Use predefined names and formats for C-locale

include/fmt/chrono.h

@@ -1396,7 +1396,30 @@ template <typename OutputIt, typename Char> class tm_writer {
  private:
   static constexpr int days_per_week = 7;
 
+  static constexpr const Char day_of_week[][10] = {
+      {'S', 'u', 'n', 'd', 'a', 'y', '\0', '\0', '\0', '\0'},
+      {'M', 'o', 'n', 'd', 'a', 'y', '\0', '\0', '\0', '\0'},
+      {'T', 'u', 'e', 's', 'd', 'a', 'y', '\0', '\0', '\0'},
+      {'W', 'e', 'd', 'n', 'e', 's', 'd', 'a', 'y', '\0'},
+      {'T', 'h', 'u', 'r', 's', 'd', 'a', 'y', '\0', '\0'},
+      {'F', 'r', 'i', 'd', 'a', 'y', '\0', '\0', '\0', '\0'},
+      {'S', 'a', 't', 'u', 'r', 'd', 'a', 'y', '\0', '\0'}};
+  static constexpr const Char month[][10] = {
+      {'J', 'a', 'n', 'u', 'a', 'r', 'y', '\0', '\0', '\0'},
+      {'F', 'e', 'b', 'r', 'u', 'a', 'r', 'y', '\0', '\0'},
+      {'M', 'a', 'r', 'c', 'h', '\0', '\0', '\0', '\0', '\0'},
+      {'A', 'p', 'r', 'i', 'l', '\0', '\0', '\0', '\0', '\0'},
+      {'M', 'a', 'y', '\0', '\0', '\0', '\0', '\0', '\0', '\0'},
+      {'J', 'u', 'n', 'e', '\0', '\0', '\0', '\0', '\0', '\0'},
+      {'J', 'u', 'l', 'y', '\0', '\0', '\0', '\0', '\0', '\0'},
+      {'A', 'u', 'g', 'u', 's', 't', '\0', '\0', '\0', '\0'},
+      {'S', 'e', 'p', 't', 'e', 'm', 'b', 'e', 'r', '\0'},
+      {'O', 'c', 't', 'o', 'b', 'e', 'r', '\0', '\0', '\0'},
+      {'N', 'o', 'v', 'e', 'm', 'b', 'e', 'r', '\0', '\0'},
+      {'D', 'e', 'c', 'e', 'm', 'b', 'e', 'r', '\0', '\0'}};
+
   const std::locale& loc_;
+  const bool is_classic_;
   OutputIt out_;
   const std::tm& tm_;
 
@@ -1482,16 +1505,33 @@ template <typename OutputIt, typename Char> class tm_writer {
 
  public:
   explicit tm_writer(const std::locale& loc, OutputIt out, const std::tm& tm)
-      : loc_(loc), out_(out), tm_(tm) {}
+      : loc_(loc),
+        is_classic_(loc_ == std::locale::classic()),
+        out_(out),
+        tm_(tm) {}
 
   OutputIt out() const { return out_; }
 
   FMT_CONSTEXPR void on_text(const Char* begin, const Char* end) {
     out_ = copy_str<Char>(begin, end, out_);
   }
 
-  void on_abbr_weekday() { format_localized('a'); }
-  void on_full_weekday() { format_localized('A'); }
+  void on_abbr_weekday() {
+    FMT_ASSERT(tm_.tm_wday >= 0 && tm_.tm_wday < 7,
+               "tm_wday not in range [0, 6]");
+    if (is_classic_)
+      out_ = write(out_, basic_string_view<Char>(day_of_week[tm_.tm_wday], 3));
+    else
+      format_localized('a');
+  }
+  void on_full_weekday() {
+    FMT_ASSERT(tm_.tm_wday >= 0 && tm_.tm_wday < 7,
+               "tm_wday not in range [0, 6]");
+    if (is_classic_)
+      out_ = write(out_, day_of_week[tm_.tm_wday]);
+    else
+      format_localized('A');
+  }
   void on_dec0_weekday(numeric_system ns) {
     if (ns != numeric_system::standard) return format_localized('w', 'O');
     write1(tm_.tm_wday);
@@ -1501,17 +1541,48 @@ template <typename OutputIt, typename Char> class tm_writer {
     write1(tm_.tm_wday == 0 ? days_per_week : tm_.tm_wday);
   }
 
-  void on_abbr_month() { format_localized('b'); }
-  void on_full_month() { format_localized('B'); }
+  void on_abbr_month() {
+    FMT_ASSERT(tm_.tm_mon >= 0 && tm_.tm_mon < 12,
+               "tm_mon not in range [0, 11]");
+    if (is_classic_)
+      out_ = write(out_, basic_string_view<Char>(month[tm_.tm_mon], 3));
+    else
+      format_localized('b');
+  }
+  void on_full_month() {
+    FMT_ASSERT(tm_.tm_mon >= 0 && tm_.tm_mon < 12,
+               "tm_mon not in range [0, 11]");
+    if (is_classic_)
+      out_ = write(out_, month[tm_.tm_mon]);
+    else
+      format_localized('B');
+  }
 
   void on_datetime(numeric_system ns) {
-    format_localized('c', ns == numeric_system::standard ? '\0' : 'E');
+    if (is_classic_) {
+      on_abbr_weekday();
+      *out_++ = ' ';
+      on_abbr_month();
+      *out_++ = ' ';
+      on_day_of_month_space(numeric_system::standard);
+      *out_++ = ' ';
+      on_iso_time();
+      *out_++ = ' ';
+      on_year(numeric_system::standard);
+    } else
+      format_localized('c', ns == numeric_system::standard ? '\0' : 'E');
   }
   void on_loc_date(numeric_system ns) {
-    format_localized('x', ns == numeric_system::standard ? '\0' : 'E');
+    if (is_classic_)
+      on_us_date();
+    else
+      format_localized('x', ns == numeric_system::standard ? '\0' : 'E');
   }
   void on_loc_time(numeric_system ns) {
-    format_localized('X', ns == numeric_system::standard ? '\0' : 'E');
+    if (is_classic_)
+      on_iso_time();
+    else
+      format_localized('X', ns == numeric_system::standard ? '\0' : 'E');
   }
   void on_us_date() {
     char buf[8];
@@ -1636,13 +1707,25 @@ template <typename OutputIt, typename Char> class tm_writer {
     out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
   }
 
-  void on_am_pm() { format_localized('p'); }
+  void on_am_pm() {
+    if (is_classic_) {
+      *out_++ = tm_.tm_hour < 12 ? 'A' : 'P';
+      *out_++ = 'M';
+    } else
+      format_localized('p');
+  }
 
   // These apply to chrono durations but not tm.
   void on_duration_value() {}
   void on_duration_unit() {}
 };
 
+template <typename OutputIt, typename Char>
+constexpr const Char tm_writer<OutputIt, Char>::day_of_week[][10];
+
+template <typename OutputIt, typename Char>
+constexpr const Char tm_writer<OutputIt, Char>::month[][10];
+
 FMT_END_DETAIL_NAMESPACE
 
 template <typename Char, typename Duration>