Overly complicated merge function

wtf-merge.hpp

template<typename InputIterator1, typename InputIterator2,
         typename OutputIterator, typename Size,
         typename Compare, typename Projection>
auto half_inplace_merge5(InputIterator1 first1, InputIterator1 last1,
                        InputIterator2 first2, InputIterator2 last2,
                        OutputIterator result, Size,
                        Compare compare, Projection projection)
    -> void
{
    using utility::iter_move;
    auto&& comp = utility::as_function(compare);
    auto&& proj = utility::as_function(projection);

    if (first1 == last1) {
        return;
    }

    if (first2 == last2) {
        detail::move(first1, last1, result);
        return;
    }

    if (comp(proj(*first2), proj(*first1))) {
        merge2:
        auto begin = first2;
        ++first2;
        if (first2 == last2) {
            *result = iter_move(begin);
            detail::move(first1, last1, ++result);
            return;
        }
        if (comp(proj(*first2), proj(*first1))) {
            do {
                ++first2;
                if (first2 == last2) {
                    result = detail::move(begin, first2, result);
                    detail::move(first1, last1, result);
                    return;
                }
            } while (comp(proj(*first2), proj(*first1)));
            result = detail::move(begin, first2, result);
            goto merge1;
        } else {
            *result = iter_move(begin);
            ++result;
            goto merge1;
        }
    } else {
        merge1:
        auto begin = first1;
        ++first1;
        if (first1 == last1) {
            *result = iter_move(begin);
            return;
        }
        if (not comp(proj(*first2), proj(*first1))) {
            do {
                ++first1;
                if (first1 == last1) {
                    detail::move(begin, first1, result);
                    return;
                }
            } while (not comp(proj(*first2), proj(*first1)));
            result = detail::move(begin, first1, result);
            goto merge2;
        } else {
            *result = iter_move(begin);
            ++result;
            goto merge2;
        }
    }
}