Use fmt::pad for faster formatting of the default format pattern

include/spdlog/details/pattern_formatter_impl.h

@@ -371,18 +371,32 @@ class full_formatter :public flag_formatter
     {
         auto duration = msg.time.time_since_epoch();
         auto millis = std::chrono::duration_cast<std::chrono::milliseconds>(duration).count() % 1000;
-        msg.formatted.write("[{:d}-{:02d}-{:02d} {:02d}:{:02d}:{:02d}.{:03d}] [{}] [{}] ",
-                            msg.tm_time.tm_year + 1900,
-                            msg.tm_time.tm_mon + 1,
-                            msg.tm_time.tm_mday,
-                            msg.tm_time.tm_hour,
-                            msg.tm_time.tm_min,
-                            msg.tm_time.tm_sec,
-                            static_cast<int>(millis),
-                            msg.logger_name,
-                            level::to_str(msg.level));
 
-        msg.formatted.write(msg.raw.str());
+
+        /* Slower version(while still very fast - about 3.2 million lines/sec),
+        msg.formatted.write("[{:d}-{:02d}-{:02d} {:02d}:{:02d}:{:02d}.{:03d}] [{}] [{}] {} ",
+        msg.tm_time.tm_year + 1900,
+        msg.tm_time.tm_mon + 1,
+        msg.tm_time.tm_mday,
+        msg.tm_time.tm_hour,
+        msg.tm_time.tm_min,
+        msg.tm_time.tm_sec,
+        static_cast<int>(millis),
+        msg.logger_name,
+        level::to_str(msg.level),
+        msg.raw.str());*/
+
+        // Faster (albeit uglier) way to format the line (5.6 million lines/sec)
+        msg.formatted << '[' << msg.tm_time.tm_year + 1900 << '-'
+                      << fmt::pad(msg.tm_time.tm_mon + 1, 2, '0') << '-'
+                      << fmt::pad(msg.tm_time.tm_mday, 2, '0') << ' '
+                      << fmt::pad(msg.tm_time.tm_hour, 2, '0') << ':'
+                      << fmt::pad(msg.tm_time.tm_min, 2, '0') << ':'
+                      << fmt::pad(msg.tm_time.tm_sec, 2, '0') << '.'
+                      << fmt::pad(static_cast<int>(millis), 3, '0') << "] ";
+
+        msg.formatted << '[' << msg.logger_name << "] [" << level::to_str(msg.level) << "] ";
+        msg.formatted.write(msg.raw.data(), msg.raw.size());
     }
 };
 
@@ -429,7 +443,7 @@ inline void spdlog::pattern_formatter::handle_flag(char flag)
 {
     switch (flag)
     {
-        // logger name
+    // logger name
     case 'n':
         _formatters.push_back(std::unique_ptr<details::flag_formatter>(new details::name_formatter()));
         break;