find_range_once.c

static bool handle_zero_word(struct range_finder *restrict range) {
  range->current_set_bit = (range->index + 1) * 64;
  if (range->current_set_bit > range->previous_set_bit + range->size_required) {
    range->selected_position =
        range->previous_set_bit + 1; // intentionally overflowing here
    range->size_available = (range->current_set_bit - range->selected_position);
    return true;
  }
  return false;
}

static bool find_range_once(struct range_finder *restrict range) {
  uint64_t bitset = range->current;

  if (bitset == ULONG_MAX) {
    // all bits are set, can skip whole thing
    range->previous_set_bit = (range->index + 1) * 64 - 1;
    return false;
  }

  if (bitset == 0) {
    return handle_zero_word(range);
  }

  while (bitset != 0) {
    int r = __builtin_ctzl(bitset);
    range->current_set_bit = (range->index) * 64 + (uint64_t)r;
    if (range->current_set_bit >
        range->previous_set_bit + range->size_required) {
      // intentionally overflowing here
      range->selected_position = range->previous_set_bit + 1;
      range->size_available =
          (range->current_set_bit - range->selected_position);
      range->previous_set_bit = range->current_set_bit;
      return true;
    }
    range->previous_set_bit = range->current_set_bit;
    bitset ^= (bitset & -bitset);
  }

  return handle_zero_word(range);
}