From 121002d7002db78c38414cf2d8e8e01e089811ca Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Sat, 18 Dec 2021 07:12:53 -0800
Subject: [PATCH] Add a map formatter

---
 include/fmt/ranges.h | 54 +++++++++++++++++++++++++++++++++++++++++++-
 test/ranges-test.cc  |  2 +-
 2 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/include/fmt/ranges.h b/include/fmt/ranges.h
index ee12bd95..3068649f 100644
--- a/include/fmt/ranges.h
+++ b/include/fmt/ranges.h
@@ -71,7 +71,7 @@ OutputIterator copy(wchar_t ch, OutputIterator out) {
   return out;
 }
 
-/// Return true value if T has std::string interface, like std::string_view.
+// Returns true if T has a std::string-like interface, like std::string_view.
 template <typename T> class is_std_string_like {
   template <typename U>
   static auto check(U* p)
@@ -86,6 +86,19 @@ template <typename T> class is_std_string_like {
 template <typename Char>
 struct is_std_string_like<fmt::basic_string_view<Char>> : std::true_type {};
 
+template <typename T> class is_map {
+  template <typename U> static auto check(U*) -> typename U::key_type;
+  template <typename> static void check(...);
+
+ public:
+#ifdef FMT_FORMAT_MAP_AS_LIST
+  static FMT_CONSTEXPR_DECL const bool value = false;
+#else
+  static FMT_CONSTEXPR_DECL const bool value =
+      !std::is_void<decltype(check<T>(nullptr))>::value;
+#endif
+};
+
 template <typename... Ts> struct conditional_helper {};
 
 template <typename T, typename _ = void> struct is_range_ : std::false_type {};
@@ -573,6 +586,7 @@ struct formatter<TupleT, Char, enable_if_t<fmt::is_tuple_like<TupleT>::value>> {
 template <typename T, typename Char> struct is_range {
   static FMT_CONSTEXPR_DECL const bool value =
       detail::is_range_<T>::value && !detail::is_std_string_like<T>::value &&
+      !detail::is_map<T>::value &&
       !std::is_convertible<T, std::basic_string<Char>>::value &&
       !std::is_constructible<detail::std_string_view<Char>, T>::value;
 };
@@ -614,6 +628,44 @@ struct formatter<
   }
 };
 
+template <typename T, typename Char>
+struct formatter<
+    T, Char,
+    enable_if_t<
+        detail::is_map<T>::value
+// Workaround a bug in MSVC 2019 and earlier.
+#if !FMT_MSC_VER
+        && (is_formattable<detail::value_type<T>, Char>::value ||
+            detail::has_fallback_formatter<detail::value_type<T>, Char>::value)
+#endif
+        >> {
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <
+      typename FormatContext, typename U,
+      FMT_ENABLE_IF(
+          std::is_same<U, conditional_t<detail::has_const_begin_end<T>::value,
+                                        const T, T>>::value)>
+  auto format(U& map, FormatContext& ctx) -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    *out++ = '{';
+    int i = 0;
+    for (const auto& item : map) {
+      if (i > 0) out = detail::write_delimiter(out);
+      out = detail::write_range_entry<Char>(out, item.first);
+      *out++ = ':';
+      *out++ = ' ';
+      out = detail::write_range_entry<Char>(out, item.second);
+      ++i;
+    }
+    *out++ = '}';
+    return out;
+  }
+};
+
 template <typename Char, typename... T> struct tuple_join_view : detail::view {
   const std::tuple<T...>& tuple;
   basic_string_view<Char> sep;
diff --git a/test/ranges-test.cc b/test/ranges-test.cc
index de785c0f..4481e2b4 100644
--- a/test/ranges-test.cc
+++ b/test/ranges-test.cc
@@ -55,7 +55,7 @@ TEST(ranges_test, format_vector2) {
 
 TEST(ranges_test, format_map) {
   auto m = std::map<std::string, int>{{"one", 1}, {"two", 2}};
-  EXPECT_EQ(fmt::format("{}", m), "[(\"one\", 1), (\"two\", 2)]");
+  EXPECT_EQ(fmt::format("{}", m), "{\"one\": 1, \"two\": 2}");
 }
 
 TEST(ranges_test, format_pair) {