typedef void (*fn)(int, int, int, int, void*);

printpos(int x, int y, int n, int i, void *arr)
{
//    printf("%2d%c", x+y*n, i%n==(n-1)?'\n':' ');
    int *a;

    a = (int*)arr;
    printf("%2d%c", a[i], i%n==(n-1)?'\n':' ');
}

unzigasgn(int x, int y, int n, int i, void *arr)
{
    int *a;

    a = (int*)arr;
    a[i] = x+y*n;
}

zigasgn(int x, int y, int n, int i, void *arr)
{
    int *a;

    a = (int*)arr;
    a[x+y*n] = i;
}

zigzag(int n, fn f1, void *arr)
{
    int i, x, y;

    i = 0;
    x = y = 0;
    f1(x, y, n, i, arr);
    for(;;) {
        /* right, or down */
        if(++i >= n*n)
            return;
        if(x+1 < n){
            x++;
            f1(x, y, n, i, arr);
        }else{
            y++;
            f1(x, y, n, i, arr);
        }

        /* left down */
        while(x-1 >= 0 && y+1 < n){
            x--;
            y++;
            if(++i >= n*n)
                return;
            f1(x, y, n, i, arr);
        }

        /* down, or right */
        if(++i >= n*n)
            return;
        if(y+1 < n){
            y++;
            f1(x, y, n, i, arr);
        }else{
            x++;
            f1(x, y, n, i, arr);
        }

        /* right up */
        while(x+1 < n && y-1 >= 0){
            x++;
            y--;
            if(++i >= n*n)
                return;
            f1(x, y, n, i, arr);
        }
    }
}

testzigzag(int n)
{
    int i;
    int n2;
    int *arr;

    n2 = n*n;
    arr = malloc(n*n*sizeof(*arr));

    zigzag(n, (fn)zigasgn, arr);
    zigzag(n, (fn)printpos, arr);
    printf("\n\n");
    zigzag(n, (fn)unzigasgn, arr);
    zigzag(n, (fn)printpos, arr);
}

main(int argc, char **argv)
{
    int n;

    n = 8;
    if(argc > 1){
        n = atoi(argv[1]);
    }
    testzigzag(n);
    return 0;
}