Add the 'n' float format specifier

include/fmt/format.h

@@ -1097,6 +1097,9 @@ FMT_CONSTEXPR void handle_float_type_spec(char spec, Handler&& handler) {
   case 'A':
     handler.on_hex();
     break;
+  case 'n':
+    handler.on_num();
+    break;
   default:
     handler.on_error();
     break;
@@ -1159,6 +1162,7 @@ class float_type_checker : private ErrorHandler {
   FMT_CONSTEXPR void on_fixed() {}
   FMT_CONSTEXPR void on_percent() {}
   FMT_CONSTEXPR void on_hex() {}
+  FMT_CONSTEXPR void on_num() {}
 
   FMT_CONSTEXPR void on_error() {
     ErrorHandler::on_error("invalid type specifier");
@@ -2642,9 +2646,14 @@ struct float_spec_handler {
   bool upper;
   bool fixed;
   bool as_percentage;
+  bool use_locale;
 
   explicit float_spec_handler(char t)
-      : type(t), upper(false), fixed(false), as_percentage(false) {}
+      : type(t),
+        upper(false),
+        fixed(false),
+        as_percentage(false),
+        use_locale(false) {}
 
   void on_general() {
     if (type == 'G') upper = true;
@@ -2668,6 +2677,8 @@ struct float_spec_handler {
     if (type == 'A') upper = true;
   }
 
+  void on_num() { use_locale = true; }
+
   FMT_NORETURN void on_error() {
     FMT_THROW(format_error("invalid type specifier"));
   }
@@ -2728,6 +2739,7 @@ void internal::basic_writer<Range>::write_double(T value,
   } else if (spec.align() == ALIGN_DEFAULT) {
     as.align_ = ALIGN_RIGHT;
   }
+  // TODO: add thousands separators if handler.use_locale is set
   if (use_grisu) {
     auto params = internal::gen_digits_params();
     params.fixed = handler.fixed;

test/format-test.cc

@@ -1424,7 +1424,7 @@ TEST(FormatterTest, FormatFloat) {
 }
 
 TEST(FormatterTest, FormatDouble) {
-  check_unknown_types(1.2, "eEfFgGaA%", "double");
+  check_unknown_types(1.2, "eEfFgGaAn%", "double");
   EXPECT_EQ("0.0", format("{:}", 0.0));
   EXPECT_EQ("0.000000", format("{:f}", 0.0));
   EXPECT_EQ("0", format("{:g}", 0.0));
@@ -1447,6 +1447,10 @@ TEST(FormatterTest, FormatDouble) {
   EXPECT_EQ(buffer, format("{:A}", -42.0));
 }
 
+TEST(FormatterTest, FormatDoubleLocale) {
+  EXPECT_EQ("1.23", format("{:n}", 1.23));
+}
+
 TEST(FormatterTest, PrecisionRounding) {
   EXPECT_EQ("0", format("{:.0f}", 0.0));
   EXPECT_EQ("0", format("{:.0f}", 0.01));