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 --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 52243eb54792470cab99806e495f7a89a5c5d1be..e963d7f79b33e5ad770f0da145e6f4066c116f50 100644 (file)
--- 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);
        };