From a5e7e7db95768bc2c39ac6c9bbd9cd0d3b87b723 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Sat, 10 Oct 2020 07:23:36 -0700
Subject: [PATCH] Fix handling of thousand separator (#1927)

---
 include/fmt/format.h | 11 ++++++-----
 test/locale-test.cc  |  1 +
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index c14d3f18..1fc57de1 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -1686,9 +1686,9 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
     // Index of a decimal digit with the least significant digit having index 0.
     int digit_index = 0;
     group = groups.cbegin();
-    auto p = buffer.data() + size;
-    for (int i = num_digits - 1; i >= 0; --i) {
-      *--p = static_cast<Char>(digits[i]);
+    auto p = buffer.data() + size - 1;
+    for (int i = num_digits - 1; i > 0; --i) {
+      *p-- = static_cast<Char>(digits[i]);
       if (*group <= 0 || ++digit_index % *group != 0 ||
           *group == max_value<char>())
         continue;
@@ -1696,11 +1696,12 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
         digit_index = 0;
         ++group;
       }
-      p -= s.size();
       std::uninitialized_copy(s.data(), s.data() + s.size(),
                               make_checked(p, s.size()));
+      p -= s.size();
     }
-    if (prefix_size != 0) p[-1] = static_cast<Char>('-');
+    *p-- = static_cast<Char>(*digits);
+    if (prefix_size != 0) *p = static_cast<Char>('-');
     auto data = buffer.data();
     out = write_padded<align::right>(
         out, specs, usize, usize,
diff --git a/test/locale-test.cc b/test/locale-test.cc
index a9ddb361..c7c30f2e 100644
--- a/test/locale-test.cc
+++ b/test/locale-test.cc
@@ -52,6 +52,7 @@ TEST(LocaleTest, Format) {
   EXPECT_EQ("1234567", fmt::format(std::locale(), "{:L}", 1234567));
   EXPECT_EQ("1~234~567", fmt::format(loc, "{:L}", 1234567));
   EXPECT_EQ("-1~234~567", fmt::format(loc, "{:L}", -1234567));
+  EXPECT_EQ("-256", fmt::format(loc, "{:L}", -256));
   fmt::format_arg_store<fmt::format_context, int> as{1234567};
   EXPECT_EQ("1~234~567", fmt::vformat(loc, "{:L}", fmt::format_args(as)));
   std::string s;