public bool Equals(BotMove other) => Figure == other.Figure && Sequence == other.Sequence;
// NegaMax algorithm with alpha/beta, source: https://en.wikipedia.org/wiki/Negamax
//TODO: append transposition tables
private BotMove Negamax(SquareBoard board, int depth, int alpha, int beta, Side side, CancellationToken branchToken)
{
if (cancellation.IsCancellationRequested ||
!CanSearchDeeper(board, depth))
{
return(BotMove.Empty(Estimate(board)));
}
var states = GetStates(board, side);
if (depth == options.MaxDepth && states.Count == 1)
{
var singleState = states[0];
return(new BotMove(singleState.Figure, singleState.MoveSequence, 0));
}
BotMove bestMove = new BotMove(Int32.MinValue);
var noMoves = true;
var cts = new CancellationTokenSource();
var workers = new List <Task>();
foreach (var state in states)
{
noMoves = false;
if (cts.IsCancellationRequested)
{
break;
}
if (branchToken.IsCancellationRequested)
{
cts.Cancel();
break;
}
if (states.Count > 1 && _runningWorkersCount < options.DegreeOfParallelism && depth <= options.MaxDepth - 1)
{
Interlocked.Increment(ref _runningWorkersCount);
workers.Add(Task.Run(() =>
{
DoNegamax(state);
Interlocked.Decrement(ref _runningWorkersCount);
}));
}
else
{
DoNegamax(state);
}
}
Task.WaitAll(workers.ToArray());
if (noMoves)
{
return(BotMove.Empty(Estimate(board)));
}
return(bestMove);
void DoNegamax(State state)
{
if (options.IsDebug)
{
Log("before", board, side, state, bestMove.Score, depth, alpha, beta);
}
var boardAfterMove = new MoveCommandChain(state.Figure, state.Board, state.MoveSequence).Execute();
var score = -Negamax(boardAfterMove, depth - 1, -beta, -alpha, SideUtil.Opposite(side), cts.Token).Score;
var shouldPrun = false;
lock (locker)
{
if (score > bestMove.Score)
{
bestMove = new BotMove(state.Figure, state.MoveSequence, score);
}
alpha = Math.Max(alpha, bestMove.Score);
//let's move cts.Cancel out of the lock scope
shouldPrun = options.AllowPrunning && alpha >= beta;
}
if (options.IsDebug)
{
Log("after", board, side, state, score, depth, alpha, beta);
}
if (shouldPrun)
{
cts.Cancel();
}
}
}
