Creation of LTS branch "lts_2020_02_25"20200225

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GitOrigin-RevId: 0033c9ea91a52ade7c6b725aa2ef3cbe15463421
Change-Id: I8a2b70063cb3ab40c6943a6db0fe40cae71ed8d7

1 files changed, 2019 insertions, 0 deletions

diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
new file mode 100644
index 00000000..d9503ae3
--- /dev/null
+++ b/absl/strings/cord.cc
@@ -0,0 +1,2019 @@
+// 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.
+
+#include "absl/strings/cord.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdio>
+#include <cstdlib>
+#include <iomanip>
+#include <limits>
+#include <ostream>
+#include <sstream>
+#include <type_traits>
+#include <unordered_set>
+#include <vector>
+
+#include "absl/base/casts.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/port.h"
+#include "absl/container/fixed_array.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_join.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+using ::absl::cord_internal::CordRep;
+using ::absl::cord_internal::CordRepConcat;
+using ::absl::cord_internal::CordRepExternal;
+using ::absl::cord_internal::CordRepSubstring;
+
+// Various representations that we allow
+enum CordRepKind {
+  CONCAT        = 0,
+  EXTERNAL      = 1,
+  SUBSTRING     = 2,
+
+  // We have different tags for different sized flat arrays,
+  // starting with FLAT
+  FLAT          = 3,
+};
+
+namespace {
+
+// Type used with std::allocator for allocating and deallocating
+// `CordRepExternal`. std::allocator is used because it opaquely handles the
+// different new / delete overloads available on a given platform.
+struct alignas(absl::cord_internal::ExternalRepAlignment()) ExternalAllocType {
+  unsigned char value[absl::cord_internal::ExternalRepAlignment()];
+};
+
+// Returns the number of objects to pass in to std::allocator<ExternalAllocType>
+// allocate() and deallocate() to create enough room for `CordRepExternal` with
+// `releaser_size` bytes on the end.
+constexpr size_t GetExternalAllocNumObjects(size_t releaser_size) {
+  // Be sure to round up since `releaser_size` could be smaller than
+  // `sizeof(ExternalAllocType)`.
+  return (sizeof(CordRepExternal) + releaser_size + sizeof(ExternalAllocType) -
+          1) /
+         sizeof(ExternalAllocType);
+}
+
+// Allocates enough memory for `CordRepExternal` and a releaser with size
+// `releaser_size` bytes.
+void* AllocateExternal(size_t releaser_size) {
+  return std::allocator<ExternalAllocType>().allocate(
+      GetExternalAllocNumObjects(releaser_size));
+}
+
+// Deallocates the memory for a `CordRepExternal` assuming it was allocated with
+// a releaser of given size and alignment.
+void DeallocateExternal(CordRepExternal* p, size_t releaser_size) {
+  std::allocator<ExternalAllocType>().deallocate(
+      reinterpret_cast<ExternalAllocType*>(p),
+      GetExternalAllocNumObjects(releaser_size));
+}
+
+// Returns a pointer to the type erased releaser for the given CordRepExternal.
+void* GetExternalReleaser(CordRepExternal* rep) {
+  return rep + 1;
+}
+
+}  // namespace
+
+namespace cord_internal {
+
+inline CordRepConcat* CordRep::concat() {
+  assert(tag == CONCAT);
+  return static_cast<CordRepConcat*>(this);
+}
+
+inline const CordRepConcat* CordRep::concat() const {
+  assert(tag == CONCAT);
+  return static_cast<const CordRepConcat*>(this);
+}
+
+inline CordRepSubstring* CordRep::substring() {
+  assert(tag == SUBSTRING);
+  return static_cast<CordRepSubstring*>(this);
+}
+
+inline const CordRepSubstring* CordRep::substring() const {
+  assert(tag == SUBSTRING);
+  return static_cast<const CordRepSubstring*>(this);
+}
+
+inline CordRepExternal* CordRep::external() {
+  assert(tag == EXTERNAL);
+  return static_cast<CordRepExternal*>(this);
+}
+
+inline const CordRepExternal* CordRep::external() const {
+  assert(tag == EXTERNAL);
+  return static_cast<const CordRepExternal*>(this);
+}
+
+}  // namespace cord_internal
+
+static const size_t kFlatOverhead = offsetof(CordRep, data);
+
+static_assert(kFlatOverhead == 13, "Unittests assume kFlatOverhead == 13");
+
+// Largest and smallest flat node lengths we are willing to allocate
+// Flat allocation size is stored in tag, which currently can encode sizes up
+// to 4K, encoded as multiple of either 8 or 32 bytes.
+// If we allow for larger sizes, we need to change this to 8/64, 16/128, etc.
+static constexpr size_t kMaxFlatSize = 4096;
+static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead;
+static constexpr size_t kMinFlatLength = 32 - kFlatOverhead;
+
+// Prefer copying blocks of at most this size, otherwise reference count.
+static const size_t kMaxBytesToCopy = 511;
+
+// Helper functions for rounded div, and rounding to exact sizes.
+static size_t DivUp(size_t n, size_t m) { return (n + m - 1) / m; }
+static size_t RoundUp(size_t n, size_t m) { return DivUp(n, m) * m; }
+
+// Returns the size to the nearest equal or larger value that can be
+// expressed exactly as a tag value.
+static size_t RoundUpForTag(size_t size) {
+  return RoundUp(size, (size <= 1024) ? 8 : 32);
+}
+
+// Converts the allocated size to a tag, rounding down if the size
+// does not exactly match a 'tag expressible' size value. The result is
+// undefined if the size exceeds the maximum size that can be encoded in
+// a tag, i.e., if size is larger than TagToAllocatedSize(<max tag>).
+static uint8_t AllocatedSizeToTag(size_t size) {
+  const size_t tag = (size <= 1024) ? size / 8 : 128 + size / 32 - 1024 / 32;
+  assert(tag <= std::numeric_limits<uint8_t>::max());
+  return tag;
+}
+
+// Converts the provided tag to the corresponding allocated size
+static constexpr size_t TagToAllocatedSize(uint8_t tag) {
+  return (tag <= 128) ? (tag * 8) : (1024 + (tag - 128) * 32);
+}
+
+// Converts the provided tag to the corresponding available data length
+static constexpr size_t TagToLength(uint8_t tag) {
+  return TagToAllocatedSize(tag) - kFlatOverhead;
+}
+
+// Enforce that kMaxFlatSize maps to a well-known exact tag value.
+static_assert(TagToAllocatedSize(224) == kMaxFlatSize, "Bad tag logic");
+
+constexpr uint64_t Fibonacci(unsigned char n, uint64_t a = 0, uint64_t b = 1) {
+  return n == 0 ? a : Fibonacci(n - 1, b, a + b);
+}
+
+static_assert(Fibonacci(63) == 6557470319842,
+              "Fibonacci values computed incorrectly");
+
+// Minimum length required for a given depth tree -- a tree is considered
+// balanced if
+//      length(t) >= min_length[depth(t)]
+// The root node depth is allowed to become twice as large to reduce rebalancing
+// for larger strings (see IsRootBalanced).
+static constexpr uint64_t min_length[] = {
+    Fibonacci(2),
+    Fibonacci(3),
+    Fibonacci(4),
+    Fibonacci(5),
+    Fibonacci(6),
+    Fibonacci(7),
+    Fibonacci(8),
+    Fibonacci(9),
+    Fibonacci(10),
+    Fibonacci(11),
+    Fibonacci(12),
+    Fibonacci(13),
+    Fibonacci(14),
+    Fibonacci(15),
+    Fibonacci(16),
+    Fibonacci(17),
+    Fibonacci(18),
+    Fibonacci(19),
+    Fibonacci(20),
+    Fibonacci(21),
+    Fibonacci(22),
+    Fibonacci(23),
+    Fibonacci(24),
+    Fibonacci(25),
+    Fibonacci(26),
+    Fibonacci(27),
+    Fibonacci(28),
+    Fibonacci(29),
+    Fibonacci(30),
+    Fibonacci(31),
+    Fibonacci(32),
+    Fibonacci(33),
+    Fibonacci(34),
+    Fibonacci(35),
+    Fibonacci(36),
+    Fibonacci(37),
+    Fibonacci(38),
+    Fibonacci(39),
+    Fibonacci(40),
+    Fibonacci(41),
+    Fibonacci(42),
+    Fibonacci(43),
+    Fibonacci(44),
+    Fibonacci(45),
+    Fibonacci(46),
+    Fibonacci(47),
+    0xffffffffffffffffull,  // Avoid overflow
+};
+
+static const int kMinLengthSize = ABSL_ARRAYSIZE(min_length);
+
+// The inlined size to use with absl::InlinedVector.
+//
+// Note: The InlinedVectors in this file (and in cord.h) do not need to use
+// the same value for their inlined size. The fact that they do is historical.
+// It may be desirable for each to use a different inlined size optimized for
+// that InlinedVector's usage.
+//
+// TODO(jgm): Benchmark to see if there's a more optimal value than 47 for
+// the inlined vector size (47 exists for backward compatibility).
+static const int kInlinedVectorSize = 47;
+
+static inline bool IsRootBalanced(CordRep* node) {
+  if (node->tag != CONCAT) {
+    return true;
+  } else if (node->concat()->depth() <= 15) {
+    return true;
+  } else if (node->concat()->depth() > kMinLengthSize) {
+    return false;
+  } else {
+    // Allow depth to become twice as large as implied by fibonacci rule to
+    // reduce rebalancing for larger strings.
+    return (node->length >= min_length[node->concat()->depth() / 2]);
+  }
+}
+
+static CordRep* Rebalance(CordRep* node);
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os);
+static bool VerifyNode(CordRep* root, CordRep* start_node,
+                       bool full_validation);
+
+static inline CordRep* VerifyTree(CordRep* node) {
+  // Verification is expensive, so only do it in debug mode.
+  // Even in debug mode we normally do only light validation.
+  // If you are debugging Cord itself, you should define the
+  // macro EXTRA_CORD_VALIDATION, e.g. by adding
+  // --copt=-DEXTRA_CORD_VALIDATION to the blaze line.
+#ifdef EXTRA_CORD_VALIDATION
+  assert(node == nullptr || VerifyNode(node, node, /*full_validation=*/true));
+#else   // EXTRA_CORD_VALIDATION
+  assert(node == nullptr || VerifyNode(node, node, /*full_validation=*/false));
+#endif  // EXTRA_CORD_VALIDATION
+  static_cast<void>(&VerifyNode);
+
+  return node;
+}
+
+// --------------------------------------------------------------------
+// Memory management
+
+inline CordRep* Ref(CordRep* rep) {
+  if (rep != nullptr) {
+    rep->refcount.Increment();
+  }
+  return rep;
+}
+
+// This internal routine is called from the cold path of Unref below. Keeping it
+// in a separate routine allows good inlining of Unref into many profitable call
+// sites. However, the call to this function can be highly disruptive to the
+// register pressure in those callers. To minimize the cost to callers, we use
+// a special LLVM calling convention that preserves most registers. This allows
+// the call to this routine in cold paths to not disrupt the caller's register
+// pressure. This calling convention is not available on all platforms; we
+// intentionally allow LLVM to ignore the attribute rather than attempting to
+// hardcode the list of supported platforms.
+#if defined(__clang__) && !defined(__i386__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wattributes"
+__attribute__((preserve_most))
+#pragma clang diagnostic pop
+#endif
+static void UnrefInternal(CordRep* rep) {
+  assert(rep != nullptr);
+
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> pending;
+  while (true) {
+    if (rep->tag == CONCAT) {
+      CordRepConcat* rep_concat = rep->concat();
+      CordRep* right = rep_concat->right;
+      if (!right->refcount.Decrement()) {
+        pending.push_back(right);
+      }
+      CordRep* left = rep_concat->left;
+      delete rep_concat;
+      rep = nullptr;
+      if (!left->refcount.Decrement()) {
+        rep = left;
+        continue;
+      }
+    } else if (rep->tag == EXTERNAL) {
+      CordRepExternal* rep_external = rep->external();
+      absl::string_view data(rep_external->base, rep->length);
+      void* releaser = GetExternalReleaser(rep_external);
+      size_t releaser_size = rep_external->releaser_invoker(releaser, data);
+      rep_external->~CordRepExternal();
+      DeallocateExternal(rep_external, releaser_size);
+      rep = nullptr;
+    } else if (rep->tag == SUBSTRING) {
+      CordRepSubstring* rep_substring = rep->substring();
+      CordRep* child = rep_substring->child;
+      delete rep_substring;
+      rep = nullptr;
+      if (!child->refcount.Decrement()) {
+        rep = child;
+        continue;
+      }
+    } else {
+      // Flat CordReps are allocated and constructed with raw ::operator new
+      // and placement new, and must be destructed and deallocated
+      // accordingly.
+#if defined(__cpp_sized_deallocation)
+      size_t size = TagToAllocatedSize(rep->tag);
+      rep->~CordRep();
+      ::operator delete(rep, size);
+#else
+      rep->~CordRep();
+      ::operator delete(rep);
+#endif
+      rep = nullptr;
+    }
+
+    if (!pending.empty()) {
+      rep = pending.back();
+      pending.pop_back();
+    } else {
+      break;
+    }
+  }
+}
+
+inline void Unref(CordRep* rep) {
+  // Fast-path for two common, hot cases: a null rep and a shared root.
+  if (ABSL_PREDICT_TRUE(rep == nullptr ||
+                        rep->refcount.DecrementExpectHighRefcount())) {
+    return;
+  }
+
+  UnrefInternal(rep);
+}
+
+// Return the depth of a node
+static int Depth(const CordRep* rep) {
+  if (rep->tag == CONCAT) {
+    return rep->concat()->depth();
+  } else {
+    return 0;
+  }
+}
+
+static void SetConcatChildren(CordRepConcat* concat, CordRep* left,
+                              CordRep* right) {
+  concat->left = left;
+  concat->right = right;
+
+  concat->length = left->length + right->length;
+  concat->set_depth(1 + std::max(Depth(left), Depth(right)));
+}
+
+// Create a concatenation of the specified nodes.
+// Does not change the refcounts of "left" and "right".
+// The returned node has a refcount of 1.
+static CordRep* RawConcat(CordRep* left, CordRep* right) {
+  // Avoid making degenerate concat nodes (one child is empty)
+  if (left == nullptr || left->length == 0) {
+    Unref(left);
+    return right;
+  }
+  if (right == nullptr || right->length == 0) {
+    Unref(right);
+    return left;
+  }
+
+  CordRepConcat* rep = new CordRepConcat();
+  rep->tag = CONCAT;
+  SetConcatChildren(rep, left, right);
+
+  return rep;
+}
+
+static CordRep* Concat(CordRep* left, CordRep* right) {
+  CordRep* rep = RawConcat(left, right);
+  if (rep != nullptr && !IsRootBalanced(rep)) {
+    rep = Rebalance(rep);
+  }
+  return VerifyTree(rep);
+}
+
+// Make a balanced tree out of an array of leaf nodes.
+static CordRep* MakeBalancedTree(CordRep** reps, size_t n) {
+  // Make repeated passes over the array, merging adjacent pairs
+  // until we are left with just a single node.
+  while (n > 1) {
+    size_t dst = 0;
+    for (size_t src = 0; src < n; src += 2) {
+      if (src + 1 < n) {
+        reps[dst] = Concat(reps[src], reps[src + 1]);
+      } else {
+        reps[dst] = reps[src];
+      }
+      dst++;
+    }
+    n = dst;
+  }
+
+  return reps[0];
+}
+
+// Create a new flat node.
+static CordRep* NewFlat(size_t length_hint) {
+  if (length_hint <= kMinFlatLength) {
+    length_hint = kMinFlatLength;
+  } else if (length_hint > kMaxFlatLength) {
+    length_hint = kMaxFlatLength;
+  }
+
+  // Round size up so it matches a size we can exactly express in a tag.
+  const size_t size = RoundUpForTag(length_hint + kFlatOverhead);
+  void* const raw_rep = ::operator new(size);
+  CordRep* rep = new (raw_rep) CordRep();
+  rep->tag = AllocatedSizeToTag(size);
+  return VerifyTree(rep);
+}
+
+// Create a new tree out of the specified array.
+// The returned node has a refcount of 1.
+static CordRep* NewTree(const char* data,
+                        size_t length,
+                        size_t alloc_hint) {
+  if (length == 0) return nullptr;
+  absl::FixedArray<CordRep*> reps((length - 1) / kMaxFlatLength + 1);
+  size_t n = 0;
+  do {
+    const size_t len = std::min(length, kMaxFlatLength);
+    CordRep* rep = NewFlat(len + alloc_hint);
+    rep->length = len;
+    memcpy(rep->data, data, len);
+    reps[n++] = VerifyTree(rep);
+    data += len;
+    length -= len;
+  } while (length != 0);
+  return MakeBalancedTree(reps.data(), n);
+}
+
+namespace cord_internal {
+
+ExternalRepReleaserPair NewExternalWithUninitializedReleaser(
+    absl::string_view data, ExternalReleaserInvoker invoker,
+    size_t releaser_size) {
+  assert(!data.empty());
+
+  void* raw_rep = AllocateExternal(releaser_size);
+  auto* rep = new (raw_rep) CordRepExternal();
+  rep->length = data.size();
+  rep->tag = EXTERNAL;
+  rep->base = data.data();
+  rep->releaser_invoker = invoker;
+  return {VerifyTree(rep), GetExternalReleaser(rep)};
+}
+
+}  // namespace cord_internal
+
+static CordRep* NewSubstring(CordRep* child, size_t offset, size_t length) {
+  // Never create empty substring nodes
+  if (length == 0) {
+    Unref(child);
+    return nullptr;
+  } else {
+    CordRepSubstring* rep = new CordRepSubstring();
+    assert((offset + length) <= child->length);
+    rep->length = length;
+    rep->tag = SUBSTRING;
+    rep->start = offset;
+    rep->child = child;
+    return VerifyTree(rep);
+  }
+}
+
+// --------------------------------------------------------------------
+// Cord::InlineRep functions
+
+// This will trigger LNK2005 in MSVC.
+#ifndef COMPILER_MSVC
+const unsigned char Cord::InlineRep::kMaxInline;
+#endif  // COMPILER_MSVC
+
+inline void Cord::InlineRep::set_data(const char* data, size_t n,
+                                      bool nullify_tail) {
+  static_assert(kMaxInline == 15, "set_data is hard-coded for a length of 15");
+
+  cord_internal::SmallMemmove(data_, data, n, nullify_tail);
+  data_[kMaxInline] = static_cast<char>(n);
+}
+
+inline char* Cord::InlineRep::set_data(size_t n) {
+  assert(n <= kMaxInline);
+  memset(data_, 0, sizeof(data_));
+  data_[kMaxInline] = static_cast<char>(n);
+  return data_;
+}
+
+inline CordRep* Cord::InlineRep::force_tree(size_t extra_hint) {
+  size_t len = data_[kMaxInline];
+  CordRep* result;
+  if (len > kMaxInline) {
+    memcpy(&result, data_, sizeof(result));
+  } else {
+    result = NewFlat(len + extra_hint);
+    result->length = len;
+    memcpy(result->data, data_, len);
+    set_tree(result);
+  }
+  return result;
+}
+
+inline void Cord::InlineRep::reduce_size(size_t n) {
+  size_t tag = data_[kMaxInline];
+  assert(tag <= kMaxInline);
+  assert(tag >= n);
+  tag -= n;
+  memset(data_ + tag, 0, n);
+  data_[kMaxInline] = static_cast<char>(tag);
+}
+
+inline void Cord::InlineRep::remove_prefix(size_t n) {
+  cord_internal::SmallMemmove(data_, data_ + n, data_[kMaxInline] - n);
+  reduce_size(n);
+}
+
+void Cord::InlineRep::AppendTree(CordRep* tree) {
+  if (tree == nullptr) return;
+  size_t len = data_[kMaxInline];
+  if (len == 0) {
+    set_tree(tree);
+  } else {
+    set_tree(Concat(force_tree(0), tree));
+  }
+}
+
+void Cord::InlineRep::PrependTree(CordRep* tree) {
+  if (tree == nullptr) return;
+  size_t len = data_[kMaxInline];
+  if (len == 0) {
+    set_tree(tree);
+  } else {
+    set_tree(Concat(tree, force_tree(0)));
+  }
+}
+
+// Searches for a non-full flat node at the rightmost leaf of the tree. If a
+// suitable leaf is found, the function will update the length field for all
+// nodes to account for the size increase. The append region address will be
+// written to region and the actual size increase will be written to size.
+static inline bool PrepareAppendRegion(CordRep* root, char** region,
+                                       size_t* size, size_t max_length) {
+  // Search down the right-hand path for a non-full FLAT node.
+  CordRep* dst = root;
+  while (dst->tag == CONCAT && dst->refcount.IsOne()) {
+    dst = dst->concat()->right;
+  }
+
+  if (dst->tag < FLAT || !dst->refcount.IsOne()) {
+    *region = nullptr;
+    *size = 0;
+    return false;
+  }
+
+  const size_t in_use = dst->length;
+  const size_t capacity = TagToLength(dst->tag);
+  if (in_use == capacity) {
+    *region = nullptr;
+    *size = 0;
+    return false;
+  }
+
+  size_t size_increase = std::min(capacity - in_use, max_length);
+
+  // We need to update the length fields for all nodes, including the leaf node.
+  for (CordRep* rep = root; rep != dst; rep = rep->concat()->right) {
+    rep->length += size_increase;
+  }
+  dst->length += size_increase;
+
+  *region = dst->data + in_use;
+  *size = size_increase;
+  return true;
+}
+
+void Cord::InlineRep::GetAppendRegion(char** region, size_t* size,
+                                      size_t max_length) {
+  if (max_length == 0) {
+    *region = nullptr;
+    *size = 0;
+    return;
+  }
+
+  // Try to fit in the inline buffer if possible.
+  size_t inline_length = data_[kMaxInline];
+  if (inline_length < kMaxInline && max_length <= kMaxInline - inline_length) {
+    *region = data_ + inline_length;
+    *size = max_length;
+    data_[kMaxInline] = static_cast<char>(inline_length + max_length);
+    return;
+  }
+
+  CordRep* root = force_tree(max_length);
+
+  if (PrepareAppendRegion(root, region, size, max_length)) {
+    return;
+  }
+
+  // Allocate new node.
+  CordRep* new_node =
+      NewFlat(std::max(static_cast<size_t>(root->length), max_length));
+  new_node->length =
+      std::min(static_cast<size_t>(TagToLength(new_node->tag)), max_length);
+  *region = new_node->data;
+  *size = new_node->length;
+  replace_tree(Concat(root, new_node));
+}
+
+void Cord::InlineRep::GetAppendRegion(char** region, size_t* size) {
+  const size_t max_length = std::numeric_limits<size_t>::max();
+
+  // Try to fit in the inline buffer if possible.
+  size_t inline_length = data_[kMaxInline];
+  if (inline_length < kMaxInline) {
+    *region = data_ + inline_length;
+    *size = kMaxInline - inline_length;
+    data_[kMaxInline] = kMaxInline;
+    return;
+  }
+
+  CordRep* root = force_tree(max_length);
+
+  if (PrepareAppendRegion(root, region, size, max_length)) {
+    return;
+  }
+
+  // Allocate new node.
+  CordRep* new_node = NewFlat(root->length);
+  new_node->length = TagToLength(new_node->tag);
+  *region = new_node->data;
+  *size = new_node->length;
+  replace_tree(Concat(root, new_node));
+}
+
+// If the rep is a leaf, this will increment the value at total_mem_usage and
+// will return true.
+static bool RepMemoryUsageLeaf(const CordRep* rep, size_t* total_mem_usage) {
+  if (rep->tag >= FLAT) {
+    *total_mem_usage += TagToAllocatedSize(rep->tag);
+    return true;
+  }
+  if (rep->tag == EXTERNAL) {
+    *total_mem_usage += sizeof(CordRepConcat) + rep->length;
+    return true;
+  }
+  return false;
+}
+
+void Cord::InlineRep::AssignSlow(const Cord::InlineRep& src) {
+  ClearSlow();
+
+  memcpy(data_, src.data_, sizeof(data_));
+  if (is_tree()) {
+    Ref(tree());
+  }
+}
+
+void Cord::InlineRep::ClearSlow() {
+  if (is_tree()) {
+    Unref(tree());
+  }
+  memset(data_, 0, sizeof(data_));
+}
+
+// --------------------------------------------------------------------
+// Constructors and destructors
+
+Cord::Cord(const Cord& src) : contents_(src.contents_) {
+  Ref(contents_.tree());  // Does nothing if contents_ has embedded data
+}
+
+Cord::Cord(absl::string_view src) {
+  const size_t n = src.size();
+  if (n <= InlineRep::kMaxInline) {
+    contents_.set_data(src.data(), n, false);
+  } else {
+    contents_.set_tree(NewTree(src.data(), n, 0));
+  }
+}
+
+// The destruction code is separate so that the compiler can determine
+// that it does not need to call the destructor on a moved-from Cord.
+void Cord::DestroyCordSlow() {
+  Unref(VerifyTree(contents_.tree()));
+}
+
+// --------------------------------------------------------------------
+// Mutators
+
+void Cord::Clear() {
+  Unref(contents_.clear());
+}
+
+Cord& Cord::operator=(absl::string_view src) {
+
+  const char* data = src.data();
+  size_t length = src.size();
+  CordRep* tree = contents_.tree();
+  if (length <= InlineRep::kMaxInline) {
+    // Embed into this->contents_
+    contents_.set_data(data, length, true);
+    Unref(tree);
+    return *this;
+  }
+  if (tree != nullptr && tree->tag >= FLAT &&
+      TagToLength(tree->tag) >= length && tree->refcount.IsOne()) {
+    // Copy in place if the existing FLAT node is reusable.
+    memmove(tree->data, data, length);
+    tree->length = length;
+    VerifyTree(tree);
+    return *this;
+  }
+  contents_.set_tree(NewTree(data, length, 0));
+  Unref(tree);
+  return *this;
+}
+
+// TODO(sanjay): Move to Cord::InlineRep section of file.  For now,
+// we keep it here to make diffs easier.
+void Cord::InlineRep::AppendArray(const char* src_data, size_t src_size) {
+  if (src_size == 0) return;  // memcpy(_, nullptr, 0) is undefined.
+  // Try to fit in the inline buffer if possible.
+  size_t inline_length = data_[kMaxInline];
+  if (inline_length < kMaxInline && src_size <= kMaxInline - inline_length) {
+    // Append new data to embedded array
+    data_[kMaxInline] = static_cast<char>(inline_length + src_size);
+    memcpy(data_ + inline_length, src_data, src_size);
+    return;
+  }
+
+  CordRep* root = tree();
+
+  size_t appended = 0;
+  if (root) {
+    char* region;
+    if (PrepareAppendRegion(root, &region, &appended, src_size)) {
+      memcpy(region, src_data, appended);
+    }
+  } else {
+    // It is possible that src_data == data_, but when we transition from an
+    // InlineRep to a tree we need to assign data_ = root via set_tree. To
+    // avoid corrupting the source data before we copy it, delay calling
+    // set_tree until after we've copied data.
+    // We are going from an inline size to beyond inline size. Make the new size
+    // either double the inlined size, or the added size + 10%.
+    const size_t size1 = inline_length * 2 + src_size;
+    const size_t size2 = inline_length + src_size / 10;
+    root = NewFlat(std::max<size_t>(size1, size2));
+    appended = std::min(src_size, TagToLength(root->tag) - inline_length);
+    memcpy(root->data, data_, inline_length);
+    memcpy(root->data + inline_length, src_data, appended);
+    root->length = inline_length + appended;
+    set_tree(root);
+  }
+
+  src_data += appended;
+  src_size -= appended;
+  if (src_size == 0) {
+    return;
+  }
+
+  // Use new block(s) for any remaining bytes that were not handled above.
+  // Alloc extra memory only if the right child of the root of the new tree is
+  // going to be a FLAT node, which will permit further inplace appends.
+  size_t length = src_size;
+  if (src_size < kMaxFlatLength) {
+    // The new length is either
+    // - old size + 10%
+    // - old_size + src_size
+    // This will cause a reasonable conservative step-up in size that is still
+    // large enough to avoid excessive amounts of small fragments being added.
+    length = std::max<size_t>(root->length / 10, src_size);
+  }
+  set_tree(Concat(root, NewTree(src_data, src_size, length - src_size)));
+}
+
+inline CordRep* Cord::TakeRep() const& {
+  return Ref(contents_.tree());
+}
+
+inline CordRep* Cord::TakeRep() && {
+  CordRep* rep = contents_.tree();
+  contents_.clear();
+  return rep;
+}
+
+template <typename C>
+inline void Cord::AppendImpl(C&& src) {
+  if (empty()) {
+    // In case of an empty destination avoid allocating a new node, do not copy
+    // data.
+    *this = std::forward<C>(src);
+    return;
+  }
+
+  // For short cords, it is faster to copy data if there is room in dst.
+  const size_t src_size = src.contents_.size();
+  if (src_size <= kMaxBytesToCopy) {
+    CordRep* src_tree = src.contents_.tree();
+    if (src_tree == nullptr) {
+      // src has embedded data.
+      contents_.AppendArray(src.contents_.data(), src_size);
+      return;
+    }
+    if (src_tree->tag >= FLAT) {
+      // src tree just has one flat node.
+      contents_.AppendArray(src_tree->data, src_size);
+      return;
+    }
+    if (&src == this) {
+      // ChunkIterator below assumes that src is not modified during traversal.
+      Append(Cord(src));
+      return;
+    }
+    // TODO(mec): Should we only do this if "dst" has space?
+    for (absl::string_view chunk : src.Chunks()) {
+      Append(chunk);
+    }
+    return;
+  }
+
+  contents_.AppendTree(std::forward<C>(src).TakeRep());
+}
+
+void Cord::Append(const Cord& src) { AppendImpl(src); }
+
+void Cord::Append(Cord&& src) { AppendImpl(std::move(src)); }
+
+void Cord::Prepend(const Cord& src) {
+  CordRep* src_tree = src.contents_.tree();
+  if (src_tree != nullptr) {
+    Ref(src_tree);
+    contents_.PrependTree(src_tree);
+    return;
+  }
+
+  // `src` cord is inlined.
+  absl::string_view src_contents(src.contents_.data(), src.contents_.size());
+  return Prepend(src_contents);
+}
+
+void Cord::Prepend(absl::string_view src) {
+  if (src.empty()) return;  // memcpy(_, nullptr, 0) is undefined.
+  size_t cur_size = contents_.size();
+  if (!contents_.is_tree() && cur_size + src.size() <= InlineRep::kMaxInline) {
+    // Use embedded storage.
+    char data[InlineRep::kMaxInline + 1] = {0};
+    data[InlineRep::kMaxInline] = cur_size + src.size();  // set size
+    memcpy(data, src.data(), src.size());
+    memcpy(data + src.size(), contents_.data(), cur_size);
+    memcpy(reinterpret_cast<void*>(&contents_), data,
+           InlineRep::kMaxInline + 1);
+  } else {
+    contents_.PrependTree(NewTree(src.data(), src.size(), 0));
+  }
+}
+
+static CordRep* RemovePrefixFrom(CordRep* node, size_t n) {
+  if (n >= node->length) return nullptr;
+  if (n == 0) return Ref(node);
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> rhs_stack;
+
+  while (node->tag == CONCAT) {
+    assert(n <= node->length);
+    if (n < node->concat()->left->length) {
+      // Push right to stack, descend left.
+      rhs_stack.push_back(node->concat()->right);
+      node = node->concat()->left;
+    } else {
+      // Drop left, descend right.
+      n -= node->concat()->left->length;
+      node = node->concat()->right;
+    }
+  }
+  assert(n <= node->length);
+
+  if (n == 0) {
+    Ref(node);
+  } else {
+    size_t start = n;
+    size_t len = node->length - n;
+    if (node->tag == SUBSTRING) {
+      // Consider in-place update of node, similar to in RemoveSuffixFrom().
+      start += node->substring()->start;
+      node = node->substring()->child;
+    }
+    node = NewSubstring(Ref(node), start, len);
+  }
+  while (!rhs_stack.empty()) {
+    node = Concat(node, Ref(rhs_stack.back()));
+    rhs_stack.pop_back();
+  }
+  return node;
+}
+
+// RemoveSuffixFrom() is very similar to RemovePrefixFrom(), with the
+// exception that removing a suffix has an optimization where a node may be
+// edited in place iff that node and all its ancestors have a refcount of 1.
+static CordRep* RemoveSuffixFrom(CordRep* node, size_t n) {
+  if (n >= node->length) return nullptr;
+  if (n == 0) return Ref(node);
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> lhs_stack;
+  bool inplace_ok = node->refcount.IsOne();
+
+  while (node->tag == CONCAT) {
+    assert(n <= node->length);
+    if (n < node->concat()->right->length) {
+      // Push left to stack, descend right.
+      lhs_stack.push_back(node->concat()->left);
+      node = node->concat()->right;
+    } else {
+      // Drop right, descend left.
+      n -= node->concat()->right->length;
+      node = node->concat()->left;
+    }
+    inplace_ok = inplace_ok && node->refcount.IsOne();
+  }
+  assert(n <= node->length);
+
+  if (n == 0) {
+    Ref(node);
+  } else if (inplace_ok && node->tag != EXTERNAL) {
+    // Consider making a new buffer if the current node capacity is much
+    // larger than the new length.
+    Ref(node);
+    node->length -= n;
+  } else {
+    size_t start = 0;
+    size_t len = node->length - n;
+    if (node->tag == SUBSTRING) {
+      start = node->substring()->start;
+      node = node->substring()->child;
+    }
+    node = NewSubstring(Ref(node), start, len);
+  }
+  while (!lhs_stack.empty()) {
+    node = Concat(Ref(lhs_stack.back()), node);
+    lhs_stack.pop_back();
+  }
+  return node;
+}
+
+void Cord::RemovePrefix(size_t n) {
+  ABSL_INTERNAL_CHECK(n <= size(),
+                      absl::StrCat("Requested prefix size ", n,
+                                   " exceeds Cord's size ", size()));
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    contents_.remove_prefix(n);
+  } else {
+    CordRep* newrep = RemovePrefixFrom(tree, n);
+    Unref(tree);
+    contents_.replace_tree(VerifyTree(newrep));
+  }
+}
+
+void Cord::RemoveSuffix(size_t n) {
+  ABSL_INTERNAL_CHECK(n <= size(),
+                      absl::StrCat("Requested suffix size ", n,
+                                   " exceeds Cord's size ", size()));
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    contents_.reduce_size(n);
+  } else {
+    CordRep* newrep = RemoveSuffixFrom(tree, n);
+    Unref(tree);
+    contents_.replace_tree(VerifyTree(newrep));
+  }
+}
+
+// Work item for NewSubRange().
+struct SubRange {
+  SubRange(CordRep* a_node, size_t a_pos, size_t a_n)
+      : node(a_node), pos(a_pos), n(a_n) {}
+  CordRep* node;  // nullptr means concat last 2 results.
+  size_t pos;
+  size_t n;
+};
+
+static CordRep* NewSubRange(CordRep* node, size_t pos, size_t n) {
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> results;
+  absl::InlinedVector<SubRange, kInlinedVectorSize> todo;
+  todo.push_back(SubRange(node, pos, n));
+  do {
+    const SubRange& sr = todo.back();
+    node = sr.node;
+    pos = sr.pos;
+    n = sr.n;
+    todo.pop_back();
+
+    if (node == nullptr) {
+      assert(results.size() >= 2);
+      CordRep* right = results.back();
+      results.pop_back();
+      CordRep* left = results.back();
+      results.pop_back();
+      results.push_back(Concat(left, right));
+    } else if (pos == 0 && n == node->length) {
+      results.push_back(Ref(node));
+    } else if (node->tag != CONCAT) {
+      if (node->tag == SUBSTRING) {
+        pos += node->substring()->start;
+        node = node->substring()->child;
+      }
+      results.push_back(NewSubstring(Ref(node), pos, n));
+    } else if (pos + n <= node->concat()->left->length) {
+      todo.push_back(SubRange(node->concat()->left, pos, n));
+    } else if (pos >= node->concat()->left->length) {
+      pos -= node->concat()->left->length;
+      todo.push_back(SubRange(node->concat()->right, pos, n));
+    } else {
+      size_t left_n = node->concat()->left->length - pos;
+      todo.push_back(SubRange(nullptr, 0, 0));  // Concat()
+      todo.push_back(SubRange(node->concat()->right, 0, n - left_n));
+      todo.push_back(SubRange(node->concat()->left, pos, left_n));
+    }
+  } while (!todo.empty());
+  assert(results.size() == 1);
+  return results[0];
+}
+
+Cord Cord::Subcord(size_t pos, size_t new_size) const {
+  Cord sub_cord;
+  size_t length = size();
+  if (pos > length) pos = length;
+  if (new_size > length - pos) new_size = length - pos;
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    // sub_cord is newly constructed, no need to re-zero-out the tail of
+    // contents_ memory.
+    sub_cord.contents_.set_data(contents_.data() + pos, new_size, false);
+  } else if (new_size == 0) {
+    // We want to return empty subcord, so nothing to do.
+  } else if (new_size <= InlineRep::kMaxInline) {
+    Cord::ChunkIterator it = chunk_begin();
+    it.AdvanceBytes(pos);
+    char* dest = sub_cord.contents_.data_;
+    size_t remaining_size = new_size;
+    while (remaining_size > it->size()) {
+      cord_internal::SmallMemmove(dest, it->data(), it->size());
+      remaining_size -= it->size();
+      dest += it->size();
+      ++it;
+    }
+    cord_internal::SmallMemmove(dest, it->data(), remaining_size);
+    sub_cord.contents_.data_[InlineRep::kMaxInline] = new_size;
+  } else {
+    sub_cord.contents_.set_tree(NewSubRange(tree, pos, new_size));
+  }
+  return sub_cord;
+}
+
+// --------------------------------------------------------------------
+// Balancing
+
+class CordForest {
+ public:
+  explicit CordForest(size_t length)
+      : root_length_(length), trees_(kMinLengthSize, nullptr) {}
+
+  void Build(CordRep* cord_root) {
+    std::vector<CordRep*> pending = {cord_root};
+
+    while (!pending.empty()) {
+      CordRep* node = pending.back();
+      pending.pop_back();
+      CheckNode(node);
+      if (ABSL_PREDICT_FALSE(node->tag != CONCAT)) {
+        AddNode(node);
+        continue;
+      }
+
+      CordRepConcat* concat_node = node->concat();
+      if (concat_node->depth() >= kMinLengthSize ||
+          concat_node->length < min_length[concat_node->depth()]) {
+        pending.push_back(concat_node->right);
+        pending.push_back(concat_node->left);
+
+        if (concat_node->refcount.IsOne()) {
+          concat_node->left = concat_freelist_;
+          concat_freelist_ = concat_node;
+        } else {
+          Ref(concat_node->right);
+          Ref(concat_node->left);
+          Unref(concat_node);
+        }
+      } else {
+        AddNode(node);
+      }
+    }
+  }
+
+  CordRep* ConcatNodes() {
+    CordRep* sum = nullptr;
+    for (auto* node : trees_) {
+      if (node == nullptr) continue;
+
+      sum = PrependNode(node, sum);
+      root_length_ -= node->length;
+      if (root_length_ == 0) break;
+    }
+    ABSL_INTERNAL_CHECK(sum != nullptr, "Failed to locate sum node");
+    return VerifyTree(sum);
+  }
+
+ private:
+  CordRep* AppendNode(CordRep* node, CordRep* sum) {
+    return (sum == nullptr) ? node : MakeConcat(sum, node);
+  }
+
+  CordRep* PrependNode(CordRep* node, CordRep* sum) {
+    return (sum == nullptr) ? node : MakeConcat(node, sum);
+  }
+
+  void AddNode(CordRep* node) {
+    CordRep* sum = nullptr;
+
+    // Collect together everything with which we will merge node
+    int i = 0;
+    for (; node->length > min_length[i + 1]; ++i) {
+      auto& tree_at_i = trees_[i];
+
+      if (tree_at_i == nullptr) continue;
+      sum = PrependNode(tree_at_i, sum);
+      tree_at_i = nullptr;
+    }
+
+    sum = AppendNode(node, sum);
+
+    // Insert sum into appropriate place in the forest
+    for (; sum->length >= min_length[i]; ++i) {
+      auto& tree_at_i = trees_[i];
+      if (tree_at_i == nullptr) continue;
+
+      sum = MakeConcat(tree_at_i, sum);
+      tree_at_i = nullptr;
+    }
+
+    // min_length[0] == 1, which means sum->length >= min_length[0]
+    assert(i > 0);
+    trees_[i - 1] = sum;
+  }
+
+  // Make concat node trying to resue existing CordRepConcat nodes we
+  // already collected in the concat_freelist_.
+  CordRep* MakeConcat(CordRep* left, CordRep* right) {
+    if (concat_freelist_ == nullptr) return RawConcat(left, right);
+
+    CordRepConcat* rep = concat_freelist_;
+    if (concat_freelist_->left == nullptr) {
+      concat_freelist_ = nullptr;
+    } else {
+      concat_freelist_ = concat_freelist_->left->concat();
+    }
+    SetConcatChildren(rep, left, right);
+
+    return rep;
+  }
+
+  static void CheckNode(CordRep* node) {
+    ABSL_INTERNAL_CHECK(node->length != 0u, "");
+    if (node->tag == CONCAT) {
+      ABSL_INTERNAL_CHECK(node->concat()->left != nullptr, "");
+      ABSL_INTERNAL_CHECK(node->concat()->right != nullptr, "");
+      ABSL_INTERNAL_CHECK(node->length == (node->concat()->left->length +
+                                           node->concat()->right->length),
+                          "");
+    }
+  }
+
+  size_t root_length_;
+
+  // use an inlined vector instead of a flat array to get bounds checking
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> trees_;
+
+  // List of concat nodes we can re-use for Cord balancing.
+  CordRepConcat* concat_freelist_ = nullptr;
+};
+
+static CordRep* Rebalance(CordRep* node) {
+  VerifyTree(node);
+  assert(node->tag == CONCAT);
+
+  if (node->length == 0) {
+    return nullptr;
+  }
+
+  CordForest forest(node->length);
+  forest.Build(node);
+  return forest.ConcatNodes();
+}
+
+// --------------------------------------------------------------------
+// Comparators
+
+namespace {
+
+int ClampResult(int memcmp_res) {
+  return static_cast<int>(memcmp_res > 0) - static_cast<int>(memcmp_res < 0);
+}
+
+int CompareChunks(absl::string_view* lhs, absl::string_view* rhs,
+                  size_t* size_to_compare) {
+  size_t compared_size = std::min(lhs->size(), rhs->size());
+  assert(*size_to_compare >= compared_size);
+  *size_to_compare -= compared_size;
+
+  int memcmp_res = ::memcmp(lhs->data(), rhs->data(), compared_size);
+  if (memcmp_res != 0) return memcmp_res;
+
+  lhs->remove_prefix(compared_size);
+  rhs->remove_prefix(compared_size);
+
+  return 0;
+}
+
+// This overload set computes comparison results from memcmp result. This
+// interface is used inside GenericCompare below. Differet implementations
+// are specialized for int and bool. For int we clamp result to {-1, 0, 1}
+// set. For bool we just interested in "value == 0".
+template <typename ResultType>
+ResultType ComputeCompareResult(int memcmp_res) {
+  return ClampResult(memcmp_res);
+}
+template <>
+bool ComputeCompareResult<bool>(int memcmp_res) {
+  return memcmp_res == 0;
+}
+
+}  // namespace
+
+// Helper routine. Locates the first flat chunk of the Cord without
+// initializing the iterator.
+inline absl::string_view Cord::InlineRep::FindFlatStartPiece() const {
+  size_t n = data_[kMaxInline];
+  if (n <= kMaxInline) {
+    return absl::string_view(data_, n);
+  }
+
+  CordRep* node = tree();
+  if (node->tag >= FLAT) {
+    return absl::string_view(node->data, node->length);
+  }
+
+  if (node->tag == EXTERNAL) {
+    return absl::string_view(node->external()->base, node->length);
+  }
+
+  // Walk down the left branches until we hit a non-CONCAT node.
+  while (node->tag == CONCAT) {
+    node = node->concat()->left;
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  assert(length != 0);
+
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  if (node->tag >= FLAT) {
+    return absl::string_view(node->data + offset, length);
+  }
+
+  assert((node->tag == EXTERNAL) && "Expect FLAT or EXTERNAL node here");
+
+  return absl::string_view(node->external()->base + offset, length);
+}
+
+inline int Cord::CompareSlowPath(absl::string_view rhs, size_t compared_size,
+                                 size_t size_to_compare) const {
+  auto advance = [](Cord::ChunkIterator* it, absl::string_view* chunk) {
+    if (!chunk->empty()) return true;
+    ++*it;
+    if (it->bytes_remaining_ == 0) return false;
+    *chunk = **it;
+    return true;
+  };
+
+  Cord::ChunkIterator lhs_it = chunk_begin();
+
+  // compared_size is inside first chunk.
+  absl::string_view lhs_chunk =
+      (lhs_it.bytes_remaining_ != 0) ? *lhs_it : absl::string_view();
+  assert(compared_size <= lhs_chunk.size());
+  assert(compared_size <= rhs.size());
+  lhs_chunk.remove_prefix(compared_size);
+  rhs.remove_prefix(compared_size);
+  size_to_compare -= compared_size;  // skip already compared size.
+
+  while (advance(&lhs_it, &lhs_chunk) && !rhs.empty()) {
+    int comparison_result = CompareChunks(&lhs_chunk, &rhs, &size_to_compare);
+    if (comparison_result != 0) return comparison_result;
+    if (size_to_compare == 0) return 0;
+  }
+
+  return static_cast<int>(rhs.empty()) - static_cast<int>(lhs_chunk.empty());
+}
+
+inline int Cord::CompareSlowPath(const Cord& rhs, size_t compared_size,
+                                 size_t size_to_compare) const {
+  auto advance = [](Cord::ChunkIterator* it, absl::string_view* chunk) {
+    if (!chunk->empty()) return true;
+    ++*it;
+    if (it->bytes_remaining_ == 0) return false;
+    *chunk = **it;
+    return true;
+  };
+
+  Cord::ChunkIterator lhs_it = chunk_begin();
+  Cord::ChunkIterator rhs_it = rhs.chunk_begin();
+
+  // compared_size is inside both first chunks.
+  absl::string_view lhs_chunk =
+      (lhs_it.bytes_remaining_ != 0) ? *lhs_it : absl::string_view();
+  absl::string_view rhs_chunk =
+      (rhs_it.bytes_remaining_ != 0) ? *rhs_it : absl::string_view();
+  assert(compared_size <= lhs_chunk.size());
+  assert(compared_size <= rhs_chunk.size());
+  lhs_chunk.remove_prefix(compared_size);
+  rhs_chunk.remove_prefix(compared_size);
+  size_to_compare -= compared_size;  // skip already compared size.
+
+  while (advance(&lhs_it, &lhs_chunk) && advance(&rhs_it, &rhs_chunk)) {
+    int memcmp_res = CompareChunks(&lhs_chunk, &rhs_chunk, &size_to_compare);
+    if (memcmp_res != 0) return memcmp_res;
+    if (size_to_compare == 0) return 0;
+  }
+
+  return static_cast<int>(rhs_chunk.empty()) -
+         static_cast<int>(lhs_chunk.empty());
+}
+
+inline absl::string_view Cord::GetFirstChunk(const Cord& c) {
+  return c.contents_.FindFlatStartPiece();
+}
+inline absl::string_view Cord::GetFirstChunk(absl::string_view sv) {
+  return sv;
+}
+
+// Compares up to 'size_to_compare' bytes of 'lhs' with 'rhs'. It is assumed
+// that 'size_to_compare' is greater that size of smallest of first chunks.
+template <typename ResultType, typename RHS>
+ResultType GenericCompare(const Cord& lhs, const RHS& rhs,
+                          size_t size_to_compare) {
+  absl::string_view lhs_chunk = Cord::GetFirstChunk(lhs);
+  absl::string_view rhs_chunk = Cord::GetFirstChunk(rhs);
+
+  size_t compared_size = std::min(lhs_chunk.size(), rhs_chunk.size());
+  assert(size_to_compare >= compared_size);
+  int memcmp_res = ::memcmp(lhs_chunk.data(), rhs_chunk.data(), compared_size);
+  if (compared_size == size_to_compare || memcmp_res != 0) {
+    return ComputeCompareResult<ResultType>(memcmp_res);
+  }
+
+  return ComputeCompareResult<ResultType>(
+      lhs.CompareSlowPath(rhs, compared_size, size_to_compare));
+}
+
+bool Cord::EqualsImpl(absl::string_view rhs, size_t size_to_compare) const {
+  return GenericCompare<bool>(*this, rhs, size_to_compare);
+}
+
+bool Cord::EqualsImpl(const Cord& rhs, size_t size_to_compare) const {
+  return GenericCompare<bool>(*this, rhs, size_to_compare);
+}
+
+template <typename RHS>
+inline int SharedCompareImpl(const Cord& lhs, const RHS& rhs) {
+  size_t lhs_size = lhs.size();
+  size_t rhs_size = rhs.size();
+  if (lhs_size == rhs_size) {
+    return GenericCompare<int>(lhs, rhs, lhs_size);
+  }
+  if (lhs_size < rhs_size) {
+    auto data_comp_res = GenericCompare<int>(lhs, rhs, lhs_size);
+    return data_comp_res == 0 ? -1 : data_comp_res;
+  }
+
+  auto data_comp_res = GenericCompare<int>(lhs, rhs, rhs_size);
+  return data_comp_res == 0 ? +1 : data_comp_res;
+}
+
+int Cord::Compare(absl::string_view rhs) const {
+  return SharedCompareImpl(*this, rhs);
+}
+
+int Cord::CompareImpl(const Cord& rhs) const {
+  return SharedCompareImpl(*this, rhs);
+}
+
+bool Cord::EndsWith(absl::string_view rhs) const {
+  size_t my_size = size();
+  size_t rhs_size = rhs.size();
+
+  if (my_size < rhs_size) return false;
+
+  Cord tmp(*this);
+  tmp.RemovePrefix(my_size - rhs_size);
+  return tmp.EqualsImpl(rhs, rhs_size);
+}
+
+bool Cord::EndsWith(const Cord& rhs) const {
+  size_t my_size = size();
+  size_t rhs_size = rhs.size();
+
+  if (my_size < rhs_size) return false;
+
+  Cord tmp(*this);
+  tmp.RemovePrefix(my_size - rhs_size);
+  return tmp.EqualsImpl(rhs, rhs_size);
+}
+
+// --------------------------------------------------------------------
+// Misc.
+
+Cord::operator std::string() const {
+  std::string s;
+  absl::CopyCordToString(*this, &s);
+  return s;
+}
+
+void CopyCordToString(const Cord& src, std::string* dst) {
+  if (!src.contents_.is_tree()) {
+    src.contents_.CopyTo(dst);
+  } else {
+    absl::strings_internal::STLStringResizeUninitialized(dst, src.size());
+    src.CopyToArraySlowPath(&(*dst)[0]);
+  }
+}
+
+void Cord::CopyToArraySlowPath(char* dst) const {
+  assert(contents_.is_tree());
+  absl::string_view fragment;
+  if (GetFlatAux(contents_.tree(), &fragment)) {
+    memcpy(dst, fragment.data(), fragment.size());
+    return;
+  }
+  for (absl::string_view chunk : Chunks()) {
+    memcpy(dst, chunk.data(), chunk.size());
+    dst += chunk.size();
+  }
+}
+
+Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
+  assert(bytes_remaining_ > 0 && "Attempted to iterate past `end()`");
+  assert(bytes_remaining_ >= current_chunk_.size());
+  bytes_remaining_ -= current_chunk_.size();
+
+  if (stack_of_right_children_.empty()) {
+    assert(!current_chunk_.empty());  // Called on invalid iterator.
+    // We have reached the end of the Cord.
+    return *this;
+  }
+
+  // Process the next node on the stack.
+  CordRep* node = stack_of_right_children_.back();
+  stack_of_right_children_.pop_back();
+
+  // Walk down the left branches until we hit a non-CONCAT node. Save the
+  // right children to the stack for subsequent traversal.
+  while (node->tag == CONCAT) {
+    stack_of_right_children_.push_back(node->concat()->right);
+    node = node->concat()->left;
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(length != 0);
+  const char* data =
+      node->tag == EXTERNAL ? node->external()->base : node->data;
+  current_chunk_ = absl::string_view(data + offset, length);
+  current_leaf_ = node;
+  return *this;
+}
+
+Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
+  assert(bytes_remaining_ >= n && "Attempted to iterate past `end()`");
+  Cord subcord;
+
+  if (n <= InlineRep::kMaxInline) {
+    // Range to read fits in inline data. Flatten it.
+    char* data = subcord.contents_.set_data(n);
+    while (n > current_chunk_.size()) {
+      memcpy(data, current_chunk_.data(), current_chunk_.size());
+      data += current_chunk_.size();
+      n -= current_chunk_.size();
+      ++*this;
+    }
+    memcpy(data, current_chunk_.data(), n);
+    if (n < current_chunk_.size()) {
+      RemoveChunkPrefix(n);
+    } else if (n > 0) {
+      ++*this;
+    }
+    return subcord;
+  }
+  if (n < current_chunk_.size()) {
+    // Range to read is a proper subrange of the current chunk.
+    assert(current_leaf_ != nullptr);
+    CordRep* subnode = Ref(current_leaf_);
+    const char* data =
+        subnode->tag == EXTERNAL ? subnode->external()->base : subnode->data;
+    subnode = NewSubstring(subnode, current_chunk_.data() - data, n);
+    subcord.contents_.set_tree(VerifyTree(subnode));
+    RemoveChunkPrefix(n);
+    return subcord;
+  }
+
+  // Range to read begins with a proper subrange of the current chunk.
+  assert(!current_chunk_.empty());
+  assert(current_leaf_ != nullptr);
+  CordRep* subnode = Ref(current_leaf_);
+  if (current_chunk_.size() < subnode->length) {
+    const char* data =
+        subnode->tag == EXTERNAL ? subnode->external()->base : subnode->data;
+    subnode = NewSubstring(subnode, current_chunk_.data() - data,
+                           current_chunk_.size());
+  }
+  n -= current_chunk_.size();
+  bytes_remaining_ -= current_chunk_.size();
+
+  // Process the next node(s) on the stack, reading whole subtrees depending on
+  // their length and how many bytes we are advancing.
+  CordRep* node = nullptr;
+  while (!stack_of_right_children_.empty()) {
+    node = stack_of_right_children_.back();
+    stack_of_right_children_.pop_back();
+    if (node->length > n) break;
+    // TODO(qrczak): This might unnecessarily recreate existing concat nodes.
+    // Avoiding that would need pretty complicated logic (instead of
+    // current_leaf_, keep current_subtree_ which points to the highest node
+    // such that the current leaf can be found on the path of left children
+    // starting from current_subtree_; delay creating subnode while node is
+    // below current_subtree_; find the proper node along the path of left
+    // children starting from current_subtree_ if this loop exits while staying
+    // below current_subtree_; etc.; alternatively, push parents instead of
+    // right children on the stack).
+    subnode = Concat(subnode, Ref(node));
+    n -= node->length;
+    bytes_remaining_ -= node->length;
+    node = nullptr;
+  }
+
+  if (node == nullptr) {
+    // We have reached the end of the Cord.
+    assert(bytes_remaining_ == 0);
+    subcord.contents_.set_tree(VerifyTree(subnode));
+    return subcord;
+  }
+
+  // Walk down the appropriate branches until we hit a non-CONCAT node. Save the
+  // right children to the stack for subsequent traversal.
+  while (node->tag == CONCAT) {
+    if (node->concat()->left->length > n) {
+      // Push right, descend left.
+      stack_of_right_children_.push_back(node->concat()->right);
+      node = node->concat()->left;
+    } else {
+      // Read left, descend right.
+      subnode = Concat(subnode, Ref(node->concat()->left));
+      n -= node->concat()->left->length;
+      bytes_remaining_ -= node->concat()->left->length;
+      node = node->concat()->right;
+    }
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  // Range to read ends with a proper (possibly empty) subrange of the current
+  // chunk.
+  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(length > n);
+  if (n > 0) subnode = Concat(subnode, NewSubstring(Ref(node), offset, n));
+  const char* data =
+      node->tag == EXTERNAL ? node->external()->base : node->data;
+  current_chunk_ = absl::string_view(data + offset + n, length - n);
+  current_leaf_ = node;
+  bytes_remaining_ -= n;
+  subcord.contents_.set_tree(VerifyTree(subnode));
+  return subcord;
+}
+
+void Cord::ChunkIterator::AdvanceBytesSlowPath(size_t n) {
+  assert(bytes_remaining_ >= n && "Attempted to iterate past `end()`");
+  assert(n >= current_chunk_.size());  // This should only be called when
+                                       // iterating to a new node.
+
+  n -= current_chunk_.size();
+  bytes_remaining_ -= current_chunk_.size();
+
+  // Process the next node(s) on the stack, skipping whole subtrees depending on
+  // their length and how many bytes we are advancing.
+  CordRep* node = nullptr;
+  while (!stack_of_right_children_.empty()) {
+    node = stack_of_right_children_.back();
+    stack_of_right_children_.pop_back();
+    if (node->length > n) break;
+    n -= node->length;
+    bytes_remaining_ -= node->length;
+    node = nullptr;
+  }
+
+  if (node == nullptr) {
+    // We have reached the end of the Cord.
+    assert(bytes_remaining_ == 0);
+    return;
+  }
+
+  // Walk down the appropriate branches until we hit a non-CONCAT node. Save the
+  // right children to the stack for subsequent traversal.
+  while (node->tag == CONCAT) {
+    if (node->concat()->left->length > n) {
+      // Push right, descend left.
+      stack_of_right_children_.push_back(node->concat()->right);
+      node = node->concat()->left;
+    } else {
+      // Skip left, descend right.
+      n -= node->concat()->left->length;
+      bytes_remaining_ -= node->concat()->left->length;
+      node = node->concat()->right;
+    }
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(length > n);
+  const char* data =
+      node->tag == EXTERNAL ? node->external()->base : node->data;
+  current_chunk_ = absl::string_view(data + offset + n, length - n);
+  current_leaf_ = node;
+  bytes_remaining_ -= n;
+}
+
+char Cord::operator[](size_t i) const {
+  assert(i < size());
+  size_t offset = i;
+  const CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    return contents_.data()[i];
+  }
+  while (true) {
+    assert(rep != nullptr);
+    assert(offset < rep->length);
+    if (rep->tag >= FLAT) {
+      // Get the "i"th character directly from the flat array.
+      return rep->data[offset];
+    } else if (rep->tag == EXTERNAL) {
+      // Get the "i"th character from the external array.
+      return rep->external()->base[offset];
+    } else if (rep->tag == CONCAT) {
+      // Recursively branch to the side of the concatenation that the "i"th
+      // character is on.
+      size_t left_length = rep->concat()->left->length;
+      if (offset < left_length) {
+        rep = rep->concat()->left;
+      } else {
+        offset -= left_length;
+        rep = rep->concat()->right;
+      }
+    } else {
+      // This must be a substring a node, so bypass it to get to the child.
+      assert(rep->tag == SUBSTRING);
+      offset += rep->substring()->start;
+      rep = rep->substring()->child;
+    }
+  }
+}
+
+absl::string_view Cord::FlattenSlowPath() {
+  size_t total_size = size();
+  CordRep* new_rep;
+  char* new_buffer;
+
+  // Try to put the contents into a new flat rep. If they won't fit in the
+  // biggest possible flat node, use an external rep instead.
+  if (total_size <= kMaxFlatLength) {
+    new_rep = NewFlat(total_size);
+    new_rep->length = total_size;
+    new_buffer = new_rep->data;
+    CopyToArraySlowPath(new_buffer);
+  } else {
+    new_buffer = std::allocator<char>().allocate(total_size);
+    CopyToArraySlowPath(new_buffer);
+    new_rep = absl::cord_internal::NewExternalRep(
+        absl::string_view(new_buffer, total_size), [](absl::string_view s) {
+          std::allocator<char>().deallocate(const_cast<char*>(s.data()),
+                                            s.size());
+        });
+  }
+  Unref(contents_.tree());
+  contents_.set_tree(new_rep);
+  return absl::string_view(new_buffer, total_size);
+}
+
+/* static */ bool Cord::GetFlatAux(CordRep* rep, absl::string_view* fragment) {
+  assert(rep != nullptr);
+  if (rep->tag >= FLAT) {
+    *fragment = absl::string_view(rep->data, rep->length);
+    return true;
+  } else if (rep->tag == EXTERNAL) {
+    *fragment = absl::string_view(rep->external()->base, rep->length);
+    return true;
+  } else if (rep->tag == SUBSTRING) {
+    CordRep* child = rep->substring()->child;
+    if (child->tag >= FLAT) {
+      *fragment =
+          absl::string_view(child->data + rep->substring()->start, rep->length);
+      return true;
+    } else if (child->tag == EXTERNAL) {
+      *fragment = absl::string_view(
+          child->external()->base + rep->substring()->start, rep->length);
+      return true;
+    }
+  }
+  return false;
+}
+
+/* static */ void Cord::ForEachChunkAux(
+    absl::cord_internal::CordRep* rep,
+    absl::FunctionRef<void(absl::string_view)> callback) {
+  assert(rep != nullptr);
+  int stack_pos = 0;
+  constexpr int stack_max = 128;
+  // Stack of right branches for tree traversal
+  absl::cord_internal::CordRep* stack[stack_max];
+  absl::cord_internal::CordRep* current_node = rep;
+  while (true) {
+    if (current_node->tag == CONCAT) {
+      if (stack_pos == stack_max) {
+        // There's no more room on our stack array to add another right branch,
+        // and the idea is to avoid allocations, so call this function
+        // recursively to navigate this subtree further.  (This is not something
+        // we expect to happen in practice).
+        ForEachChunkAux(current_node, callback);
+
+        // Pop the next right branch and iterate.
+        current_node = stack[--stack_pos];
+        continue;
+      } else {
+        // Save the right branch for later traversal and continue down the left
+        // branch.
+        stack[stack_pos++] = current_node->concat()->right;
+        current_node = current_node->concat()->left;
+        continue;
+      }
+    }
+    // This is a leaf node, so invoke our callback.
+    absl::string_view chunk;
+    bool success = GetFlatAux(current_node, &chunk);
+    assert(success);
+    if (success) {
+      callback(chunk);
+    }
+    if (stack_pos == 0) {
+      // end of traversal
+      return;
+    }
+    current_node = stack[--stack_pos];
+  }
+}
+
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os) {
+  const int kIndentStep = 1;
+  int indent = 0;
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> stack;
+  absl::InlinedVector<int, kInlinedVectorSize> indents;
+  for (;;) {
+    *os << std::setw(3) << rep->refcount.Get();
+    *os << " " << std::setw(7) << rep->length;
+    *os << " [";
+    if (include_data) *os << static_cast<void*>(rep);
+    *os << "]";
+    *os << " " << (IsRootBalanced(rep) ? 'b' : 'u');
+    *os << " " << std::setw(indent) << "";
+    if (rep->tag == CONCAT) {
+      *os << "CONCAT depth=" << Depth(rep) << "\n";
+      indent += kIndentStep;
+      indents.push_back(indent);
+      stack.push_back(rep->concat()->right);
+      rep = rep->concat()->left;
+    } else if (rep->tag == SUBSTRING) {
+      *os << "SUBSTRING @ " << rep->substring()->start << "\n";
+      indent += kIndentStep;
+      rep = rep->substring()->child;
+    } else {  // Leaf
+      if (rep->tag == EXTERNAL) {
+        *os << "EXTERNAL [";
+        if (include_data)
+          *os << absl::CEscape(std::string(rep->external()->base, rep->length));
+        *os << "]\n";
+      } else {
+        *os << "FLAT cap=" << TagToLength(rep->tag) << " [";
+        if (include_data)
+          *os << absl::CEscape(std::string(rep->data, rep->length));
+        *os << "]\n";
+      }
+      if (stack.empty()) break;
+      rep = stack.back();
+      stack.pop_back();
+      indent = indents.back();
+      indents.pop_back();
+    }
+  }
+  ABSL_INTERNAL_CHECK(indents.empty(), "");
+}
+
+static std::string ReportError(CordRep* root, CordRep* node) {
+  std::ostringstream buf;
+  buf << "Error at node " << node << " in:";
+  DumpNode(root, true, &buf);
+  return buf.str();
+}
+
+static bool VerifyNode(CordRep* root, CordRep* start_node,
+                       bool full_validation) {
+  absl::InlinedVector<CordRep*, 2> worklist;
+  worklist.push_back(start_node);
+  do {
+    CordRep* node = worklist.back();
+    worklist.pop_back();
+
+    ABSL_INTERNAL_CHECK(node != nullptr, ReportError(root, node));
+    if (node != root) {
+      ABSL_INTERNAL_CHECK(node->length != 0, ReportError(root, node));
+    }
+
+    if (node->tag == CONCAT) {
+      ABSL_INTERNAL_CHECK(node->concat()->left != nullptr,
+                          ReportError(root, node));
+      ABSL_INTERNAL_CHECK(node->concat()->right != nullptr,
+                          ReportError(root, node));
+      ABSL_INTERNAL_CHECK((node->length == node->concat()->left->length +
+                                               node->concat()->right->length),
+                          ReportError(root, node));
+      if (full_validation) {
+        worklist.push_back(node->concat()->right);
+        worklist.push_back(node->concat()->left);
+      }
+    } else if (node->tag >= FLAT) {
+      ABSL_INTERNAL_CHECK(node->length <= TagToLength(node->tag),
+                          ReportError(root, node));
+    } else if (node->tag == EXTERNAL) {
+      ABSL_INTERNAL_CHECK(node->external()->base != nullptr,
+                          ReportError(root, node));
+    } else if (node->tag == SUBSTRING) {
+      ABSL_INTERNAL_CHECK(
+          node->substring()->start < node->substring()->child->length,
+          ReportError(root, node));
+      ABSL_INTERNAL_CHECK(node->substring()->start + node->length <=
+                              node->substring()->child->length,
+                          ReportError(root, node));
+    }
+  } while (!worklist.empty());
+  return true;
+}
+
+// Traverses the tree and computes the total memory allocated.
+/* static */ size_t Cord::MemoryUsageAux(const CordRep* rep) {
+  size_t total_mem_usage = 0;
+
+  // Allow a quick exit for the common case that the root is a leaf.
+  if (RepMemoryUsageLeaf(rep, &total_mem_usage)) {
+    return total_mem_usage;
+  }
+
+  // Iterate over the tree. cur_node is never a leaf node and leaf nodes will
+  // never be appended to tree_stack. This reduces overhead from manipulating
+  // tree_stack.
+  absl::InlinedVector<const CordRep*, kInlinedVectorSize> tree_stack;
+  const CordRep* cur_node = rep;
+  while (true) {
+    const CordRep* next_node = nullptr;
+
+    if (cur_node->tag == CONCAT) {
+      total_mem_usage += sizeof(CordRepConcat);
+      const CordRep* left = cur_node->concat()->left;
+      if (!RepMemoryUsageLeaf(left, &total_mem_usage)) {
+        next_node = left;
+      }
+
+      const CordRep* right = cur_node->concat()->right;
+      if (!RepMemoryUsageLeaf(right, &total_mem_usage)) {
+        if (next_node) {
+          tree_stack.push_back(next_node);
+        }
+        next_node = right;
+      }
+    } else {
+      // Since cur_node is not a leaf or a concat node it must be a substring.
+      assert(cur_node->tag == SUBSTRING);
+      total_mem_usage += sizeof(CordRepSubstring);
+      next_node = cur_node->substring()->child;
+      if (RepMemoryUsageLeaf(next_node, &total_mem_usage)) {
+        next_node = nullptr;
+      }
+    }
+
+    if (!next_node) {
+      if (tree_stack.empty()) {
+        return total_mem_usage;
+      }
+      next_node = tree_stack.back();
+      tree_stack.pop_back();
+    }
+    cur_node = next_node;
+  }
+}
+
+std::ostream& operator<<(std::ostream& out, const Cord& cord) {
+  for (absl::string_view chunk : cord.Chunks()) {
+    out.write(chunk.data(), chunk.size());
+  }
+  return out;
+}
+
+namespace strings_internal {
+size_t CordTestAccess::FlatOverhead() { return kFlatOverhead; }
+size_t CordTestAccess::MaxFlatLength() { return kMaxFlatLength; }
+size_t CordTestAccess::FlatTagToLength(uint8_t tag) {
+  return TagToLength(tag);
+}
+uint8_t CordTestAccess::LengthToTag(size_t s) {
+  ABSL_INTERNAL_CHECK(s <= kMaxFlatLength, absl::StrCat("Invalid length ", s));
+  return AllocatedSizeToTag(s + kFlatOverhead);
+}
+size_t CordTestAccess::SizeofCordRepConcat() { return sizeof(CordRepConcat); }
+size_t CordTestAccess::SizeofCordRepExternal() {
+  return sizeof(CordRepExternal);
+}
+size_t CordTestAccess::SizeofCordRepSubstring() {
+  return sizeof(CordRepSubstring);
+}
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl