Tic-Tac-Toe in C

ttt.c

#include <assert.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

/// TicTacToe ==========================================================================================================
typedef char Player;
const Player EMPTY = ' ';
const Player CROSS = 'x';
const Player CIRCL = 'o';

typedef struct
{
    union {
        Player arr[9];
        Player mat[3][3];
    };
    int empty;
} __attribute__((aligned(16))) TicTacToe;

void init_game(TicTacToe *game)
{
    memset(game->arr, EMPTY, 9);
    game->empty = 9;
}

void clear(TicTacToe *game, int k)
{
    assert(game->arr[k] != EMPTY);
    game->arr[k] = EMPTY;
    game->empty += 1;
}

// returns 2 if player won
// returns 1 if the game is a draw
// returns 0 if the game must go on
int play(TicTacToe *game, Player player, int k)
{
    assert(game->arr[k] == EMPTY);
    game->arr[k] = player;
    game->empty -= 1;

    int r = k / 3;
    int c = k % 3;
    if (((game->mat[r][0] == player) & (game->mat[r][1] == player) & (game->mat[r][2] == player)) |
        ((game->mat[0][c] == player) & (game->mat[1][c] == player) & (game->mat[2][c] == player)) |
        ((game->mat[0][0] == player) & (game->mat[1][1] == player) & (game->mat[2][2] == player)) |
        ((game->mat[0][2] == player) & (game->mat[1][1] == player) & (game->mat[2][0] == player)))
    {
        return 2;
    }
    else if (game->empty == 0)
    {
        return 1;
    }
    return 0;
}

void print(TicTacToe *game)
{
    printf(" %c | %c | %c\n"
           "---+---+---\n"
           " %c | %c | %c\n"
           "---+---+---\n"
           " %c | %c | %c\n",
           game->arr[0], game->arr[1], game->arr[2],
           game->arr[3], game->arr[4], game->arr[5],
           game->arr[6], game->arr[7], game->arr[8]);
}

///=====================================================================================================================

int best_outcome(TicTacToe *game, Player player, int *move)
{
    assert(player != EMPTY);
    Player opponent = (player == CIRCL) ? CROSS : CIRCL;

    // this array represents the outcomes from various moves
    // table[0] -> moves that lose the game
    // table[1] -> moves that draw the game
    // table[2] -> moves that win the game
    int table_size[3] = {0};
    int table[3][9];

    for (int k = 0; k < 9; ++k)
    {
        if (game->arr[k] != EMPTY)
        {
            continue;
        }

        int outcome = play(game, player, k);
        if (outcome == 0) // we don't know if it's a win/draw/loss yet
        {
            outcome = 2 - best_outcome(game, opponent, NULL);
        }

        int index = table_size[outcome]++;
        table[outcome][index] = k;
        clear(game, k);
    }

    int outcome = table_size[2] ? 2 : (table_size[1] ? 1 : 0);

    if (move != NULL)
    {
        int index = rand() % table_size[outcome];
        *move = table[outcome][index];
    }

    return outcome;
}

char read_character()
{
    char c = 0;
    char value = 0;
    bool read = true;
    while ((c = getchar()) != '\n' && c != EOF)
    {
        if (read)
        {
            value = c;
            read = false;
        }
    }
    return value;
}

void get_user_input(int *k)
{
    printf("move? ");
    *k = read_character() - '0';
}

int main()
{
    srand(time(NULL));

    TicTacToe game;

    while (true)
    {
        printf("Choose player [ox]: ");
        char symbol = read_character();
        init_game(&game);
        print(&game);

        Player player = CROSS;
        while (true)
        {
            int k = 0;
            player = (Player)((int)CIRCL + (int)CROSS - (int)player);

            printf("\n");
            if (player == symbol)
            {
                get_user_input(&k);
            }
            else
            {
                best_outcome(&game, player, &k);
            }

            int result = play(&game, player, k);
            print(&game);
            if (result == 2)
            {
                printf("%c wins\n", player);
                break;
            }
            else if (result == 1)
            {
                printf("it's a draw\n");
                break;
            }
        }
    }

    return EXIT_SUCCESS;
}