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Fix constexpr remainder and make fmod exact for large quotients - #208
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Summary
remainderwas using thefmodformula (x - trunc(x / y) * y), which truncates the quotient instead of rounding to nearest, so it was wrong even for small inputs.fmod's constexpr kernel used the same formula and lost bits onceabs(x / y)hit 2^53.remaindernow delegates toremquo.Fixes #136 and #138.
How tested
Clang and gcc presets, Debug/Release, C++17/20, warnings-as-errors. The kernel is bit-identical to MPFR
mpfr_fmodacross 278Mfloatand 60Mdoublepairs (large-quotient and subnormal bands included).