tor-browser

cord.h (65385B)

---

// Copyright 2020 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// File: cord.h
// -----------------------------------------------------------------------------
//
// This file defines the `absl::Cord` data structure and operations on that data
// structure. A Cord is a string-like sequence of characters optimized for
// specific use cases. Unlike a `std::string`, which stores an array of
// contiguous characters, Cord data is stored in a structure consisting of
// separate, reference-counted "chunks."
//
// Because a Cord consists of these chunks, data can be added to or removed from
// a Cord during its lifetime. Chunks may also be shared between Cords. Unlike a
// `std::string`, a Cord can therefore accommodate data that changes over its
// lifetime, though it's not quite "mutable"; it can change only in the
// attachment, detachment, or rearrangement of chunks of its constituent data.
//
// A Cord provides some benefit over `std::string` under the following (albeit
// narrow) circumstances:
//
//   * Cord data is designed to grow and shrink over a Cord's lifetime. Cord
//     provides efficient insertions and deletions at the start and end of the
//     character sequences, avoiding copies in those cases. Static data should
//     generally be stored as strings.
//   * External memory consisting of string-like data can be directly added to
//     a Cord without requiring copies or allocations.
//   * Cord data may be shared and copied cheaply. Cord provides a copy-on-write
//     implementation and cheap sub-Cord operations. Copying a Cord is an O(1)
//     operation.
//
// As a consequence to the above, Cord data is generally large. Small data
// should generally use strings, as construction of a Cord requires some
// overhead. Small Cords (<= 15 bytes) are represented inline, but most small
// Cords are expected to grow over their lifetimes.
//
// Note that because a Cord is made up of separate chunked data, random access
// to character data within a Cord is slower than within a `std::string`.
//
// Thread Safety
//
// Cord has the same thread-safety properties as many other types like
// std::string, std::vector<>, int, etc -- it is thread-compatible. In
// particular, if threads do not call non-const methods, then it is safe to call
// const methods without synchronization. Copying a Cord produces a new instance
// that can be used concurrently with the original in arbitrary ways.

#ifndef ABSL_STRINGS_CORD_H_
#define ABSL_STRINGS_CORD_H_

#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <iosfwd>
#include <iterator>
#include <string>
#include <type_traits>
#include <utility>

#include "absl/base/attributes.h"
#include "absl/base/config.h"
#include "absl/base/internal/endian.h"
#include "absl/base/macros.h"
#include "absl/base/nullability.h"
#include "absl/base/optimization.h"
#include "absl/crc/internal/crc_cord_state.h"
#include "absl/functional/function_ref.h"
#include "absl/meta/type_traits.h"
#include "absl/strings/cord_analysis.h"
#include "absl/strings/cord_buffer.h"
#include "absl/strings/internal/cord_data_edge.h"
#include "absl/strings/internal/cord_internal.h"
#include "absl/strings/internal/cord_rep_btree.h"
#include "absl/strings/internal/cord_rep_btree_reader.h"
#include "absl/strings/internal/cord_rep_crc.h"
#include "absl/strings/internal/cord_rep_flat.h"
#include "absl/strings/internal/cordz_info.h"
#include "absl/strings/internal/cordz_update_scope.h"
#include "absl/strings/internal/cordz_update_tracker.h"
#include "absl/strings/internal/string_constant.h"
#include "absl/strings/string_view.h"
#include "absl/types/compare.h"
#include "absl/types/optional.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
class Cord;
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 {
 // Counts the *approximate* number of bytes held in full or in part by this
 // Cord (which may not remain the same between invocations). Cords that share
 // memory could each be "charged" independently for the same shared memory.
 // See also comment on `kTotalMorePrecise` on internally shared memory.
 kTotal,

 // Counts the *approximate* number of bytes held in full or in part by this
 // Cord for the distinct memory held by this cord. This option is similar
 // to `kTotal`, except that if the cord has multiple references to the same
 // memory, that memory is only counted once.
 //
 // For example:
 //   absl::Cord cord;
 //   cord.Append(some_other_cord);
 //   cord.Append(some_other_cord);
 //   // Counts `some_other_cord` twice:
 //   cord.EstimatedMemoryUsage(kTotal);
 //   // Counts `some_other_cord` once:
 //   cord.EstimatedMemoryUsage(kTotalMorePrecise);
 //
 // The `kTotalMorePrecise` number is more expensive to compute as it requires
 // deduplicating all memory references. Applications should prefer to use
 // `kFairShare` or `kTotal` unless they really need a more precise estimate
 // on "how much memory is potentially held / kept alive by this cord?"
 kTotalMorePrecise,

 // Counts the *approximate* number of bytes held in full or in part by this
 // Cord weighted by the sharing ratio of that data. For example, if some data
 // edge is shared by 4 different Cords, then each cord is attributed 1/4th of
 // the total memory usage as a 'fair share' of the total memory usage.
 kFairShare,
};

// Cord
//
// A Cord is a sequence of characters, designed to be more efficient than a
// `std::string` in certain circumstances: namely, large string data that needs
// to change over its lifetime or shared, especially when such data is shared
// across API boundaries.
//
// A Cord stores its character data in a structure that allows efficient prepend
// and append operations. This makes a Cord useful for large string data sent
// over in a wire format that may need to be prepended or appended at some point
// during the data exchange (e.g. HTTP, protocol buffers). For example, a
// Cord is useful for storing an HTTP request, and prepending an HTTP header to
// such a request.
//
// Cords should not be used for storing general string data, however. They
// require overhead to construct and are slower than strings for random access.
//
// The Cord API provides the following common API operations:
//
// * Create or assign Cords out of existing string data, memory, or other Cords
// * Append and prepend data to an existing Cord
// * Create new Sub-Cords from existing Cord data
// * Swap Cord data and compare Cord equality
// * Write out Cord data by constructing a `std::string`
//
// Additionally, the API provides iterator utilities to iterate through Cord
// data via chunks or character bytes.
//
class Cord {
private:
 template <typename T>
 using EnableIfString =
     absl::enable_if_t<std::is_same<T, std::string>::value, int>;

public:
 // Cord::Cord() Constructors.

 // Creates an empty Cord.
 constexpr Cord() noexcept;

 // Creates a Cord from an existing Cord. Cord is copyable and efficiently
 // movable. The moved-from state is valid but unspecified.
 Cord(const Cord& src);
 Cord(Cord&& src) noexcept;
 Cord& operator=(const Cord& x);
 Cord& operator=(Cord&& x) noexcept;

 // Creates a Cord from a `src` string. This constructor is marked explicit to
 // prevent implicit Cord constructions from arguments convertible to an
 // `absl::string_view`.
 explicit Cord(absl::string_view src);
 Cord& operator=(absl::string_view src);

 // Creates a Cord from a `std::string&&` rvalue. These constructors are
 // templated to avoid ambiguities for types that are convertible to both
 // `absl::string_view` and `std::string`, such as `const char*`.
 template <typename T, EnableIfString<T> = 0>
 explicit Cord(T&& src);
 template <typename T, EnableIfString<T> = 0>
 Cord& operator=(T&& src);

 // Cord::~Cord()
 //
 // Destructs the Cord.
 ~Cord() {
   if (contents_.is_tree()) DestroyCordSlow();
 }

 // MakeCordFromExternal()
 //
 // Creates a Cord that takes ownership of external string memory. The
 // contents of `data` are not copied to the Cord; instead, the external
 // memory is added to the Cord and reference-counted. This data may not be
 // changed for the life of the Cord, though it may be prepended or appended
 // to.
 //
 // `MakeCordFromExternal()` takes a callable "releaser" that is invoked when
 // the reference count for `data` reaches zero. As noted above, this data must
 // remain live until the releaser is invoked. The callable releaser also must:
 //
 //   * be move constructible
 //   * support `void operator()(absl::string_view) const` or `void operator()`
 //
 // Example:
 //
 // Cord MakeCord(BlockPool* pool) {
 //   Block* block = pool->NewBlock();
 //   FillBlock(block);
 //   return absl::MakeCordFromExternal(
 //       block->ToStringView(),
 //       [pool, block](absl::string_view v) {
 //         pool->FreeBlock(block, v);
 //       });
 // }
 //
 // WARNING: Because a Cord can be reference-counted, it's likely a bug if your
 // releaser doesn't do anything. For example, consider the following:
 //
 // void Foo(const char* buffer, int len) {
 //   auto c = absl::MakeCordFromExternal(absl::string_view(buffer, len),
 //                                       [](absl::string_view) {});
 //
 //   // BUG: If Bar() copies its cord for any reason, including keeping a
 //   // substring of it, the lifetime of buffer might be extended beyond
 //   // when Foo() returns.
 //   Bar(c);
 // }
 template <typename Releaser>
 friend Cord MakeCordFromExternal(absl::string_view data, Releaser&& releaser);

 // Cord::Clear()
 //
 // Releases the Cord data. Any nodes that share data with other Cords, if
 // applicable, will have their reference counts reduced by 1.
 ABSL_ATTRIBUTE_REINITIALIZES void Clear();

 // Cord::Append()
 //
 // Appends data to the Cord, which may come from another Cord or other string
 // data.
 void Append(const Cord& src);
 void Append(Cord&& src);
 void Append(absl::string_view src);
 template <typename T, EnableIfString<T> = 0>
 void Append(T&& src);

 // Appends `buffer` to this cord, unless `buffer` has a zero length in which
 // case this method has no effect on this cord instance.
 // This method is guaranteed to consume `buffer`.
 void Append(CordBuffer buffer);

 // Returns a CordBuffer, re-using potential existing capacity in this cord.
 //
 // Cord instances may have additional unused capacity in the last (or first)
 // nodes of the underlying tree to facilitate amortized growth. This method
 // allows applications to explicitly use this spare capacity if available,
 // or create a new CordBuffer instance otherwise.
 // If this cord has a final non-shared node with at least `min_capacity`
 // available, then this method will return that buffer including its data
 // contents. I.e.; the returned buffer will have a non-zero length, and
 // a capacity of at least `buffer.length + min_capacity`. Otherwise, this
 // method will return `CordBuffer::CreateWithDefaultLimit(capacity)`.
 //
 // Below an example of using GetAppendBuffer. Notice that in this example we
 // use `GetAppendBuffer()` only on the first iteration. As we know nothing
 // about any initial extra capacity in `cord`, we may be able to use the extra
 // capacity. But as we add new buffers with fully utilized contents after that
 // we avoid calling `GetAppendBuffer()` on subsequent iterations: while this
 // works fine, it results in an unnecessary inspection of cord contents:
 //
 //   void AppendRandomDataToCord(absl::Cord &cord, size_t n) {
 //     bool first = true;
 //     while (n > 0) {
 //       CordBuffer buffer = first ? cord.GetAppendBuffer(n)
 //                                 : CordBuffer::CreateWithDefaultLimit(n);
 //       absl::Span<char> data = buffer.available_up_to(n);
 //       FillRandomValues(data.data(), data.size());
 //       buffer.IncreaseLengthBy(data.size());
 //       cord.Append(std::move(buffer));
 //       n -= data.size();
 //       first = false;
 //     }
 //   }
 CordBuffer GetAppendBuffer(size_t capacity, size_t min_capacity = 16);

 // Returns a CordBuffer, re-using potential existing capacity in this cord.
 //
 // This function is identical to `GetAppendBuffer`, except that in the case
 // where a new `CordBuffer` is allocated, it is allocated using the provided
 // custom limit instead of the default limit. `GetAppendBuffer` will default
 // to `CordBuffer::CreateWithDefaultLimit(capacity)` whereas this method
 // will default to `CordBuffer::CreateWithCustomLimit(block_size, capacity)`.
 // This method is equivalent to `GetAppendBuffer` if `block_size` is zero.
 // See the documentation for `CreateWithCustomLimit` for more details on the
 // restrictions and legal values for `block_size`.
 CordBuffer GetCustomAppendBuffer(size_t block_size, size_t capacity,
                                  size_t min_capacity = 16);

 // Cord::Prepend()
 //
 // Prepends data to the Cord, which may come from another Cord or other string
 // data.
 void Prepend(const Cord& src);
 void Prepend(absl::string_view src);
 template <typename T, EnableIfString<T> = 0>
 void Prepend(T&& src);

 // Prepends `buffer` to this cord, unless `buffer` has a zero length in which
 // case this method has no effect on this cord instance.
 // This method is guaranteed to consume `buffer`.
 void Prepend(CordBuffer buffer);

 // Cord::RemovePrefix()
 //
 // Removes the first `n` bytes of a Cord.
 void RemovePrefix(size_t n);
 void RemoveSuffix(size_t n);

 // Cord::Subcord()
 //
 // Returns a new Cord representing the subrange [pos, pos + new_size) of
 // *this. If pos >= size(), the result is empty(). If
 // (pos + new_size) >= size(), the result is the subrange [pos, size()).
 Cord Subcord(size_t pos, size_t new_size) const;

 // Cord::swap()
 //
 // Swaps the contents of the Cord with `other`.
 void swap(Cord& other) noexcept;

 // swap()
 //
 // Swaps the contents of two Cords.
 friend void swap(Cord& x, Cord& y) noexcept { x.swap(y); }

 // Cord::size()
 //
 // Returns the size of the Cord.
 size_t size() const;

 // Cord::empty()
 //
 // Determines whether the given Cord is empty, returning `true` if so.
 bool empty() const;

 // Cord::EstimatedMemoryUsage()
 //
 // Returns the *approximate* number of bytes held by this cord.
 // See CordMemoryAccounting for more information on the accounting method.
 size_t EstimatedMemoryUsage(CordMemoryAccounting accounting_method =
                                 CordMemoryAccounting::kTotal) const;

 // Cord::Compare()
 //
 // Compares 'this' Cord with rhs. This function and its relatives treat Cords
 // as sequences of unsigned bytes. The comparison is a straightforward
 // lexicographic comparison. `Cord::Compare()` returns values as follows:
 //
 //   -1  'this' Cord is smaller
 //    0  two Cords are equal
 //    1  'this' Cord is larger
 int Compare(absl::string_view rhs) const;
 int Compare(const Cord& rhs) const;

 // Cord::StartsWith()
 //
 // Determines whether the Cord starts with the passed string data `rhs`.
 bool StartsWith(const Cord& rhs) const;
 bool StartsWith(absl::string_view rhs) const;

 // Cord::EndsWith()
 //
 // Determines whether the Cord ends with the passed string data `rhs`.
 bool EndsWith(absl::string_view rhs) const;
 bool EndsWith(const Cord& rhs) const;

 // Cord::Contains()
 //
 // Determines whether the Cord contains the passed string data `rhs`.
 bool Contains(absl::string_view rhs) const;
 bool Contains(const Cord& rhs) const;

 // Cord::operator std::string()
 //
 // Converts a Cord into a `std::string()`. This operator is marked explicit to
 // prevent unintended Cord usage in functions that take a string.
 explicit operator std::string() const;

 // CopyCordToString()
 //
 // Copies the contents of a `src` Cord into a `*dst` string.
 //
 // This function optimizes the case of reusing the destination string since it
 // can reuse previously allocated capacity. However, this function does not
 // guarantee that pointers previously returned by `dst->data()` remain valid
 // even if `*dst` had enough capacity to hold `src`. If `*dst` is a new
 // object, prefer to simply use the conversion operator to `std::string`.
 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;

 //----------------------------------------------------------------------------
 // Cord::ChunkIterator
 //----------------------------------------------------------------------------
 //
 // A `Cord::ChunkIterator` allows iteration over the constituent chunks of its
 // Cord. Such iteration allows you to perform non-const operations on the data
 // of a Cord without modifying it.
 //
 // Generally, you do not instantiate a `Cord::ChunkIterator` directly;
 // instead, you create one implicitly through use of the `Cord::Chunks()`
 // member function.
 //
 // The `Cord::ChunkIterator` has the following properties:
 //
 //   * The iterator is invalidated after any non-const operation on the
 //     Cord object over which it iterates.
 //   * The `string_view` returned by dereferencing a valid, non-`end()`
 //     iterator is guaranteed to be non-empty.
 //   * Two `ChunkIterator` objects can be compared equal if and only if they
 //     remain valid and iterate over the same Cord.
 //   * The iterator in this case is a proxy iterator; the `string_view`
 //     returned by the iterator does not live inside the Cord, and its
 //     lifetime is limited to the lifetime of the iterator itself. To help
 //     prevent lifetime issues, `ChunkIterator::reference` is not a true
 //     reference type and is equivalent to `value_type`.
 //   * The iterator keeps state that can grow for Cords that contain many
 //     nodes and are imbalanced due to sharing. Prefer to pass this type by
 //     const reference instead of by value.
 class ChunkIterator {
  public:
   using iterator_category = std::input_iterator_tag;
   using value_type = absl::string_view;
   using difference_type = ptrdiff_t;
   using pointer = absl::Nonnull<const value_type*>;
   using reference = value_type;

   ChunkIterator() = default;

   ChunkIterator& operator++();
   ChunkIterator operator++(int);
   bool operator==(const ChunkIterator& other) const;
   bool operator!=(const ChunkIterator& other) const;
   reference operator*() const;
   pointer operator->() const;

   friend class Cord;
   friend class CharIterator;

  private:
   using CordRep = absl::cord_internal::CordRep;
   using CordRepBtree = absl::cord_internal::CordRepBtree;
   using CordRepBtreeReader = absl::cord_internal::CordRepBtreeReader;

   // Constructs a `begin()` iterator from `tree`.
   explicit ChunkIterator(absl::Nonnull<cord_internal::CordRep*> tree);

   // Constructs a `begin()` iterator from `cord`.
   explicit ChunkIterator(absl::Nonnull<const Cord*> cord);

   // Initializes this instance from a tree. Invoked by constructors.
   void InitTree(absl::Nonnull<cord_internal::CordRep*> tree);

   // Removes `n` bytes from `current_chunk_`. Expects `n` to be smaller than
   // `current_chunk_.size()`.
   void RemoveChunkPrefix(size_t n);
   Cord AdvanceAndReadBytes(size_t n);
   void AdvanceBytes(size_t n);

   // Btree specific operator++
   ChunkIterator& AdvanceBtree();
   void AdvanceBytesBtree(size_t n);

   // A view into bytes of the current `CordRep`. It may only be a view to a
   // suffix of bytes if this is being used by `CharIterator`.
   absl::string_view current_chunk_;
   // The current leaf, or `nullptr` if the iterator points to short data.
   // If the current chunk is a substring node, current_leaf_ points to the
   // underlying flat or external node.
   absl::Nullable<absl::cord_internal::CordRep*> current_leaf_ = nullptr;
   // The number of bytes left in the `Cord` over which we are iterating.
   size_t bytes_remaining_ = 0;

   // Cord reader for cord btrees. Empty if not traversing a btree.
   CordRepBtreeReader btree_reader_;
 };

 // Cord::chunk_begin()
 //
 // Returns an iterator to the first chunk of the `Cord`.
 //
 // Generally, prefer using `Cord::Chunks()` within a range-based for loop for
 // iterating over the chunks of a Cord. This method may be useful for getting
 // a `ChunkIterator` where range-based for-loops are not useful.
 //
 // Example:
 //
 //   absl::Cord::ChunkIterator FindAsChunk(const absl::Cord& c,
 //                                         absl::string_view s) {
 //     return std::find(c.chunk_begin(), c.chunk_end(), s);
 //   }
 ChunkIterator chunk_begin() const ABSL_ATTRIBUTE_LIFETIME_BOUND;

 // Cord::chunk_end()
 //
 // Returns an iterator one increment past the last chunk of the `Cord`.
 //
 // Generally, prefer using `Cord::Chunks()` within a range-based for loop for
 // iterating over the chunks of a Cord. This method may be useful for getting
 // a `ChunkIterator` where range-based for-loops may not be available.
 ChunkIterator chunk_end() const ABSL_ATTRIBUTE_LIFETIME_BOUND;

 //----------------------------------------------------------------------------
 // Cord::ChunkRange
 //----------------------------------------------------------------------------
 //
 // `ChunkRange` is a helper class for iterating over the chunks of the `Cord`,
 // producing an iterator which can be used within a range-based for loop.
 // Construction of a `ChunkRange` will return an iterator pointing to the
 // first chunk of the Cord. Generally, do not construct a `ChunkRange`
 // directly; instead, prefer to use the `Cord::Chunks()` method.
 //
 // Implementation note: `ChunkRange` is simply a convenience wrapper over
 // `Cord::chunk_begin()` and `Cord::chunk_end()`.
 class ChunkRange {
  public:
   // Fulfill minimum c++ container requirements [container.requirements]
   // These (partial) container type definitions allow ChunkRange to be used
   // in various utilities expecting a subset of [container.requirements].
   // For example, the below enables using `::testing::ElementsAre(...)`
   using value_type = absl::string_view;
   using reference = value_type&;
   using const_reference = const value_type&;
   using iterator = ChunkIterator;
   using const_iterator = ChunkIterator;

   explicit ChunkRange(absl::Nonnull<const Cord*> cord) : cord_(cord) {}

   ChunkIterator begin() const;
   ChunkIterator end() const;

  private:
   absl::Nonnull<const Cord*> cord_;
 };

 // Cord::Chunks()
 //
 // Returns a `Cord::ChunkRange` for iterating over the chunks of a `Cord` with
 // a range-based for-loop. For most iteration tasks on a Cord, use
 // `Cord::Chunks()` to retrieve this iterator.
 //
 // Example:
 //
 //   void ProcessChunks(const Cord& cord) {
 //     for (absl::string_view chunk : cord.Chunks()) { ... }
 //   }
 //
 // Note that the ordinary caveats of temporary lifetime extension apply:
 //
 //   void Process() {
 //     for (absl::string_view chunk : CordFactory().Chunks()) {
 //       // The temporary Cord returned by CordFactory has been destroyed!
 //     }
 //   }
 ChunkRange Chunks() const ABSL_ATTRIBUTE_LIFETIME_BOUND;

 //----------------------------------------------------------------------------
 // Cord::CharIterator
 //----------------------------------------------------------------------------
 //
 // A `Cord::CharIterator` allows iteration over the constituent characters of
 // a `Cord`.
 //
 // Generally, you do not instantiate a `Cord::CharIterator` directly; instead,
 // you create one implicitly through use of the `Cord::Chars()` member
 // function.
 //
 // A `Cord::CharIterator` has the following properties:
 //
 //   * The iterator is invalidated after any non-const operation on the
 //     Cord object over which it iterates.
 //   * Two `CharIterator` objects can be compared equal if and only if they
 //     remain valid and iterate over the same Cord.
 //   * The iterator keeps state that can grow for Cords that contain many
 //     nodes and are imbalanced due to sharing. Prefer to pass this type by
 //     const reference instead of by value.
 //   * This type cannot act as a forward iterator because a `Cord` can reuse
 //     sections of memory. This fact violates the requirement for forward
 //     iterators to compare equal if dereferencing them returns the same
 //     object.
 class CharIterator {
  public:
   using iterator_category = std::input_iterator_tag;
   using value_type = char;
   using difference_type = ptrdiff_t;
   using pointer = absl::Nonnull<const char*>;
   using reference = const char&;

   CharIterator() = default;

   CharIterator& operator++();
   CharIterator operator++(int);
   bool operator==(const CharIterator& other) const;
   bool operator!=(const CharIterator& other) const;
   reference operator*() const;

   friend Cord;

  private:
   explicit CharIterator(absl::Nonnull<const Cord*> cord)
       : chunk_iterator_(cord) {}

   ChunkIterator chunk_iterator_;
 };

 // Cord::AdvanceAndRead()
 //
 // Advances the `Cord::CharIterator` by `n_bytes` and returns the bytes
 // advanced as a separate `Cord`. `n_bytes` must be less than or equal to the
 // number of bytes within the Cord; otherwise, behavior is undefined. It is
 // valid to pass `char_end()` and `0`.
 static Cord AdvanceAndRead(absl::Nonnull<CharIterator*> it, size_t n_bytes);

 // Cord::Advance()
 //
 // Advances the `Cord::CharIterator` by `n_bytes`. `n_bytes` must be less than
 // or equal to the number of bytes remaining within the Cord; otherwise,
 // behavior is undefined. It is valid to pass `char_end()` and `0`.
 static void Advance(absl::Nonnull<CharIterator*> it, size_t n_bytes);

 // Cord::ChunkRemaining()
 //
 // Returns the longest contiguous view starting at the iterator's position.
 //
 // `it` must be dereferenceable.
 static absl::string_view ChunkRemaining(const CharIterator& it);

 // Cord::char_begin()
 //
 // Returns an iterator to the first character of the `Cord`.
 //
 // Generally, prefer using `Cord::Chars()` within a range-based for loop for
 // iterating over the chunks of a Cord. This method may be useful for getting
 // a `CharIterator` where range-based for-loops may not be available.
 CharIterator char_begin() const ABSL_ATTRIBUTE_LIFETIME_BOUND;

 // Cord::char_end()
 //
 // Returns an iterator to one past the last character of the `Cord`.
 //
 // Generally, prefer using `Cord::Chars()` within a range-based for loop for
 // iterating over the chunks of a Cord. This method may be useful for getting
 // a `CharIterator` where range-based for-loops are not useful.
 CharIterator char_end() const ABSL_ATTRIBUTE_LIFETIME_BOUND;

 // Cord::CharRange
 //
 // `CharRange` is a helper class for iterating over the characters of a
 // producing an iterator which can be used within a range-based for loop.
 // Construction of a `CharRange` will return an iterator pointing to the first
 // character of the Cord. Generally, do not construct a `CharRange` directly;
 // instead, prefer to use the `Cord::Chars()` method shown below.
 //
 // Implementation note: `CharRange` is simply a convenience wrapper over
 // `Cord::char_begin()` and `Cord::char_end()`.
 class CharRange {
  public:
   // Fulfill minimum c++ container requirements [container.requirements]
   // These (partial) container type definitions allow CharRange to be used
   // in various utilities expecting a subset of [container.requirements].
   // For example, the below enables using `::testing::ElementsAre(...)`
   using value_type = char;
   using reference = value_type&;
   using const_reference = const value_type&;
   using iterator = CharIterator;
   using const_iterator = CharIterator;

   explicit CharRange(absl::Nonnull<const Cord*> cord) : cord_(cord) {}

   CharIterator begin() const;
   CharIterator end() const;

  private:
   absl::Nonnull<const Cord*> cord_;
 };

 // Cord::Chars()
 //
 // Returns a `Cord::CharRange` for iterating over the characters of a `Cord`
 // with a range-based for-loop. For most character-based iteration tasks on a
 // Cord, use `Cord::Chars()` to retrieve this iterator.
 //
 // Example:
 //
 //   void ProcessCord(const Cord& cord) {
 //     for (char c : cord.Chars()) { ... }
 //   }
 //
 // Note that the ordinary caveats of temporary lifetime extension apply:
 //
 //   void Process() {
 //     for (char c : CordFactory().Chars()) {
 //       // The temporary Cord returned by CordFactory has been destroyed!
 //     }
 //   }
 CharRange Chars() const ABSL_ATTRIBUTE_LIFETIME_BOUND;

 // Cord::operator[]
 //
 // Gets the "i"th character of the Cord and returns it, provided that
 // 0 <= i < Cord.size().
 //
 // NOTE: This routine is reasonably efficient. It is roughly
 // logarithmic based on the number of chunks that make up the cord. Still,
 // if you need to iterate over the contents of a cord, you should
 // use a CharIterator/ChunkIterator rather than call operator[] or Get()
 // repeatedly in a loop.
 char operator[](size_t i) const;

 // Cord::TryFlat()
 //
 // If this cord's representation is a single flat array, returns a
 // string_view referencing that array.  Otherwise returns nullopt.
 absl::optional<absl::string_view> TryFlat() const
     ABSL_ATTRIBUTE_LIFETIME_BOUND;

 // Cord::Flatten()
 //
 // Flattens the cord into a single array and returns a view of the data.
 //
 // If the cord was already flat, the contents are not modified.
 absl::string_view Flatten() ABSL_ATTRIBUTE_LIFETIME_BOUND;

 // Cord::Find()
 //
 // Returns an iterator to the first occurrence of the substring `needle`.
 //
 // If the substring `needle` does not occur, `Cord::char_end()` is returned.
 CharIterator Find(absl::string_view needle) const;
 CharIterator Find(const absl::Cord& needle) const;

 // Supports absl::Cord as a sink object for absl::Format().
 friend void AbslFormatFlush(absl::Nonnull<absl::Cord*> cord,
                             absl::string_view part) {
   cord->Append(part);
 }

 // Support automatic stringification with absl::StrCat and absl::StrFormat.
 template <typename Sink>
 friend void AbslStringify(Sink& sink, const absl::Cord& cord) {
   for (absl::string_view chunk : cord.Chunks()) {
     sink.Append(chunk);
   }
 }

 // Cord::SetExpectedChecksum()
 //
 // Stores a checksum value with this non-empty cord instance, for later
 // retrieval.
 //
 // The expected checksum is a number stored out-of-band, alongside the data.
 // It is preserved across copies and assignments, but any mutations to a cord
 // will cause it to lose its expected checksum.
 //
 // The expected checksum is not part of a Cord's value, and does not affect
 // operations such as equality or hashing.
 //
 // This field is intended to store a CRC32C checksum for later validation, to
 // help support end-to-end checksum workflows.  However, the Cord API itself
 // does no CRC validation, and assigns no meaning to this number.
 //
 // This call has no effect if this cord is empty.
 void SetExpectedChecksum(uint32_t crc);

 // Returns this cord's expected checksum, if it has one.  Otherwise, returns
 // nullopt.
 absl::optional<uint32_t> ExpectedChecksum() const;

 template <typename H>
 friend H AbslHashValue(H hash_state, const absl::Cord& c) {
   absl::optional<absl::string_view> maybe_flat = c.TryFlat();
   if (maybe_flat.has_value()) {
     return H::combine(std::move(hash_state), *maybe_flat);
   }
   return c.HashFragmented(std::move(hash_state));
 }

 // Create a Cord with the contents of StringConstant<T>::value.
 // No allocations will be done and no data will be copied.
 // This is an INTERNAL API and subject to change or removal. This API can only
 // be used by spelling absl::strings_internal::MakeStringConstant, which is
 // also an internal API.
 template <typename T>
 // NOLINTNEXTLINE(google-explicit-constructor)
 constexpr Cord(strings_internal::StringConstant<T>);

private:
 using CordRep = absl::cord_internal::CordRep;
 using CordRepFlat = absl::cord_internal::CordRepFlat;
 using CordzInfo = cord_internal::CordzInfo;
 using CordzUpdateScope = cord_internal::CordzUpdateScope;
 using CordzUpdateTracker = cord_internal::CordzUpdateTracker;
 using InlineData = cord_internal::InlineData;
 using MethodIdentifier = CordzUpdateTracker::MethodIdentifier;

 // Creates a cord instance with `method` representing the originating
 // public API call causing the cord to be created.
 explicit Cord(absl::string_view src, MethodIdentifier method);

 friend class CordTestPeer;
 friend bool operator==(const Cord& lhs, const Cord& rhs);
 friend bool operator==(const Cord& lhs, absl::string_view rhs);

#ifdef __cpp_impl_three_way_comparison

 // Cords support comparison with other Cords and string_views via operator<
 // and others; here we provide a wrapper for the C++20 three-way comparison
 // <=> operator.

 static inline std::strong_ordering ConvertCompareResultToStrongOrdering(
     int c) {
   if (c == 0) {
     return std::strong_ordering::equal;
   } else if (c < 0) {
     return std::strong_ordering::less;
   } else {
     return std::strong_ordering::greater;
   }
 }

 friend inline std::strong_ordering operator<=>(const Cord& x, const Cord& y) {
   return ConvertCompareResultToStrongOrdering(x.Compare(y));
 }

 friend inline std::strong_ordering operator<=>(const Cord& lhs,
                                                absl::string_view rhs) {
   return ConvertCompareResultToStrongOrdering(lhs.Compare(rhs));
 }

 friend inline std::strong_ordering operator<=>(absl::string_view lhs,
                                                const Cord& rhs) {
   return ConvertCompareResultToStrongOrdering(-rhs.Compare(lhs));
 }
#endif

 friend absl::Nullable<const CordzInfo*> GetCordzInfoForTesting(
     const Cord& cord);

 // Calls the provided function once for each cord chunk, in order.  Unlike
 // Chunks(), this API will not allocate memory.
 void ForEachChunk(absl::FunctionRef<void(absl::string_view)>) const;

 // Allocates new contiguous storage for the contents of the cord. This is
 // called by Flatten() when the cord was not already flat.
 absl::string_view FlattenSlowPath();

 // Actual cord contents are hidden inside the following simple
 // class so that we can isolate the bulk of cord.cc from changes
 // to the representation.
 //
 // InlineRep holds either a tree pointer, or an array of kMaxInline bytes.
 class InlineRep {
  public:
   static constexpr unsigned char kMaxInline = cord_internal::kMaxInline;
   static_assert(kMaxInline >= sizeof(absl::cord_internal::CordRep*), "");

   constexpr InlineRep() : data_() {}
   explicit InlineRep(InlineData::DefaultInitType init) : data_(init) {}
   InlineRep(const InlineRep& src);
   InlineRep(InlineRep&& src);
   InlineRep& operator=(const InlineRep& src);
   InlineRep& operator=(InlineRep&& src) noexcept;

   explicit constexpr InlineRep(absl::string_view sv,
                                absl::Nullable<CordRep*> rep);

   void Swap(absl::Nonnull<InlineRep*> rhs);
   size_t size() const;
   // Returns nullptr if holding pointer
   absl::Nullable<const char*> data() const;
   // Discards pointer, if any
   void set_data(absl::Nonnull<const char*> data, size_t n);
   absl::Nonnull<char*> set_data(size_t n);  // Write data to the result
   // Returns nullptr if holding bytes
   absl::Nullable<absl::cord_internal::CordRep*> tree() const;
   absl::Nonnull<absl::cord_internal::CordRep*> as_tree() const;
   absl::Nonnull<const char*> as_chars() const;
   // Returns non-null iff was holding a pointer
   absl::Nullable<absl::cord_internal::CordRep*> clear();
   // Converts to pointer if necessary.
   void reduce_size(size_t n);    // REQUIRES: holding data
   void remove_prefix(size_t n);  // REQUIRES: holding data
   void AppendArray(absl::string_view src, MethodIdentifier method);
   absl::string_view FindFlatStartPiece() const;

   // Creates a CordRepFlat instance from the current inlined data with `extra'
   // bytes of desired additional capacity.
   absl::Nonnull<CordRepFlat*> MakeFlatWithExtraCapacity(size_t extra);

   // Sets the tree value for this instance. `rep` must not be null.
   // Requires the current instance to hold a tree, and a lock to be held on
   // any CordzInfo referenced by this instance. The latter is enforced through
   // the CordzUpdateScope argument. If the current instance is sampled, then
   // the CordzInfo instance is updated to reference the new `rep` value.
   void SetTree(absl::Nonnull<CordRep*> rep, const CordzUpdateScope& scope);

   // Identical to SetTree(), except that `rep` is allowed to be null, in
   // which case the current instance is reset to an empty value.
   void SetTreeOrEmpty(absl::Nullable<CordRep*> rep,
                       const CordzUpdateScope& scope);

   // Sets the tree value for this instance, and randomly samples this cord.
   // This function disregards existing contents in `data_`, and should be
   // called when a Cord is 'promoted' from an 'uninitialized' or 'inlined'
   // value to a non-inlined (tree / ring) value.
   void EmplaceTree(absl::Nonnull<CordRep*> rep, MethodIdentifier method);

   // Identical to EmplaceTree, except that it copies the parent stack from
   // the provided `parent` data if the parent is sampled.
   void EmplaceTree(absl::Nonnull<CordRep*> rep, const InlineData& parent,
                    MethodIdentifier method);

   // Commits the change of a newly created, or updated `rep` root value into
   // this cord. `old_rep` indicates the old (inlined or tree) value of the
   // cord, and determines if the commit invokes SetTree() or EmplaceTree().
   void CommitTree(absl::Nullable<const CordRep*> old_rep,
                   absl::Nonnull<CordRep*> rep, const CordzUpdateScope& scope,
                   MethodIdentifier method);

   void AppendTreeToInlined(absl::Nonnull<CordRep*> tree,
                            MethodIdentifier method);
   void AppendTreeToTree(absl::Nonnull<CordRep*> tree,
                         MethodIdentifier method);
   void AppendTree(absl::Nonnull<CordRep*> tree, MethodIdentifier method);
   void PrependTreeToInlined(absl::Nonnull<CordRep*> tree,
                             MethodIdentifier method);
   void PrependTreeToTree(absl::Nonnull<CordRep*> tree,
                          MethodIdentifier method);
   void PrependTree(absl::Nonnull<CordRep*> tree, MethodIdentifier method);

   bool IsSame(const InlineRep& other) const { return data_ == other.data_; }

   // Copies the inline contents into `dst`. Assumes the cord is not empty.
   void CopyTo(absl::Nonnull<std::string*> dst) const {
     data_.CopyInlineToString(dst);
   }

   // Copies the inline contents into `dst`. Assumes the cord is not empty.
   void CopyToArray(absl::Nonnull<char*> dst) const;

   bool is_tree() const { return data_.is_tree(); }

   // Returns true if the Cord is being profiled by cordz.
   bool is_profiled() const { return data_.is_tree() && data_.is_profiled(); }

   // Returns the available inlined capacity, or 0 if is_tree() == true.
   size_t remaining_inline_capacity() const {
     return data_.is_tree() ? 0 : kMaxInline - data_.inline_size();
   }

   // Returns the profiled CordzInfo, or nullptr if not sampled.
   absl::Nullable<absl::cord_internal::CordzInfo*> cordz_info() const {
     return data_.cordz_info();
   }

   // Sets the profiled CordzInfo.
   void set_cordz_info(absl::Nonnull<cord_internal::CordzInfo*> cordz_info) {
     assert(cordz_info != nullptr);
     data_.set_cordz_info(cordz_info);
   }

   // Resets the current cordz_info to null / empty.
   void clear_cordz_info() { data_.clear_cordz_info(); }

  private:
   friend class Cord;

   void AssignSlow(const InlineRep& src);
   // Unrefs the tree and stops profiling.
   void UnrefTree();

   void ResetToEmpty() { data_ = {}; }

   void set_inline_size(size_t size) { data_.set_inline_size(size); }
   size_t inline_size() const { return data_.inline_size(); }

   // Empty cords that carry a checksum have a CordRepCrc node with a null
   // child node. The code can avoid lots of special cases where it would
   // otherwise transition from tree to inline storage if we just remove the
   // CordRepCrc node before mutations. Must never be called inside a
   // CordzUpdateScope since it untracks the cordz info.
   void MaybeRemoveEmptyCrcNode();

   cord_internal::InlineData data_;
 };
 InlineRep contents_;

 // Helper for GetFlat() and TryFlat().
 static bool GetFlatAux(absl::Nonnull<absl::cord_internal::CordRep*> rep,
                        absl::Nonnull<absl::string_view*> fragment);

 // Helper for ForEachChunk().
 static void ForEachChunkAux(
     absl::Nonnull<absl::cord_internal::CordRep*> rep,
     absl::FunctionRef<void(absl::string_view)> callback);

 // The destructor for non-empty Cords.
 void DestroyCordSlow();

 // Out-of-line implementation of slower parts of logic.
 void CopyToArraySlowPath(absl::Nonnull<char*> dst) const;
 int CompareSlowPath(absl::string_view rhs, size_t compared_size,
                     size_t size_to_compare) const;
 int CompareSlowPath(const Cord& rhs, size_t compared_size,
                     size_t size_to_compare) const;
 bool EqualsImpl(absl::string_view rhs, size_t size_to_compare) const;
 bool EqualsImpl(const Cord& rhs, size_t size_to_compare) const;
 int CompareImpl(const Cord& rhs) const;

 template <typename ResultType, typename RHS>
 friend ResultType GenericCompare(const Cord& lhs, const RHS& rhs,
                                  size_t size_to_compare);
 static absl::string_view GetFirstChunk(const Cord& c);
 static absl::string_view GetFirstChunk(absl::string_view sv);

 // Returns a new reference to contents_.tree(), or steals an existing
 // reference if called on an rvalue.
 absl::Nonnull<absl::cord_internal::CordRep*> TakeRep() const&;
 absl::Nonnull<absl::cord_internal::CordRep*> TakeRep() &&;

 // Helper for Append().
 template <typename C>
 void AppendImpl(C&& src);

 // Appends / Prepends `src` to this instance, using precise sizing.
 // This method does explicitly not attempt to use any spare capacity
 // in any pending last added private owned flat.
 // Requires `src` to be <= kMaxFlatLength.
 void AppendPrecise(absl::string_view src, MethodIdentifier method);
 void PrependPrecise(absl::string_view src, MethodIdentifier method);

 CordBuffer GetAppendBufferSlowPath(size_t block_size, size_t capacity,
                                    size_t min_capacity);

 // Prepends the provided data to this instance. `method` contains the public
 // API method for this action which is tracked for Cordz sampling purposes.
 void PrependArray(absl::string_view src, MethodIdentifier method);

 // Assigns the value in 'src' to this instance, 'stealing' its contents.
 // Requires src.length() > kMaxBytesToCopy.
 Cord& AssignLargeString(std::string&& src);

 // Helper for AbslHashValue().
 template <typename H>
 H HashFragmented(H hash_state) const {
   typename H::AbslInternalPiecewiseCombiner combiner;
   ForEachChunk([&combiner, &hash_state](absl::string_view chunk) {
     hash_state = combiner.add_buffer(std::move(hash_state), chunk.data(),
                                      chunk.size());
   });
   return H::combine(combiner.finalize(std::move(hash_state)), size());
 }

 friend class CrcCord;
 void SetCrcCordState(crc_internal::CrcCordState state);
 absl::Nullable<const crc_internal::CrcCordState*> MaybeGetCrcCordState()
     const;

 CharIterator FindImpl(CharIterator it, absl::string_view needle) const;

 void CopyToArrayImpl(absl::Nonnull<char*> dst) const;
};

ABSL_NAMESPACE_END
}  // namespace absl

namespace absl {
ABSL_NAMESPACE_BEGIN

// allow a Cord to be logged
extern std::ostream& operator<<(std::ostream& out, const Cord& cord);

// ------------------------------------------------------------------
// Internal details follow.  Clients should ignore.

namespace cord_internal {

// Does non-template-specific `CordRepExternal` initialization.
// Requires `data` to be non-empty.
void InitializeCordRepExternal(absl::string_view data,
                              absl::Nonnull<CordRepExternal*> rep);

// Creates a new `CordRep` that owns `data` and `releaser` and returns a pointer
// to it. Requires `data` to be non-empty.
template <typename Releaser>
// NOLINTNEXTLINE - suppress clang-tidy raw pointer return.
absl::Nonnull<CordRep*> NewExternalRep(absl::string_view data,
                                      Releaser&& releaser) {
 assert(!data.empty());
 using ReleaserType = absl::decay_t<Releaser>;
 CordRepExternal* rep = new CordRepExternalImpl<ReleaserType>(
     std::forward<Releaser>(releaser), 0);
 InitializeCordRepExternal(data, rep);
 return rep;
}

// Overload for function reference types that dispatches using a function
// pointer because there are no `alignof()` or `sizeof()` a function reference.
// NOLINTNEXTLINE - suppress clang-tidy raw pointer return.
inline absl::Nonnull<CordRep*> NewExternalRep(
   absl::string_view data, void (&releaser)(absl::string_view)) {
 return NewExternalRep(data, &releaser);
}

}  // namespace cord_internal

template <typename Releaser>
Cord MakeCordFromExternal(absl::string_view data, Releaser&& releaser) {
 Cord cord;
 if (ABSL_PREDICT_TRUE(!data.empty())) {
   cord.contents_.EmplaceTree(::absl::cord_internal::NewExternalRep(
                                  data, std::forward<Releaser>(releaser)),
                              Cord::MethodIdentifier::kMakeCordFromExternal);
 } else {
   using ReleaserType = absl::decay_t<Releaser>;
   cord_internal::InvokeReleaser(
       cord_internal::Rank1{}, ReleaserType(std::forward<Releaser>(releaser)),
       data);
 }
 return cord;
}

constexpr Cord::InlineRep::InlineRep(absl::string_view sv,
                                    absl::Nullable<CordRep*> rep)
   : data_(sv, rep) {}

inline Cord::InlineRep::InlineRep(const Cord::InlineRep& src)
   : data_(InlineData::kDefaultInit) {
 if (CordRep* tree = src.tree()) {
   EmplaceTree(CordRep::Ref(tree), src.data_,
               CordzUpdateTracker::kConstructorCord);
 } else {
   data_ = src.data_;
 }
}

inline Cord::InlineRep::InlineRep(Cord::InlineRep&& src) : data_(src.data_) {
 src.ResetToEmpty();
}

inline Cord::InlineRep& Cord::InlineRep::operator=(const Cord::InlineRep& src) {
 if (this == &src) {
   return *this;
 }
 if (!is_tree() && !src.is_tree()) {
   data_ = src.data_;
   return *this;
 }
 AssignSlow(src);
 return *this;
}

inline Cord::InlineRep& Cord::InlineRep::operator=(
   Cord::InlineRep&& src) noexcept {
 if (is_tree()) {
   UnrefTree();
 }
 data_ = src.data_;
 src.ResetToEmpty();
 return *this;
}

inline void Cord::InlineRep::Swap(absl::Nonnull<Cord::InlineRep*> rhs) {
 if (rhs == this) {
   return;
 }
 using std::swap;
 swap(data_, rhs->data_);
}

inline absl::Nullable<const char*> Cord::InlineRep::data() const {
 return is_tree() ? nullptr : data_.as_chars();
}

inline absl::Nonnull<const char*> Cord::InlineRep::as_chars() const {
 assert(!data_.is_tree());
 return data_.as_chars();
}

inline absl::Nonnull<absl::cord_internal::CordRep*> Cord::InlineRep::as_tree()
   const {
 assert(data_.is_tree());
 return data_.as_tree();
}

inline absl::Nullable<absl::cord_internal::CordRep*> Cord::InlineRep::tree()
   const {
 if (is_tree()) {
   return as_tree();
 } else {
   return nullptr;
 }
}

inline size_t Cord::InlineRep::size() const {
 return is_tree() ? as_tree()->length : inline_size();
}

inline absl::Nonnull<cord_internal::CordRepFlat*>
Cord::InlineRep::MakeFlatWithExtraCapacity(size_t extra) {
 static_assert(cord_internal::kMinFlatLength >= sizeof(data_), "");
 size_t len = data_.inline_size();
 auto* result = CordRepFlat::New(len + extra);
 result->length = len;
 data_.copy_max_inline_to(result->Data());
 return result;
}

inline void Cord::InlineRep::EmplaceTree(absl::Nonnull<CordRep*> rep,
                                        MethodIdentifier method) {
 assert(rep);
 data_.make_tree(rep);
 CordzInfo::MaybeTrackCord(data_, method);
}

inline void Cord::InlineRep::EmplaceTree(absl::Nonnull<CordRep*> rep,
                                        const InlineData& parent,
                                        MethodIdentifier method) {
 data_.make_tree(rep);
 CordzInfo::MaybeTrackCord(data_, parent, method);
}

inline void Cord::InlineRep::SetTree(absl::Nonnull<CordRep*> rep,
                                    const CordzUpdateScope& scope) {
 assert(rep);
 assert(data_.is_tree());
 data_.set_tree(rep);
 scope.SetCordRep(rep);
}

inline void Cord::InlineRep::SetTreeOrEmpty(absl::Nullable<CordRep*> rep,
                                           const CordzUpdateScope& scope) {
 assert(data_.is_tree());
 if (rep) {
   data_.set_tree(rep);
 } else {
   data_ = {};
 }
 scope.SetCordRep(rep);
}

inline void Cord::InlineRep::CommitTree(absl::Nullable<const CordRep*> old_rep,
                                       absl::Nonnull<CordRep*> rep,
                                       const CordzUpdateScope& scope,
                                       MethodIdentifier method) {
 if (old_rep) {
   SetTree(rep, scope);
 } else {
   EmplaceTree(rep, method);
 }
}

inline absl::Nullable<absl::cord_internal::CordRep*> Cord::InlineRep::clear() {
 if (is_tree()) {
   CordzInfo::MaybeUntrackCord(cordz_info());
 }
 absl::cord_internal::CordRep* result = tree();
 ResetToEmpty();
 return result;
}

inline void Cord::InlineRep::CopyToArray(absl::Nonnull<char*> dst) const {
 assert(!is_tree());
 size_t n = inline_size();
 assert(n != 0);
 cord_internal::SmallMemmove(dst, data_.as_chars(), n);
}

inline void Cord::InlineRep::MaybeRemoveEmptyCrcNode() {
 CordRep* rep = tree();
 if (rep == nullptr || ABSL_PREDICT_TRUE(rep->length > 0)) {
   return;
 }
 assert(rep->IsCrc());
 assert(rep->crc()->child == nullptr);
 CordzInfo::MaybeUntrackCord(cordz_info());
 CordRep::Unref(rep);
 ResetToEmpty();
}

constexpr inline Cord::Cord() noexcept {}

inline Cord::Cord(absl::string_view src)
   : Cord(src, CordzUpdateTracker::kConstructorString) {}

template <typename T>
constexpr Cord::Cord(strings_internal::StringConstant<T>)
   : contents_(strings_internal::StringConstant<T>::value,
               strings_internal::StringConstant<T>::value.size() <=
                       cord_internal::kMaxInline
                   ? nullptr
                   : &cord_internal::ConstInitExternalStorage<
                         strings_internal::StringConstant<T>>::value) {}

inline Cord& Cord::operator=(const Cord& x) {
 contents_ = x.contents_;
 return *this;
}

template <typename T, Cord::EnableIfString<T>>
Cord& Cord::operator=(T&& src) {
 if (src.size() <= cord_internal::kMaxBytesToCopy) {
   return operator=(absl::string_view(src));
 } else {
   return AssignLargeString(std::forward<T>(src));
 }
}

inline Cord::Cord(const Cord& src) : contents_(src.contents_) {}

inline Cord::Cord(Cord&& src) noexcept : contents_(std::move(src.contents_)) {}

inline void Cord::swap(Cord& other) noexcept {
 contents_.Swap(&other.contents_);
}

inline Cord& Cord::operator=(Cord&& x) noexcept {
 contents_ = std::move(x.contents_);
 return *this;
}

extern template Cord::Cord(std::string&& src);

inline size_t Cord::size() const {
 // Length is 1st field in str.rep_
 return contents_.size();
}

inline bool Cord::empty() const { return size() == 0; }

inline size_t Cord::EstimatedMemoryUsage(
   CordMemoryAccounting accounting_method) const {
 size_t result = sizeof(Cord);
 if (const absl::cord_internal::CordRep* rep = contents_.tree()) {
   switch (accounting_method) {
     case CordMemoryAccounting::kFairShare:
       result += cord_internal::GetEstimatedFairShareMemoryUsage(rep);
       break;
     case CordMemoryAccounting::kTotalMorePrecise:
       result += cord_internal::GetMorePreciseMemoryUsage(rep);
       break;
     case CordMemoryAccounting::kTotal:
       result += cord_internal::GetEstimatedMemoryUsage(rep);
       break;
   }
 }
 return result;
}

inline absl::optional<absl::string_view> Cord::TryFlat() const
   ABSL_ATTRIBUTE_LIFETIME_BOUND {
 absl::cord_internal::CordRep* rep = contents_.tree();
 if (rep == nullptr) {
   return absl::string_view(contents_.data(), contents_.size());
 }
 absl::string_view fragment;
 if (GetFlatAux(rep, &fragment)) {
   return fragment;
 }
 return absl::nullopt;
}

inline absl::string_view Cord::Flatten() ABSL_ATTRIBUTE_LIFETIME_BOUND {
 absl::cord_internal::CordRep* rep = contents_.tree();
 if (rep == nullptr) {
   return absl::string_view(contents_.data(), contents_.size());
 } else {
   absl::string_view already_flat_contents;
   if (GetFlatAux(rep, &already_flat_contents)) {
     return already_flat_contents;
   }
 }
 return FlattenSlowPath();
}

inline void Cord::Append(absl::string_view src) {
 contents_.AppendArray(src, CordzUpdateTracker::kAppendString);
}

inline void Cord::Prepend(absl::string_view src) {
 PrependArray(src, CordzUpdateTracker::kPrependString);
}

inline void Cord::Append(CordBuffer buffer) {
 if (ABSL_PREDICT_FALSE(buffer.length() == 0)) return;
 contents_.MaybeRemoveEmptyCrcNode();
 absl::string_view short_value;
 if (CordRep* rep = buffer.ConsumeValue(short_value)) {
   contents_.AppendTree(rep, CordzUpdateTracker::kAppendCordBuffer);
 } else {
   AppendPrecise(short_value, CordzUpdateTracker::kAppendCordBuffer);
 }
}

inline void Cord::Prepend(CordBuffer buffer) {
 if (ABSL_PREDICT_FALSE(buffer.length() == 0)) return;
 contents_.MaybeRemoveEmptyCrcNode();
 absl::string_view short_value;
 if (CordRep* rep = buffer.ConsumeValue(short_value)) {
   contents_.PrependTree(rep, CordzUpdateTracker::kPrependCordBuffer);
 } else {
   PrependPrecise(short_value, CordzUpdateTracker::kPrependCordBuffer);
 }
}

inline CordBuffer Cord::GetAppendBuffer(size_t capacity, size_t min_capacity) {
 if (empty()) return CordBuffer::CreateWithDefaultLimit(capacity);
 return GetAppendBufferSlowPath(0, capacity, min_capacity);
}

inline CordBuffer Cord::GetCustomAppendBuffer(size_t block_size,
                                             size_t capacity,
                                             size_t min_capacity) {
 if (empty()) {
   return block_size ? CordBuffer::CreateWithCustomLimit(block_size, capacity)
                     : CordBuffer::CreateWithDefaultLimit(capacity);
 }
 return GetAppendBufferSlowPath(block_size, capacity, min_capacity);
}

extern template void Cord::Append(std::string&& src);
extern template void Cord::Prepend(std::string&& src);

inline int Cord::Compare(const Cord& rhs) const {
 if (!contents_.is_tree() && !rhs.contents_.is_tree()) {
   return contents_.data_.Compare(rhs.contents_.data_);
 }

 return CompareImpl(rhs);
}

// Does 'this' cord start/end with rhs
inline bool Cord::StartsWith(const Cord& rhs) const {
 if (contents_.IsSame(rhs.contents_)) return true;
 size_t rhs_size = rhs.size();
 if (size() < rhs_size) return false;
 return EqualsImpl(rhs, rhs_size);
}

inline bool Cord::StartsWith(absl::string_view rhs) const {
 size_t rhs_size = rhs.size();
 if (size() < rhs_size) return false;
 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);
 if (tree->tag == cord_internal::BTREE) {
   current_chunk_ = btree_reader_.Init(tree->btree());
 } else {
   current_leaf_ = tree;
   current_chunk_ = cord_internal::EdgeData(tree);
 }
}

inline Cord::ChunkIterator::ChunkIterator(
   absl::Nonnull<cord_internal::CordRep*> tree) {
 bytes_remaining_ = tree->length;
 InitTree(tree);
}

inline Cord::ChunkIterator::ChunkIterator(absl::Nonnull<const Cord*> cord) {
 if (CordRep* tree = cord->contents_.tree()) {
   bytes_remaining_ = tree->length;
   if (ABSL_PREDICT_TRUE(bytes_remaining_ != 0)) {
     InitTree(tree);
   } else {
     current_chunk_ = {};
   }
 } else {
   bytes_remaining_ = cord->contents_.inline_size();
   current_chunk_ = {cord->contents_.data(), bytes_remaining_};
 }
}

inline Cord::ChunkIterator& Cord::ChunkIterator::AdvanceBtree() {
 current_chunk_ = btree_reader_.Next();
 return *this;
}

inline void Cord::ChunkIterator::AdvanceBytesBtree(size_t n) {
 assert(n >= current_chunk_.size());
 bytes_remaining_ -= n;
 if (bytes_remaining_) {
   if (n == current_chunk_.size()) {
     current_chunk_ = btree_reader_.Next();
   } else {
     size_t offset = btree_reader_.length() - bytes_remaining_;
     current_chunk_ = btree_reader_.Seek(offset);
   }
 } else {
   current_chunk_ = {};
 }
}

inline Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
 ABSL_HARDENING_ASSERT(bytes_remaining_ > 0 &&
                       "Attempted to iterate past `end()`");
 assert(bytes_remaining_ >= current_chunk_.size());
 bytes_remaining_ -= current_chunk_.size();
 if (bytes_remaining_ > 0) {
   if (btree_reader_) {
     return AdvanceBtree();
   } else {
     assert(!current_chunk_.empty());  // Called on invalid iterator.
   }
   current_chunk_ = {};
 }
 return *this;
}

inline Cord::ChunkIterator Cord::ChunkIterator::operator++(int) {
 ChunkIterator tmp(*this);
 operator++();
 return tmp;
}

inline bool Cord::ChunkIterator::operator==(const ChunkIterator& other) const {
 return bytes_remaining_ == other.bytes_remaining_;
}

inline bool Cord::ChunkIterator::operator!=(const ChunkIterator& other) const {
 return !(*this == other);
}

inline Cord::ChunkIterator::reference Cord::ChunkIterator::operator*() const {
 ABSL_HARDENING_ASSERT(bytes_remaining_ != 0);
 return current_chunk_;
}

inline Cord::ChunkIterator::pointer Cord::ChunkIterator::operator->() const {
 ABSL_HARDENING_ASSERT(bytes_remaining_ != 0);
 return &current_chunk_;
}

inline void Cord::ChunkIterator::RemoveChunkPrefix(size_t n) {
 assert(n < current_chunk_.size());
 current_chunk_.remove_prefix(n);
 bytes_remaining_ -= n;
}

inline void Cord::ChunkIterator::AdvanceBytes(size_t n) {
 assert(bytes_remaining_ >= n);
 if (ABSL_PREDICT_TRUE(n < current_chunk_.size())) {
   RemoveChunkPrefix(n);
 } else if (n != 0) {
   if (btree_reader_) {
     AdvanceBytesBtree(n);
   } else {
     bytes_remaining_ = 0;
   }
 }
}

inline Cord::ChunkIterator Cord::chunk_begin() const {
 return ChunkIterator(this);
}

inline Cord::ChunkIterator Cord::chunk_end() const { return ChunkIterator(); }

inline Cord::ChunkIterator Cord::ChunkRange::begin() const {
 return cord_->chunk_begin();
}

inline Cord::ChunkIterator Cord::ChunkRange::end() const {
 return cord_->chunk_end();
}

inline Cord::ChunkRange Cord::Chunks() const { return ChunkRange(this); }

inline Cord::CharIterator& Cord::CharIterator::operator++() {
 if (ABSL_PREDICT_TRUE(chunk_iterator_->size() > 1)) {
   chunk_iterator_.RemoveChunkPrefix(1);
 } else {
   ++chunk_iterator_;
 }
 return *this;
}

inline Cord::CharIterator Cord::CharIterator::operator++(int) {
 CharIterator tmp(*this);
 operator++();
 return tmp;
}

inline bool Cord::CharIterator::operator==(const CharIterator& other) const {
 return chunk_iterator_ == other.chunk_iterator_;
}

inline bool Cord::CharIterator::operator!=(const CharIterator& other) const {
 return !(*this == other);
}

inline Cord::CharIterator::reference Cord::CharIterator::operator*() const {
 return *chunk_iterator_->data();
}

inline Cord Cord::AdvanceAndRead(absl::Nonnull<CharIterator*> it,
                                size_t n_bytes) {
 assert(it != nullptr);
 return it->chunk_iterator_.AdvanceAndReadBytes(n_bytes);
}

inline void Cord::Advance(absl::Nonnull<CharIterator*> it, size_t n_bytes) {
 assert(it != nullptr);
 it->chunk_iterator_.AdvanceBytes(n_bytes);
}

inline absl::string_view Cord::ChunkRemaining(const CharIterator& it) {
 return *it.chunk_iterator_;
}

inline Cord::CharIterator Cord::char_begin() const {
 return CharIterator(this);
}

inline Cord::CharIterator Cord::char_end() const { return CharIterator(); }

inline Cord::CharIterator Cord::CharRange::begin() const {
 return cord_->char_begin();
}

inline Cord::CharIterator Cord::CharRange::end() const {
 return cord_->char_end();
}

inline Cord::CharRange Cord::Chars() const { return CharRange(this); }

inline void Cord::ForEachChunk(
   absl::FunctionRef<void(absl::string_view)> callback) const {
 absl::cord_internal::CordRep* rep = contents_.tree();
 if (rep == nullptr) {
   callback(absl::string_view(contents_.data(), contents_.size()));
 } else {
   ForEachChunkAux(rep, callback);
 }
}

// Nonmember Cord-to-Cord relational operators.
inline bool operator==(const Cord& lhs, const Cord& rhs) {
 if (lhs.contents_.IsSame(rhs.contents_)) return true;
 size_t rhs_size = rhs.size();
 if (lhs.size() != rhs_size) return false;
 return lhs.EqualsImpl(rhs, rhs_size);
}

inline bool operator!=(const Cord& x, const Cord& y) { return !(x == y); }
inline bool operator<(const Cord& x, const Cord& y) { return x.Compare(y) < 0; }
inline bool operator>(const Cord& x, const Cord& y) { return x.Compare(y) > 0; }
inline bool operator<=(const Cord& x, const Cord& y) {
 return x.Compare(y) <= 0;
}
inline bool operator>=(const Cord& x, const Cord& y) {
 return x.Compare(y) >= 0;
}

// Nonmember Cord-to-absl::string_view relational operators.
//
// Due to implicit conversions, these also enable comparisons of Cord with
// std::string and const char*.
inline bool operator==(const Cord& lhs, absl::string_view rhs) {
 size_t lhs_size = lhs.size();
 size_t rhs_size = rhs.size();
 if (lhs_size != rhs_size) return false;
 return lhs.EqualsImpl(rhs, rhs_size);
}

inline bool operator==(absl::string_view x, const Cord& y) { return y == x; }
inline bool operator!=(const Cord& x, absl::string_view y) { return !(x == y); }
inline bool operator!=(absl::string_view x, const Cord& y) { return !(x == y); }
inline bool operator<(const Cord& x, absl::string_view y) {
 return x.Compare(y) < 0;
}
inline bool operator<(absl::string_view x, const Cord& y) {
 return y.Compare(x) > 0;
}
inline bool operator>(const Cord& x, absl::string_view y) { return y < x; }
inline bool operator>(absl::string_view x, const Cord& y) { return y < x; }
inline bool operator<=(const Cord& x, absl::string_view y) { return !(y < x); }
inline bool operator<=(absl::string_view x, const Cord& y) { return !(y < x); }
inline bool operator>=(const Cord& x, absl::string_view y) { return !(x < y); }
inline bool operator>=(absl::string_view x, const Cord& y) { return !(x < y); }

// Some internals exposed to test code.
namespace strings_internal {
class CordTestAccess {
public:
 static size_t FlatOverhead();
 static size_t MaxFlatLength();
 static size_t SizeofCordRepExternal();
 static size_t SizeofCordRepSubstring();
 static size_t FlatTagToLength(uint8_t tag);
 static uint8_t LengthToTag(size_t s);
};
}  // namespace strings_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STRINGS_CORD_H_