// The function minimax() calculates the best possible move that Chappie can make by
// recursing as far as it can within the time limit and keeping track of what the best move and
// score are as it recurses. It is also implemented with AB pruning, meaning that if it
// starts to go down a path where the outcome will inevitable be worse that our best values,
// it prunes off that path, therefore saving time to go down a better path.
private MoveResult minimax(Board b, int d, Stopwatch w, int alpha, int beta)
{
// check to see if the time limit is up
if (w.ElapsedMilliseconds > getTimePerMove())
{
throw new MoveTimedOutException();
}
// base case
if (b.gameOver() || d == 0)
{
return(new MoveResult(0, evaluate(b), b.gameOver()));
}
// initialization of trackers
int bestMove = 0;
int bestVal;
bool gameCompleted = false;
// check all the the moves that top could make, and act as if it is the MAX in minimax
if (b.whoseMove() == Position.Top) // TOP is MAX
{
// smallest possible value so that it can only get better
bestVal = Int32.MinValue;
for (int move = 7; move <= 12 && alpha < beta; move++)
{
if (b.legalMove(move))
{
Board b1 = new Board(b); // duplicate board
b1.makeMove(move, false); // make the move
MoveResult val = minimax(b1, d - 1, w, alpha, beta); // find its value
if (val.getScore() > bestVal) // remember if best
{
bestVal = val.getScore();
bestMove = move;
// track the current condition of the game
gameCompleted = val.isEndGame();
}
// prune
if (bestVal > alpha)
{
alpha = bestVal;
}
}
}
}
// check all the the moves that bottom could make, and act as if it is the MIN in minimax
else // BOTTOM is MIN
{
// lergest possible value so that it can only get better
bestVal = Int32.MaxValue;
for (int move = 0; move <= 5 && alpha < beta; move++)
{
if (b.legalMove(move))
{
Board b1 = new Board(b); // duplicate board
b1.makeMove(move, false); // make the move
MoveResult val = minimax(b1, d - 1, w, alpha, beta); // find its value
if (val.getScore() < bestVal) // remember if best
{
bestVal = val.getScore();
bestMove = move;
// track the current condition of the game
gameCompleted = val.isEndGame();
}
// prune
if (bestVal < beta)
{
beta = bestVal;
}
}
}
}
return(new MoveResult(bestMove, bestVal, gameCompleted));
}
