[ape4]

Perhaps nicer to avoid the comment and write:

    const C1 = 505054698555331      // floor(2^64*4/146097)

as

    constexpr int C1 = floor(2^64*4/146097);

[aw1621107]

std::floor was made constexpr in C++23, which is pretty recent as far as C++ standards go. It's possible the author didn't think using C++23 was worth the constraints it places on who could use the code.

[CodesInChaos]

That's a mathematical expression, not a C++ expression. And floor here isn't the C++ floor function, it's just describing the usual integer division semantics. The challenge here is that you need 128-bit integers to avoid overflowing.

[glitchdout]

    const C1 = 505054698555331      // floor(2^64*4/146097)

is faster