Function random_interval

numpy/random/src/distributions/distributions.c:1048–1073

uint64_t random_interval(bitgen_t *bitgen_state, uint64_t max) {
  uint64_t mask, value;
  if (max == 0) {
    return 0;
  }

  mask = max;

  /* Smallest bit mask >= max */
  mask |= mask >> 1;
  mask |= mask >> 2;
  mask |= mask >> 4;
  mask |= mask >> 8;
  mask |= mask >> 16;
  mask |= mask >> 32;

  /* Search a random value in [0..mask] <= max */
  if (max <= 0xffffffffUL) {
    while ((value = (next_uint32(bitgen_state) & mask)) > max)
      ;
  } else {
    while ((value = (next_uint64(bitgen_state) & mask)) > max)
      ;
  }
  return value;
}

/* Bounded generators */
static inline uint64_t gen_mask(uint64_t max) {