private int ComputeDelta(int src, int dst, int mapSize)
{
    int increasing, decreasing;
    if (dst >= src)
    {
        increasing = dst - src;               // increasing direction is direct
        decreasing = (mapSize + src) - dst;   // decreasing direction is toroidal
    }
    else
    {
        increasing = (mapSize + dst) - src;   // increasing direction is toroidal
        decreasing = src - dst;               // decreasing direction is direct
    }

    if (increasing <= decreasing) { return  increasing; }
    else                          { return -decreasing; }
}

public Position GetNextStepTowards(Position origin, Position target)
{
    Position nextStep = new Position(0, 0);

    // compute the distances
    int dx = ComputeDelta(origin.X, target.X, mapXSize);
    int dy = ComputeDelta(origin.Y, target.Y, mapYSize);

    // keep the dominant distance, and clear the other distance
    if      (dx*dx > dy*dy) { dy = 0; }
    else if (dx*dx < dy*dy) { dx = 0; }

    // normalize the distances so they are -1, 0, or 1
    nextStep.X = dx.CompareTo(0);
    nextStep.Y = dy.CompareTo(0);

    return nextStep;
}