Non-recursive merge sort with two nested loops - how?

Question

First question here, and yes this is a homework question. We are tasked with performing merge sort on an array (which I am familiar with), but in a way I am unsure of how to do.

Answer 1

Here's my lazy, iterative/bottom-up merge-sort implementation that uses std::merge:

template
OutIt mergesort(InIt begin, InIt const end, OutIt o /* auxiliary buffer */)
{
    ptrdiff_t j;
    for (j = 0; begin != end; ++begin, ++j)
    {
        for (ptrdiff_t n = 1; n <= j && j % (n * 2) == 0; n *= 2)
        {
            o = std::merge(o - n * 2, o - n, o - n, o, begin - n * 2);
            o = std::swap_ranges(begin - n * 2, begin, o - n * 2);
        }
        *o = *begin;
        ++o;
    }
    --j;
    for (ptrdiff_t m = 1, n = 1; n <= j; n *= 2)
    {
        if (j & n)
        {
            o = std::merge(o - (m + n), o - m, o - m, o, o - (m + n));
            o = std::swap_ranges(begin - (m + n), begin, o - (m + n));
            m += n;
        }
    }
    return o;
}

Here's an in-place version that uses std::inplace_merge:

template
InIt inplace_mergesort(InIt begin, InIt const end)
{
    ptrdiff_t j;
    for (j = 0; begin != end; ++begin, ++j)
    {
        for (ptrdiff_t n = 1; n <= j && j % (n * 2) == 0; n *= 2)
        { std::inplace_merge(begin - n * 2, begin - n, begin); }
    }
    --j;
    for (ptrdiff_t m = 1, n = 1; n <= j; n *= 2)
    {
        if (j & n)
        {
            std::inplace_merge(begin - (m + n), begin - m, begin);
            m += n;
        }
    }
    return begin;
}