We can take advantage of the fact that tic-tac-toe is a zero-sum game. This means that at the end of the game, the sum of the scores of the players will equal zero. For a two-player game, this means that one player's score will always be the negative of the other players. This is convenient for us, since minimizing the other player's score is then identical to maximizing one's own score. So instead of one player maximizing his score and one player minimizing the other player's score, we can just have both players attempt to maximize their own score.
Change EvaluateStaticPosition back to its original form, so that it gives a score based on how good the board state is for the current player.
int EvaluateStaticPosition(stateT state)
{
if(CheckForWin(state, state.whoseTurn))
{
return WINNING_POSITION;
}
if(CheckForWin(state, Opponent(state.whoseTurn)))
{
return LOSING_POSITION;
}
return NEUTRAL_POSITION;
}
Delete MinMove, since we only care about maximizing. Rewrite MaxMove so that it chooses the move that gives the opponent the worst possible score. The score for the best move is the negative of the other player's worst score.
int MaxMove(stateT state, moveT &bestMove)
{
if(GameIsOver(state))
{
return EvaluateStaticPosition(state);
}
vector<moveT> moveList;
GenerateMoveList(state, moveList);
int nMoves = moveList.size();
int v = -1000;
for(int i = 0 ;i<nMoves; i++)
{
moveT move = moveList[i];
MakeMove(state, move);
moveT opponentsBestMove;
int curRating = -MaxMove(state, opponentsBestMove);
if (curRating > v)
{
v = curRating;
bestMove = move;
}
RetractMove(state, move);
}
return v;
}
Since MaxMove is used for both the players, we no longer need to differentiate among players in the MiniMax function.
moveT MiniMax(stateT state)
{
moveT bestMove;
int i = 0;
i = MaxMove(state, bestMove);
cout<<"i is "<<i<<endl;
return bestMove;
}
I did intentionally put bestMove = move in the MinMove function, inside the curRating < v block. That is the logical place for it; if a move's rating is the lowest found so far, then that move is the best move. At least until another even lower rating is found.
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