range_finder.cs

struct range_finder {
  // input
  uint64_t *bitmap;
  size_t bitmap_size;
  size_t size_required;
  size_t index;

  // output
  size_t size_available;
  size_t selected_position;

  // the current word we are working on
  uint64_t current;

  // state
  uint64_t current_set_bit;
  uint64_t previous_set_bit;
};

static void init_range(uint64_t *bitmap, size_t bitmap_size,
                       size_t size_required,
                       struct range_finder *restrict range);

static bool find_next(struct range_finder *restrict range);