Function mergesort_impl

src/fl/stl/algorithm.h:534–545

// Merge sort implementation (stable, in-place)
template <typename Iterator, typename Compare>
void mergesort_impl(Iterator first, Iterator last, Compare comp) FL_NOEXCEPT {
    auto size = last - first;
    if (size <= 16) {  // Use insertion sort for small arrays (it's stable)
        insertion_sort(first, last, comp);
        return;
    }
    
    Iterator middle = first + size / 2;
    mergesort_impl(first, middle, comp);
    mergesort_impl(middle, last, comp);
    merge_inplace(first, middle, last, comp);
}

} // namespace detail