Low-quality zig-zagging through a 2-dimensional array in C.

/*
 * Zig-Zag through an array that is w columns and h rows.  Find the stepth item
 * in the series.
 */
int *
zigzag(int **a, int w, int h, int step)
{
    int i, j;
    i = 0;
    j = 0;
    if (step-- == 0)
        goto done;
    ++i;
    for (;;) {
        if (step-- == 0)
            break;
        while (j + 1 < h && i > 0) {
            --i;
            ++j;
            if (step-- == 0)
                goto done;
        }
        if (j + 1 == h) {
            ++i;
        } else {
            ++j;
        }
        if (step-- == 0)
            goto done;
        while (i + 1 < w && j > 0) {
            --j;
            ++i;
            if (step-- == 0)
                goto done;
        }
        if (i + 1 == w) {
            ++j;
        } else {
            ++i;
        }
    }
done:
    return &a[j][i];
}