Specialize formatter for chrono durations

include/fmt/time.h

@@ -11,6 +11,13 @@
 #include "format.h"
 #include <ctime>
 
+#ifndef FMT_USE_CHRONO
+# define FMT_USE_CHRONO 0
+#endif
+#if FMT_USE_CHRONO
+# include <chrono>
+#endif
+
 FMT_BEGIN_NAMESPACE
 
 // Prevents expansion of a preceding token as a function-style macro.
@@ -24,11 +31,9 @@ inline null<> gmtime_r(...) { return null<>(); }
 inline null<> gmtime_s(...) { return null<>(); }
 
 // Parses a put_time-like format string and invokes handler actions.
-template <bool IS_CONSTEXPR, typename Char, typename Handler>
-FMT_CONSTEXPR void parse_datetime_format(
-    basic_string_view<Char> format_str, Handler &&handler) {
-  auto begin = format_str.data();
-  auto end = begin + format_str.size();
+template <typename Char, typename Handler>
+FMT_CONSTEXPR const Char *parse_chrono_format(
+    const Char *begin, const Char *end, Handler &&handler) {
   auto ptr = begin;
   while (ptr != end) {
     auto c = *ptr;
@@ -65,13 +70,94 @@ FMT_CONSTEXPR void parse_datetime_format(
     case 'B':
       handler.on_full_month();
       break;
+    // Hour, minute, second:
+    case 'S':
+      handler.on_second();
+      break;
       // TODO: parse more format specifiers
     }
   }
   if (begin != ptr)
     handler.on_text(begin, ptr);
+  return ptr;
 }
-} // namespace internal
+
+struct chrono_format_checker {
+  template <typename Char>
+  void on_text(const Char *, const Char *) {}
+  void on_abbr_weekday() {}
+  void on_full_weekday() {}
+  void on_dec0_weekday() {}
+  void on_dec1_weekday() {}
+  void on_abbr_month() {}
+  void on_full_month() {}
+  void on_second() {}
+};
+}  // namespace internal
+
+#if FMT_USE_CHRONO
+namespace internal {
+
+template <typename OutputIt, typename Char>
+struct chrono_formatter {
+  OutputIt out;
+  std::chrono::seconds s;
+  std::chrono::milliseconds ms;
+
+  explicit chrono_formatter(OutputIt o) : out(o) {}
+
+  template <typename Int>
+  void write(Int value, int width) {
+    typedef typename int_traits<Int>::main_type main_type;
+    main_type n = value;
+    auto num_digits = internal::count_digits(n);
+    if (width > num_digits)
+      out = std::fill_n(out, width - num_digits, '0');
+    out = format_decimal<Char>(out, n, num_digits);
+  }
+
+  void on_text(const Char *, const Char *) {}
+  void on_abbr_weekday() {}
+  void on_full_weekday() {}
+  void on_dec0_weekday() {}
+  void on_dec1_weekday() {}
+  void on_abbr_month() {}
+  void on_full_month() {}
+
+  void on_second() {
+    write(s.count(), 2);
+    if (ms != std::chrono::milliseconds()) {
+      *out++ = '.';
+      write(ms.count(), 3);
+    }
+  }
+};
+}  // namespace internal
+
+template <typename Rep, typename Period, typename Char>
+struct formatter<std::chrono::duration<Rep, Period>, Char> {
+  mutable basic_string_view<Char> format_str;
+  using Duration = std::chrono::duration<Rep, Period>;
+
+  FMT_CONSTEXPR auto parse(basic_parse_context<Char> &ctx)
+      -> decltype(ctx.begin()) {
+    auto begin = ctx.begin(), end = ctx.end();
+    end = parse_chrono_format(begin, end, internal::chrono_format_checker());
+    format_str = basic_string_view<Char>(&*begin, end - begin);
+    return end;
+  }
+
+  template <typename FormatContext>
+  auto format(const Duration &d, FormatContext &ctx)
+      -> decltype(ctx.out()) {
+    internal::chrono_formatter<decltype(ctx.out()), Char> f(ctx.out());
+    f.s = std::chrono::duration_cast<std::chrono::seconds>(d);
+    f.ms = std::chrono::duration_cast<std::chrono::milliseconds>(d - f.s);
+    parse_chrono_format(format_str.begin(), format_str.end(), f);
+    return f.out;
+  }
+};
+#endif  // FMT_USE_CHRONO
 
 // Thread-safe replacement for std::localtime
 inline std::tm localtime(std::time_t time) {

test/time-test.cc

@@ -57,3 +57,10 @@ TEST(TimeTest, GMTime) {
   std::tm tm = *std::gmtime(&t);
   EXPECT_TRUE(EqualTime(tm, fmt::gmtime(t)));
 }
+
+#if FMT_USE_CHRONO
+TEST(TimeTest, Chrono) {
+  EXPECT_EQ("42", fmt::format("{:%S}", std::chrono::seconds(42)));
+  EXPECT_EQ("01.234", fmt::format("{:%S}", std::chrono::milliseconds(1234)));
+}
+#endif