Add `absl::AppendCordToString`

PiperOrigin-RevId: 626039936
Change-Id: I6e791363bab06d66029a8565c42d158a2fe176f0

absl/strings/cord.cc

@@ -1062,6 +1062,15 @@ void CopyCordToString(const Cord& src, absl::Nonnull<std::string*> dst) {
   }
 }
 
+void AppendCordToString(const Cord& src, absl::Nonnull<std::string*> dst) {
+  const size_t cur_dst_size = dst->size();
+  const size_t new_dst_size = cur_dst_size + src.size();
+  absl::strings_internal::STLStringResizeUninitializedAmortized(dst,
+                                                                new_dst_size);
+  char* append_ptr = &(*dst)[cur_dst_size];
+  src.CopyToArrayImpl(append_ptr);
+}
+
 void Cord::CopyToArraySlowPath(absl::Nonnull<char*> dst) const {
   assert(contents_.is_tree());
   absl::string_view fragment;

absl/strings/cord.h

@@ -105,6 +105,7 @@ class CordTestPeer;
 template <typename Releaser>
 Cord MakeCordFromExternal(absl::string_view, Releaser&&);
 void CopyCordToString(const Cord& src, absl::Nonnull<std::string*> dst);
+void AppendCordToString(const Cord& src, absl::Nonnull<std::string*> dst);
 
 // Cord memory accounting modes
 enum class CordMemoryAccounting {
@@ -421,6 +422,18 @@ class Cord {
   friend void CopyCordToString(const Cord& src,
                                absl::Nonnull<std::string*> dst);
 
+  // AppendCordToString()
+  //
+  // Appends the contents of a `src` Cord to a `*dst` string.
+  //
+  // This function optimizes the case of appending to a non-empty destination
+  // string. If `*dst` already has capacity to store the contents of the cord,
+  // this function does not invalidate pointers previously returned by
+  // `dst->data()`. If `*dst` is a new object, prefer to simply use the
+  // conversion operator to `std::string`.
+  friend void AppendCordToString(const Cord& src,
+                                 absl::Nonnull<std::string*> dst);
+
   class CharIterator;
 
   //----------------------------------------------------------------------------
@@ -1066,6 +1079,8 @@ class Cord {
       const;
 
   CharIterator FindImpl(CharIterator it, absl::string_view needle) const;
+
+  void CopyToArrayImpl(absl::Nonnull<char*> dst) const;
 };
 
 ABSL_NAMESPACE_END
@@ -1450,6 +1465,14 @@ inline bool Cord::StartsWith(absl::string_view rhs) const {
   return EqualsImpl(rhs, rhs_size);
 }
 
+inline void Cord::CopyToArrayImpl(absl::Nonnull<char*> dst) const {
+  if (!contents_.is_tree()) {
+    if (!empty()) contents_.CopyToArray(dst);
+  } else {
+    CopyToArraySlowPath(dst);
+  }
+}
+
 inline void Cord::ChunkIterator::InitTree(
     absl::Nonnull<cord_internal::CordRep*> tree) {
   tree = cord_internal::SkipCrcNode(tree);

absl/strings/cord_test.cc

@@ -703,6 +703,38 @@ TEST_P(CordTest, CopyToString) {
                                 "copying ", "to ", "a ", "string."})));
 }
 
+static void VerifyAppendCordToString(const absl::Cord& cord) {
+  std::string initially_empty;
+  absl::AppendCordToString(cord, &initially_empty);
+  EXPECT_EQ(initially_empty, cord);
+
+  const absl::string_view kInitialContents = "initial contents.";
+  std::string expected_after_append =
+      absl::StrCat(kInitialContents, std::string(cord));
+
+  std::string no_reserve(kInitialContents);
+  absl::AppendCordToString(cord, &no_reserve);
+  EXPECT_EQ(no_reserve, expected_after_append);
+
+  std::string has_reserved_capacity(kInitialContents);
+  has_reserved_capacity.reserve(has_reserved_capacity.size() + cord.size());
+  const char* address_before_copy = has_reserved_capacity.data();
+  absl::AppendCordToString(cord, &has_reserved_capacity);
+  EXPECT_EQ(has_reserved_capacity, expected_after_append);
+  EXPECT_EQ(has_reserved_capacity.data(), address_before_copy)
+      << "AppendCordToString allocated new string storage; "
+         "has_reserved_capacity = \""
+      << has_reserved_capacity << "\"";
+}
+
+TEST_P(CordTest, AppendToString) {
+  VerifyAppendCordToString(absl::Cord());  // empty cords cannot carry CRCs
+  VerifyAppendCordToString(MaybeHardened(absl::Cord("small cord")));
+  VerifyAppendCordToString(MaybeHardened(
+      absl::MakeFragmentedCord({"fragmented ", "cord ", "to ", "test ",
+                                "appending ", "to ", "a ", "string."})));
+}
+
 TEST_P(CordTest, AppendEmptyBuffer) {
   absl::Cord cord;
   cord.Append(absl::CordBuffer());