private int MiniMax(Move move, GridData gridData, int turn, int depth)
{
// The MiniMax method gives the best possible move for the current grid data.
// This works by, effectively, playing each possible game. If that game results
// in a CPU win, then return a positive score; if it results in a CPU loss,
// then return a negative score.
// The CPU and Player alternate turns. The Player is aiming to get the lowest
// possible score (i.e. a CPU loss / a Player win). Therefore, when the lowest
// score possible for the Player is positive, then a perfect-playing CPU will
// eventually win, and that is the move it should take.
// We limit the depth, in order to create a beatable CPU.
eState moveState = new StatePlayerConverter().GetPlayerState(turn);
gridData.PlaceMove(move, moveState);
if(gridData.HasWinner())
{
int winScore = 10 - depth;
return IsCPUMove(turn) ? winScore: -winScore;
}
else if(gridData.IsStalemate() || depth == _maxDepth)
{
return 0;
}
else
{
// Game is neither won nor a stalemate. Therefore, we'll keep playing.
int nextTurn = (turn + 1) % 2;
int bestScore = -20;
List<Move> nextMoves = gridData.GetPossibleMoves();
foreach(Move nextMove in nextMoves)
{
int score = MiniMax(nextMove, gridData.Copy(), nextTurn, depth + 1);
// Player move will return a negative score if it has won. Negate it for
// now so that it is positive and will be seen as the player's best move.
score = IsCPUMove(nextTurn) ? score : -score;
if(score > bestScore)
{
bestScore = score;
}
}
// Player move has negated to find the best score. Turn it back, so that it
// gives an accurate score for CPU move.
return IsCPUMove(nextTurn) ? bestScore: -bestScore;
}
}
public CPUMoveFinder(GridData gridData, bool isBeatable)
{
_gridData = gridData;
_maxDepth = isBeatable ? 4 : -1;
}
public CPUMoveFinder(GridData gridData, bool isBeatable)
{
_gridData = gridData;
_maxDepth = isBeatable ? 4 : -1;
}