From 96930161f918d08a689076f500f128522a237a75 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Sun, 27 Mar 2022 08:07:45 -0700
Subject: [PATCH] Implement 128-bit operator+= for uint128_fallback

---
 include/fmt/format.h |  9 ++++++---
 test/format-test.cc  | 15 ++++++++++++---
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 60e9b3d7..59b8077b 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -363,9 +363,12 @@ class uint128_fallback {
   FMT_CONSTEXPR auto operator>>=(int shift) -> uint128_fallback& {
     return *this = *this >> shift;
   }
-  FMT_CONSTEXPR void operator+=(uint64_t n) {
-    lo_ += n;
-    if (lo_ < n) ++hi_;
+  FMT_CONSTEXPR void operator+=(uint128_fallback n) {
+    uint64_t new_lo = lo_ + n.lo_;
+    uint64_t new_hi = hi_ + n.hi_ + (new_lo < lo_ ? 1 : 0);
+    FMT_ASSERT(new_hi >= hi_, "");
+    lo_ = new_lo;
+    hi_ = new_hi;
   }
 };
 
diff --git a/test/format-test.cc b/test/format-test.cc
index 1bee1aa4..b5ecd03c 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -33,6 +33,7 @@ using fmt::memory_buffer;
 using fmt::runtime;
 using fmt::string_view;
 using fmt::detail::max_value;
+using fmt::detail::uint128_fallback;
 
 using testing::Return;
 using testing::StrictMock;
@@ -40,7 +41,6 @@ using testing::StrictMock;
 enum { buffer_size = 256 };
 
 TEST(uint128_test, ctor) {
-  using fmt::detail::uint128_fallback;
   auto n = uint128_fallback();
   EXPECT_EQ(n, 0);
   n = uint128_fallback(42);
@@ -49,7 +49,7 @@ TEST(uint128_test, ctor) {
 }
 
 TEST(uint128_test, shift) {
-  auto n = fmt::detail::uint128_fallback(42);
+  auto n = uint128_fallback(42);
   n = n << 64;
   EXPECT_EQ(static_cast<uint64_t>(n), 0);
   n = n >> 64;
@@ -62,10 +62,19 @@ TEST(uint128_test, shift) {
 }
 
 TEST(uint128_test, minus) {
-  auto n = fmt::detail::uint128_fallback(42);
+  auto n = uint128_fallback(42);
   EXPECT_EQ(n - 2, 40);
 }
 
+TEST(uint128_test, plus_assign) {
+  auto n = uint128_fallback(32);
+  n += uint128_fallback(10);
+  EXPECT_EQ(n, 42);
+  n = uint128_fallback(max_value<uint64_t>());
+  n += uint128_fallback(1);
+  EXPECT_EQ(n, uint128_fallback(1) << 64);
+}
+
 template <typename Float> void check_isfinite() {
   using fmt::detail::isfinite;
   EXPECT_TRUE(isfinite(Float(0.0)));