeuristic(state)));
}
// Initialize the best move for this recursive call.
var bestMove = new MinimaxMove(isLightPlayer ? int.MinValue : int.MaxValue);
// Get the valid moves for this recursive call.
var validMoves = GetValidMoves(state, isLightPlayer);
var consideredLocalMoves = false;
var workers = new Queue <Task>();
var bigLock = new object();
var cts = new CancellationTokenSource();
foreach (var move in validMoves)
{
// SHARED STATE
// The local variables (bestMove, alpha, beta) are protected by a lock.
// The non-local variables (m_taskCount) are modified using Interlocked
consideredLocalMoves = true;
// If the pruning token is signaled, stop this loop.
if (token.IsCancellationRequested)
{
cts.Cancel();
break;
}
var currentMove = move;
if (_taskCount < _degOfParallelism && depth <= _maxDepth - 1)
{
Interlocked.Increment(ref _taskCount);
workers.Enqueue(Task.Run(() =>
{
currentMove.Value = InternalSearchTPL(GetInsight(state, currentMove, isLightPlayer), !isLightPlayer, alpha, beta, depth + 1, cts.Token).Value;
lock (bigLock)
{
if (isLightPlayer)
{
if (currentMove.Value > bestMove.Value)
{
bestMove = currentMove;
}
if (bestMove.Value >= beta)
{
cts.Cancel();
}
alpha = Math.Max(alpha, bestMove.Value.Value);
}
else
{
if (currentMove.Value < bestMove.Value)
{
bestMove = currentMove;
}
if (bestMove.Value <= alpha)
{
cts.Cancel();
}
beta = Math.Min(beta, bestMove.Value.Value);
}
}
Interlocked.Decrement(ref _taskCount);
}));
}
else
{
bool isPruning = false;
currentMove.Value = InternalSearchTPL(GetInsight(state, currentMove, isLightPlayer), !isLightPlayer, alpha, beta, depth + 1, cts.Token).Value;
// If there are no tasks, no need to lock.
bool lockTaken = false;
try
{
if (workers.Count > 0)
{
Monitor.Enter(bigLock, ref lockTaken);
}
if (isLightPlayer)
{
if (currentMove.Value > bestMove.Value)
{
bestMove = currentMove;
}
if (bestMove.Value >= beta)
{
isPruning = true;
}
alpha = Math.Max(alpha, bestMove.Value.Value);
}
else
{
if (currentMove.Value < bestMove.Value)
{
bestMove = currentMove;
}
if (bestMove.Value <= alpha)
{
isPruning = true;
}
beta = Math.Min(beta, bestMove.Value.Value);
}
}
finally
{
if (lockTaken)
{
Monitor.Exit(bigLock);
}
}
if (isPruning)
{
cts.Cancel();
break;
}
}
}
Task.WaitAll(workers.ToArray());
// If there were no valid moves, still calculate the value.
if (!consideredLocalMoves)
{
bestMove.Value =
InternalSearchTPL(state, !isLightPlayer, alpha, beta, depth + 1, token).Value;
}
return(bestMove);
}
