private void GetBestMoveImpl(Game game, int depth, out Move bestMove, out Estimate bestEstimate)
{
Player player = game.State.NextMovePlayer;
bool isMaximizeStage = player == Player.Maximizing;
Move curBestMove = null;
Estimate curBestEstimate = isMaximizeStage
? Estimate.MinInf
: Estimate.MaxInf;
foreach (Move move in game.GetAllowedMoves())
{
MovesChecked += 1;
game.DoMove(move);
Estimate estimate = game.State.StaticEstimate;
// force AI to win faster
if (game.State.IsTerminate && estimate != Estimate.Zero)
{
EstimateHelper.AdjustTerminalStateEstimate(depth, ref estimate);
}
if (depth < 3)
{
Trace.WriteLine($"{Tab(depth - 1)}Consider {move}");
}
if (game.State.IsTerminate || depth == MaxDepth)
{
// no longer go deeper
UpdateBestMove(isMaximizeStage, move, estimate, ref curBestMove, ref curBestEstimate);
}
else
{
// go deeper
Move bestMoveInternal;
Estimate bestEstimateInternal;
GetBestMoveImpl(game, depth + 1, out bestMoveInternal, out bestEstimateInternal);
UpdateBestMove(isMaximizeStage, move, bestEstimateInternal, ref curBestMove, ref curBestEstimate);
}
game.UndoMove(move);
}
if (curBestMove == null)
{
throw new InvalidOperationException("No moves found");
}
bestMove = curBestMove;
bestEstimate = curBestEstimate;
#if DEBUG
if (depth <= 3)
{
Trace.WriteLine($"{Tab(depth-1)}RET: {depth}; BM: {bestMove}; E: {bestEstimate} ({(isMaximizeStage ? "max" : "min")})");
}
#endif
}
private Estimate FindImpl(Game game, Estimate alpha, Estimate beta, int depth, int maxDepth, Metadata meta)
{
Player player = game.State.NextMovePlayer;
bool isMax = player == Player.Maximizing;
if (maxDepth - depth == 0)
{
return(game.State.StaticEstimate);
}
if (game.State.IsTerminate)
{
// make AI pick shortest path to terminate state
// otherwise there will be funny situation then AI seeing that it won
// w/o chances for other side will not purse the winning
Estimate terminateEstimate = game.State.StaticEstimate;
EstimateHelper.AdjustTerminalStateEstimate(depth, ref terminateEstimate);
return(terminateEstimate);
}
Estimate v = isMax
? Estimate.MinInf
: Estimate.MaxInf;
List <Move> moves = game.GetAllowedMoves()
.OrderByDescending(x => x.Priority)
.ToList();
foreach (Move move in moves)
{
meta.MovesChecked += 1;
using (DisposableMoveHandle.New(game, move))
{
Estimate curEstimate = FindImpl(game, alpha, beta, depth + 1, maxDepth, meta);
bool betterMoveFound = isMax
? curEstimate > v
: curEstimate < v;
if (betterMoveFound)
{
v = curEstimate;
if (depth == 1)
{
meta.BestMove = move;
}
}
if (isMax)
{
alpha = Estimate.Max(alpha, v);
}
else
{
beta = Estimate.Min(beta, v);
}
#if DEBUG
if (depth <= 3)
{
Trace.WriteLine($"{ new string(' ', depth) } - Move {move} for {player} -- {curEstimate} -- Term? {game.State.IsTerminate}");
}
#endif
if (alpha >= beta)
{
// alpha/beta cut-off
//Trace.WriteLine($"cut-off on depth {depth}");
break;
}
}
}
return(v);
}