On Aug 7, 2025, at 11:45 AM, Hans Åberg <haberg_1@icloud.com> wrote:

On 7 Aug 2025, at 19:53, Oliver Hunt <oliver@apple.com> wrote:

On Aug 7, 2025, at 5:44 AM, Hans Åberg via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

I made a low-level multiprecision division function, similar in intent to the proposal https://isocpp.org/files/papers/P3161R4.html:

For an unsigned type Word, dividend a[] of size n, divisor b[] of size b, the quotient is written into q[] and the remainder into a[]:
template<class Word>
inline void div(Word a[], size_t m, const Word b[], size_t n, Word q[])
By this structure, there is no need for internal allocations, which is important for speed. The “const” part is done via virtual shifts, which can be avoided by passing a b[] with the high bit set.

Rather than Word a[] these would really need to be at least std::span<Word> - we can harden span, but not raw pointers.

A lot of APIs seem to be written generically over iterators, but I find those APIs obnoxiously verbose - but also in practice I doubt anyone plans to make a multi precision library using std::set<word>s :D

One reason I choose a[] is to avoid overhead given the low programming level. This excludes iterators, as division goes top down, and reverse iterators are offset by one, with lots of hidden additions.

Another is that I could not find a good C++ construct that admits switching between dynamic and static allocations. A quick test on std::span shows that problem, but it could be that it can be made to work. Otherwise, I would prefer to keep size and value together, as in std::span.