Find nth SET bit in an int

Question

Instead of just the lowest set bit, I want to find the position of the nth lowest set bit. (I\'m NOT talking about value on the nt

Answer 1

Based on a method by Juha Järvi published in the famous Bit Twiddling Hacks, I tested this implementation where n and i are used as in the question:

    a = i - (i >> 1 & 0x55555555);
    b = (a & 0x33333333) + (a >> 2 & 0x33333333);
    c = b + (b >> 4) & 0x0f0f0f0f;

    r = n + 1;
    s = 0;
    t = c + (c >> 8) & 0xff;

    if (r > t) {
        s += 16;
        r -= t;
    }

    t = c >> s & 0xf;

    if (r > t) {
        s += 8;
        r -= t;
    }

    t = b >> s & 0x7;

    if (r > t) {
        s += 4;
        r -= t;
    }

    t = a >> s & 0x3;

    if (r > t) {
        s += 2;
        r -= t;
    }

    t = i >> s & 0x1;

    if (r > t)
        s++;

    return (s);

Based on my own tests, this is about as fast as the loop on x86, whereas it is 20% faster on arm64 and probably a lot faster on arm due to the fast conditional instructions, but I can't test this right now.