Half-open interval overlap (mod N), fully symmetric

gistfile1.txt

// do [a,b) and [c,d) overlap?
// (assumes N <= INT_MAX/2)
static bool half_open_intervals_overlap(int a, int b, int c, int d)
{
    int w0 = modN(b - a);
    int w1 = modN(d - c);

    return (w1 && modN(c - a) < w0) || (w0 && modN(a - c) < w1);
}