From d870468159f2d9a4b8ad1ba5244d1c850a8abb8c Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Sat, 11 Jul 2020 06:53:25 -0700
Subject: [PATCH] Make append work with fixed-size buffer

---
 include/fmt/core.h   |  6 +++---
 include/fmt/format.h | 14 +++++++++-----
 test/core-test.cc    | 31 +++++++++++++++++++++++--------
 3 files changed, 35 insertions(+), 16 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 3a7b1433..80d93dff 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -1811,9 +1811,9 @@ OutputIt vformat_to(
     basic_format_args<buffer_context<type_identity_t<Char>>> args) {
   auto& c = detail::get_container(out);
   using container = remove_reference_t<decltype(c)>;
-  typename std::conditional<
-      std::is_same<container, detail::buffer<Char>>::value,
-      detail::buffer<Char>&, detail::container_buffer<container>>::type buf(c);
+  conditional_t<std::is_same<container, detail::buffer<Char>>::value,
+                detail::buffer<Char>&, detail::container_buffer<container>>
+      buf(c);
   detail::vformat_to(buf, to_string_view(format_str), args);
   return out;
 }
diff --git a/include/fmt/format.h b/include/fmt/format.h
index 3fd7769d..0e2a8da1 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -571,11 +571,15 @@ template <typename T> constexpr bool use_grisu() {
 template <typename T>
 template <typename U>
 void buffer<T>::append(const U* begin, const U* end) {
-  size_t new_size = size_ + to_unsigned(end - begin);
-  try_reserve(new_size);
-  std::uninitialized_copy(begin, end,
-                          make_checked(ptr_ + size_, capacity_ - size_));
-  size_ = new_size;
+  do {
+    auto count = to_unsigned(end - begin);
+    try_reserve(size_ + count);
+    auto free_cap = capacity_ - size_;
+    if (free_cap < count) count = free_cap;
+    std::uninitialized_copy_n(begin, count, make_checked(ptr_ + size_, count));
+    size_ += count;
+    begin += count;
+  } while (begin != end);
 }
 }  // namespace detail
 
diff --git a/test/core-test.cc b/test/core-test.cc
index d8157359..78d54bb1 100644
--- a/test/core-test.cc
+++ b/test/core-test.cc
@@ -35,6 +35,7 @@ using fmt::detail::make_arg;
 using fmt::detail::value;
 
 using testing::_;
+using testing::Invoke;
 using testing::Return;
 using testing::StrictMock;
 
@@ -84,9 +85,9 @@ template <typename T> struct mock_buffer : buffer<T> {
 
   mock_buffer(T* data = nullptr, size_t capacity = 0) {
     this->set(data, capacity);
-    ON_CALL(*this, do_grow(_))
-        .WillByDefault(
-            testing::Invoke([](size_t capacity) { return capacity; }));
+    ON_CALL(*this, do_grow(_)).WillByDefault(Invoke([](size_t capacity) {
+      return capacity;
+    }));
   }
 };
 
@@ -178,6 +179,20 @@ TEST(BufferTest, Append) {
   EXPECT_EQ(12u, buffer.size());
 }
 
+TEST(BufferTest, AppendPartial) {
+  char data[10];
+  mock_buffer<char> buffer(data, sizeof(data));
+  testing::InSequence seq;
+  EXPECT_CALL(buffer, do_grow(15)).WillOnce(Return(10));
+  EXPECT_CALL(buffer, do_grow(15)).WillOnce(Invoke([&buffer](size_t) {
+    EXPECT_EQ(fmt::string_view(buffer.data(), buffer.size()), "0123456789");
+    buffer.clear();
+    return 10;
+  }));
+  auto test = "0123456789abcde";
+  buffer.append(test, test + 15);
+}
+
 TEST(BufferTest, AppendAllocatesEnoughStorage) {
   char data[19];
   mock_buffer<char> buffer(data, 10);
@@ -292,10 +307,10 @@ VISIT_TYPE(unsigned long, unsigned long long);
 
 template <typename T> class NumericArgTest : public testing::Test {};
 
-using types = ::testing::Types<bool, signed char, unsigned char, signed,
-                         unsigned short, int, unsigned, long, unsigned long,
-                         long long, unsigned long long, float, double,
-                         long double>;
+using types =
+    ::testing::Types<bool, signed char, unsigned char, signed, unsigned short,
+                     int, unsigned, long, unsigned long, long long,
+                     unsigned long long, float, double, long double>;
 TYPED_TEST_CASE(NumericArgTest, types);
 
 template <typename T>
@@ -372,7 +387,7 @@ TEST(ArgTest, CustomArg) {
   using visitor =
       mock_visitor<fmt::basic_format_arg<fmt::format_context>::handle>;
   testing::StrictMock<visitor> v;
-  EXPECT_CALL(v, visit(_)).WillOnce(testing::Invoke(check_custom()));
+  EXPECT_CALL(v, visit(_)).WillOnce(Invoke(check_custom()));
   fmt::visit_format_arg(v, make_arg<fmt::format_context>(test));
 }