Implement locale-specific integer formatting

cppformat/format.h

@@ -29,6 +29,7 @@
 #define FMT_FORMAT_H_
 
 #include <cassert>
+#include <clocale>
 #include <cmath>
 #include <cstdio>
 #include <cstring>
@@ -871,9 +872,38 @@ inline unsigned count_digits(uint32_t n) {
 }
 #endif
 
+// A functor that doesn't add a thousands separator.
+struct NoThousandsSep {
+  template <typename Char>
+  void operator()(Char *) {}
+};
+
+// A functor that adds a thousands separator.
+class ThousandsSep {
+ private:
+  fmt::StringRef sep_;
+
+  // Index of a decimal digit with the least significant digit having index 0.
+  unsigned digit_index_;
+
+ public:
+  explicit ThousandsSep(fmt::StringRef sep) : sep_(sep), digit_index_(0) {}
+
+  template <typename Char>
+  void operator()(Char *&buffer) {
+    if (++digit_index_ % 3 != 0)
+      return;
+    buffer -= sep_.size();
+    std::uninitialized_copy(sep_.data(), sep_.data() + sep_.size(), buffer);
+  }
+};
+
 // Formats a decimal unsigned integer value writing into buffer.
-template <typename UInt, typename Char>
-inline void format_decimal(Char *buffer, UInt value, unsigned num_digits) {
+// thousands_sep is a functor that is called after writing each char to
+// add a thousands separator if necessary.
+template <typename UInt, typename Char, typename ThousandsSep>
+inline void format_decimal(Char *buffer, UInt value, unsigned num_digits,
+                           ThousandsSep thousands_sep) {
   buffer += num_digits;
   while (value >= 100) {
     // Integer division is slow so do it for a group of two digits instead
@@ -882,7 +912,9 @@ inline void format_decimal(Char *buffer, UInt value, unsigned num_digits) {
     unsigned index = static_cast<unsigned>((value % 100) * 2);
     value /= 100;
     *--buffer = Data::DIGITS[index + 1];
+    thousands_sep(buffer);
     *--buffer = Data::DIGITS[index];
+    thousands_sep(buffer);
   }
   if (value < 10) {
     *--buffer = static_cast<char>('0' + value);
@@ -893,6 +925,11 @@ inline void format_decimal(Char *buffer, UInt value, unsigned num_digits) {
   *--buffer = Data::DIGITS[index];
 }
 
+template <typename UInt, typename Char>
+inline void format_decimal(Char *buffer, UInt value, unsigned num_digits) {
+  return format_decimal(buffer, value, num_digits, NoThousandsSep());
+}
+
 #ifndef _WIN32
 # define FMT_USE_WINDOWS_H 0
 #elif !defined(FMT_USE_WINDOWS_H)
@@ -2627,9 +2664,8 @@ void BasicWriter<Char>::write_int(T value, Spec spec) {
   switch (spec.type()) {
   case 0: case 'd': {
     unsigned num_digits = internal::count_digits(abs_value);
-    CharPtr p = prepare_int_buffer(
-      num_digits, spec, prefix, prefix_size) + 1 - num_digits;
-    internal::format_decimal(get(p), abs_value, num_digits);
+    CharPtr p = prepare_int_buffer(num_digits, spec, prefix, prefix_size) + 1;
+    internal::format_decimal(get(p), abs_value, 0);
     break;
   }
   case 'x': case 'X': {
@@ -2684,6 +2720,14 @@ void BasicWriter<Char>::write_int(T value, Spec spec) {
     } while ((n >>= 3) != 0);
     break;
   }
+  case 'n': {
+    unsigned num_digits = internal::count_digits(abs_value);
+    fmt::StringRef sep = std::localeconv()->thousands_sep;
+    std::size_t size = num_digits + sep.size() * (num_digits - 1) / 3;
+    CharPtr p = prepare_int_buffer(size, spec, prefix, prefix_size) + 1;
+    internal::format_decimal(get(p), abs_value, 0, internal::ThousandsSep(sep));
+    break;
+  }
   default:
     internal::report_unknown_type(
       spec.type(), spec.flag(CHAR_FLAG) ? "char" : "integer");

test/format-test.cc

@@ -28,6 +28,7 @@
 #include <cctype>
 #include <cfloat>
 #include <climits>
+#include <clocale>
 #include <cmath>
 #include <cstring>
 #include <fstream>
@@ -1164,7 +1165,7 @@ TEST(FormatterTest, FormatShort) {
 TEST(FormatterTest, FormatInt) {
   EXPECT_THROW_MSG(format("{0:v", 42),
       FormatError, "missing '}' in format string");
-  check_unknown_types(42, "bBdoxX", "integer");
+  check_unknown_types(42, "bBdoxXn", "integer");
 }
 
 TEST(FormatterTest, FormatBin) {
@@ -1248,6 +1249,16 @@ TEST(FormatterTest, FormatOct) {
   EXPECT_EQ(buffer, format("{0:o}", ULONG_MAX));
 }
 
+TEST(FormatterTest, FormatIntLocale) {
+#ifndef _WIN32
+  const char *locale = "en_US.utf-8";
+#else
+  const char *locale = "English_United States";
+#endif
+  std::setlocale(LC_ALL, locale);
+  EXPECT_EQ("1,234,567", format("{:n}", 1234567));
+}
+
 TEST(FormatterTest, FormatFloat) {
   EXPECT_EQ("392.500000", format("{0:f}", 392.5f));
 }
@@ -1311,7 +1322,7 @@ TEST(FormatterTest, FormatLongDouble) {
 }
 
 TEST(FormatterTest, FormatChar) {
-  const char types[] = "cbBdoxX";
+  const char types[] = "cbBdoxXn";
   check_unknown_types('a', types, "char");
   EXPECT_EQ("a", format("{0}", 'a'));
   EXPECT_EQ("z", format("{0:c}", 'z'));