nice "play with bits" question I'm struggling with

[misterT]

I stumbled to this thread while searching the forum and because there's no (extremely good) solutions, I decided to give it a go. Using the tricks in the book "Hacker's Delight".
So, the goal is to find out the index of the Nth rightmost (least significant) bit that is set.

From the book:
Use the following formula to isolate the rightmost 1-bit, producing 0 if none (e.g., 01011000 -> 00001000):
x & (-x)

Complete function:

int find_nth_bit(unsigned int x, int n) {

    while(--n) {
        x = x ^ (x & (-x));  // Turn off the rightmost bit
    }

    x = x & (-x);  // we have now isolated the Nth rightmost bit
  
    // Find the isolated bit position.
    // This can be optimized many ways. Using a look-up-table or switch-case for example.
    while(x) {
        n++;
        x = x >> 1;
    }

    return n;
}

 

[electroRF]

Hi T
Nice function Thank you for sharing.

just to note, it seems 'n' in this function starts counting from 1 (not from 0)