[tcbrindle]

> Why on earth would you do this?

Good question! :)

It actually started out as a learning exercise -- I didn't (and still don't) know much about ray tracing, and so I thought it would be good to study a simple example by translating it from a language I didn't know (TypeScript) to one I was more familiar with.

This involved writing a simple vec3 class, and it seemed natural to make that constexpr. Then as I went through I realised that many more functions could be made constexpr too. And then if I hacked together some (admittedly very poor) replacement maths functions, even more of it could be constexpr...

At this point, it became a challenge to see whether I could make the whole thing run at compile-time. The only tricky part was avoiding virtual functions -- I'd originally naturally translated TypeScript "interface"s into C++ abstract classes. I ended up using two different approaches: firstly using std::variant and std::visit (the any_thing class in the source code), and secondly using a struct of function pointers (for the surfaces) -- because while virtual calls aren't allowed in constexpr code, calls via a function pointer are just fine.

In the end, I was pretty satisfied that I'd managed to push constexpr as far as I could take it. I had intended to blog about it, but never got round to it ... and now 18 months later someone has saved me the job by posting it to HN anyway :)