barrier_get_direction(int x1, int y1, int x2, int y2)
{
    int direction = 0;

    /* which way are we trying to go */
    if (x2 > x1)
        direction |= BarrierPositiveX;
    if (x2 < x1)
        direction |= BarrierNegativeX;
    if (y2 > y1)
        direction |= BarrierPositiveY;
    if (y2 < y1)
        direction |= BarrierNegativeY;

    return direction;
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.