[BUG]: constexpr remainder computes fmod instead of IEEE remainder #136

Due Diligence

• I have searched for similar issues before starting this bug report

Describe the bug

The constexpr remainder path uses x - trunc(x/y)*y (truncated quotient, which is fmod), but IEEE remainder must round the quotient to nearest-even. Wrong whenever the fractional part of x/y exceeds 0.5.

Steps to reproduce

#include <ccmath/ccmath.hpp>
#include <cmath>
#include <cstdio>
int main() {
  constexpr double v = ccm::remainder(5.0, 3.0);
  printf("ccm=%g std=%g\n", v, std::remainder(5.0, 3.0));
}

Output: ccm=2 std=-1.

Expected behavior

remainder(5,3) should be -1 (quotient rounds to 2, so 5 - 2*3 = -1).

Compiler version and type

Apple clang 17.0.0 (clang-1700.4.4.1)

Operating System

No response

Screenshots

Not applicable