Use predefined names and formats for C-locale

include/fmt/chrono.h

@@ -1393,11 +1393,38 @@ struct tm_format_checker : null_chrono_spec_handler<tm_format_checker> {
   FMT_CONSTEXPR void on_tz_name() {}
 };
 
+inline const char* tm_wday_full_name(int wday) {
+  static constexpr const char* full_name_list[] = {
+      "Sunday",   "Monday", "Tuesday", "Wednesday",
+      "Thursday", "Friday", "Saturday"};
+  return wday >= 0 && wday <= 6 ? full_name_list[wday] : "?";
+}
+inline const char* tm_wday_short_name(int wday) {
+  static constexpr const char* short_name_list[] = {"Sun", "Mon", "Tue", "Wed",
+                                                    "Thu", "Fri", "Sat"};
+  return wday >= 0 && wday <= 6 ? short_name_list[wday] : "???";
+}
+
+inline const char* tm_mon_full_name(int mon) {
+  static constexpr const char* full_name_list[] = {
+      "January", "February", "March",     "April",   "May",      "June",
+      "July",    "August",   "September", "October", "November", "December"};
+  return mon >= 0 && mon <= 11 ? full_name_list[mon] : "?";
+}
+inline const char* tm_mon_short_name(int mon) {
+  static constexpr const char* short_name_list[] = {
+      "Jan", "Feb", "Mar", "Apr", "May", "Jun",
+      "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
+  };
+  return mon >= 0 && mon <= 11 ? short_name_list[mon] : "???";
+}
+
 template <typename OutputIt, typename Char> class tm_writer {
  private:
   static constexpr int days_per_week = 7;
 
   const std::locale& loc_;
+  const bool is_classic_;
   OutputIt out_;
   const std::tm& tm_;
 
@@ -1514,7 +1541,10 @@ 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_; }
 
@@ -1522,8 +1552,18 @@ template <typename OutputIt, typename Char> class tm_writer {
     out_ = copy_str<Char>(begin, end, out_);
   }
 
-  void on_abbr_weekday() { format_localized('a'); }
+  void on_abbr_weekday() {
-  void on_full_weekday() { format_localized('A'); }
+    if (is_classic_)
+      out_ = write(out_, tm_wday_short_name(tm_wday()));
+    else
+      format_localized('a');
+  }
+  void on_full_weekday() {
+    if (is_classic_)
+      out_ = write(out_, tm_wday_full_name(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_wday());
@@ -1534,17 +1574,51 @@ template <typename OutputIt, typename Char> class tm_writer {
     write1(wday == 0 ? days_per_week : wday);
   }
 
-  void on_abbr_month() { format_localized('b'); }
+  void on_abbr_month() {
-  void on_full_month() { format_localized('B'); }
+    if (is_classic_)
+      out_ = write(out_, tm_mon_short_name(tm_mon()));
+    else
+      format_localized('b');
+  }
+  void on_full_month() {
+    if (is_classic_)
+      out_ = write(out_, tm_mon_full_name(tm_mon()));
+    else
+      format_localized('B');
+  }
 
   void on_datetime(numeric_system ns) {
-    format_localized('c', ns == numeric_system::standard ? '\0' : 'E');
+    if (is_classic_) {
+#ifdef _WIN32
+      on_us_date();
+      *out_++ = ' ';
+      on_iso_time();
+#else
+      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);
+#endif
+    } 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];
@@ -1658,7 +1732,22 @@ template <typename OutputIt, typename Char> class tm_writer {
     write2(tm_sec());
   }
 
-  void on_12_hour_time() { format_localized('r'); }
+  void on_12_hour_time() {
+    if (is_classic_) {
+#ifdef _WIN32
+      on_iso_time();
+#else
+      char buf[8];
+      write_digit2_separated(buf, to_unsigned(tm_hour12()),
+                             to_unsigned(tm_min()), to_unsigned(tm_sec()), ':');
+      out_ = copy_str<Char>(std::begin(buf), std::end(buf), out_);
+      *out_++ = ' ';
+      on_am_pm();
+#endif
+    } else {
+      format_localized('r');
+    }
+  }
   void on_24_hour_time() {
     write2(tm_hour());
     *out_++ = ':';
@@ -1671,7 +1760,14 @@ 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_hour() < 12 ? 'A' : 'P';
+      *out_++ = 'M';
+    } else {
+      format_localized('p');
+    }
+  }
 
   // These apply to chrono durations but not tm.
   void on_duration_value() {}