From 75c59ed3bacec12b307ebd167b73c507168c1174 Mon Sep 17 00:00:00 2001
From: Adam Burgess <adamburgess@users.noreply.github.com>
Date: Wed, 26 Aug 2020 04:45:06 +1000
Subject: [PATCH] Add formatters for chrono::time_point and helper overloads
 for localtime/gmtime(time_point) Fixes #1819

---
 include/fmt/chrono.h | 21 +++++++++++++++++++++
 test/chrono-test.cc  | 22 ++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index e70b8053..07af8d68 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -351,6 +351,11 @@ inline std::tm localtime(std::time_t time) {
   return lt.tm_;
 }
 
+inline std::tm localtime(
+    std::chrono::time_point<std::chrono::system_clock> time_point) {
+  return localtime(std::chrono::system_clock::to_time_t(time_point));
+}
+
 // Thread-safe replacement for std::gmtime
 inline std::tm gmtime(std::time_t time) {
   struct dispatcher {
@@ -387,6 +392,11 @@ inline std::tm gmtime(std::time_t time) {
   return gt.tm_;
 }
 
+inline std::tm gmtime(
+    std::chrono::time_point<std::chrono::system_clock> time_point) {
+  return gmtime(std::chrono::system_clock::to_time_t(time_point));
+}
+
 namespace detail {
 inline size_t strftime(char* str, size_t count, const char* format,
                        const std::tm* time) {
@@ -399,6 +409,17 @@ inline size_t strftime(wchar_t* str, size_t count, const wchar_t* format,
 }
 }  // namespace detail
 
+template <typename Char>
+struct formatter<std::chrono::time_point<std::chrono::system_clock>, Char>
+    : formatter<std::tm, Char> {
+  template <typename FormatContext>
+  auto format(std::chrono::time_point<std::chrono::system_clock> val,
+              FormatContext& ctx) -> decltype(ctx.out()) {
+    std::tm time = localtime(val);
+    return formatter<std::tm, Char>::format(time, ctx);
+  }
+};
+
 template <typename Char> struct formatter<std::tm, Char> {
   template <typename ParseContext>
   auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
diff --git a/test/chrono-test.cc b/test/chrono-test.cc
index b876c151..a7065f76 100644
--- a/test/chrono-test.cc
+++ b/test/chrono-test.cc
@@ -95,6 +95,28 @@ TEST(TimeTest, GMTime) {
   EXPECT_TRUE(EqualTime(tm, fmt::gmtime(t)));
 }
 
+TEST(TimeTest, TimePoint) {
+  setenv("TZ", "EST+5", 1);
+  tzset();
+  std::chrono::system_clock::time_point point(std::chrono::seconds(1598412345));
+  EXPECT_EQ("It is 2020-08-25 22:25:45.",
+            fmt::format("It is {:%Y-%m-%d %H:%M:%S}.", point));
+}
+
+TEST(TimeTest, LocalTimeWithTimePoint) {
+  setenv("TZ", "EST+5", 1);
+  tzset();
+  std::chrono::system_clock::time_point point(std::chrono::seconds(1598412345));
+  EXPECT_EQ("It is 2020-08-25 22:25:45.",
+            fmt::format("It is {:%Y-%m-%d %H:%M:%S}.", fmt::localtime(point)));
+}
+
+TEST(TimeTest, GMTimeWithTimePoint) {
+  std::chrono::system_clock::time_point point(std::chrono::seconds(1598412345));
+  EXPECT_EQ("It is 2020-08-26 03:25:45 UTC.",
+            fmt::format("It is {:%Y-%m-%d %H:%M:%S} UTC.", fmt::gmtime(point)));
+}
+
 #define EXPECT_TIME(spec, time, duration)                 \
   {                                                       \
     std::locale loc("ja_JP.utf8");                        \