
Task

Play a game of tic-tac-toe.

Ensure that legal moves are played and that a winning position is notified.


Tic-tac-toe   is also known as:

   naughts and crosses 
   tic tac toe 
   tick tack toe 
   three in a row 
   tres en rayo       and
   Xs  and  Os


See also
   MathWorld™, Tic-Tac-Toe game.
   Wikipedia tic-tac-toe.


#include <stdio.h>
#include <stdlib.h>
 
int b[3][3]; /* board. 0: blank; -1: computer; 1: human */
 
int check_winner()
{
	int i;
	for (i = 0; i < 3; i++) {
		if (b[i][0] && b[i][1] == b[i][0] && b[i][2] == b[i][0])
			return b[i][0];
		if (b[0][i] && b[1][i] == b[0][i] && b[2][i] == b[0][i])
			return b[0][i];
	}
	if (!b[1][1]) return 0;
 
	if (b[1][1] == b[0][0] && b[2][2] == b[0][0]) return b[0][0];
	if (b[1][1] == b[2][0] && b[0][2] == b[1][1]) return b[1][1];
 
	return 0;
}
 
void showboard()
{
	const char *t = "X O";
	int i, j;
	for (i = 0; i < 3; i++, putchar('\n'))
		for (j = 0; j < 3; j++)
			printf("%c ", t[ b[i][j] + 1 ]);
	printf("-----\n");
}
 
#define for_ij for (i = 0; i < 3; i++) for (j = 0; j < 3; j++)
int best_i, best_j;
int test_move(int val, int depth)
{
	int i, j, score;
	int best = -1, changed = 0;
 
	if ((score = check_winner())) return (score == val) ? 1 : -1;
 
	for_ij {
		if (b[i][j]) continue;
 
		changed = b[i][j] = val;
		score = -test_move(-val, depth + 1);
		b[i][j] = 0;
 
		if (score <= best) continue;
		if (!depth) {
			best_i = i;
			best_j = j;
		}
		best = score;
	}
 
	return changed ? best : 0;
}
 
const char* game(int user)
{
	int i, j, k, move, win = 0;
	for_ij b[i][j] = 0;
 
	printf("Board postions are numbered so:\n1 2 3\n4 5 6\n7 8 9\n");
	printf("You have O, I have X.\n\n");
	for (k = 0; k < 9; k++, user = !user) {
		while(user) {
			printf("your move: ");
			if (!scanf("%d", &move)) {
				scanf("%*s");
				continue;
			}
			if (--move < 0 || move >= 9) continue;
			if (b[i = move / 3][j = move % 3]) continue;
 
			b[i][j] = 1;
			break;
		}
		if (!user) {
			if (!k) { /* randomize if computer opens, less boring */
				best_i = rand() % 3;
				best_j = rand() % 3;
			} else
				test_move(-1, 0);
 
			b[best_i][best_j] = -1;
			printf("My move: %d\n", best_i * 3 + best_j + 1);
		}
 
		showboard();
		if ((win = check_winner())) 
			return win == 1 ? "You win.\n\n": "I win.\n\n";
	}
	return "A draw.\n\n";
}
 
int main()
{
	int first = 0;
	while (1) printf("%s", game(first = !first));
	return 0;
}