Why does finding a day distance from weekday y to weekday x in future direction have to be this complicated?

Question

In libc++, this is the way we find days from weekday y (rhs) to weekday x (lhs) in future direction.

constexpr days operator-(const weekday& __lhs, const weekday& __rhs) noexcept
{
    const int __wdu = __lhs.c_encoding() - __rhs.c_encoding();
    const int __wk = (__wdu >= 0 ? __wdu : __wdu-6) / 7;
    return days{__wdu - __wk * 7};
}

The same as in date.h library

CONSTCD14
inline
days
operator-(const weekday& x, const weekday& y) NOEXCEPT
{
    auto const wdu = x.wd_ - y.wd_;
    auto const wk = (wdu >= 0 ? wdu : wdu-6) / 7;
    return days{wdu - wk * 7};
}

I wonder why we can't do just only this?

return days{x.wd_ - y.wd_ >= 0 ? x.wd_ - y.wd_ : x.wd_ - y.wd_ + 7};

Answer 1

The rationale is that I wanted to offer some support for weekdays that are !ok(). For example:

auto x = weekday{13} - weekday{5};  // x == 1d

I.e., for some range greater than [0, 6], the arithmetic is modulo 7. This was an experiment, that I suppose is still in progress. This support did not make it into the C++20 spec.