Here's the one I use. Hope it helps.

static unsigned long rodent(unsigned long v)
{
    register long t = 1L<<30, r = 0, s;
#define STEP(k) s = t + r; r >>= 1; if (s <= v) { v -= s; r |= t;}

    STEP(15);   t >>= 2;
    STEP(14);   t >>= 2;
    STEP(13);   t >>= 2;
    STEP(12);   t >>= 2;
    STEP(11);   t >>= 2;
    STEP(10);   t >>= 2;
    STEP(9);    t >>= 2;
    STEP(8);    t >>= 2;
    STEP(7);    t >>= 2;
    STEP(6);    t >>= 2;
    STEP(5);    t >>= 2;
    STEP(4);    t >>= 2;
    STEP(3);    t >>= 2;
    STEP(2);    t >>= 2;
    STEP(1);    t >>= 2;
    STEP(0);
     if (r * Origin: The Cave/2, Wellington, New Zealand (3:771/130)

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