[libc++][format][chrono] Fix incorrect subsecond part of `duration` in format string "%S"

[Mordante]

Summary

associated github issue: #62082

for example:

#include <chrono>
#include <format>
#include <iostream>

int main()
{
    std::cout << std::format("{0:%M:%S} {0}\n", std::chrono::duration<int, std::ratio<101, 103>>{40});
    std::cout << std::format("{0:%M:%S} {0}\n", std::chrono::duration<int, std::ratio<1, 1024>>{1511});
}

current output:

00:39.000023 40[101/103]s
00:01.0000000487 1511[1/1024]s

fixed:

00:39.223300 40[101/103]s
00:01.4755850000 1511[1/1024]s

[foad]

Not my area.

[Mordante]

Thanks for working on this! For your next upload can you either use the arc tool or a diff with a high context. That way it's easier to review the patch in context.

Can you also add tests for this change, for example the ones in the bug report. (I think using microseconds does not give the proper resolution, but that's easier to validate with tests and the CI.)

[jwakely]

In D149868#4324218, @Mordante wrote:

(I think using microseconds does not give the proper resolution, but that's easier to validate with tests and the CI.)

Yeah, I think the second value should be formatted as 00:01.4755859375

[Mordante]

Are you still interested to work on this patch?

[Mordante]

The original author seems to be MIA, I've created D154851 instead. Commandeering to abandon this patch.

libcxx/include/__chrono/formatter.h

 
   using __duration = chrono::duration<_Rep, _Period>;
 
-  auto __fraction = __value - chrono::duration_cast<chrono::seconds>(__value);
+  constexpr auto __one_sec = chrono::duration_cast<chrono::microseconds>(chrono::seconds(1u));
+  auto __micro_seconds = chrono::duration_cast<chrono::microseconds>(__value).count() % __one_sec.count();
   if constexpr (chrono::treat_as_floating_point_v<_Rep>)
     // When the floating-point value has digits itself they are ignored based
     // on the wording in [tab:time.format.spec]

     // differently and uses 3 decimals.
     // https://godbolt.org/z/6dsbnW8ba
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
-                   _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}.0f}"),
-                   __fraction.count(),
+                   _LIBCPP_STATICALLY_WIDEN(_CharT, "{:{}.0f}"),
+                   __micro_seconds,
                    chrono::hh_mm_ss<__duration>::fractional_width);
   else
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
-                   _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}}"),
-                   __fraction.count(),
+                   _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0<{}}"),
+                   __micro_seconds,
                    chrono::hh_mm_ss<__duration>::fractional_width);
 }