gcc tak.c -o tak; ./tak

tak.c

#include <stdio.h>

/* each line along which a player can win */
enum axes {
    HORIZ_TOP,
    HORIZ_MID,
    HORIZ_BOT,
    VERT_LEFT,
    VERT_MID,
    VERT_RIGHT,
    DIAG_TOP_LEFT, /* from top left to bottom right */
    DIAG_BOT_LEFT, /* from bottom left to top right */
    AXES_MAX
};

/* if any rank hits (plus or minus) three, the game is over */
static int ranks[AXES_MAX] = { 0 };
static char board[3][3] = {
    { ' ', ' ', ' ' },
    { ' ', ' ', ' ' },
    { ' ', ' ', ' ' },
};

static int check_game_state(void)
{
    int i;

    /* if any rank hits (plus or minus) three, the game is over.
     * - positive indicates 'x' wins
     * - negative indicates 'o' wins
     */
    for (i = 0; i < AXES_MAX; i++)
    {
        switch (ranks[i])
        {
        case 3:
            return 1;
        case -3:
            return -1;
        }
    }

    return 0;
}

static void draw_board(void)
{
    int i;
    printf("\n      [1] [2] [3]\n\n");
    printf("[A]    %c | %c | %c \n",
           board[0][0], board[0][1], board[0][2]);
    for (i = 1; i < 3; i++)
    {
        printf("      ---|---|---\n");
        printf("[%c]    %c | %c | %c \n", 'A' + i,
               board[i][0], board[i][1], board[i][2]);
    }
}

static int get_input(int *row, int *col)
{
    char tmp = 0;
    char r = 0;
    unsigned int c = 0;
    int matches = 0;

    matches = scanf("%c%u", &r, &c);
    while (tmp != '\n' && tmp != EOF)
    {
        /* scanf doesn't remove extra leftovers from the buffer */
        tmp = getchar();
    }

    if (matches != 2 || c < 1 || c > 3)
    {
        return -1;
    }

    switch (r)
    {
    case 'a':
    case 'b':
    case 'c':
        *row = r - 'a';
        break;
    case 'A':
    case 'B':
    case 'C':
        *row = r - 'A';
        break;
    default:
        return -1;
    }

    *col = (int) c - 1;
    return 0;
}

static void make_move(int row, int col, char player)
{
    int rank_factor = (player == 'x') ? 1 : -1;

    /* the game ends if any rank hits (plus or minus) three.
     * there is a rank for each "axis" on which you can win:
     * - the three horizontal rows (top, middle, and bottom)
     * - the three vertical columns (left, middle, and right)
     * - the two diagonal axes (from top left, from bottom left)
     */

    /* update rank for each axis this move affected */
    ranks[HORIZ_TOP + row] += rank_factor;
    ranks[VERT_LEFT + col] += rank_factor;
    ranks[DIAG_TOP_LEFT] += ((row == 0 && col == 0) ||
                             (row == 1 && col == 1) ||
                             (row == 2 && col == 2)) ? rank_factor : 0;
    ranks[DIAG_BOT_LEFT] +=  ((row == 2 && col == 0) ||
                              (row == 1 && col == 1) ||
                              (row == 0 && col == 2)) ? rank_factor : 0;

    /* then update the board itself */
    board[row][col] = player;
}

int main(void)
{
    int spaces_available = 9;
    char player = 'o';

    while (spaces_available > 0 && check_game_state() == 0)
    {
        player = (player == 'x') ? 'o' : 'x';

        draw_board();

        int row;
        int col;
        while (get_input(&row, &col) != 0 || board[row][col] != ' ')
        {
            /* do nothing */
        }

        make_move(row, col, player);
        spaces_available--;
    }

    draw_board();
    return 0;
}