[Generate the next number with a given number of set bits] #c

twiddle.c

#include <stdint.h> // uint64_t
#include <stdio.h>  // printf

uint64_t twiddle(uint64_t x) {
  uint64_t smallest, ripple, new_smallest, ones;

  if (x == 0)
    return 0;
  smallest = (x & -x);
  ripple = x + smallest;
  new_smallest = (ripple & -ripple);
  ones = ((new_smallest / smallest) >> 1) - 1;
  return ripple | ones;
}

int main() {
  uint64_t bits = 4;
  uint64_t a = 0xF; // a = 0b0000....1111

  for (int i = 0; i < 100; i++) {
    printf("Next number is %lu\n", (a = twiddle(a)));
  }

#if 0
	while (a <= 0xF000000000000000) { 				// a = 0b11110000...0000
		printf("Next number is %lu\n", (a=twiddle(a)));
	}
#endif

  return 0;
}

From Hacker's Delight