private MinMaxScore MinMaxPlay(Board board, int depth, Play play)
{
Board newBoard = new Board(board);
if (play != null)
{
// make move.
newBoard.MarkBoard(play);
// see if play results in final state.
MinMaxScore minMaxScore = GetMinMaxScore(newBoard, play, depth);
// if so, return score.
if (minMaxScore != null)
{
return(minMaxScore);
}
}
// update marker type.
Marker marker = play == null
? Marker.O : (play.PlayerId == Marker.O)
? Marker.X : Marker.O;
depth++;
List <Play> possiblePlays = PossiblePlaysInCurrentBoardState(newBoard, marker);
// Compute every possible move's score and add to list
List <MinMaxScore> moves = possiblePlays.Select(possibleMove => MinMaxPlay(newBoard, depth, possibleMove)).ToList();
// Find maximum score on list.
MinMaxScore maxMinMaxScoreMove = FindMaxScore(moves);
return(maxMinMaxScoreMove);
}
private MinMaxScore GetMinMaxScore(Board board, Play play, int depth)
{
MinMaxScore minMaxScore = new MinMaxScore
{
Play = play,
Points = 0
};
// Check win condition for player X
if (board.IsVictoryCondition(Marker.X))
{
minMaxScore.Points = play.PlayerId == Marker.X ? 10 - depth : depth - 10;
return(minMaxScore);
}
// Check win condition for player O
if (board.IsVictoryCondition(Marker.O))
{
minMaxScore.Points = play.PlayerId == Marker.O ? 10 - depth : depth - 10;
return(minMaxScore);
}
// Check draw condition
if (board.AreAllPositionMarked())
{
return(minMaxScore);
}
// otherwise...
return(null);
}
private Play DecidePlay(Board board)
{
// Ground Rule #1: start marking center position.
int center = (board.Size - 1) / 2;
if (board.GetMarkAtPosition(center, center) == Marker.Empty)
{
return(PlayAtCenter(center));
}
// Ground Rule #2: if center's already marked and board's only marked once, mark a corner.
if (board.GetTotalMarks() == 1)
{
return(PlayAtAnyCorner(board));
}
// Generic move.
MinMaxScore maximumScore = MinMaxPlay(board, 0, null);
return(maximumScore.Play);
}