// state : The root of the (sub)-tree this search expands
// alpha : Value used for AlphaBeta pruning - maximum value maximizing player can definitely get
// beta : Value used for AlphaBeta pruning - minimum value minimizing player can definitely get
// depthLeft : How many more depths we are allowed to explore
private double AlphaBetaSearch(UTTTState state, double alpha, double beta, int depthLeft)
{
// throw an exception if our player is forced to stop
if (sw.ElapsedMilliseconds > msGiven)
{
throw new OutOfTimeException();
}
// if final depth reached, then return the value of this leaf
if (depthLeft == 0 || state.IsFinal())
{
return(state.Evaluate());
}
// if not final depth, then generate all possible branches
List <UTTTMove> moves = state.GetPossibleMoves();
//
if (moves.Count == 0)
{
throw new OutOfTimeException();
}
// if only a single move is possible, don't decrease depth
if (moves.Count == 1)
{
depthLeft++;
}
// if the state has been seen before, then continue
// tracing the best move of the previous iteration by putting the old move
// in the first position to be evaluated. This leads to better pruning.
StateRecord rec = null;
statesSeen.TryGetValue(state.GetHashCode(), out rec);
if (rec != null)
{
if (rec.iteration == iteration)
{
return(rec.value);
}
if (rec.bestMove != null)
{
FindCopyAndSwap(rec.bestMove, moves, moves.First());
}
statesSeen.Remove(state.GetHashCode());
}
else
{
rec = new StateRecord();
}
// keep track of the best move
// update the StateRecord in the dictionary after we have finished analyzing the state
Boolean min = state.MinimizingHasTurn();
UTTTMove bestMove = null;
foreach (UTTTMove move in moves)
{
state.DoMove(move);
double result = AlphaBetaSearch(state, alpha, beta, depthLeft - 1);
state.UndoMove(move);
if ((min && result < beta) || (!min && result > alpha))
{
bestMove = move;
if (min)
{
beta = result;
}
else
{
alpha = result;
}
}
if (alpha >= beta)
{
// update record and return
double res = (alpha + beta) / 2;
rec.Update(res, bestMove, state, statesSeen, iteration);
return(res);
}
}
// update record and return
if (min)
{
rec.Update(beta, bestMove, state, statesSeen, iteration);
return(beta);
}
else
{
rec.Update(alpha, bestMove, state, statesSeen, iteration);
return(alpha);
}
}