[gcc r15-3640] libstdc++: Tweak localized formatting for floating-point types

[Jonathan Wakely via Libstdc++-cvs]

https://gcc.gnu.org/g:99b8be43d7c548db127ee4f4d0918c55edc68b3f

commit r15-3640-g99b8be43d7c548db127ee4f4d0918c55edc68b3f
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Apr 29 18:16:29 2024 +0100

    libstdc++: Tweak localized formatting for floating-point types
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format (__formatter_fp::_M_localize): Add comments
            and micro-optimize string copy.

Diff:
---
 libstdc++-v3/include/std/format | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 52243eb54792..e963d7f79b33 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -1886,25 +1886,28 @@ namespace __format
        if (__grp.empty() && __point == __dot)
          return __lstr; // Locale uses '.' and no grouping.
 
-       size_t __d = __str.find(__dot);
-       size_t __e = min(__d, __str.find(__exp));
+       size_t __d = __str.find(__dot); // Index of radix character (if any).
+       size_t __e = min(__d, __str.find(__exp)); // First of radix or exponent
        if (__e == __str.npos)
          __e = __str.size();
-       const size_t __r = __str.size() - __e;
+       const size_t __r = __str.size() - __e; // Length of remainder.
        auto __overwrite = [&](_CharT* __p, size_t) {
+         // Apply grouping to the digits before the radix or exponent.
          auto __end = std::__add_grouping(__p, __np.thousands_sep(),
                                           __grp.data(), __grp.size(),
                                           __str.data(), __str.data() + __e);
-         if (__r)
+         if (__r) // If there's a fractional part or exponent
            {
              if (__d != __str.npos)
                {
-                 *__end = __point;
+                 *__end = __point; // Add the locale's radix character.
                  ++__end;
                  ++__e;
                }
-             if (__r > 1)
-               __end += __str.copy(__end, __str.npos, __e);
+             const size_t __rlen = __str.size() - __e;
+             // Append fractional digits and/or exponent:
+             char_traits<_CharT>::copy(__end, __str.data() + __e, __rlen);
+             __end += __rlen;
            }
          return (__end - __p);
        };