chore(3p/abseil_cpp): unvendor abseil_cpp r/3786

we weren't actually using these sources anymore, okay?

Change-Id: If701571d9716de308d3512e1eb22c35db0877a66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5248
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>

1 files changed, 0 insertions, 2587 deletions

diff --git a/third_party/abseil_cpp/absl/container/internal/btree.h b/third_party/abseil_cpp/absl/container/internal/btree.h
deleted file mode 100644
index f2fc31df8d41..000000000000
--- a/third_party/abseil_cpp/absl/container/internal/btree.h
+++ /dev/null
@@ -1,2587 +0,0 @@
-// Copyright 2018 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.
-
-// A btree implementation of the STL set and map interfaces. A btree is smaller
-// and generally also faster than STL set/map (refer to the benchmarks below).
-// The red-black tree implementation of STL set/map has an overhead of 3
-// pointers (left, right and parent) plus the node color information for each
-// stored value. So a set<int32_t> consumes 40 bytes for each value stored in
-// 64-bit mode. This btree implementation stores multiple values on fixed
-// size nodes (usually 256 bytes) and doesn't store child pointers for leaf
-// nodes. The result is that a btree_set<int32_t> may use much less memory per
-// stored value. For the random insertion benchmark in btree_bench.cc, a
-// btree_set<int32_t> with node-size of 256 uses 5.1 bytes per stored value.
-//
-// The packing of multiple values on to each node of a btree has another effect
-// besides better space utilization: better cache locality due to fewer cache
-// lines being accessed. Better cache locality translates into faster
-// operations.
-//
-// CAVEATS
-//
-// Insertions and deletions on a btree can cause splitting, merging or
-// rebalancing of btree nodes. And even without these operations, insertions
-// and deletions on a btree will move values around within a node. In both
-// cases, the result is that insertions and deletions can invalidate iterators
-// pointing to values other than the one being inserted/deleted. Therefore, this
-// container does not provide pointer stability. This is notably different from
-// STL set/map which takes care to not invalidate iterators on insert/erase
-// except, of course, for iterators pointing to the value being erased.  A
-// partial workaround when erasing is available: erase() returns an iterator
-// pointing to the item just after the one that was erased (or end() if none
-// exists).
-
-#ifndef ABSL_CONTAINER_INTERNAL_BTREE_H_
-#define ABSL_CONTAINER_INTERNAL_BTREE_H_
-
-#include <algorithm>
-#include <cassert>
-#include <cstddef>
-#include <cstdint>
-#include <cstring>
-#include <functional>
-#include <iterator>
-#include <limits>
-#include <new>
-#include <string>
-#include <type_traits>
-#include <utility>
-
-#include "absl/base/macros.h"
-#include "absl/container/internal/common.h"
-#include "absl/container/internal/compressed_tuple.h"
-#include "absl/container/internal/container_memory.h"
-#include "absl/container/internal/layout.h"
-#include "absl/memory/memory.h"
-#include "absl/meta/type_traits.h"
-#include "absl/strings/cord.h"
-#include "absl/strings/string_view.h"
-#include "absl/types/compare.h"
-#include "absl/utility/utility.h"
-
-namespace absl {
-ABSL_NAMESPACE_BEGIN
-namespace container_internal {
-
-// A helper class that indicates if the Compare parameter is a key-compare-to
-// comparator.
-template <typename Compare, typename T>
-using btree_is_key_compare_to =
-    std::is_convertible<absl::result_of_t<Compare(const T &, const T &)>,
-                        absl::weak_ordering>;
-
-struct StringBtreeDefaultLess {
-  using is_transparent = void;
-
-  StringBtreeDefaultLess() = default;
-
-  // Compatibility constructor.
-  StringBtreeDefaultLess(std::less<std::string>) {}  // NOLINT
-  StringBtreeDefaultLess(std::less<string_view>) {}  // NOLINT
-
-  absl::weak_ordering operator()(absl::string_view lhs,
-                                 absl::string_view rhs) const {
-    return compare_internal::compare_result_as_ordering(lhs.compare(rhs));
-  }
-  StringBtreeDefaultLess(std::less<absl::Cord>) {}  // NOLINT
-  absl::weak_ordering operator()(const absl::Cord &lhs,
-                                 const absl::Cord &rhs) const {
-    return compare_internal::compare_result_as_ordering(lhs.Compare(rhs));
-  }
-  absl::weak_ordering operator()(const absl::Cord &lhs,
-                                 absl::string_view rhs) const {
-    return compare_internal::compare_result_as_ordering(lhs.Compare(rhs));
-  }
-  absl::weak_ordering operator()(absl::string_view lhs,
-                                 const absl::Cord &rhs) const {
-    return compare_internal::compare_result_as_ordering(-rhs.Compare(lhs));
-  }
-};
-
-struct StringBtreeDefaultGreater {
-  using is_transparent = void;
-
-  StringBtreeDefaultGreater() = default;
-
-  StringBtreeDefaultGreater(std::greater<std::string>) {}  // NOLINT
-  StringBtreeDefaultGreater(std::greater<string_view>) {}  // NOLINT
-
-  absl::weak_ordering operator()(absl::string_view lhs,
-                                 absl::string_view rhs) const {
-    return compare_internal::compare_result_as_ordering(rhs.compare(lhs));
-  }
-  StringBtreeDefaultGreater(std::greater<absl::Cord>) {}  // NOLINT
-  absl::weak_ordering operator()(const absl::Cord &lhs,
-                                 const absl::Cord &rhs) const {
-    return compare_internal::compare_result_as_ordering(rhs.Compare(lhs));
-  }
-  absl::weak_ordering operator()(const absl::Cord &lhs,
-                                 absl::string_view rhs) const {
-    return compare_internal::compare_result_as_ordering(-lhs.Compare(rhs));
-  }
-  absl::weak_ordering operator()(absl::string_view lhs,
-                                 const absl::Cord &rhs) const {
-    return compare_internal::compare_result_as_ordering(rhs.Compare(lhs));
-  }
-};
-
-// A helper class to convert a boolean comparison into a three-way "compare-to"
-// comparison that returns an `absl::weak_ordering`. This helper
-// class is specialized for less<std::string>, greater<std::string>,
-// less<string_view>, greater<string_view>, less<absl::Cord>, and
-// greater<absl::Cord>.
-//
-// key_compare_to_adapter is provided so that btree users
-// automatically get the more efficient compare-to code when using common
-// Abseil string types with common comparison functors.
-// These string-like specializations also turn on heterogeneous lookup by
-// default.
-template <typename Compare>
-struct key_compare_to_adapter {
-  using type = Compare;
-};
-
-template <>
-struct key_compare_to_adapter<std::less<std::string>> {
-  using type = StringBtreeDefaultLess;
-};
-
-template <>
-struct key_compare_to_adapter<std::greater<std::string>> {
-  using type = StringBtreeDefaultGreater;
-};
-
-template <>
-struct key_compare_to_adapter<std::less<absl::string_view>> {
-  using type = StringBtreeDefaultLess;
-};
-
-template <>
-struct key_compare_to_adapter<std::greater<absl::string_view>> {
-  using type = StringBtreeDefaultGreater;
-};
-
-template <>
-struct key_compare_to_adapter<std::less<absl::Cord>> {
-  using type = StringBtreeDefaultLess;
-};
-
-template <>
-struct key_compare_to_adapter<std::greater<absl::Cord>> {
-  using type = StringBtreeDefaultGreater;
-};
-
-// Detects an 'absl_btree_prefer_linear_node_search' member. This is
-// a protocol used as an opt-in or opt-out of linear search.
-//
-//  For example, this would be useful for key types that wrap an integer
-//  and define their own cheap operator<(). For example:
-//
-//   class K {
-//    public:
-//     using absl_btree_prefer_linear_node_search = std::true_type;
-//     ...
-//    private:
-//     friend bool operator<(K a, K b) { return a.k_ < b.k_; }
-//     int k_;
-//   };
-//
-//   btree_map<K, V> m;  // Uses linear search
-//
-// If T has the preference tag, then it has a preference.
-// Btree will use the tag's truth value.
-template <typename T, typename = void>
-struct has_linear_node_search_preference : std::false_type {};
-template <typename T, typename = void>
-struct prefers_linear_node_search : std::false_type {};
-template <typename T>
-struct has_linear_node_search_preference<
-    T, absl::void_t<typename T::absl_btree_prefer_linear_node_search>>
-    : std::true_type {};
-template <typename T>
-struct prefers_linear_node_search<
-    T, absl::void_t<typename T::absl_btree_prefer_linear_node_search>>
-    : T::absl_btree_prefer_linear_node_search {};
-
-template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
-          bool Multi, typename SlotPolicy>
-struct common_params {
-  // If Compare is a common comparator for a string-like type, then we adapt it
-  // to use heterogeneous lookup and to be a key-compare-to comparator.
-  using key_compare = typename key_compare_to_adapter<Compare>::type;
-  // True when key_compare has been adapted to StringBtreeDefault{Less,Greater}.
-  using is_key_compare_adapted =
-      absl::negation<std::is_same<key_compare, Compare>>;
-  // A type which indicates if we have a key-compare-to functor or a plain old
-  // key-compare functor.
-  using is_key_compare_to = btree_is_key_compare_to<key_compare, Key>;
-
-  using allocator_type = Alloc;
-  using key_type = Key;
-  using size_type = std::make_signed<size_t>::type;
-  using difference_type = ptrdiff_t;
-
-  // True if this is a multiset or multimap.
-  using is_multi_container = std::integral_constant<bool, Multi>;
-
-  using slot_policy = SlotPolicy;
-  using slot_type = typename slot_policy::slot_type;
-  using value_type = typename slot_policy::value_type;
-  using init_type = typename slot_policy::mutable_value_type;
-  using pointer = value_type *;
-  using const_pointer = const value_type *;
-  using reference = value_type &;
-  using const_reference = const value_type &;
-
-  enum {
-    kTargetNodeSize = TargetNodeSize,
-
-    // Upper bound for the available space for values. This is largest for leaf
-    // nodes, which have overhead of at least a pointer + 4 bytes (for storing
-    // 3 field_types and an enum).
-    kNodeValueSpace =
-        TargetNodeSize - /*minimum overhead=*/(sizeof(void *) + 4),
-  };
-
-  // This is an integral type large enough to hold as many
-  // ValueSize-values as will fit a node of TargetNodeSize bytes.
-  using node_count_type =
-      absl::conditional_t<(kNodeValueSpace / sizeof(value_type) >
-                           (std::numeric_limits<uint8_t>::max)()),
-                          uint16_t, uint8_t>;  // NOLINT
-
-  // The following methods are necessary for passing this struct as PolicyTraits
-  // for node_handle and/or are used within btree.
-  static value_type &element(slot_type *slot) {
-    return slot_policy::element(slot);
-  }
-  static const value_type &element(const slot_type *slot) {
-    return slot_policy::element(slot);
-  }
-  template <class... Args>
-  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
-    slot_policy::construct(alloc, slot, std::forward<Args>(args)...);
-  }
-  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
-    slot_policy::construct(alloc, slot, other);
-  }
-  static void destroy(Alloc *alloc, slot_type *slot) {
-    slot_policy::destroy(alloc, slot);
-  }
-  static void transfer(Alloc *alloc, slot_type *new_slot, slot_type *old_slot) {
-    construct(alloc, new_slot, old_slot);
-    destroy(alloc, old_slot);
-  }
-  static void swap(Alloc *alloc, slot_type *a, slot_type *b) {
-    slot_policy::swap(alloc, a, b);
-  }
-  static void move(Alloc *alloc, slot_type *src, slot_type *dest) {
-    slot_policy::move(alloc, src, dest);
-  }
-};
-
-// A parameters structure for holding the type parameters for a btree_map.
-// Compare and Alloc should be nothrow copy-constructible.
-template <typename Key, typename Data, typename Compare, typename Alloc,
-          int TargetNodeSize, bool Multi>
-struct map_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
-                                  map_slot_policy<Key, Data>> {
-  using super_type = typename map_params::common_params;
-  using mapped_type = Data;
-  // This type allows us to move keys when it is safe to do so. It is safe
-  // for maps in which value_type and mutable_value_type are layout compatible.
-  using slot_policy = typename super_type::slot_policy;
-  using slot_type = typename super_type::slot_type;
-  using value_type = typename super_type::value_type;
-  using init_type = typename super_type::init_type;
-
-  using key_compare = typename super_type::key_compare;
-  // Inherit from key_compare for empty base class optimization.
-  struct value_compare : private key_compare {
-    value_compare() = default;
-    explicit value_compare(const key_compare &cmp) : key_compare(cmp) {}
-
-    template <typename T, typename U>
-    auto operator()(const T &left, const U &right) const
-        -> decltype(std::declval<key_compare>()(left.first, right.first)) {
-      return key_compare::operator()(left.first, right.first);
-    }
-  };
-  using is_map_container = std::true_type;
-
-  template <typename V>
-  static auto key(const V &value) -> decltype(value.first) {
-    return value.first;
-  }
-  static const Key &key(const slot_type *s) { return slot_policy::key(s); }
-  static const Key &key(slot_type *s) { return slot_policy::key(s); }
-  // For use in node handle.
-  static auto mutable_key(slot_type *s)
-      -> decltype(slot_policy::mutable_key(s)) {
-    return slot_policy::mutable_key(s);
-  }
-  static mapped_type &value(value_type *value) { return value->second; }
-};
-
-// This type implements the necessary functions from the
-// absl::container_internal::slot_type interface.
-template <typename Key>
-struct set_slot_policy {
-  using slot_type = Key;
-  using value_type = Key;
-  using mutable_value_type = Key;
-
-  static value_type &element(slot_type *slot) { return *slot; }
-  static const value_type &element(const slot_type *slot) { return *slot; }
-
-  template <typename Alloc, class... Args>
-  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
-    absl::allocator_traits<Alloc>::construct(*alloc, slot,
-                                             std::forward<Args>(args)...);
-  }
-
-  template <typename Alloc>
-  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
-    absl::allocator_traits<Alloc>::construct(*alloc, slot, std::move(*other));
-  }
-
-  template <typename Alloc>
-  static void destroy(Alloc *alloc, slot_type *slot) {
-    absl::allocator_traits<Alloc>::destroy(*alloc, slot);
-  }
-
-  template <typename Alloc>
-  static void swap(Alloc * /*alloc*/, slot_type *a, slot_type *b) {
-    using std::swap;
-    swap(*a, *b);
-  }
-
-  template <typename Alloc>
-  static void move(Alloc * /*alloc*/, slot_type *src, slot_type *dest) {
-    *dest = std::move(*src);
-  }
-};
-
-// A parameters structure for holding the type parameters for a btree_set.
-// Compare and Alloc should be nothrow copy-constructible.
-template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
-          bool Multi>
-struct set_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
-                                  set_slot_policy<Key>> {
-  using value_type = Key;
-  using slot_type = typename set_params::common_params::slot_type;
-  using value_compare = typename set_params::common_params::key_compare;
-  using is_map_container = std::false_type;
-
-  template <typename V>
-  static const V &key(const V &value) { return value; }
-  static const Key &key(const slot_type *slot) { return *slot; }
-  static const Key &key(slot_type *slot) { return *slot; }
-};
-
-// An adapter class that converts a lower-bound compare into an upper-bound
-// compare. Note: there is no need to make a version of this adapter specialized
-// for key-compare-to functors because the upper-bound (the first value greater
-// than the input) is never an exact match.
-template <typename Compare>
-struct upper_bound_adapter {
-  explicit upper_bound_adapter(const Compare &c) : comp(c) {}
-  template <typename K1, typename K2>
-  bool operator()(const K1 &a, const K2 &b) const {
-    // Returns true when a is not greater than b.
-    return !compare_internal::compare_result_as_less_than(comp(b, a));
-  }
-
- private:
-  Compare comp;
-};
-
-enum class MatchKind : uint8_t { kEq, kNe };
-
-template <typename V, bool IsCompareTo>
-struct SearchResult {
-  V value;
-  MatchKind match;
-
-  static constexpr bool HasMatch() { return true; }
-  bool IsEq() const { return match == MatchKind::kEq; }
-};
-
-// When we don't use CompareTo, `match` is not present.
-// This ensures that callers can't use it accidentally when it provides no
-// useful information.
-template <typename V>
-struct SearchResult<V, false> {
-  SearchResult() {}
-  explicit SearchResult(V value) : value(value) {}
-  SearchResult(V value, MatchKind /*match*/) : value(value) {}
-
-  V value;
-
-  static constexpr bool HasMatch() { return false; }
-  static constexpr bool IsEq() { return false; }
-};
-
-// A node in the btree holding. The same node type is used for both internal
-// and leaf nodes in the btree, though the nodes are allocated in such a way
-// that the children array is only valid in internal nodes.
-template <typename Params>
-class btree_node {
-  using is_key_compare_to = typename Params::is_key_compare_to;
-  using is_multi_container = typename Params::is_multi_container;
-  using field_type = typename Params::node_count_type;
-  using allocator_type = typename Params::allocator_type;
-  using slot_type = typename Params::slot_type;
-
- public:
-  using params_type = Params;
-  using key_type = typename Params::key_type;
-  using value_type = typename Params::value_type;
-  using pointer = typename Params::pointer;
-  using const_pointer = typename Params::const_pointer;
-  using reference = typename Params::reference;
-  using const_reference = typename Params::const_reference;
-  using key_compare = typename Params::key_compare;
-  using size_type = typename Params::size_type;
-  using difference_type = typename Params::difference_type;
-
-  // Btree decides whether to use linear node search as follows:
-  //   - If the comparator expresses a preference, use that.
-  //   - If the key expresses a preference, use that.
-  //   - If the key is arithmetic and the comparator is std::less or
-  //     std::greater, choose linear.
-  //   - Otherwise, choose binary.
-  // TODO(ezb): Might make sense to add condition(s) based on node-size.
-  using use_linear_search = std::integral_constant<
-      bool,
-      has_linear_node_search_preference<key_compare>::value
-          ? prefers_linear_node_search<key_compare>::value
-          : has_linear_node_search_preference<key_type>::value
-                ? prefers_linear_node_search<key_type>::value
-                : std::is_arithmetic<key_type>::value &&
-                      (std::is_same<std::less<key_type>, key_compare>::value ||
-                       std::is_same<std::greater<key_type>,
-                                    key_compare>::value)>;
-
-  // This class is organized by gtl::Layout as if it had the following
-  // structure:
-  //   // A pointer to the node's parent.
-  //   btree_node *parent;
-  //
-  //   // The position of the node in the node's parent.
-  //   field_type position;
-  //   // The index of the first populated value in `values`.
-  //   // TODO(ezb): right now, `start` is always 0. Update insertion/merge
-  //   // logic to allow for floating storage within nodes.
-  //   field_type start;
-  //   // The index after the last populated value in `values`. Currently, this
-  //   // is the same as the count of values.
-  //   field_type finish;
-  //   // The maximum number of values the node can hold. This is an integer in
-  //   // [1, kNodeValues] for root leaf nodes, kNodeValues for non-root leaf
-  //   // nodes, and kInternalNodeMaxCount (as a sentinel value) for internal
-  //   // nodes (even though there are still kNodeValues values in the node).
-  //   // TODO(ezb): make max_count use only 4 bits and record log2(capacity)
-  //   // to free extra bits for is_root, etc.
-  //   field_type max_count;
-  //
-  //   // The array of values. The capacity is `max_count` for leaf nodes and
-  //   // kNodeValues for internal nodes. Only the values in
-  //   // [start, finish) have been initialized and are valid.
-  //   slot_type values[max_count];
-  //
-  //   // The array of child pointers. The keys in children[i] are all less
-  //   // than key(i). The keys in children[i + 1] are all greater than key(i).
-  //   // There are 0 children for leaf nodes and kNodeValues + 1 children for
-  //   // internal nodes.
-  //   btree_node *children[kNodeValues + 1];
-  //
-  // This class is only constructed by EmptyNodeType. Normally, pointers to the
-  // layout above are allocated, cast to btree_node*, and de-allocated within
-  // the btree implementation.
-  ~btree_node() = default;
-  btree_node(btree_node const &) = delete;
-  btree_node &operator=(btree_node const &) = delete;
-
-  // Public for EmptyNodeType.
-  constexpr static size_type Alignment() {
-    static_assert(LeafLayout(1).Alignment() == InternalLayout().Alignment(),
-                  "Alignment of all nodes must be equal.");
-    return InternalLayout().Alignment();
-  }
-
- protected:
-  btree_node() = default;
-
- private:
-  using layout_type = absl::container_internal::Layout<btree_node *, field_type,
-                                                       slot_type, btree_node *>;
-  constexpr static size_type SizeWithNValues(size_type n) {
-    return layout_type(/*parent*/ 1,
-                       /*position, start, finish, max_count*/ 4,
-                       /*values*/ n,
-                       /*children*/ 0)
-        .AllocSize();
-  }
-  // A lower bound for the overhead of fields other than values in a leaf node.
-  constexpr static size_type MinimumOverhead() {
-    return SizeWithNValues(1) - sizeof(value_type);
-  }
-
-  // Compute how many values we can fit onto a leaf node taking into account
-  // padding.
-  constexpr static size_type NodeTargetValues(const int begin, const int end) {
-    return begin == end ? begin
-                        : SizeWithNValues((begin + end) / 2 + 1) >
-                                  params_type::kTargetNodeSize
-                              ? NodeTargetValues(begin, (begin + end) / 2)
-                              : NodeTargetValues((begin + end) / 2 + 1, end);
-  }
-
-  enum {
-    kTargetNodeSize = params_type::kTargetNodeSize,
-    kNodeTargetValues = NodeTargetValues(0, params_type::kTargetNodeSize),
-
-    // We need a minimum of 3 values per internal node in order to perform
-    // splitting (1 value for the two nodes involved in the split and 1 value
-    // propagated to the parent as the delimiter for the split).
-    kNodeValues = kNodeTargetValues >= 3 ? kNodeTargetValues : 3,
-
-    // The node is internal (i.e. is not a leaf node) if and only if `max_count`
-    // has this value.
-    kInternalNodeMaxCount = 0,
-  };
-
-  // Leaves can have less than kNodeValues values.
-  constexpr static layout_type LeafLayout(const int max_values = kNodeValues) {
-    return layout_type(/*parent*/ 1,
-                       /*position, start, finish, max_count*/ 4,
-                       /*values*/ max_values,
-                       /*children*/ 0);
-  }
-  constexpr static layout_type InternalLayout() {
-    return layout_type(/*parent*/ 1,
-                       /*position, start, finish, max_count*/ 4,
-                       /*values*/ kNodeValues,
-                       /*children*/ kNodeValues + 1);
-  }
-  constexpr static size_type LeafSize(const int max_values = kNodeValues) {
-    return LeafLayout(max_values).AllocSize();
-  }
-  constexpr static size_type InternalSize() {
-    return InternalLayout().AllocSize();
-  }
-
-  // N is the index of the type in the Layout definition.
-  // ElementType<N> is the Nth type in the Layout definition.
-  template <size_type N>
-  inline typename layout_type::template ElementType<N> *GetField() {
-    // We assert that we don't read from values that aren't there.
-    assert(N < 3 || !leaf());
-    return InternalLayout().template Pointer<N>(reinterpret_cast<char *>(this));
-  }
-  template <size_type N>
-  inline const typename layout_type::template ElementType<N> *GetField() const {
-    assert(N < 3 || !leaf());
-    return InternalLayout().template Pointer<N>(
-        reinterpret_cast<const char *>(this));
-  }
-  void set_parent(btree_node *p) { *GetField<0>() = p; }
-  field_type &mutable_finish() { return GetField<1>()[2]; }
-  slot_type *slot(int i) { return &GetField<2>()[i]; }
-  slot_type *start_slot() { return slot(start()); }
-  slot_type *finish_slot() { return slot(finish()); }
-  const slot_type *slot(int i) const { return &GetField<2>()[i]; }
-  void set_position(field_type v) { GetField<1>()[0] = v; }
-  void set_start(field_type v) { GetField<1>()[1] = v; }
-  void set_finish(field_type v) { GetField<1>()[2] = v; }
-  // This method is only called by the node init methods.
-  void set_max_count(field_type v) { GetField<1>()[3] = v; }
-
- public:
-  // Whether this is a leaf node or not. This value doesn't change after the
-  // node is created.
-  bool leaf() const { return GetField<1>()[3] != kInternalNodeMaxCount; }
-
-  // Getter for the position of this node in its parent.
-  field_type position() const { return GetField<1>()[0]; }
-
-  // Getter for the offset of the first value in the `values` array.
-  field_type start() const {
-    // TODO(ezb): when floating storage is implemented, return GetField<1>()[1];
-    assert(GetField<1>()[1] == 0);
-    return 0;
-  }
-
-  // Getter for the offset after the last value in the `values` array.
-  field_type finish() const { return GetField<1>()[2]; }
-
-  // Getters for the number of values stored in this node.
-  field_type count() const {
-    assert(finish() >= start());
-    return finish() - start();
-  }
-  field_type max_count() const {
-    // Internal nodes have max_count==kInternalNodeMaxCount.
-    // Leaf nodes have max_count in [1, kNodeValues].
-    const field_type max_count = GetField<1>()[3];
-    return max_count == field_type{kInternalNodeMaxCount}
-               ? field_type{kNodeValues}
-               : max_count;
-  }
-
-  // Getter for the parent of this node.
-  btree_node *parent() const { return *GetField<0>(); }
-  // Getter for whether the node is the root of the tree. The parent of the
-  // root of the tree is the leftmost node in the tree which is guaranteed to
-  // be a leaf.
-  bool is_root() const { return parent()->leaf(); }
-  void make_root() {
-    assert(parent()->is_root());
-    set_parent(parent()->parent());
-  }
-
-  // Getters for the key/value at position i in the node.
-  const key_type &key(int i) const { return params_type::key(slot(i)); }
-  reference value(int i) { return params_type::element(slot(i)); }
-  const_reference value(int i) const { return params_type::element(slot(i)); }
-
-  // Getters/setter for the child at position i in the node.
-  btree_node *child(int i) const { return GetField<3>()[i]; }
-  btree_node *start_child() const { return child(start()); }
-  btree_node *&mutable_child(int i) { return GetField<3>()[i]; }
-  void clear_child(int i) {
-    absl::container_internal::SanitizerPoisonObject(&mutable_child(i));
-  }
-  void set_child(int i, btree_node *c) {
-    absl::container_internal::SanitizerUnpoisonObject(&mutable_child(i));
-    mutable_child(i) = c;
-    c->set_position(i);
-  }
-  void init_child(int i, btree_node *c) {
-    set_child(i, c);
-    c->set_parent(this);
-  }
-
-  // Returns the position of the first value whose key is not less than k.
-  template <typename K>
-  SearchResult<int, is_key_compare_to::value> lower_bound(
-      const K &k, const key_compare &comp) const {
-    return use_linear_search::value ? linear_search(k, comp)
-                                    : binary_search(k, comp);
-  }
-  // Returns the position of the first value whose key is greater than k.
-  template <typename K>
-  int upper_bound(const K &k, const key_compare &comp) const {
-    auto upper_compare = upper_bound_adapter<key_compare>(comp);
-    return use_linear_search::value ? linear_search(k, upper_compare).value
-                                    : binary_search(k, upper_compare).value;
-  }
-
-  template <typename K, typename Compare>
-  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
-  linear_search(const K &k, const Compare &comp) const {
-    return linear_search_impl(k, start(), finish(), comp,
-                              btree_is_key_compare_to<Compare, key_type>());
-  }
-
-  template <typename K, typename Compare>
-  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
-  binary_search(const K &k, const Compare &comp) const {
-    return binary_search_impl(k, start(), finish(), comp,
-                              btree_is_key_compare_to<Compare, key_type>());
-  }
-
-  // Returns the position of the first value whose key is not less than k using
-  // linear search performed using plain compare.
-  template <typename K, typename Compare>
-  SearchResult<int, false> linear_search_impl(
-      const K &k, int s, const int e, const Compare &comp,
-      std::false_type /* IsCompareTo */) const {
-    while (s < e) {
-      if (!comp(key(s), k)) {
-        break;
-      }
-      ++s;
-    }
-    return SearchResult<int, false>{s};
-  }
-
-  // Returns the position of the first value whose key is not less than k using
-  // linear search performed using compare-to.
-  template <typename K, typename Compare>
-  SearchResult<int, true> linear_search_impl(
-      const K &k, int s, const int e, const Compare &comp,
-      std::true_type /* IsCompareTo */) const {
-    while (s < e) {
-      const absl::weak_ordering c = comp(key(s), k);
-      if (c == 0) {
-        return {s, MatchKind::kEq};
-      } else if (c > 0) {
-        break;
-      }
-      ++s;
-    }
-    return {s, MatchKind::kNe};
-  }
-
-  // Returns the position of the first value whose key is not less than k using
-  // binary search performed using plain compare.
-  template <typename K, typename Compare>
-  SearchResult<int, false> binary_search_impl(
-      const K &k, int s, int e, const Compare &comp,
-      std::false_type /* IsCompareTo */) const {
-    while (s != e) {
-      const int mid = (s + e) >> 1;
-      if (comp(key(mid), k)) {
-        s = mid + 1;
-      } else {
-        e = mid;
-      }
-    }
-    return SearchResult<int, false>{s};
-  }
-
-  // Returns the position of the first value whose key is not less than k using
-  // binary search performed using compare-to.
-  template <typename K, typename CompareTo>
-  SearchResult<int, true> binary_search_impl(
-      const K &k, int s, int e, const CompareTo &comp,
-      std::true_type /* IsCompareTo */) const {
-    if (is_multi_container::value) {
-      MatchKind exact_match = MatchKind::kNe;
-      while (s != e) {
-        const int mid = (s + e) >> 1;
-        const absl::weak_ordering c = comp(key(mid), k);
-        if (c < 0) {
-          s = mid + 1;
-        } else {
-          e = mid;
-          if (c == 0) {
-            // Need to return the first value whose key is not less than k,
-            // which requires continuing the binary search if this is a
-            // multi-container.
-            exact_match = MatchKind::kEq;
-          }
-        }
-      }
-      return {s, exact_match};
-    } else {  // Not a multi-container.
-      while (s != e) {
-        const int mid = (s + e) >> 1;
-        const absl::weak_ordering c = comp(key(mid), k);
-        if (c < 0) {
-          s = mid + 1;
-        } else if (c > 0) {
-          e = mid;
-        } else {
-          return {mid, MatchKind::kEq};
-        }
-      }
-      return {s, MatchKind::kNe};
-    }
-  }
-
-  // Emplaces a value at position i, shifting all existing values and
-  // children at positions >= i to the right by 1.
-  template <typename... Args>
-  void emplace_value(size_type i, allocator_type *alloc, Args &&... args);
-
-  // Removes the values at positions [i, i + to_erase), shifting all existing
-  // values and children after that range to the left by to_erase. Clears all
-  // children between [i, i + to_erase).
-  void remove_values(field_type i, field_type to_erase, allocator_type *alloc);
-
-  // Rebalances a node with its right sibling.
-  void rebalance_right_to_left(int to_move, btree_node *right,
-                               allocator_type *alloc);
-  void rebalance_left_to_right(int to_move, btree_node *right,
-                               allocator_type *alloc);
-
-  // Splits a node, moving a portion of the node's values to its right sibling.
-  void split(int insert_position, btree_node *dest, allocator_type *alloc);
-
-  // Merges a node with its right sibling, moving all of the values and the
-  // delimiting key in the parent node onto itself, and deleting the src node.
-  void merge(btree_node *src, allocator_type *alloc);
-
-  // Node allocation/deletion routines.
-  void init_leaf(btree_node *parent, int max_count) {
-    set_parent(parent);
-    set_position(0);
-    set_start(0);
-    set_finish(0);
-    set_max_count(max_count);
-    absl::container_internal::SanitizerPoisonMemoryRegion(
-        start_slot(), max_count * sizeof(slot_type));
-  }
-  void init_internal(btree_node *parent) {
-    init_leaf(parent, kNodeValues);
-    // Set `max_count` to a sentinel value to indicate that this node is
-    // internal.
-    set_max_count(kInternalNodeMaxCount);
-    absl::container_internal::SanitizerPoisonMemoryRegion(
-        &mutable_child(start()), (kNodeValues + 1) * sizeof(btree_node *));
-  }
-
-  static void deallocate(const size_type size, btree_node *node,
-                         allocator_type *alloc) {
-    absl::container_internal::Deallocate<Alignment()>(alloc, node, size);
-  }
-
-  // Deletes a node and all of its children.
-  static void clear_and_delete(btree_node *node, allocator_type *alloc);
-
- private:
-  template <typename... Args>
-  void value_init(const field_type i, allocator_type *alloc, Args &&... args) {
-    absl::container_internal::SanitizerUnpoisonObject(slot(i));
-    params_type::construct(alloc, slot(i), std::forward<Args>(args)...);
-  }
-  void value_destroy(const field_type i, allocator_type *alloc) {
-    params_type::destroy(alloc, slot(i));
-    absl::container_internal::SanitizerPoisonObject(slot(i));
-  }
-  void value_destroy_n(const field_type i, const field_type n,
-                       allocator_type *alloc) {
-    for (slot_type *s = slot(i), *end = slot(i + n); s != end; ++s) {
-      params_type::destroy(alloc, s);
-      absl::container_internal::SanitizerPoisonObject(s);
-    }
-  }
-
-  static void transfer(slot_type *dest, slot_type *src, allocator_type *alloc) {
-    absl::container_internal::SanitizerUnpoisonObject(dest);
-    params_type::transfer(alloc, dest, src);
-    absl::container_internal::SanitizerPoisonObject(src);
-  }
-
-  // Transfers value from slot `src_i` in `src_node` to slot `dest_i` in `this`.
-  void transfer(const size_type dest_i, const size_type src_i,
-                btree_node *src_node, allocator_type *alloc) {
-    transfer(slot(dest_i), src_node->slot(src_i), alloc);
-  }
-
-  // Transfers `n` values starting at value `src_i` in `src_node` into the
-  // values starting at value `dest_i` in `this`.
-  void transfer_n(const size_type n, const size_type dest_i,
-                  const size_type src_i, btree_node *src_node,
-                  allocator_type *alloc) {
-    for (slot_type *src = src_node->slot(src_i), *end = src + n,
-                   *dest = slot(dest_i);
-         src != end; ++src, ++dest) {
-      transfer(dest, src, alloc);
-    }
-  }
-
-  // Same as above, except that we start at the end and work our way to the
-  // beginning.
-  void transfer_n_backward(const size_type n, const size_type dest_i,
-                           const size_type src_i, btree_node *src_node,
-                           allocator_type *alloc) {
-    for (slot_type *src = src_node->slot(src_i + n - 1), *end = src - n,
-                   *dest = slot(dest_i + n - 1);
-         src != end; --src, --dest) {
-      transfer(dest, src, alloc);
-    }
-  }
-
-  template <typename P>
-  friend class btree;
-  template <typename N, typename R, typename P>
-  friend struct btree_iterator;
-  friend class BtreeNodePeer;
-};
-
-template <typename Node, typename Reference, typename Pointer>
-struct btree_iterator {
- private:
-  using key_type = typename Node::key_type;
-  using size_type = typename Node::size_type;
-  using params_type = typename Node::params_type;
-
-  using node_type = Node;
-  using normal_node = typename std::remove_const<Node>::type;
-  using const_node = const Node;
-  using normal_pointer = typename params_type::pointer;
-  using normal_reference = typename params_type::reference;
-  using const_pointer = typename params_type::const_pointer;
-  using const_reference = typename params_type::const_reference;
-  using slot_type = typename params_type::slot_type;
-
-  using iterator =
-      btree_iterator<normal_node, normal_reference, normal_pointer>;
-  using const_iterator =
-      btree_iterator<const_node, const_reference, const_pointer>;
-
- public:
-  // These aliases are public for std::iterator_traits.
-  using difference_type = typename Node::difference_type;
-  using value_type = typename params_type::value_type;
-  using pointer = Pointer;
-  using reference = Reference;
-  using iterator_category = std::bidirectional_iterator_tag;
-
-  btree_iterator() : node(nullptr), position(-1) {}
-  explicit btree_iterator(Node *n) : node(n), position(n->start()) {}
-  btree_iterator(Node *n, int p) : node(n), position(p) {}
-
-  // NOTE: this SFINAE allows for implicit conversions from iterator to
-  // const_iterator, but it specifically avoids defining copy constructors so
-  // that btree_iterator can be trivially copyable. This is for performance and
-  // binary size reasons.
-  template <typename N, typename R, typename P,
-            absl::enable_if_t<
-                std::is_same<btree_iterator<N, R, P>, iterator>::value &&
-                    std::is_same<btree_iterator, const_iterator>::value,
-                int> = 0>
-  btree_iterator(const btree_iterator<N, R, P> &other)  // NOLINT
-      : node(other.node), position(other.position) {}
-
- private:
-  // This SFINAE allows explicit conversions from const_iterator to
-  // iterator, but also avoids defining a copy constructor.
-  // NOTE: the const_cast is safe because this constructor is only called by
-  // non-const methods and the container owns the nodes.
-  template <typename N, typename R, typename P,
-            absl::enable_if_t<
-                std::is_same<btree_iterator<N, R, P>, const_iterator>::value &&
-                    std::is_same<btree_iterator, iterator>::value,
-                int> = 0>
-  explicit btree_iterator(const btree_iterator<N, R, P> &other)
-      : node(const_cast<node_type *>(other.node)), position(other.position) {}
-
-  // Increment/decrement the iterator.
-  void increment() {
-    if (node->leaf() && ++position < node->finish()) {
-      return;
-    }
-    increment_slow();
-  }
-  void increment_slow();
-
-  void decrement() {
-    if (node->leaf() && --position >= node->start()) {
-      return;
-    }
-    decrement_slow();
-  }
-  void decrement_slow();
-
- public:
-  bool operator==(const iterator &other) const {
-    return node == other.node && position == other.position;
-  }
-  bool operator==(const const_iterator &other) const {
-    return node == other.node && position == other.position;
-  }
-  bool operator!=(const iterator &other) const {
-    return node != other.node || position != other.position;
-  }
-  bool operator!=(const const_iterator &other) const {
-    return node != other.node || position != other.position;
-  }
-
-  // Accessors for the key/value the iterator is pointing at.
-  reference operator*() const {
-    ABSL_HARDENING_ASSERT(node != nullptr);
-    ABSL_HARDENING_ASSERT(node->start() <= position);
-    ABSL_HARDENING_ASSERT(node->finish() > position);
-    return node->value(position);
-  }
-  pointer operator->() const { return &operator*(); }
-
-  btree_iterator &operator++() {
-    increment();
-    return *this;
-  }
-  btree_iterator &operator--() {
-    decrement();
-    return *this;
-  }
-  btree_iterator operator++(int) {
-    btree_iterator tmp = *this;
-    ++*this;
-    return tmp;
-  }
-  btree_iterator operator--(int) {
-    btree_iterator tmp = *this;
-    --*this;
-    return tmp;
-  }
-
- private:
-  template <typename Params>
-  friend class btree;
-  template <typename Tree>
-  friend class btree_container;
-  template <typename Tree>
-  friend class btree_set_container;
-  template <typename Tree>
-  friend class btree_map_container;
-  template <typename Tree>
-  friend class btree_multiset_container;
-  template <typename N, typename R, typename P>
-  friend struct btree_iterator;
-  template <typename TreeType, typename CheckerType>
-  friend class base_checker;
-
-  const key_type &key() const { return node->key(position); }
-  slot_type *slot() { return node->slot(position); }
-
-  // The node in the tree the iterator is pointing at.
-  Node *node;
-  // The position within the node of the tree the iterator is pointing at.
-  // NOTE: this is an int rather than a field_type because iterators can point
-  // to invalid positions (such as -1) in certain circumstances.
-  int position;
-};
-
-template <typename Params>
-class btree {
-  using node_type = btree_node<Params>;
-  using is_key_compare_to = typename Params::is_key_compare_to;
-  using init_type = typename Params::init_type;
-  using field_type = typename node_type::field_type;
-  using is_multi_container = typename Params::is_multi_container;
-  using is_key_compare_adapted = typename Params::is_key_compare_adapted;
-
-  // We use a static empty node for the root/leftmost/rightmost of empty btrees
-  // in order to avoid branching in begin()/end().
-  struct alignas(node_type::Alignment()) EmptyNodeType : node_type {
-    using field_type = typename node_type::field_type;
-    node_type *parent;
-    field_type position = 0;
-    field_type start = 0;
-    field_type finish = 0;
-    // max_count must be != kInternalNodeMaxCount (so that this node is regarded
-    // as a leaf node). max_count() is never called when the tree is empty.
-    field_type max_count = node_type::kInternalNodeMaxCount + 1;
-
-#ifdef _MSC_VER
-    // MSVC has constexpr code generations bugs here.
-    EmptyNodeType() : parent(this) {}
-#else
-    constexpr EmptyNodeType(node_type *p) : parent(p) {}
-#endif
-  };
-
-  static node_type *EmptyNode() {
-#ifdef _MSC_VER
-    static EmptyNodeType *empty_node = new EmptyNodeType;
-    // This assert fails on some other construction methods.
-    assert(empty_node->parent == empty_node);
-    return empty_node;
-#else
-    static constexpr EmptyNodeType empty_node(
-        const_cast<EmptyNodeType *>(&empty_node));
-    return const_cast<EmptyNodeType *>(&empty_node);
-#endif
-  }
-
-  enum : uint32_t {
-    kNodeValues = node_type::kNodeValues,
-    kMinNodeValues = kNodeValues / 2,
-  };
-
-  struct node_stats {
-    using size_type = typename Params::size_type;
-
-    node_stats(size_type l, size_type i) : leaf_nodes(l), internal_nodes(i) {}
-
-    node_stats &operator+=(const node_stats &other) {
-      leaf_nodes += other.leaf_nodes;
-      internal_nodes += other.internal_nodes;
-      return *this;
-    }
-
-    size_type leaf_nodes;
-    size_type internal_nodes;
-  };
-
- public:
-  using key_type = typename Params::key_type;
-  using value_type = typename Params::value_type;
-  using size_type = typename Params::size_type;
-  using difference_type = typename Params::difference_type;
-  using key_compare = typename Params::key_compare;
-  using value_compare = typename Params::value_compare;
-  using allocator_type = typename Params::allocator_type;
-  using reference = typename Params::reference;
-  using const_reference = typename Params::const_reference;
-  using pointer = typename Params::pointer;
-  using const_pointer = typename Params::const_pointer;
-  using iterator = btree_iterator<node_type, reference, pointer>;
-  using const_iterator = typename iterator::const_iterator;
-  using reverse_iterator = std::reverse_iterator<iterator>;
-  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
-  using node_handle_type = node_handle<Params, Params, allocator_type>;
-
-  // Internal types made public for use by btree_container types.
-  using params_type = Params;
-  using slot_type = typename Params::slot_type;
-
- private:
-  // For use in copy_or_move_values_in_order.
-  const value_type &maybe_move_from_iterator(const_iterator it) { return *it; }
-  value_type &&maybe_move_from_iterator(iterator it) { return std::move(*it); }
-
-  // Copies or moves (depending on the template parameter) the values in
-  // other into this btree in their order in other. This btree must be empty
-  // before this method is called. This method is used in copy construction,
-  // copy assignment, and move assignment.
-  template <typename Btree>
-  void copy_or_move_values_in_order(Btree &other);
-
-  // Validates that various assumptions/requirements are true at compile time.
-  constexpr static bool static_assert_validation();
-
- public:
-  btree(const key_compare &comp, const allocator_type &alloc)
-      : root_(comp, alloc, EmptyNode()), rightmost_(EmptyNode()), size_(0) {}
-
-  btree(const btree &other) : btree(other, other.allocator()) {}
-  btree(const btree &other, const allocator_type &alloc)
-      : btree(other.key_comp(), alloc) {
-    copy_or_move_values_in_order(other);
-  }
-  btree(btree &&other) noexcept
-      : root_(std::move(other.root_)),
-        rightmost_(absl::exchange(other.rightmost_, EmptyNode())),
-        size_(absl::exchange(other.size_, 0)) {
-    other.mutable_root() = EmptyNode();
-  }
-  btree(btree &&other, const allocator_type &alloc)
-      : btree(other.key_comp(), alloc) {
-    if (alloc == other.allocator()) {
-      swap(other);
-    } else {
-      // Move values from `other` one at a time when allocators are different.
-      copy_or_move_values_in_order(other);
-    }
-  }
-
-  ~btree() {
-    // Put static_asserts in destructor to avoid triggering them before the type
-    // is complete.
-    static_assert(static_assert_validation(), "This call must be elided.");
-    clear();
-  }
-
-  // Assign the contents of other to *this.
-  btree &operator=(const btree &other);
-  btree &operator=(btree &&other) noexcept;
-
-  iterator begin() { return iterator(leftmost()); }
-  const_iterator begin() const { return const_iterator(leftmost()); }
-  iterator end() { return iterator(rightmost_, rightmost_->finish()); }
-  const_iterator end() const {
-    return const_iterator(rightmost_, rightmost_->finish());
-  }
-  reverse_iterator rbegin() { return reverse_iterator(end()); }
-  const_reverse_iterator rbegin() const {
-    return const_reverse_iterator(end());
-  }
-  reverse_iterator rend() { return reverse_iterator(begin()); }
-  const_reverse_iterator rend() const {
-    return const_reverse_iterator(begin());
-  }
-
-  // Finds the first element whose key is not less than key.
-  template <typename K>
-  iterator lower_bound(const K &key) {
-    return internal_end(internal_lower_bound(key).value);
-  }
-  template <typename K>
-  const_iterator lower_bound(const K &key) const {
-    return internal_end(internal_lower_bound(key).value);
-  }
-
-  // Finds the first element whose key is greater than key.
-  template <typename K>
-  iterator upper_bound(const K &key) {
-    return internal_end(internal_upper_bound(key));
-  }
-  template <typename K>
-  const_iterator upper_bound(const K &key) const {
-    return internal_end(internal_upper_bound(key));
-  }
-
-  // Finds the range of values which compare equal to key. The first member of
-  // the returned pair is equal to lower_bound(key). The second member of the
-  // pair is equal to upper_bound(key).
-  template <typename K>
-  std::pair<iterator, iterator> equal_range(const K &key);
-  template <typename K>
-  std::pair<const_iterator, const_iterator> equal_range(const K &key) const {
-    return const_cast<btree *>(this)->equal_range(key);
-  }
-
-  // Inserts a value into the btree only if it does not already exist. The
-  // boolean return value indicates whether insertion succeeded or failed.
-  // Requirement: if `key` already exists in the btree, does not consume `args`.
-  // Requirement: `key` is never referenced after consuming `args`.
-  template <typename K, typename... Args>
-  std::pair<iterator, bool> insert_unique(const K &key, Args &&... args);
-
-  // Inserts with hint. Checks to see if the value should be placed immediately
-  // before `position` in the tree. If so, then the insertion will take
-  // amortized constant time. If not, the insertion will take amortized
-  // logarithmic time as if a call to insert_unique() were made.
-  // Requirement: if `key` already exists in the btree, does not consume `args`.
-  // Requirement: `key` is never referenced after consuming `args`.
-  template <typename K, typename... Args>
-  std::pair<iterator, bool> insert_hint_unique(iterator position,
-                                               const K &key,
-                                               Args &&... args);
-
-  // Insert a range of values into the btree.
-  // Note: the first overload avoids constructing a value_type if the key
-  // already exists in the btree.
-  template <typename InputIterator,
-            typename = decltype(std::declval<const key_compare &>()(
-                params_type::key(*std::declval<InputIterator>()),
-                std::declval<const key_type &>()))>
-  void insert_iterator_unique(InputIterator b, InputIterator e, int);
-  // We need the second overload for cases in which we need to construct a
-  // value_type in order to compare it with the keys already in the btree.
-  template <typename InputIterator>
-  void insert_iterator_unique(InputIterator b, InputIterator e, char);
-
-  // Inserts a value into the btree.
-  template <typename ValueType>
-  iterator insert_multi(const key_type &key, ValueType &&v);
-
-  // Inserts a value into the btree.
-  template <typename ValueType>
-  iterator insert_multi(ValueType &&v) {
-    return insert_multi(params_type::key(v), std::forward<ValueType>(v));
-  }
-
-  // Insert with hint. Check to see if the value should be placed immediately
-  // before position in the tree. If it does, then the insertion will take
-  // amortized constant time. If not, the insertion will take amortized
-  // logarithmic time as if a call to insert_multi(v) were made.
-  template <typename ValueType>
-  iterator insert_hint_multi(iterator position, ValueType &&v);
-
-  // Insert a range of values into the btree.
-  template <typename InputIterator>
-  void insert_iterator_multi(InputIterator b, InputIterator e);
-
-  // Erase the specified iterator from the btree. The iterator must be valid
-  // (i.e. not equal to end()).  Return an iterator pointing to the node after
-  // the one that was erased (or end() if none exists).
-  // Requirement: does not read the value at `*iter`.
-  iterator erase(iterator iter);
-
-  // Erases range. Returns the number of keys erased and an iterator pointing
-  // to the element after the last erased element.
-  std::pair<size_type, iterator> erase_range(iterator begin, iterator end);
-
-  // Finds the iterator corresponding to a key or returns end() if the key is
-  // not present.
-  template <typename K>
-  iterator find(const K &key) {
-    return internal_end(internal_find(key));
-  }
-  template <typename K>
-  const_iterator find(const K &key) const {
-    return internal_end(internal_find(key));
-  }
-
-  // Clear the btree, deleting all of the values it contains.
-  void clear();
-
-  // Swaps the contents of `this` and `other`.
-  void swap(btree &other);
-
-  const key_compare &key_comp() const noexcept {
-    return root_.template get<0>();
-  }
-  template <typename K1, typename K2>
-  bool compare_keys(const K1 &a, const K2 &b) const {
-    return compare_internal::compare_result_as_less_than(key_comp()(a, b));
-  }
-
-  value_compare value_comp() const { return value_compare(key_comp()); }
-
-  // Verifies the structure of the btree.
-  void verify() const;
-
-  // Size routines.
-  size_type size() const { return size_; }
-  size_type max_size() const { return (std::numeric_limits<size_type>::max)(); }
-  bool empty() const { return size_ == 0; }
-
-  // The height of the btree. An empty tree will have height 0.
-  size_type height() const {
-    size_type h = 0;
-    if (!empty()) {
-      // Count the length of the chain from the leftmost node up to the
-      // root. We actually count from the root back around to the level below
-      // the root, but the calculation is the same because of the circularity
-      // of that traversal.
-      const node_type *n = root();
-      do {
-        ++h;
-        n = n->parent();
-      } while (n != root());
-    }
-    return h;
-  }
-
-  // The number of internal, leaf and total nodes used by the btree.
-  size_type leaf_nodes() const { return internal_stats(root()).leaf_nodes; }
-  size_type internal_nodes() const {
-    return internal_stats(root()).internal_nodes;
-  }
-  size_type nodes() const {
-    node_stats stats = internal_stats(root());
-    return stats.leaf_nodes + stats.internal_nodes;
-  }
-
-  // The total number of bytes used by the btree.
-  size_type bytes_used() const {
-    node_stats stats = internal_stats(root());
-    if (stats.leaf_nodes == 1 && stats.internal_nodes == 0) {
-      return sizeof(*this) + node_type::LeafSize(root()->max_count());
-    } else {
-      return sizeof(*this) + stats.leaf_nodes * node_type::LeafSize() +
-             stats.internal_nodes * node_type::InternalSize();
-    }
-  }
-
-  // The average number of bytes used per value stored in the btree.
-  static double average_bytes_per_value() {
-    // Returns the number of bytes per value on a leaf node that is 75%
-    // full. Experimentally, this matches up nicely with the computed number of
-    // bytes per value in trees that had their values inserted in random order.
-    return node_type::LeafSize() / (kNodeValues * 0.75);
-  }
-
-  // The fullness of the btree. Computed as the number of elements in the btree
-  // divided by the maximum number of elements a tree with the current number
-  // of nodes could hold. A value of 1 indicates perfect space
-  // utilization. Smaller values indicate space wastage.
-  // Returns 0 for empty trees.
-  double fullness() const {
-    if (empty()) return 0.0;
-    return static_cast<double>(size()) / (nodes() * kNodeValues);
-  }
-  // The overhead of the btree structure in bytes per node. Computed as the
-  // total number of bytes used by the btree minus the number of bytes used for
-  // storing elements divided by the number of elements.
-  // Returns 0 for empty trees.
-  double overhead() const {
-    if (empty()) return 0.0;
-    return (bytes_used() - size() * sizeof(value_type)) /
-           static_cast<double>(size());
-  }
-
-  // The allocator used by the btree.
-  allocator_type get_allocator() const { return allocator(); }
-
- private:
-  // Internal accessor routines.
-  node_type *root() { return root_.template get<2>(); }
-  const node_type *root() const { return root_.template get<2>(); }
-  node_type *&mutable_root() noexcept { return root_.template get<2>(); }
-  key_compare *mutable_key_comp() noexcept { return &root_.template get<0>(); }
-
-  // The leftmost node is stored as the parent of the root node.
-  node_type *leftmost() { return root()->parent(); }
-  const node_type *leftmost() const { return root()->parent(); }
-
-  // Allocator routines.
-  allocator_type *mutable_allocator() noexcept {
-    return &root_.template get<1>();
-  }
-  const allocator_type &allocator() const noexcept {
-    return root_.template get<1>();
-  }
-
-  // Allocates a correctly aligned node of at least size bytes using the
-  // allocator.
-  node_type *allocate(const size_type size) {
-    return reinterpret_cast<node_type *>(
-        absl::container_internal::Allocate<node_type::Alignment()>(
-            mutable_allocator(), size));
-  }
-
-  // Node creation/deletion routines.
-  node_type *new_internal_node(node_type *parent) {
-    node_type *n = allocate(node_type::InternalSize());
-    n->init_internal(parent);
-    return n;
-  }
-  node_type *new_leaf_node(node_type *parent) {
-    node_type *n = allocate(node_type::LeafSize());
-    n->init_leaf(parent, kNodeValues);
-    return n;
-  }
-  node_type *new_leaf_root_node(const int max_count) {
-    node_type *n = allocate(node_type::LeafSize(max_count));
-    n->init_leaf(/*parent=*/n, max_count);
-    return n;
-  }
-
-  // Deletion helper routines.
-  iterator rebalance_after_delete(iterator iter);
-
-  // Rebalances or splits the node iter points to.
-  void rebalance_or_split(iterator *iter);
-
-  // Merges the values of left, right and the delimiting key on their parent
-  // onto left, removing the delimiting key and deleting right.
-  void merge_nodes(node_type *left, node_type *right);
-
-  // Tries to merge node with its left or right sibling, and failing that,
-  // rebalance with its left or right sibling. Returns true if a merge
-  // occurred, at which point it is no longer valid to access node. Returns
-  // false if no merging took place.
-  bool try_merge_or_rebalance(iterator *iter);
-
-  // Tries to shrink the height of the tree by 1.
-  void try_shrink();
-
-  iterator internal_end(iterator iter) {
-    return iter.node != nullptr ? iter : end();
-  }
-  const_iterator internal_end(const_iterator iter) const {
-    return iter.node != nullptr ? iter : end();
-  }
-
-  // Emplaces a value into the btree immediately before iter. Requires that
-  // key(v) <= iter.key() and (--iter).key() <= key(v).
-  template <typename... Args>
-  iterator internal_emplace(iterator iter, Args &&... args);
-
-  // Returns an iterator pointing to the first value >= the value "iter" is
-  // pointing at. Note that "iter" might be pointing to an invalid location such
-  // as iter.position == iter.node->finish(). This routine simply moves iter up
-  // in the tree to a valid location.
-  // Requires: iter.node is non-null.
-  template <typename IterType>
-  static IterType internal_last(IterType iter);
-
-  // Returns an iterator pointing to the leaf position at which key would
-  // reside in the tree, unless there is an exact match - in which case, the
-  // result may not be on a leaf. When there's a three-way comparator, we can
-  // return whether there was an exact match. This allows the caller to avoid a
-  // subsequent comparison to determine if an exact match was made, which is
-  // important for keys with expensive comparison, such as strings.
-  template <typename K>
-  SearchResult<iterator, is_key_compare_to::value> internal_locate(
-      const K &key) const;
-
-  // Internal routine which implements lower_bound().
-  template <typename K>
-  SearchResult<iterator, is_key_compare_to::value> internal_lower_bound(
-      const K &key) const;
-
-  // Internal routine which implements upper_bound().
-  template <typename K>
-  iterator internal_upper_bound(const K &key) const;
-
-  // Internal routine which implements find().
-  template <typename K>
-  iterator internal_find(const K &key) const;
-
-  // Verifies the tree structure of node.
-  int internal_verify(const node_type *node, const key_type *lo,
-                      const key_type *hi) const;
-
-  node_stats internal_stats(const node_type *node) const {
-    // The root can be a static empty node.
-    if (node == nullptr || (node == root() && empty())) {
-      return node_stats(0, 0);
-    }
-    if (node->leaf()) {
-      return node_stats(1, 0);
-    }
-    node_stats res(0, 1);
-    for (int i = node->start(); i <= node->finish(); ++i) {
-      res += internal_stats(node->child(i));
-    }
-    return res;
-  }
-
-  // We use compressed tuple in order to save space because key_compare and
-  // allocator_type are usually empty.
-  absl::container_internal::CompressedTuple<key_compare, allocator_type,
-                                            node_type *>
-      root_;
-
-  // A pointer to the rightmost node. Note that the leftmost node is stored as
-  // the root's parent.
-  node_type *rightmost_;
-
-  // Number of values.
-  size_type size_;
-};
-
-////
-// btree_node methods
-template <typename P>
-template <typename... Args>
-inline void btree_node<P>::emplace_value(const size_type i,
-                                         allocator_type *alloc,
-                                         Args &&... args) {
-  assert(i >= start());
-  assert(i <= finish());
-  // Shift old values to create space for new value and then construct it in
-  // place.
-  if (i < finish()) {
-    transfer_n_backward(finish() - i, /*dest_i=*/i + 1, /*src_i=*/i, this,
-                        alloc);
-  }
-  value_init(i, alloc, std::forward<Args>(args)...);
-  set_finish(finish() + 1);
-
-  if (!leaf() && finish() > i + 1) {
-    for (int j = finish(); j > i + 1; --j) {
-      set_child(j, child(j - 1));
-    }
-    clear_child(i + 1);
-  }
-}
-
-template <typename P>
-inline void btree_node<P>::remove_values(const field_type i,
-                                         const field_type to_erase,
-                                         allocator_type *alloc) {
-  // Transfer values after the removed range into their new places.
-  value_destroy_n(i, to_erase, alloc);
-  const field_type orig_finish = finish();
-  const field_type src_i = i + to_erase;
-  transfer_n(orig_finish - src_i, i, src_i, this, alloc);
-
-  if (!leaf()) {
-    // Delete all children between begin and end.
-    for (int j = 0; j < to_erase; ++j) {
-      clear_and_delete(child(i + j + 1), alloc);
-    }
-    // Rotate children after end into new positions.
-    for (int j = i + to_erase + 1; j <= orig_finish; ++j) {
-      set_child(j - to_erase, child(j));
-      clear_child(j);
-    }
-  }
-  set_finish(orig_finish - to_erase);
-}
-
-template <typename P>
-void btree_node<P>::rebalance_right_to_left(const int to_move,
-                                            btree_node *right,
-                                            allocator_type *alloc) {
-  assert(parent() == right->parent());
-  assert(position() + 1 == right->position());
-  assert(right->count() >= count());
-  assert(to_move >= 1);
-  assert(to_move <= right->count());
-
-  // 1) Move the delimiting value in the parent to the left node.
-  transfer(finish(), position(), parent(), alloc);
-
-  // 2) Move the (to_move - 1) values from the right node to the left node.
-  transfer_n(to_move - 1, finish() + 1, right->start(), right, alloc);
-
-  // 3) Move the new delimiting value to the parent from the right node.
-  parent()->transfer(position(), right->start() + to_move - 1, right, alloc);
-
-  // 4) Shift the values in the right node to their correct positions.
-  right->transfer_n(right->count() - to_move, right->start(),
-                    right->start() + to_move, right, alloc);
-
-  if (!leaf()) {
-    // Move the child pointers from the right to the left node.
-    for (int i = 0; i < to_move; ++i) {
-      init_child(finish() + i + 1, right->child(i));
-    }
-    for (int i = right->start(); i <= right->finish() - to_move; ++i) {
-      assert(i + to_move <= right->max_count());
-      right->init_child(i, right->child(i + to_move));
-      right->clear_child(i + to_move);
-    }
-  }
-
-  // Fixup `finish` on the left and right nodes.
-  set_finish(finish() + to_move);
-  right->set_finish(right->finish() - to_move);
-}
-
-template <typename P>
-void btree_node<P>::rebalance_left_to_right(const int to_move,
-                                            btree_node *right,
-                                            allocator_type *alloc) {
-  assert(parent() == right->parent());
-  assert(position() + 1 == right->position());
-  assert(count() >= right->count());
-  assert(to_move >= 1);
-  assert(to_move <= count());
-
-  // Values in the right node are shifted to the right to make room for the
-  // new to_move values. Then, the delimiting value in the parent and the
-  // other (to_move - 1) values in the left node are moved into the right node.
-  // Lastly, a new delimiting value is moved from the left node into the
-  // parent, and the remaining empty left node entries are destroyed.
-
-  // 1) Shift existing values in the right node to their correct positions.
-  right->transfer_n_backward(right->count(), right->start() + to_move,
-                             right->start(), right, alloc);
-
-  // 2) Move the delimiting value in the parent to the right node.
-  right->transfer(right->start() + to_move - 1, position(), parent(), alloc);
-
-  // 3) Move the (to_move - 1) values from the left node to the right node.
-  right->transfer_n(to_move - 1, right->start(), finish() - (to_move - 1), this,
-                    alloc);
-
-  // 4) Move the new delimiting value to the parent from the left node.
-  parent()->transfer(position(), finish() - to_move, this, alloc);
-
-  if (!leaf()) {
-    // Move the child pointers from the left to the right node.
-    for (int i = right->finish(); i >= right->start(); --i) {
-      right->init_child(i + to_move, right->child(i));
-      right->clear_child(i);
-    }
-    for (int i = 1; i <= to_move; ++i) {
-      right->init_child(i - 1, child(finish() - to_move + i));
-      clear_child(finish() - to_move + i);
-    }
-  }
-
-  // Fixup the counts on the left and right nodes.
-  set_finish(finish() - to_move);
-  right->set_finish(right->finish() + to_move);
-}
-
-template <typename P>
-void btree_node<P>::split(const int insert_position, btree_node *dest,
-                          allocator_type *alloc) {
-  assert(dest->count() == 0);
-  assert(max_count() == kNodeValues);
-
-  // We bias the split based on the position being inserted. If we're
-  // inserting at the beginning of the left node then bias the split to put
-  // more values on the right node. If we're inserting at the end of the
-  // right node then bias the split to put more values on the left node.
-  if (insert_position == start()) {
-    dest->set_finish(dest->start() + finish() - 1);
-  } else if (insert_position == kNodeValues) {
-    dest->set_finish(dest->start());
-  } else {
-    dest->set_finish(dest->start() + count() / 2);
-  }
-  set_finish(finish() - dest->count());
-  assert(count() >= 1);
-
-  // Move values from the left sibling to the right sibling.
-  dest->transfer_n(dest->count(), dest->start(), finish(), this, alloc);
-
-  // The split key is the largest value in the left sibling.
-  --mutable_finish();
-  parent()->emplace_value(position(), alloc, finish_slot());
-  value_destroy(finish(), alloc);
-  parent()->init_child(position() + 1, dest);
-
-  if (!leaf()) {
-    for (int i = dest->start(), j = finish() + 1; i <= dest->finish();
-         ++i, ++j) {
-      assert(child(j) != nullptr);
-      dest->init_child(i, child(j));
-      clear_child(j);
-    }
-  }
-}
-
-template <typename P>
-void btree_node<P>::merge(btree_node *src, allocator_type *alloc) {
-  assert(parent() == src->parent());
-  assert(position() + 1 == src->position());
-
-  // Move the delimiting value to the left node.
-  value_init(finish(), alloc, parent()->slot(position()));
-
-  // Move the values from the right to the left node.
-  transfer_n(src->count(), finish() + 1, src->start(), src, alloc);
-
-  if (!leaf()) {
-    // Move the child pointers from the right to the left node.
-    for (int i = src->start(), j = finish() + 1; i <= src->finish(); ++i, ++j) {
-      init_child(j, src->child(i));
-      src->clear_child(i);
-    }
-  }
-
-  // Fixup `finish` on the src and dest nodes.
-  set_finish(start() + 1 + count() + src->count());
-  src->set_finish(src->start());
-
-  // Remove the value on the parent node and delete the src node.
-  parent()->remove_values(position(), /*to_erase=*/1, alloc);
-}
-
-template <typename P>
-void btree_node<P>::clear_and_delete(btree_node *node, allocator_type *alloc) {
-  if (node->leaf()) {
-    node->value_destroy_n(node->start(), node->count(), alloc);
-    deallocate(LeafSize(node->max_count()), node, alloc);
-    return;
-  }
-  if (node->count() == 0) {
-    deallocate(InternalSize(), node, alloc);
-    return;
-  }
-
-  // The parent of the root of the subtree we are deleting.
-  btree_node *delete_root_parent = node->parent();
-
-  // Navigate to the leftmost leaf under node, and then delete upwards.
-  while (!node->leaf()) node = node->start_child();
-  // Use `int` because `pos` needs to be able to hold `kNodeValues+1`, which
-  // isn't guaranteed to be a valid `field_type`.
-  int pos = node->position();
-  btree_node *parent = node->parent();
-  for (;;) {
-    // In each iteration of the next loop, we delete one leaf node and go right.
-    assert(pos <= parent->finish());
-    do {
-      node = parent->child(pos);
-      if (!node->leaf()) {
-        // Navigate to the leftmost leaf under node.
-        while (!node->leaf()) node = node->start_child();
-        pos = node->position();
-        parent = node->parent();
-      }
-      node->value_destroy_n(node->start(), node->count(), alloc);
-      deallocate(LeafSize(node->max_count()), node, alloc);
-      ++pos;
-    } while (pos <= parent->finish());
-
-    // Once we've deleted all children of parent, delete parent and go up/right.
-    assert(pos > parent->finish());
-    do {
-      node = parent;
-      pos = node->position();
-      parent = node->parent();
-      node->value_destroy_n(node->start(), node->count(), alloc);
-      deallocate(InternalSize(), node, alloc);
-      if (parent == delete_root_parent) return;
-      ++pos;
-    } while (pos > parent->finish());
-  }
-}
-
-////
-// btree_iterator methods
-template <typename N, typename R, typename P>
-void btree_iterator<N, R, P>::increment_slow() {
-  if (node->leaf()) {
-    assert(position >= node->finish());
-    btree_iterator save(*this);
-    while (position == node->finish() && !node->is_root()) {
-      assert(node->parent()->child(node->position()) == node);
-      position = node->position();
-      node = node->parent();
-    }
-    // TODO(ezb): assert we aren't incrementing end() instead of handling.
-    if (position == node->finish()) {
-      *this = save;
-    }
-  } else {
-    assert(position < node->finish());
-    node = node->child(position + 1);
-    while (!node->leaf()) {
-      node = node->start_child();
-    }
-    position = node->start();
-  }
-}
-
-template <typename N, typename R, typename P>
-void btree_iterator<N, R, P>::decrement_slow() {
-  if (node->leaf()) {
-    assert(position <= -1);
-    btree_iterator save(*this);
-    while (position < node->start() && !node->is_root()) {
-      assert(node->parent()->child(node->position()) == node);
-      position = node->position() - 1;
-      node = node->parent();
-    }
-    // TODO(ezb): assert we aren't decrementing begin() instead of handling.
-    if (position < node->start()) {
-      *this = save;
-    }
-  } else {
-    assert(position >= node->start());
-    node = node->child(position);
-    while (!node->leaf()) {
-      node = node->child(node->finish());
-    }
-    position = node->finish() - 1;
-  }
-}
-
-////
-// btree methods
-template <typename P>
-template <typename Btree>
-void btree<P>::copy_or_move_values_in_order(Btree &other) {
-  static_assert(std::is_same<btree, Btree>::value ||
-                    std::is_same<const btree, Btree>::value,
-                "Btree type must be same or const.");
-  assert(empty());
-
-  // We can avoid key comparisons because we know the order of the
-  // values is the same order we'll store them in.
-  auto iter = other.begin();
-  if (iter == other.end()) return;
-  insert_multi(maybe_move_from_iterator(iter));
-  ++iter;
-  for (; iter != other.end(); ++iter) {
-    // If the btree is not empty, we can just insert the new value at the end
-    // of the tree.
-    internal_emplace(end(), maybe_move_from_iterator(iter));
-  }
-}
-
-template <typename P>
-constexpr bool btree<P>::static_assert_validation() {
-  static_assert(std::is_nothrow_copy_constructible<key_compare>::value,
-                "Key comparison must be nothrow copy constructible");
-  static_assert(std::is_nothrow_copy_constructible<allocator_type>::value,
-                "Allocator must be nothrow copy constructible");
-  static_assert(type_traits_internal::is_trivially_copyable<iterator>::value,
-                "iterator not trivially copyable.");
-
-  // Note: We assert that kTargetValues, which is computed from
-  // Params::kTargetNodeSize, must fit the node_type::field_type.
-  static_assert(
-      kNodeValues < (1 << (8 * sizeof(typename node_type::field_type))),
-      "target node size too large");
-
-  // Verify that key_compare returns an absl::{weak,strong}_ordering or bool.
-  using compare_result_type =
-      absl::result_of_t<key_compare(key_type, key_type)>;
-  static_assert(
-      std::is_same<compare_result_type, bool>::value ||
-          std::is_convertible<compare_result_type, absl::weak_ordering>::value,
-      "key comparison function must return absl::{weak,strong}_ordering or "
-      "bool.");
-
-  // Test the assumption made in setting kNodeValueSpace.
-  static_assert(node_type::MinimumOverhead() >= sizeof(void *) + 4,
-                "node space assumption incorrect");
-
-  return true;
-}
-
-template <typename P>
-template <typename K>
-auto btree<P>::equal_range(const K &key) -> std::pair<iterator, iterator> {
-  const SearchResult<iterator, is_key_compare_to::value> res =
-      internal_lower_bound(key);
-  const iterator lower = internal_end(res.value);
-  if (res.HasMatch() ? !res.IsEq()
-                     : lower == end() || compare_keys(key, lower.key())) {
-    return {lower, lower};
-  }
-
-  const iterator next = std::next(lower);
-  // When the comparator is heterogeneous, we can't assume that comparison with
-  // non-`key_type` will be equivalent to `key_type` comparisons so there
-  // could be multiple equivalent keys even in a unique-container. But for
-  // heterogeneous comparisons from the default string adapted comparators, we
-  // don't need to worry about this.
-  if (!is_multi_container::value &&
-      (std::is_same<K, key_type>::value || is_key_compare_adapted::value)) {
-    // The next iterator after lower must point to a key greater than `key`.
-    // Note: if this assert fails, then it may indicate that the comparator does
-    // not meet the equivalence requirements for Compare
-    // (see https://en.cppreference.com/w/cpp/named_req/Compare).
-    assert(next == end() || compare_keys(key, next.key()));
-    return {lower, next};
-  }
-  // Try once more to avoid the call to upper_bound() if there's only one
-  // equivalent key. This should prevent all calls to upper_bound() in cases of
-  // unique-containers with heterogeneous comparators in which all comparison
-  // operators have the same equivalence classes.
-  if (next == end() || compare_keys(key, next.key())) return {lower, next};
-
-  // In this case, we need to call upper_bound() to avoid worst case O(N)
-  // behavior if we were to iterate over equal keys.
-  return {lower, upper_bound(key)};
-}
-
-template <typename P>
-template <typename K, typename... Args>
-auto btree<P>::insert_unique(const K &key, Args &&... args)
-    -> std::pair<iterator, bool> {
-  if (empty()) {
-    mutable_root() = rightmost_ = new_leaf_root_node(1);
-  }
-
-  SearchResult<iterator, is_key_compare_to::value> res = internal_locate(key);
-  iterator iter = res.value;
-
-  if (res.HasMatch()) {
-    if (res.IsEq()) {
-      // The key already exists in the tree, do nothing.
-      return {iter, false};
-    }
-  } else {
-    iterator last = internal_last(iter);
-    if (last.node && !compare_keys(key, last.key())) {
-      // The key already exists in the tree, do nothing.
-      return {last, false};
-    }
-  }
-  return {internal_emplace(iter, std::forward<Args>(args)...), true};
-}
-
-template <typename P>
-template <typename K, typename... Args>
-inline auto btree<P>::insert_hint_unique(iterator position, const K &key,
-                                         Args &&... args)
-    -> std::pair<iterator, bool> {
-  if (!empty()) {
-    if (position == end() || compare_keys(key, position.key())) {
-      if (position == begin() || compare_keys(std::prev(position).key(), key)) {
-        // prev.key() < key < position.key()
-        return {internal_emplace(position, std::forward<Args>(args)...), true};
-      }
-    } else if (compare_keys(position.key(), key)) {
-      ++position;
-      if (position == end() || compare_keys(key, position.key())) {
-        // {original `position`}.key() < key < {current `position`}.key()
-        return {internal_emplace(position, std::forward<Args>(args)...), true};
-      }
-    } else {
-      // position.key() == key
-      return {position, false};
-    }
-  }
-  return insert_unique(key, std::forward<Args>(args)...);
-}
-
-template <typename P>
-template <typename InputIterator, typename>
-void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e, int) {
-  for (; b != e; ++b) {
-    insert_hint_unique(end(), params_type::key(*b), *b);
-  }
-}
-
-template <typename P>
-template <typename InputIterator>
-void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e, char) {
-  for (; b != e; ++b) {
-    init_type value(*b);
-    insert_hint_unique(end(), params_type::key(value), std::move(value));
-  }
-}
-
-template <typename P>
-template <typename ValueType>
-auto btree<P>::insert_multi(const key_type &key, ValueType &&v) -> iterator {
-  if (empty()) {
-    mutable_root() = rightmost_ = new_leaf_root_node(1);
-  }
-
-  iterator iter = internal_upper_bound(key);
-  if (iter.node == nullptr) {
-    iter = end();
-  }
-  return internal_emplace(iter, std::forward<ValueType>(v));
-}
-
-template <typename P>
-template <typename ValueType>
-auto btree<P>::insert_hint_multi(iterator position, ValueType &&v) -> iterator {
-  if (!empty()) {
-    const key_type &key = params_type::key(v);
-    if (position == end() || !compare_keys(position.key(), key)) {
-      if (position == begin() ||
-          !compare_keys(key, std::prev(position).key())) {
-        // prev.key() <= key <= position.key()
-        return internal_emplace(position, std::forward<ValueType>(v));
-      }
-    } else {
-      ++position;
-      if (position == end() || !compare_keys(position.key(), key)) {
-        // {original `position`}.key() < key < {current `position`}.key()
-        return internal_emplace(position, std::forward<ValueType>(v));
-      }
-    }
-  }
-  return insert_multi(std::forward<ValueType>(v));
-}
-
-template <typename P>
-template <typename InputIterator>
-void btree<P>::insert_iterator_multi(InputIterator b, InputIterator e) {
-  for (; b != e; ++b) {
-    insert_hint_multi(end(), *b);
-  }
-}
-
-template <typename P>
-auto btree<P>::operator=(const btree &other) -> btree & {
-  if (this != &other) {
-    clear();
-
-    *mutable_key_comp() = other.key_comp();
-    if (absl::allocator_traits<
-            allocator_type>::propagate_on_container_copy_assignment::value) {
-      *mutable_allocator() = other.allocator();
-    }
-
-    copy_or_move_values_in_order(other);
-  }
-  return *this;
-}
-
-template <typename P>
-auto btree<P>::operator=(btree &&other) noexcept -> btree & {
-  if (this != &other) {
-    clear();
-
-    using std::swap;
-    if (absl::allocator_traits<
-            allocator_type>::propagate_on_container_copy_assignment::value) {
-      // Note: `root_` also contains the allocator and the key comparator.
-      swap(root_, other.root_);
-      swap(rightmost_, other.rightmost_);
-      swap(size_, other.size_);
-    } else {
-      if (allocator() == other.allocator()) {
-        swap(mutable_root(), other.mutable_root());
-        swap(*mutable_key_comp(), *other.mutable_key_comp());
-        swap(rightmost_, other.rightmost_);
-        swap(size_, other.size_);
-      } else {
-        // We aren't allowed to propagate the allocator and the allocator is
-        // different so we can't take over its memory. We must move each element
-        // individually. We need both `other` and `this` to have `other`s key
-        // comparator while moving the values so we can't swap the key
-        // comparators.
-        *mutable_key_comp() = other.key_comp();
-        copy_or_move_values_in_order(other);
-      }
-    }
-  }
-  return *this;
-}
-
-template <typename P>
-auto btree<P>::erase(iterator iter) -> iterator {
-  bool internal_delete = false;
-  if (!iter.node->leaf()) {
-    // Deletion of a value on an internal node. First, move the largest value
-    // from our left child here, then delete that position (in remove_values()
-    // below). We can get to the largest value from our left child by
-    // decrementing iter.
-    iterator internal_iter(iter);
-    --iter;
-    assert(iter.node->leaf());
-    params_type::move(mutable_allocator(), iter.node->slot(iter.position),
-                      internal_iter.node->slot(internal_iter.position));
-    internal_delete = true;
-  }
-
-  // Delete the key from the leaf.
-  iter.node->remove_values(iter.position, /*to_erase=*/1, mutable_allocator());
-  --size_;
-
-  // We want to return the next value after the one we just erased. If we
-  // erased from an internal node (internal_delete == true), then the next
-  // value is ++(++iter). If we erased from a leaf node (internal_delete ==
-  // false) then the next value is ++iter. Note that ++iter may point to an
-  // internal node and the value in the internal node may move to a leaf node
-  // (iter.node) when rebalancing is performed at the leaf level.
-
-  iterator res = rebalance_after_delete(iter);
-
-  // If we erased from an internal node, advance the iterator.
-  if (internal_delete) {
-    ++res;
-  }
-  return res;
-}
-
-template <typename P>
-auto btree<P>::rebalance_after_delete(iterator iter) -> iterator {
-  // Merge/rebalance as we walk back up the tree.
-  iterator res(iter);
-  bool first_iteration = true;
-  for (;;) {
-    if (iter.node == root()) {
-      try_shrink();
-      if (empty()) {
-        return end();
-      }
-      break;
-    }
-    if (iter.node->count() >= kMinNodeValues) {
-      break;
-    }
-    bool merged = try_merge_or_rebalance(&iter);
-    // On the first iteration, we should update `res` with `iter` because `res`
-    // may have been invalidated.
-    if (first_iteration) {
-      res = iter;
-      first_iteration = false;
-    }
-    if (!merged) {
-      break;
-    }
-    iter.position = iter.node->position();
-    iter.node = iter.node->parent();
-  }
-
-  // Adjust our return value. If we're pointing at the end of a node, advance
-  // the iterator.
-  if (res.position == res.node->finish()) {
-    res.position = res.node->finish() - 1;
-    ++res;
-  }
-
-  return res;
-}
-
-template <typename P>
-auto btree<P>::erase_range(iterator begin, iterator end)
-    -> std::pair<size_type, iterator> {
-  difference_type count = std::distance(begin, end);
-  assert(count >= 0);
-
-  if (count == 0) {
-    return {0, begin};
-  }
-
-  if (count == size_) {
-    clear();
-    return {count, this->end()};
-  }
-
-  if (begin.node == end.node) {
-    assert(end.position > begin.position);
-    begin.node->remove_values(begin.position, end.position - begin.position,
-                              mutable_allocator());
-    size_ -= count;
-    return {count, rebalance_after_delete(begin)};
-  }
-
-  const size_type target_size = size_ - count;
-  while (size_ > target_size) {
-    if (begin.node->leaf()) {
-      const size_type remaining_to_erase = size_ - target_size;
-      const size_type remaining_in_node = begin.node->finish() - begin.position;
-      const size_type to_erase =
-          (std::min)(remaining_to_erase, remaining_in_node);
-      begin.node->remove_values(begin.position, to_erase, mutable_allocator());
-      size_ -= to_erase;
-      begin = rebalance_after_delete(begin);
-    } else {
-      begin = erase(begin);
-    }
-  }
-  return {count, begin};
-}
-
-template <typename P>
-void btree<P>::clear() {
-  if (!empty()) {
-    node_type::clear_and_delete(root(), mutable_allocator());
-  }
-  mutable_root() = EmptyNode();
-  rightmost_ = EmptyNode();
-  size_ = 0;
-}
-
-template <typename P>
-void btree<P>::swap(btree &other) {
-  using std::swap;
-  if (absl::allocator_traits<
-          allocator_type>::propagate_on_container_swap::value) {
-    // Note: `root_` also contains the allocator and the key comparator.
-    swap(root_, other.root_);
-  } else {
-    // It's undefined behavior if the allocators are unequal here.
-    assert(allocator() == other.allocator());
-    swap(mutable_root(), other.mutable_root());
-    swap(*mutable_key_comp(), *other.mutable_key_comp());
-  }
-  swap(rightmost_, other.rightmost_);
-  swap(size_, other.size_);
-}
-
-template <typename P>
-void btree<P>::verify() const {
-  assert(root() != nullptr);
-  assert(leftmost() != nullptr);
-  assert(rightmost_ != nullptr);
-  assert(empty() || size() == internal_verify(root(), nullptr, nullptr));
-  assert(leftmost() == (++const_iterator(root(), -1)).node);
-  assert(rightmost_ == (--const_iterator(root(), root()->finish())).node);
-  assert(leftmost()->leaf());
-  assert(rightmost_->leaf());
-}
-
-template <typename P>
-void btree<P>::rebalance_or_split(iterator *iter) {
-  node_type *&node = iter->node;
-  int &insert_position = iter->position;
-  assert(node->count() == node->max_count());
-  assert(kNodeValues == node->max_count());
-
-  // First try to make room on the node by rebalancing.
-  node_type *parent = node->parent();
-  if (node != root()) {
-    if (node->position() > parent->start()) {
-      // Try rebalancing with our left sibling.
-      node_type *left = parent->child(node->position() - 1);
-      assert(left->max_count() == kNodeValues);
-      if (left->count() < kNodeValues) {
-        // We bias rebalancing based on the position being inserted. If we're
-        // inserting at the end of the right node then we bias rebalancing to
-        // fill up the left node.
-        int to_move = (kNodeValues - left->count()) /
-                      (1 + (insert_position < static_cast<int>(kNodeValues)));
-        to_move = (std::max)(1, to_move);
-
-        if (insert_position - to_move >= node->start() ||
-            left->count() + to_move < static_cast<int>(kNodeValues)) {
-          left->rebalance_right_to_left(to_move, node, mutable_allocator());
-
-          assert(node->max_count() - node->count() == to_move);
-          insert_position = insert_position - to_move;
-          if (insert_position < node->start()) {
-            insert_position = insert_position + left->count() + 1;
-            node = left;
-          }
-
-          assert(node->count() < node->max_count());
-          return;
-        }
-      }
-    }
-
-    if (node->position() < parent->finish()) {
-      // Try rebalancing with our right sibling.
-      node_type *right = parent->child(node->position() + 1);
-      assert(right->max_count() == kNodeValues);
-      if (right->count() < kNodeValues) {
-        // We bias rebalancing based on the position being inserted. If we're
-        // inserting at the beginning of the left node then we bias rebalancing
-        // to fill up the right node.
-        int to_move = (static_cast<int>(kNodeValues) - right->count()) /
-                      (1 + (insert_position > node->start()));
-        to_move = (std::max)(1, to_move);
-
-        if (insert_position <= node->finish() - to_move ||
-            right->count() + to_move < static_cast<int>(kNodeValues)) {
-          node->rebalance_left_to_right(to_move, right, mutable_allocator());
-
-          if (insert_position > node->finish()) {
-            insert_position = insert_position - node->count() - 1;
-            node = right;
-          }
-
-          assert(node->count() < node->max_count());
-          return;
-        }
-      }
-    }
-
-    // Rebalancing failed, make sure there is room on the parent node for a new
-    // value.
-    assert(parent->max_count() == kNodeValues);
-    if (parent->count() == kNodeValues) {
-      iterator parent_iter(node->parent(), node->position());
-      rebalance_or_split(&parent_iter);
-    }
-  } else {
-    // Rebalancing not possible because this is the root node.
-    // Create a new root node and set the current root node as the child of the
-    // new root.
-    parent = new_internal_node(parent);
-    parent->init_child(parent->start(), root());
-    mutable_root() = parent;
-    // If the former root was a leaf node, then it's now the rightmost node.
-    assert(!parent->start_child()->leaf() ||
-           parent->start_child() == rightmost_);
-  }
-
-  // Split the node.
-  node_type *split_node;
-  if (node->leaf()) {
-    split_node = new_leaf_node(parent);
-    node->split(insert_position, split_node, mutable_allocator());
-    if (rightmost_ == node) rightmost_ = split_node;
-  } else {
-    split_node = new_internal_node(parent);
-    node->split(insert_position, split_node, mutable_allocator());
-  }
-
-  if (insert_position > node->finish()) {
-    insert_position = insert_position - node->count() - 1;
-    node = split_node;
-  }
-}
-
-template <typename P>
-void btree<P>::merge_nodes(node_type *left, node_type *right) {
-  left->merge(right, mutable_allocator());
-  if (rightmost_ == right) rightmost_ = left;
-}
-
-template <typename P>
-bool btree<P>::try_merge_or_rebalance(iterator *iter) {
-  node_type *parent = iter->node->parent();
-  if (iter->node->position() > parent->start()) {
-    // Try merging with our left sibling.
-    node_type *left = parent->child(iter->node->position() - 1);
-    assert(left->max_count() == kNodeValues);
-    if (1U + left->count() + iter->node->count() <= kNodeValues) {
-      iter->position += 1 + left->count();
-      merge_nodes(left, iter->node);
-      iter->node = left;
-      return true;
-    }
-  }
-  if (iter->node->position() < parent->finish()) {
-    // Try merging with our right sibling.
-    node_type *right = parent->child(iter->node->position() + 1);
-    assert(right->max_count() == kNodeValues);
-    if (1U + iter->node->count() + right->count() <= kNodeValues) {
-      merge_nodes(iter->node, right);
-      return true;
-    }
-    // Try rebalancing with our right sibling. We don't perform rebalancing if
-    // we deleted the first element from iter->node and the node is not
-    // empty. This is a small optimization for the common pattern of deleting
-    // from the front of the tree.
-    if (right->count() > kMinNodeValues &&
-        (iter->node->count() == 0 || iter->position > iter->node->start())) {
-      int to_move = (right->count() - iter->node->count()) / 2;
-      to_move = (std::min)(to_move, right->count() - 1);
-      iter->node->rebalance_right_to_left(to_move, right, mutable_allocator());
-      return false;
-    }
-  }
-  if (iter->node->position() > parent->start()) {
-    // Try rebalancing with our left sibling. We don't perform rebalancing if
-    // we deleted the last element from iter->node and the node is not
-    // empty. This is a small optimization for the common pattern of deleting
-    // from the back of the tree.
-    node_type *left = parent->child(iter->node->position() - 1);
-    if (left->count() > kMinNodeValues &&
-        (iter->node->count() == 0 || iter->position < iter->node->finish())) {
-      int to_move = (left->count() - iter->node->count()) / 2;
-      to_move = (std::min)(to_move, left->count() - 1);
-      left->rebalance_left_to_right(to_move, iter->node, mutable_allocator());
-      iter->position += to_move;
-      return false;
-    }
-  }
-  return false;
-}
-
-template <typename P>
-void btree<P>::try_shrink() {
-  node_type *orig_root = root();
-  if (orig_root->count() > 0) {
-    return;
-  }
-  // Deleted the last item on the root node, shrink the height of the tree.
-  if (orig_root->leaf()) {
-    assert(size() == 0);
-    mutable_root() = rightmost_ = EmptyNode();
-  } else {
-    node_type *child = orig_root->start_child();
-    child->make_root();
-    mutable_root() = child;
-  }
-  node_type::clear_and_delete(orig_root, mutable_allocator());
-}
-
-template <typename P>
-template <typename IterType>
-inline IterType btree<P>::internal_last(IterType iter) {
-  assert(iter.node != nullptr);
-  while (iter.position == iter.node->finish()) {
-    iter.position = iter.node->position();
-    iter.node = iter.node->parent();
-    if (iter.node->leaf()) {
-      iter.node = nullptr;
-      break;
-    }
-  }
-  return iter;
-}
-
-template <typename P>
-template <typename... Args>
-inline auto btree<P>::internal_emplace(iterator iter, Args &&... args)
-    -> iterator {
-  if (!iter.node->leaf()) {
-    // We can't insert on an internal node. Instead, we'll insert after the
-    // previous value which is guaranteed to be on a leaf node.
-    --iter;
-    ++iter.position;
-  }
-  const field_type max_count = iter.node->max_count();
-  allocator_type *alloc = mutable_allocator();
-  if (iter.node->count() == max_count) {
-    // Make room in the leaf for the new item.
-    if (max_count < kNodeValues) {
-      // Insertion into the root where the root is smaller than the full node
-      // size. Simply grow the size of the root node.
-      assert(iter.node == root());
-      iter.node =
-          new_leaf_root_node((std::min<int>)(kNodeValues, 2 * max_count));
-      // Transfer the values from the old root to the new root.
-      node_type *old_root = root();
-      node_type *new_root = iter.node;
-      new_root->transfer_n(old_root->count(), new_root->start(),
-                           old_root->start(), old_root, alloc);
-      new_root->set_finish(old_root->finish());
-      old_root->set_finish(old_root->start());
-      node_type::clear_and_delete(old_root, alloc);
-      mutable_root() = rightmost_ = new_root;
-    } else {
-      rebalance_or_split(&iter);
-    }
-  }
-  iter.node->emplace_value(iter.position, alloc, std::forward<Args>(args)...);
-  ++size_;
-  return iter;
-}
-
-template <typename P>
-template <typename K>
-inline auto btree<P>::internal_locate(const K &key) const
-    -> SearchResult<iterator, is_key_compare_to::value> {
-  iterator iter(const_cast<node_type *>(root()));
-  for (;;) {
-    SearchResult<int, is_key_compare_to::value> res =
-        iter.node->lower_bound(key, key_comp());
-    iter.position = res.value;
-    if (res.IsEq()) {
-      return {iter, MatchKind::kEq};
-    }
-    // Note: in the non-key-compare-to case, we don't need to walk all the way
-    // down the tree if the keys are equal, but determining equality would
-    // require doing an extra comparison on each node on the way down, and we
-    // will need to go all the way to the leaf node in the expected case.
-    if (iter.node->leaf()) {
-      break;
-    }
-    iter.node = iter.node->child(iter.position);
-  }
-  // Note: in the non-key-compare-to case, the key may actually be equivalent
-  // here (and the MatchKind::kNe is ignored).
-  return {iter, MatchKind::kNe};
-}
-
-template <typename P>
-template <typename K>
-auto btree<P>::internal_lower_bound(const K &key) const
-    -> SearchResult<iterator, is_key_compare_to::value> {
-  iterator iter(const_cast<node_type *>(root()));
-  SearchResult<int, is_key_compare_to::value> res;
-  bool seen_eq = false;
-  for (;;) {
-    res = iter.node->lower_bound(key, key_comp());
-    iter.position = res.value;
-    // TODO(ezb): we should be able to terminate early on IsEq() if there can't
-    // be multiple equivalent keys in container for this lookup type.
-    if (iter.node->leaf()) {
-      break;
-    }
-    seen_eq = seen_eq || res.IsEq();
-    iter.node = iter.node->child(iter.position);
-  }
-  if (res.IsEq()) return {iter, MatchKind::kEq};
-  return {internal_last(iter), seen_eq ? MatchKind::kEq : MatchKind::kNe};
-}
-
-template <typename P>
-template <typename K>
-auto btree<P>::internal_upper_bound(const K &key) const -> iterator {
-  iterator iter(const_cast<node_type *>(root()));
-  for (;;) {
-    iter.position = iter.node->upper_bound(key, key_comp());
-    if (iter.node->leaf()) {
-      break;
-    }
-    iter.node = iter.node->child(iter.position);
-  }
-  return internal_last(iter);
-}
-
-template <typename P>
-template <typename K>
-auto btree<P>::internal_find(const K &key) const -> iterator {
-  SearchResult<iterator, is_key_compare_to::value> res = internal_locate(key);
-  if (res.HasMatch()) {
-    if (res.IsEq()) {
-      return res.value;
-    }
-  } else {
-    const iterator iter = internal_last(res.value);
-    if (iter.node != nullptr && !compare_keys(key, iter.key())) {
-      return iter;
-    }
-  }
-  return {nullptr, 0};
-}
-
-template <typename P>
-int btree<P>::internal_verify(const node_type *node, const key_type *lo,
-                              const key_type *hi) const {
-  assert(node->count() > 0);
-  assert(node->count() <= node->max_count());
-  if (lo) {
-    assert(!compare_keys(node->key(node->start()), *lo));
-  }
-  if (hi) {
-    assert(!compare_keys(*hi, node->key(node->finish() - 1)));
-  }
-  for (int i = node->start() + 1; i < node->finish(); ++i) {
-    assert(!compare_keys(node->key(i), node->key(i - 1)));
-  }
-  int count = node->count();
-  if (!node->leaf()) {
-    for (int i = node->start(); i <= node->finish(); ++i) {
-      assert(node->child(i) != nullptr);
-      assert(node->child(i)->parent() == node);
-      assert(node->child(i)->position() == i);
-      count += internal_verify(node->child(i),
-                               i == node->start() ? lo : &node->key(i - 1),
-                               i == node->finish() ? hi : &node->key(i));
-    }
-  }
-  return count;
-}
-
-}  // namespace container_internal
-ABSL_NAMESPACE_END
-}  // namespace absl
-
-#endif  // ABSL_CONTAINER_INTERNAL_BTREE_H_