Minimax.c

#include <stdio.h>

char gridChar(int i) {
    switch (i) {
        case -1:
            return 'X';
        case 0:
            return ' ';
        case 1:
            return 'O';
    }
}

void draw(int b[9]) {
    printf(" %c | %c | %c\n", gridChar(b[0]), gridChar(b[1]), gridChar(b[2]));
    printf("---+---+---\n");
    printf(" %c | %c | %c\n", gridChar(b[3]), gridChar(b[4]), gridChar(b[5]));
    printf("---+---+---\n");
    printf(" %c | %c | %c\n", gridChar(b[6]), gridChar(b[7]), gridChar(b[8]));
}

int win(const int board[9]) {
    unsigned wins[8][3] = {{0, 1, 2}, {3, 4, 5}, {6, 7, 8}, {0, 3, 6}, {1, 4, 7}, {2, 5, 8}, {0, 4, 8}, {2, 4, 6}};
    int i;
    for (i = 0; i < 8; ++i) {
        if (board[wins[i][0]] != 0 &&
            board[wins[i][0]] == board[wins[i][1]] &&
            board[wins[i][0]] == board[wins[i][2]])
            return board[wins[i][2]];
    }
    return 0;
}

int minimax(int board[9], int player) {
    int winner = win(board);
    if (winner != 0) return winner * player;

    int move = -1;
    int score = -2;
    int i;
    for (i = 0; i < 9; ++i) {
        if (board[i] == 0) {
            board[i] = player;
            int thisScore = -minimax(board, player * -1);
            if (thisScore > score) {
                score = thisScore;
                move = i;
            }
            board[i] = 0;
        }
    }
    if (move == -1) return 0;
    return score;
}

void computerMove(int board[9]) {
    int move = -1;
    int score = -2;
    int i;
    for (i = 0; i < 9; ++i) {
        if (board[i] == 0) {
            board[i] = 1;
            int tempScore = -minimax(board, -1);
            board[i] = 0;
            if (tempScore > score) {
                score = tempScore;
                move = i;
            }
        }
    }
    board[move] = 1;
}

void playerMove(int board[9]) {
    int move = 0;
    do {
        printf("\nInput move (1 - 9): ");
        scanf("%d", &move);
        move--;
        printf("\n");
    } while (move >= 9 || move < 0 || board[move] != 0);
    board[move] = -1;
}

int main() {
    int board[9] = {0, 0, 0, 0, 0, 0, 0, 0, 0};
    printf("Computer: O, You: X\nPlay (1)st or (2)nd? ");
    int player = 0;
    scanf("%d", &player);
    printf("\n");
    unsigned turn;
    for (turn = 0; turn < 9 && win(board) == 0; ++turn) {
        if ((turn + player) % 2 == 0)
            computerMove(board);
        else {
            draw(board);
            playerMove(board);
        }
    }
    switch (win(board)) {
        case 0:
            printf("A draw. How droll.\n");
            break;
        case 1:
            draw(board);
            printf("You lose.\n");
            break;
        case -1:
            printf("You win. Inconceivable!\n");
            break;
    }
}