/*
 * We represent a game board by an array of 9 squares.
 * Each square is either 'x', 'o', or ' '.
 * So a game position is an array of 9 chars, but is not a string.
 *
 * Players, whose turn it is, etc, are represented by 'x' or 'o'.
 */

#include <stdio.h>

extern void clearboard(char *b);
extern void printboard(char *b);
extern char isover(char *b);  /* returns winner or ' ', or 0 if not over */
extern void computermove(char *b, char player);
extern void usermove(char *b, char player);
extern char otherplayer(char player);


int main()
{
    char board[9];
    char whoseturn = 'x';
    char personplayer = 'x';

    clearboard(board);

    while (!isover(board)) {
	printboard(board);
	if (whoseturn == personplayer)
	    usermove(board, whoseturn);
	else
	    computermove(board, whoseturn);
	whoseturn = otherplayer(whoseturn);
    }
    if (isover(board) == ' ')
	printf("draw\n");
    else
	printf("%c won\n", isover(board));

    return 0;
}


char isover(char *b)
{
    extern int iswinner(char *b, char player), isdraw(char *b);
    if (iswinner(b, 'x'))
	return 'x';
    if (iswinner(b, 'o'))
	return 'o';
    if (isdraw(b))
	return ' ';
    return 0;
}


int iswinner(char *b, char player)
{
    int i;
    extern int horizwin(char *b, char player, int row);
    extern int vertwin(char *b, char player, int col);
    extern int diagwin(char *b, char player, int ismaindiag);

    for (i = 0; i < 3; i++)
	if (horizwin(b, player, i) || vertwin(b, player, i))
	    return 1;
    return diagwin(b, player, 0) || diagwin(b, player, 1);
}


int isdraw(char *b)
{
    int i;
    for (i = 0; i < 9; i++)
	if (b[i] == ' ')
	    return 0;
    return 1;
}


int horizwin(char *b, char player, int row)
{
    int i;
    for (i = 0; i < 3; i++)
	if (b[row * 3 + i] != player)
	    return 0;
    return 1;
}


int vertwin(char *b, char player, int col)
{
    int i;
    for (i = 0; i < 3; i++)
	if (b[i * 3 + col] != player)
	    return 0;
    return 1;
}


int diagwin(char *b, char player, int ismaindiag)
{
    int i;
    if (ismaindiag) {
	for (i = 0; i <= 8; i += 4)
	    if (b[i] != player)
		return 0;
    } else {
	for (i = 2; i <= 6; i += 2)
	    if (b[i] != player)
		return 0;
    }
    return 1;
}


void computermove(char *b, char player)
{
    int i;
    int bestsofar = -2;
    int bestmove;
    extern int howgood(char *pos, char player);

    for (i = 0; i < 9; i++) {
	/* how good is it if we move at loc 'i'? */
	if (b[i] == ' ') {
	    int rhowgood;
	    b[i] = player;
	    rhowgood = -howgood(b, otherplayer(player));
	    b[i] = ' ';
	    if (rhowgood > bestsofar) {
		bestsofar = rhowgood;
		bestmove = i;
	    }
	}
    }
    printf("I move at %d\n", bestmove);
    b[bestmove] = player;
}


/*
 * howgood(position, player) says how good that position is for that player,
 * if it's that player's move.
 *
 * Return value:
 *     -1: player is going to lose (if both players play optimally)
 *      0: tie (if both players play optimally)
 *      1: player is going to win (if both players play optimally)
 */

int howgood(char *pos, char player)
{
    int i, bestsofar, rhowgood;

    if (isdraw(pos))
	return 0;
    if (iswinner(pos, player))
	return 1;
    if (iswinner(pos, otherplayer(player)))
	return -1;

    bestsofar = -1;
    for (i = 0; i < 9; i++) {
	/* how good is it if we move at loc 'i'? */
	if (pos[i] == ' ') {
	    pos[i] = player;
	    rhowgood = -howgood(pos, otherplayer(player));
	    pos[i] = ' ';
	    if (rhowgood > bestsofar)
		bestsofar = rhowgood;
	}
    }
    return bestsofar;
}


void usermove(char *b, char player)
{
    int i;
    printf("Enter your move, and I don't have time for error checking\n");
    scanf("%d", &i);
    b[i] = player;
}


char otherplayer(char player)
{
    return (player == 'x') ? 'o' : 'x';
}


void clearboard(char *b)
{
    int i;
    for (i = 0; i < 9; i++)
	b[i] = ' ';
}


void printboard(char *b)
{
    extern void printrow(char *b);
    printrow(b);
    printf("---+---+---\n");
    printrow(b + 3);
    printf("---+---+---\n");
    printrow(b + 6);
}


void printrow(char *b)
{
    printf(" %c | %c | %c\n", b[0], b[1], b[2]);
}