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diff --git a/xdelta3/cpp-btree/btree.h b/xdelta3/cpp-btree/btree.h
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+// Copyright 2013 Google Inc. All Rights Reserved.
+//
+// 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
+//
+//     http://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 both
+// smaller and faster than STL set/map. 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> consumes 20
+// bytes for each value stored. 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> may use much
+// less memory per stored value. For the random insertion benchmark in
+// btree_test.cc, a btree_set<int32> with node-size of 256 uses 4.9 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. 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).  See also safe_btree.
+
+// PERFORMANCE
+//
+//   btree_bench --benchmarks=. 2>&1 | ./benchmarks.awk
+//
+// Run on pmattis-warp.nyc (4 X 2200 MHz CPUs); 2010/03/04-15:23:06
+// Benchmark                 STL(ns) B-Tree(ns) @    <size>
+// --------------------------------------------------------
+// BM_set_int32_insert        1516      608  +59.89%  <256>    [40.0,  5.2]
+// BM_set_int32_lookup        1160      414  +64.31%  <256>    [40.0,  5.2]
+// BM_set_int32_fulllookup     960      410  +57.29%  <256>    [40.0,  4.4]
+// BM_set_int32_delete        1741      528  +69.67%  <256>    [40.0,  5.2]
+// BM_set_int32_queueaddrem   3078     1046  +66.02%  <256>    [40.0,  5.5]
+// BM_set_int32_mixedaddrem   3600     1384  +61.56%  <256>    [40.0,  5.3]
+// BM_set_int32_fifo           227      113  +50.22%  <256>    [40.0,  4.4]
+// BM_set_int32_fwditer        158       26  +83.54%  <256>    [40.0,  5.2]
+// BM_map_int32_insert        1551      636  +58.99%  <256>    [48.0, 10.5]
+// BM_map_int32_lookup        1200      508  +57.67%  <256>    [48.0, 10.5]
+// BM_map_int32_fulllookup     989      487  +50.76%  <256>    [48.0,  8.8]
+// BM_map_int32_delete        1794      628  +64.99%  <256>    [48.0, 10.5]
+// BM_map_int32_queueaddrem   3189     1266  +60.30%  <256>    [48.0, 11.6]
+// BM_map_int32_mixedaddrem   3822     1623  +57.54%  <256>    [48.0, 10.9]
+// BM_map_int32_fifo           151      134  +11.26%  <256>    [48.0,  8.8]
+// BM_map_int32_fwditer        161       32  +80.12%  <256>    [48.0, 10.5]
+// BM_set_int64_insert        1546      636  +58.86%  <256>    [40.0, 10.5]
+// BM_set_int64_lookup        1200      512  +57.33%  <256>    [40.0, 10.5]
+// BM_set_int64_fulllookup     971      487  +49.85%  <256>    [40.0,  8.8]
+// BM_set_int64_delete        1745      616  +64.70%  <256>    [40.0, 10.5]
+// BM_set_int64_queueaddrem   3163     1195  +62.22%  <256>    [40.0, 11.6]
+// BM_set_int64_mixedaddrem   3760     1564  +58.40%  <256>    [40.0, 10.9]
+// BM_set_int64_fifo           146      103  +29.45%  <256>    [40.0,  8.8]
+// BM_set_int64_fwditer        162       31  +80.86%  <256>    [40.0, 10.5]
+// BM_map_int64_insert        1551      720  +53.58%  <256>    [48.0, 20.7]
+// BM_map_int64_lookup        1214      612  +49.59%  <256>    [48.0, 20.7]
+// BM_map_int64_fulllookup     994      592  +40.44%  <256>    [48.0, 17.2]
+// BM_map_int64_delete        1778      764  +57.03%  <256>    [48.0, 20.7]
+// BM_map_int64_queueaddrem   3189     1547  +51.49%  <256>    [48.0, 20.9]
+// BM_map_int64_mixedaddrem   3779     1887  +50.07%  <256>    [48.0, 21.6]
+// BM_map_int64_fifo           147      145   +1.36%  <256>    [48.0, 17.2]
+// BM_map_int64_fwditer        162       41  +74.69%  <256>    [48.0, 20.7]
+// BM_set_string_insert       1989     1966   +1.16%  <256>    [64.0, 44.5]
+// BM_set_string_lookup       1709     1600   +6.38%  <256>    [64.0, 44.5]
+// BM_set_string_fulllookup   1573     1529   +2.80%  <256>    [64.0, 35.4]
+// BM_set_string_delete       2520     1920  +23.81%  <256>    [64.0, 44.5]
+// BM_set_string_queueaddrem  4706     4309   +8.44%  <256>    [64.0, 48.3]
+// BM_set_string_mixedaddrem  5080     4654   +8.39%  <256>    [64.0, 46.7]
+// BM_set_string_fifo          318      512  -61.01%  <256>    [64.0, 35.4]
+// BM_set_string_fwditer       182       93  +48.90%  <256>    [64.0, 44.5]
+// BM_map_string_insert       2600     2227  +14.35%  <256>    [72.0, 55.8]
+// BM_map_string_lookup       2068     1730  +16.34%  <256>    [72.0, 55.8]
+// BM_map_string_fulllookup   1859     1618  +12.96%  <256>    [72.0, 44.0]
+// BM_map_string_delete       3168     2080  +34.34%  <256>    [72.0, 55.8]
+// BM_map_string_queueaddrem  5840     4701  +19.50%  <256>    [72.0, 59.4]
+// BM_map_string_mixedaddrem  6400     5200  +18.75%  <256>    [72.0, 57.8]
+// BM_map_string_fifo          398      596  -49.75%  <256>    [72.0, 44.0]
+// BM_map_string_fwditer       243      113  +53.50%  <256>    [72.0, 55.8]
+
+#ifndef UTIL_BTREE_BTREE_H__
+#define UTIL_BTREE_BTREE_H__
+
+#include <assert.h>
+#include <stddef.h>
+#include <string.h>
+#include <sys/types.h>
+#include <algorithm>
+#include <functional>
+#include <iostream>
+#include <iterator>
+#include <limits>
+#include <type_traits>
+#include <new>
+#include <ostream>
+#include <string>
+#include <utility>
+
+#ifndef NDEBUG
+#define NDEBUG 1
+#endif
+
+namespace btree {
+
+// Inside a btree method, if we just call swap(), it will choose the
+// btree::swap method, which we don't want. And we can't say ::swap
+// because then MSVC won't pickup any std::swap() implementations. We
+// can't just use std::swap() directly because then we don't get the
+// specialization for types outside the std namespace. So the solution
+// is to have a special swap helper function whose name doesn't
+// collide with other swap functions defined by the btree classes.
+template <typename T>
+inline void btree_swap_helper(T &a, T &b) {
+  using std::swap;
+  swap(a, b);
+}
+
+// A template helper used to select A or B based on a condition.
+template<bool cond, typename A, typename B>
+struct if_{
+  typedef A type;
+};
+
+template<typename A, typename B>
+struct if_<false, A, B> {
+  typedef B type;
+};
+
+// Types small_ and big_ are promise that sizeof(small_) < sizeof(big_)
+typedef char small_;
+
+struct big_ {
+  char dummy[2];
+};
+
+// A compile-time assertion.
+template <bool>
+struct CompileAssert {
+};
+
+#define COMPILE_ASSERT(expr, msg) \
+  typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1]
+
+// A helper type used to indicate that a key-compare-to functor has been
+// provided. A user can specify a key-compare-to functor by doing:
+//
+//  struct MyStringComparer
+//      : public util::btree::btree_key_compare_to_tag {
+//    int operator()(const string &a, const string &b) const {
+//      return a.compare(b);
+//    }
+//  };
+//
+// Note that the return type is an int and not a bool. There is a
+// COMPILE_ASSERT which enforces this return type.
+struct btree_key_compare_to_tag {
+};
+
+// A helper class that indicates if the Compare parameter is derived from
+// btree_key_compare_to_tag.
+template <typename Compare>
+struct btree_is_key_compare_to
+    : public std::is_convertible<Compare, btree_key_compare_to_tag> {
+};
+
+// A helper class to convert a boolean comparison into a three-way
+// "compare-to" comparison that returns a negative value to indicate
+// less-than, zero to indicate equality and a positive value to
+// indicate greater-than. This helper class is specialized for
+// less<string> and greater<string>. The btree_key_compare_to_adapter
+// class is provided so that btree users automatically get the more
+// efficient compare-to code when using common google string types
+// with common comparison functors.
+template <typename Compare>
+struct btree_key_compare_to_adapter : Compare {
+  btree_key_compare_to_adapter() { }
+  btree_key_compare_to_adapter(const Compare &c) : Compare(c) { }
+  btree_key_compare_to_adapter(const btree_key_compare_to_adapter<Compare> &c)
+      : Compare(c) {
+  }
+};
+
+template <>
+struct btree_key_compare_to_adapter<std::less<std::string> >
+    : public btree_key_compare_to_tag {
+  btree_key_compare_to_adapter() {}
+  btree_key_compare_to_adapter(const std::less<std::string>&) {}
+  btree_key_compare_to_adapter(
+      const btree_key_compare_to_adapter<std::less<std::string> >&) {}
+  int operator()(const std::string &a, const std::string &b) const {
+    return a.compare(b);
+  }
+};
+
+template <>
+struct btree_key_compare_to_adapter<std::greater<std::string> >
+    : public btree_key_compare_to_tag {
+  btree_key_compare_to_adapter() {}
+  btree_key_compare_to_adapter(const std::greater<std::string>&) {}
+  btree_key_compare_to_adapter(
+      const btree_key_compare_to_adapter<std::greater<std::string> >&) {}
+  int operator()(const std::string &a, const std::string &b) const {
+    return b.compare(a);
+  }
+};
+
+// A helper class that allows a compare-to functor to behave like a plain
+// compare functor. This specialization is used when we do not have a
+// compare-to functor.
+template <typename Key, typename Compare, bool HaveCompareTo>
+struct btree_key_comparer {
+  btree_key_comparer() {}
+  btree_key_comparer(Compare c) : comp(c) {}
+  static bool bool_compare(const Compare &comp, const Key &x, const Key &y) {
+    return comp(x, y);
+  }
+  bool operator()(const Key &x, const Key &y) const {
+    return bool_compare(comp, x, y);
+  }
+  Compare comp;
+};
+
+// A specialization of btree_key_comparer when a compare-to functor is
+// present. We need a plain (boolean) comparison in some parts of the btree
+// code, such as insert-with-hint.
+template <typename Key, typename Compare>
+struct btree_key_comparer<Key, Compare, true> {
+  btree_key_comparer() {}
+  btree_key_comparer(Compare c) : comp(c) {}
+  static bool bool_compare(const Compare &comp, const Key &x, const Key &y) {
+    return comp(x, y) < 0;
+  }
+  bool operator()(const Key &x, const Key &y) const {
+    return bool_compare(comp, x, y);
+  }
+  Compare comp;
+};
+
+// A helper function to compare to keys using the specified compare
+// functor. This dispatches to the appropriate btree_key_comparer comparison,
+// depending on whether we have a compare-to functor or not (which depends on
+// whether Compare is derived from btree_key_compare_to_tag).
+template <typename Key, typename Compare>
+static bool btree_compare_keys(
+    const Compare &comp, const Key &x, const Key &y) {
+  typedef btree_key_comparer<Key, Compare,
+      btree_is_key_compare_to<Compare>::value> key_comparer;
+  return key_comparer::bool_compare(comp, x, y);
+}
+
+template <typename Key, typename Compare,
+          typename Alloc, int TargetNodeSize, int ValueSize>
+struct btree_common_params {
+  // If Compare is derived from btree_key_compare_to_tag then use it as the
+  // key_compare type. Otherwise, use btree_key_compare_to_adapter<> which will
+  // fall-back to Compare if we don't have an appropriate specialization.
+  typedef typename if_<
+    btree_is_key_compare_to<Compare>::value,
+    Compare, btree_key_compare_to_adapter<Compare> >::type key_compare;
+  // A type which indicates if we have a key-compare-to functor or a plain old
+  // key-compare functor.
+  typedef btree_is_key_compare_to<key_compare> is_key_compare_to;
+
+  typedef Alloc allocator_type;
+  typedef Key key_type;
+  typedef ssize_t size_type;
+  typedef ptrdiff_t difference_type;
+
+  enum {
+    kTargetNodeSize = TargetNodeSize,
+
+    // Available space for values.  This is largest for leaf nodes,
+    // which has overhead no fewer than two pointers.
+    kNodeValueSpace = TargetNodeSize - 2 * sizeof(void*),
+  };
+
+  // This is an integral type large enough to hold as many
+  // ValueSize-values as will fit a node of TargetNodeSize bytes.
+  typedef typename if_<
+    (kNodeValueSpace / ValueSize) >= 256,
+    uint16_t,
+    uint8_t>::type node_count_type;
+};
+
+// A parameters structure for holding the type parameters for a btree_map.
+template <typename Key, typename Data, typename Compare,
+          typename Alloc, int TargetNodeSize>
+struct btree_map_params
+    : public btree_common_params<Key, Compare, Alloc, TargetNodeSize,
+                                 sizeof(Key) + sizeof(Data)> {
+  typedef Data data_type;
+  typedef Data mapped_type;
+  typedef std::pair<const Key, data_type> value_type;
+  typedef std::pair<Key, data_type> mutable_value_type;
+  typedef value_type* pointer;
+  typedef const value_type* const_pointer;
+  typedef value_type& reference;
+  typedef const value_type& const_reference;
+
+  enum {
+    kValueSize = sizeof(Key) + sizeof(data_type),
+  };
+
+  static const Key& key(const value_type &x) { return x.first; }
+  static const Key& key(const mutable_value_type &x) { return x.first; }
+  static void swap(mutable_value_type *a, mutable_value_type *b) {
+    btree_swap_helper(a->first, b->first);
+    btree_swap_helper(a->second, b->second);
+  }
+};
+
+// A parameters structure for holding the type parameters for a btree_set.
+template <typename Key, typename Compare, typename Alloc, int TargetNodeSize>
+struct btree_set_params
+    : public btree_common_params<Key, Compare, Alloc, TargetNodeSize,
+                                 sizeof(Key)> {
+  typedef std::false_type data_type;
+  typedef std::false_type mapped_type;
+  typedef Key value_type;
+  typedef value_type mutable_value_type;
+  typedef value_type* pointer;
+  typedef const value_type* const_pointer;
+  typedef value_type& reference;
+  typedef const value_type& const_reference;
+
+  enum {
+    kValueSize = sizeof(Key),
+  };
+
+  static const Key& key(const value_type &x) { return x; }
+  static void swap(mutable_value_type *a, mutable_value_type *b) {
+    btree_swap_helper<mutable_value_type>(*a, *b);
+  }
+};
+
+// An adapter class that converts a lower-bound compare into an upper-bound
+// compare.
+template <typename Key, typename Compare>
+struct btree_upper_bound_adapter : public Compare {
+  btree_upper_bound_adapter(Compare c) : Compare(c) {}
+  bool operator()(const Key &a, const Key &b) const {
+    return !static_cast<const Compare&>(*this)(b, a);
+  }
+};
+
+template <typename Key, typename CompareTo>
+struct btree_upper_bound_compare_to_adapter : public CompareTo {
+  btree_upper_bound_compare_to_adapter(CompareTo c) : CompareTo(c) {}
+  int operator()(const Key &a, const Key &b) const {
+    return static_cast<const CompareTo&>(*this)(b, a);
+  }
+};
+
+// Dispatch helper class for using linear search with plain compare.
+template <typename K, typename N, typename Compare>
+struct btree_linear_search_plain_compare {
+  static int lower_bound(const K &k, const N &n, Compare comp)  {
+    return n.linear_search_plain_compare(k, 0, n.count(), comp);
+  }
+  static int upper_bound(const K &k, const N &n, Compare comp)  {
+    typedef btree_upper_bound_adapter<K, Compare> upper_compare;
+    return n.linear_search_plain_compare(k, 0, n.count(), upper_compare(comp));
+  }
+};
+
+// Dispatch helper class for using linear search with compare-to
+template <typename K, typename N, typename CompareTo>
+struct btree_linear_search_compare_to {
+  static int lower_bound(const K &k, const N &n, CompareTo comp)  {
+    return n.linear_search_compare_to(k, 0, n.count(), comp);
+  }
+  static int upper_bound(const K &k, const N &n, CompareTo comp)  {
+    typedef btree_upper_bound_adapter<K,
+        btree_key_comparer<K, CompareTo, true> > upper_compare;
+    return n.linear_search_plain_compare(k, 0, n.count(), upper_compare(comp));
+  }
+};
+
+// Dispatch helper class for using binary search with plain compare.
+template <typename K, typename N, typename Compare>
+struct btree_binary_search_plain_compare {
+  static int lower_bound(const K &k, const N &n, Compare comp)  {
+    return n.binary_search_plain_compare(k, 0, n.count(), comp);
+  }
+  static int upper_bound(const K &k, const N &n, Compare comp)  {
+    typedef btree_upper_bound_adapter<K, Compare> upper_compare;
+    return n.binary_search_plain_compare(k, 0, n.count(), upper_compare(comp));
+  }
+};
+
+// Dispatch helper class for using binary search with compare-to.
+template <typename K, typename N, typename CompareTo>
+struct btree_binary_search_compare_to {
+  static int lower_bound(const K &k, const N &n, CompareTo comp)  {
+    return n.binary_search_compare_to(k, 0, n.count(), CompareTo());
+  }
+  static int upper_bound(const K &k, const N &n, CompareTo comp)  {
+    typedef btree_upper_bound_adapter<K,
+        btree_key_comparer<K, CompareTo, true> > upper_compare;
+    return n.linear_search_plain_compare(k, 0, n.count(), upper_compare(comp));
+  }
+};
+
+// 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 {
+ public:
+  typedef Params params_type;
+  typedef btree_node<Params> self_type;
+  typedef typename Params::key_type key_type;
+  typedef typename Params::data_type data_type;
+  typedef typename Params::value_type value_type;
+  typedef typename Params::mutable_value_type mutable_value_type;
+  typedef typename Params::pointer pointer;
+  typedef typename Params::const_pointer const_pointer;
+  typedef typename Params::reference reference;
+  typedef typename Params::const_reference const_reference;
+  typedef typename Params::key_compare key_compare;
+  typedef typename Params::size_type size_type;
+  typedef typename Params::difference_type difference_type;
+  // Typedefs for the various types of node searches.
+  typedef btree_linear_search_plain_compare<
+    key_type, self_type, key_compare> linear_search_plain_compare_type;
+  typedef btree_linear_search_compare_to<
+    key_type, self_type, key_compare> linear_search_compare_to_type;
+  typedef btree_binary_search_plain_compare<
+    key_type, self_type, key_compare> binary_search_plain_compare_type;
+  typedef btree_binary_search_compare_to<
+    key_type, self_type, key_compare> binary_search_compare_to_type;
+  // If we have a valid key-compare-to type, use linear_search_compare_to,
+  // otherwise use linear_search_plain_compare.
+  typedef typename if_<
+    Params::is_key_compare_to::value,
+    linear_search_compare_to_type,
+    linear_search_plain_compare_type>::type linear_search_type;
+  // If we have a valid key-compare-to type, use binary_search_compare_to,
+  // otherwise use binary_search_plain_compare.
+  typedef typename if_<
+    Params::is_key_compare_to::value,
+    binary_search_compare_to_type,
+    binary_search_plain_compare_type>::type binary_search_type;
+  // If the key is an integral or floating point type, use linear search which
+  // is faster than binary search for such types. Might be wise to also
+  // configure linear search based on node-size.
+  typedef typename if_<
+    std::is_integral<key_type>::value ||
+    std::is_floating_point<key_type>::value,
+    linear_search_type, binary_search_type>::type search_type;
+
+  struct base_fields {
+    typedef typename Params::node_count_type field_type;
+
+    // A boolean indicating whether the node is a leaf or not.
+    bool leaf;
+    // The position of the node in the node's parent.
+    field_type position;
+    // The maximum number of values the node can hold.
+    field_type max_count;
+    // The count of the number of values in the node.
+    field_type count;
+    // A pointer to the node's parent.
+    btree_node *parent;
+  };
+
+  enum {
+    kValueSize = params_type::kValueSize,
+    kTargetNodeSize = params_type::kTargetNodeSize,
+
+    // Compute how many values we can fit onto a leaf node.
+    kNodeTargetValues = (kTargetNodeSize - sizeof(base_fields)) / kValueSize,
+    // 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,
+
+    kExactMatch = 1 << 30,
+    kMatchMask = kExactMatch - 1,
+  };
+
+  struct leaf_fields : public base_fields {
+    // The array of values. Only the first count of these values have been
+    // constructed and are valid.
+    mutable_value_type values[kNodeValues];
+  };
+
+  struct internal_fields : public leaf_fields {
+    // 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 always count + 1 children.
+    btree_node *children[kNodeValues + 1];
+  };
+
+  struct root_fields : public internal_fields {
+    btree_node *rightmost;
+    size_type size;
+  };
+
+ public:
+  // Getter/setter for whether this is a leaf node or not. This value doesn't
+  // change after the node is created.
+  bool leaf() const { return fields_.leaf; }
+
+  // Getter for the position of this node in its parent.
+  int position() const { return fields_.position; }
+  void set_position(int v) { fields_.position = v; }
+
+  // Getter/setter for the number of values stored in this node.
+  int count() const { return fields_.count; }
+  void set_count(int v) { fields_.count = v; }
+  int max_count() const { return fields_.max_count; }
+
+  // Getter for the parent of this node.
+  btree_node* parent() const { return fields_.parent; }
+  // 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());
+    fields_.parent = fields_.parent->parent();
+  }
+
+  // Getter for the rightmost root node field. Only valid on the root node.
+  btree_node* rightmost() const { return fields_.rightmost; }
+  btree_node** mutable_rightmost() { return &fields_.rightmost; }
+
+  // Getter for the size root node field. Only valid on the root node.
+  size_type size() const { return fields_.size; }
+  size_type* mutable_size() { return &fields_.size; }
+
+  // Getters for the key/value at position i in the node.
+  const key_type& key(int i) const {
+    return params_type::key(fields_.values[i]);
+  }
+  reference value(int i) {
+    return reinterpret_cast<reference>(fields_.values[i]);
+  }
+  const_reference value(int i) const {
+    return reinterpret_cast<const_reference>(fields_.values[i]);
+  }
+  mutable_value_type* mutable_value(int i) {
+    return &fields_.values[i];
+  }
+
+  // Swap value i in this node with value j in node x.
+  void value_swap(int i, btree_node *x, int j) {
+    params_type::swap(mutable_value(i), x->mutable_value(j));
+  }
+
+  // Getters/setter for the child at position i in the node.
+  btree_node* child(int i) const { return fields_.children[i]; }
+  btree_node** mutable_child(int i) { return &fields_.children[i]; }
+  void set_child(int i, btree_node *c) {
+    *mutable_child(i) = c;
+    c->fields_.parent = this;
+    c->fields_.position = i;
+  }
+
+  // Returns the position of the first value whose key is not less than k.
+  template <typename Compare>
+  int lower_bound(const key_type &k, const Compare &comp) const {
+    return search_type::lower_bound(k, *this, comp);
+  }
+  // Returns the position of the first value whose key is greater than k.
+  template <typename Compare>
+  int upper_bound(const key_type &k, const Compare &comp) const {
+    return search_type::upper_bound(k, *this, comp);
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // linear search performed using plain compare.
+  template <typename Compare>
+  int linear_search_plain_compare(
+      const key_type &k, int s, int e, const Compare &comp) const {
+    while (s < e) {
+      if (!btree_compare_keys(comp, key(s), k)) {
+        break;
+      }
+      ++s;
+    }
+    return s;
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // linear search performed using compare-to.
+  template <typename Compare>
+  int linear_search_compare_to(
+      const key_type &k, int s, int e, const Compare &comp) const {
+    while (s < e) {
+      int c = comp(key(s), k);
+      if (c == 0) {
+        return s | kExactMatch;
+      } else if (c > 0) {
+        break;
+      }
+      ++s;
+    }
+    return s;
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // binary search performed using plain compare.
+  template <typename Compare>
+  int binary_search_plain_compare(
+      const key_type &k, int s, int e, const Compare &comp) const {
+    while (s != e) {
+      int mid = (s + e) / 2;
+      if (btree_compare_keys(comp, key(mid), k)) {
+        s = mid + 1;
+      } else {
+        e = mid;
+      }
+    }
+    return s;
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // binary search performed using compare-to.
+  template <typename CompareTo>
+  int binary_search_compare_to(
+      const key_type &k, int s, int e, const CompareTo &comp) const {
+    while (s != e) {
+      int mid = (s + e) / 2;
+      int c = comp(key(mid), k);
+      if (c < 0) {
+        s = mid + 1;
+      } else if (c > 0) {
+        e = mid;
+      } else {
+        // Need to return the first value whose key is not less than k, which
+        // requires continuing the binary search. Note that we are guaranteed
+        // that the result is an exact match because if "key(mid-1) < k" the
+        // call to binary_search_compare_to() will return "mid".
+        s = binary_search_compare_to(k, s, mid, comp);
+        return s | kExactMatch;
+      }
+    }
+    return s;
+  }
+
+  // Inserts the value x at position i, shifting all existing values and
+  // children at positions >= i to the right by 1.
+  void insert_value(int i, const value_type &x);
+
+  // Removes the value at position i, shifting all existing values and children
+  // at positions > i to the left by 1.
+  void remove_value(int i);
+
+  // Rebalances a node with its right sibling.
+  void rebalance_right_to_left(btree_node *sibling, int to_move);
+  void rebalance_left_to_right(btree_node *sibling, int to_move);
+
+  // Splits a node, moving a portion of the node's values to its right sibling.
+  void split(btree_node *sibling, int insert_position);
+
+  // Merges a node with its right sibling, moving all of the values and the
+  // delimiting key in the parent node onto itself.
+  void merge(btree_node *sibling);
+
+  // Swap the contents of "this" and "src".
+  void swap(btree_node *src);
+
+  // Node allocation/deletion routines.
+  static btree_node* init_leaf(
+      leaf_fields *f, btree_node *parent, int max_count) {
+    btree_node *n = reinterpret_cast<btree_node*>(f);
+    f->leaf = 1;
+    f->position = 0;
+    f->max_count = max_count;
+    f->count = 0;
+    f->parent = parent;
+    if (!NDEBUG) {
+      memset(&f->values, 0, max_count * sizeof(value_type));
+    }
+    return n;
+  }
+  static btree_node* init_internal(internal_fields *f, btree_node *parent) {
+    btree_node *n = init_leaf(f, parent, kNodeValues);
+    f->leaf = 0;
+    if (!NDEBUG) {
+      memset(f->children, 0, sizeof(f->children));
+    }
+    return n;
+  }
+  static btree_node* init_root(root_fields *f, btree_node *parent) {
+    btree_node *n = init_internal(f, parent);
+    f->rightmost = parent;
+    f->size = parent->count();
+    return n;
+  }
+  void destroy() {
+    for (int i = 0; i < count(); ++i) {
+      value_destroy(i);
+    }
+  }
+
+ private:
+  void value_init(int i) {
+    new (&fields_.values[i]) mutable_value_type;
+  }
+  void value_init(int i, const value_type &x) {
+    new (&fields_.values[i]) mutable_value_type(x);
+  }
+  void value_destroy(int i) {
+    fields_.values[i].~mutable_value_type();
+  }
+
+ private:
+  root_fields fields_;
+
+ private:
+  btree_node(const btree_node&);
+  void operator=(const btree_node&);
+};
+
+template <typename Node, typename Reference, typename Pointer>
+struct btree_iterator {
+  typedef typename Node::key_type key_type;
+  typedef typename Node::size_type size_type;
+  typedef typename Node::difference_type difference_type;
+  typedef typename Node::params_type params_type;
+
+  typedef Node node_type;
+  typedef typename std::remove_const<Node>::type normal_node;
+  typedef const Node const_node;
+  typedef typename params_type::value_type value_type;
+  typedef typename params_type::pointer normal_pointer;
+  typedef typename params_type::reference normal_reference;
+  typedef typename params_type::const_pointer const_pointer;
+  typedef typename params_type::const_reference const_reference;
+
+  typedef Pointer pointer;
+  typedef Reference reference;
+  typedef std::bidirectional_iterator_tag iterator_category;
+
+  typedef btree_iterator<
+    normal_node, normal_reference, normal_pointer> iterator;
+  typedef btree_iterator<
+    const_node, const_reference, const_pointer> const_iterator;
+  typedef btree_iterator<Node, Reference, Pointer> self_type;
+
+  btree_iterator()
+      : node(NULL),
+        position(-1) {
+  }
+  btree_iterator(Node *n, int p)
+      : node(n),
+        position(p) {
+  }
+  btree_iterator(const iterator &x)
+      : node(x.node),
+        position(x.position) {
+  }
+
+  // Increment/decrement the iterator.
+  void increment() {
+    if (node->leaf() && ++position < node->count()) {
+      return;
+    }
+    increment_slow();
+  }
+  void increment_by(int count);
+  void increment_slow();
+
+  void decrement() {
+    if (node->leaf() && --position >= 0) {
+      return;
+    }
+    decrement_slow();
+  }
+  void decrement_slow();
+
+  bool operator==(const const_iterator &x) const {
+    return node == x.node && position == x.position;
+  }
+  bool operator!=(const const_iterator &x) const {
+    return node != x.node || position != x.position;
+  }
+
+  // Accessors for the key/value the iterator is pointing at.
+  const key_type& key() const {
+    return node->key(position);
+  }
+  reference operator*() const {
+    return node->value(position);
+  }
+  pointer operator->() const {
+    return &node->value(position);
+  }
+
+  self_type& operator++() {
+    increment();
+    return *this;
+  }
+  self_type& operator--() {
+    decrement();
+    return *this;
+  }
+  self_type operator++(int) {
+    self_type tmp = *this;
+    ++*this;
+    return tmp;
+  }
+  self_type operator--(int) {
+    self_type tmp = *this;
+    --*this;
+    return tmp;
+  }
+
+  // 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.
+  int position;
+};
+
+// Dispatch helper class for using btree::internal_locate with plain compare.
+struct btree_internal_locate_plain_compare {
+  template <typename K, typename T, typename Iter>
+  static std::pair<Iter, int> dispatch(const K &k, const T &t, Iter iter) {
+    return t.internal_locate_plain_compare(k, iter);
+  }
+};
+
+// Dispatch helper class for using btree::internal_locate with compare-to.
+struct btree_internal_locate_compare_to {
+  template <typename K, typename T, typename Iter>
+  static std::pair<Iter, int> dispatch(const K &k, const T &t, Iter iter) {
+    return t.internal_locate_compare_to(k, iter);
+  }
+};
+
+template <typename Params>
+class btree : public Params::key_compare {
+  typedef btree<Params> self_type;
+  typedef btree_node<Params> node_type;
+  typedef typename node_type::base_fields base_fields;
+  typedef typename node_type::leaf_fields leaf_fields;
+  typedef typename node_type::internal_fields internal_fields;
+  typedef typename node_type::root_fields root_fields;
+  typedef typename Params::is_key_compare_to is_key_compare_to;
+
+  friend struct btree_internal_locate_plain_compare;
+  friend struct btree_internal_locate_compare_to;
+  typedef typename if_<
+    is_key_compare_to::value,
+    btree_internal_locate_compare_to,
+    btree_internal_locate_plain_compare>::type internal_locate_type;
+
+  enum {
+    kNodeValues = node_type::kNodeValues,
+    kMinNodeValues = kNodeValues / 2,
+    kValueSize = node_type::kValueSize,
+    kExactMatch = node_type::kExactMatch,
+    kMatchMask = node_type::kMatchMask,
+  };
+
+  // A helper class to get the empty base class optimization for 0-size
+  // allocators. Base is internal_allocator_type.
+  // (e.g. empty_base_handle<internal_allocator_type, node_type*>). If Base is
+  // 0-size, the compiler doesn't have to reserve any space for it and
+  // sizeof(empty_base_handle) will simply be sizeof(Data). Google [empty base
+  // class optimization] for more details.
+  template <typename Base, typename Data>
+  struct empty_base_handle : public Base {
+    empty_base_handle(const Base &b, const Data &d)
+        : Base(b),
+          data(d) {
+    }
+    Data data;
+  };
+
+  struct node_stats {
+    node_stats(ssize_t l, ssize_t i)
+        : leaf_nodes(l),
+          internal_nodes(i) {
+    }
+
+    node_stats& operator+=(const node_stats &x) {
+      leaf_nodes += x.leaf_nodes;
+      internal_nodes += x.internal_nodes;
+      return *this;
+    }
+
+    ssize_t leaf_nodes;
+    ssize_t internal_nodes;
+  };
+
+ public:
+  typedef Params params_type;
+  typedef typename Params::key_type key_type;
+  typedef typename Params::data_type data_type;
+  typedef typename Params::mapped_type mapped_type;
+  typedef typename Params::value_type value_type;
+  typedef typename Params::key_compare key_compare;
+  typedef typename Params::pointer pointer;
+  typedef typename Params::const_pointer const_pointer;
+  typedef typename Params::reference reference;
+  typedef typename Params::const_reference const_reference;
+  typedef typename Params::size_type size_type;
+  typedef typename Params::difference_type difference_type;
+  typedef btree_iterator<node_type, reference, pointer> iterator;
+  typedef typename iterator::const_iterator const_iterator;
+  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+  typedef std::reverse_iterator<iterator> reverse_iterator;
+
+  typedef typename Params::allocator_type allocator_type;
+  typedef typename allocator_type::template rebind<char>::other
+    internal_allocator_type;
+
+ public:
+  // Default constructor.
+  btree(const key_compare &comp, const allocator_type &alloc);
+
+  // Copy constructor.
+  btree(const self_type &x);
+
+  // Destructor.
+  ~btree() {
+    clear();
+  }
+
+  // Iterator routines.
+  iterator begin() {
+    return iterator(leftmost(), 0);
+  }
+  const_iterator begin() const {
+    return const_iterator(leftmost(), 0);
+  }
+  iterator end() {
+    return iterator(rightmost(), rightmost() ? rightmost()->count() : 0);
+  }
+  const_iterator end() const {
+    return const_iterator(rightmost(), rightmost() ? rightmost()->count() : 0);
+  }
+  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.
+  iterator lower_bound(const key_type &key) {
+    return internal_end(
+        internal_lower_bound(key, iterator(root(), 0)));
+  }
+  const_iterator lower_bound(const key_type &key) const {
+    return internal_end(
+        internal_lower_bound(key, const_iterator(root(), 0)));
+  }
+
+  // Finds the first element whose key is greater than key.
+  iterator upper_bound(const key_type &key) {
+    return internal_end(
+        internal_upper_bound(key, iterator(root(), 0)));
+  }
+  const_iterator upper_bound(const key_type &key) const {
+    return internal_end(
+        internal_upper_bound(key, const_iterator(root(), 0)));
+  }
+
+  // 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 pair of
+  // the pair is equal to upper_bound(key).
+  std::pair<iterator,iterator> equal_range(const key_type &key) {
+    return std::make_pair(lower_bound(key), upper_bound(key));
+  }
+  std::pair<const_iterator,const_iterator> equal_range(const key_type &key) const {
+    return std::make_pair(lower_bound(key), upper_bound(key));
+  }
+
+  // Inserts a value into the btree only if it does not already exist. The
+  // boolean return value indicates whether insertion succeeded or failed. The
+  // ValuePointer type is used to avoid instatiating the value unless the key
+  // is being inserted. Value is not dereferenced if the key already exists in
+  // the btree. See btree_map::operator[].
+  template <typename ValuePointer>
+  std::pair<iterator,bool> insert_unique(const key_type &key, ValuePointer value);
+
+  // Inserts a value into the btree only if it does not already exist. The
+  // boolean return value indicates whether insertion succeeded or failed.
+  std::pair<iterator,bool> insert_unique(const value_type &v) {
+    return insert_unique(params_type::key(v), &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_unique(v) were made.
+  iterator insert_unique(iterator position, const value_type &v);
+
+  // Insert a range of values into the btree.
+  template <typename InputIterator>
+  void insert_unique(InputIterator b, InputIterator e);
+
+  // Inserts a value into the btree. The ValuePointer type is used to avoid
+  // instatiating the value unless the key is being inserted. Value is not
+  // dereferenced if the key already exists in the btree. See
+  // btree_map::operator[].
+  template <typename ValuePointer>
+  iterator insert_multi(const key_type &key, ValuePointer value);
+
+  // Inserts a value into the btree.
+  iterator insert_multi(const value_type &v) {
+    return insert_multi(params_type::key(v), &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.
+  iterator insert_multi(iterator position, const value_type &v);
+
+  // Insert a range of values into the btree.
+  template <typename InputIterator>
+  void insert_multi(InputIterator b, InputIterator e);
+
+  void assign(const self_type &x);
+
+  // 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).
+  iterator erase(iterator iter);
+
+  // Erases range. Returns the number of keys erased.
+  int erase(iterator begin, iterator end);
+
+  // Erases the specified key from the btree. Returns 1 if an element was
+  // erased and 0 otherwise.
+  int erase_unique(const key_type &key);
+
+  // Erases all of the entries matching the specified key from the
+  // btree. Returns the number of elements erased.
+  int erase_multi(const key_type &key);
+
+  // Finds the iterator corresponding to a key or returns end() if the key is
+  // not present.
+  iterator find_unique(const key_type &key) {
+    return internal_end(
+        internal_find_unique(key, iterator(root(), 0)));
+  }
+  const_iterator find_unique(const key_type &key) const {
+    return internal_end(
+        internal_find_unique(key, const_iterator(root(), 0)));
+  }
+  iterator find_multi(const key_type &key) {
+    return internal_end(
+        internal_find_multi(key, iterator(root(), 0)));
+  }
+  const_iterator find_multi(const key_type &key) const {
+    return internal_end(
+        internal_find_multi(key, const_iterator(root(), 0)));
+  }
+
+  // Returns a count of the number of times the key appears in the btree.
+  size_type count_unique(const key_type &key) const {
+    const_iterator b = internal_find_unique(
+        key, const_iterator(root(), 0));
+    if (!b.node) {
+      // The key doesn't exist in the tree.
+      return 0;
+    }
+    return 1;
+  }
+  // Returns a count of the number of times the key appears in the btree.
+  size_type count_multi(const key_type &key) const {
+    return distance(lower_bound(key), upper_bound(key));
+  }
+
+  // Clear the btree, deleting all of the values it contains.
+  void clear();
+
+  // Swap the contents of *this and x.
+  void swap(self_type &x);
+
+  // Assign the contents of x to *this.
+  self_type& operator=(const self_type &x) {
+    if (&x == this) {
+      // Don't copy onto ourselves.
+      return *this;
+    }
+    assign(x);
+    return *this;
+  }
+
+  key_compare* mutable_key_comp() {
+    return this;
+  }
+  const key_compare& key_comp() const {
+    return *this;
+  }
+  bool compare_keys(const key_type &x, const key_type &y) const {
+    return btree_compare_keys(key_comp(), x, y);
+  }
+
+  // Dump the btree to the specified ostream. Requires that operator<< is
+  // defined for Key and Value.
+  void dump(std::ostream &os) const {
+    if (root() != NULL) {
+      internal_dump(os, root(), 0);
+    }
+  }
+
+  // Verifies the structure of the btree.
+  void verify() const;
+
+  // Size routines. Note that empty() is slightly faster than doing size()==0.
+  size_type size() const {
+    if (empty()) return 0;
+    if (root()->leaf()) return root()->count();
+    return root()->size();
+  }
+  size_type max_size() const { return std::numeric_limits<size_type>::max(); }
+  bool empty() const { return root() == NULL; }
+
+  // The height of the btree. An empty tree will have height 0.
+  size_type height() const {
+    size_type h = 0;
+    if (root()) {
+      // 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) +
+          sizeof(base_fields) + root()->max_count() * sizeof(value_type);
+    } else {
+      return sizeof(*this) +
+          sizeof(root_fields) - sizeof(internal_fields) +
+          stats.leaf_nodes * sizeof(leaf_fields) +
+          stats.internal_nodes * sizeof(internal_fields);
+    }
+  }
+
+  // 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 sizeof(leaf_fields) / (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.
+  double fullness() const {
+    return 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.
+  double overhead() const {
+    if (empty()) {
+      return 0.0;
+    }
+    return (bytes_used() - size() * kValueSize) / double(size());
+  }
+
+ private:
+  // Internal accessor routines.
+  node_type* root() { return root_.data; }
+  const node_type* root() const { return root_.data; }
+  node_type** mutable_root() { return &root_.data; }
+
+  // The rightmost node is stored in the root node.
+  node_type* rightmost() {
+    return (!root() || root()->leaf()) ? root() : root()->rightmost();
+  }
+  const node_type* rightmost() const {
+    return (!root() || root()->leaf()) ? root() : root()->rightmost();
+  }
+  node_type** mutable_rightmost() { return root()->mutable_rightmost(); }
+
+  // The leftmost node is stored as the parent of the root node.
+  node_type* leftmost() { return root() ? root()->parent() : NULL; }
+  const node_type* leftmost() const { return root() ? root()->parent() : NULL; }
+
+  // The size of the tree is stored in the root node.
+  size_type* mutable_size() { return root()->mutable_size(); }
+
+  // Allocator routines.
+  internal_allocator_type* mutable_internal_allocator() {
+    return static_cast<internal_allocator_type*>(&root_);
+  }
+  const internal_allocator_type& internal_allocator() const {
+    return *static_cast<const internal_allocator_type*>(&root_);
+  }
+
+  // Node creation/deletion routines.
+  node_type* new_internal_node(node_type *parent) {
+    internal_fields *p = reinterpret_cast<internal_fields*>(
+        mutable_internal_allocator()->allocate(sizeof(internal_fields)));
+    return node_type::init_internal(p, parent);
+  }
+  node_type* new_internal_root_node() {
+    root_fields *p = reinterpret_cast<root_fields*>(
+        mutable_internal_allocator()->allocate(sizeof(root_fields)));
+    return node_type::init_root(p, root()->parent());
+  }
+  node_type* new_leaf_node(node_type *parent) {
+    leaf_fields *p = reinterpret_cast<leaf_fields*>(
+        mutable_internal_allocator()->allocate(sizeof(leaf_fields)));
+    return node_type::init_leaf(p, parent, kNodeValues);
+  }
+  node_type* new_leaf_root_node(int max_count) {
+    leaf_fields *p = reinterpret_cast<leaf_fields*>(
+        mutable_internal_allocator()->allocate(
+            sizeof(base_fields) + max_count * sizeof(value_type)));
+    return node_type::init_leaf(p, reinterpret_cast<node_type*>(p), max_count);
+  }
+  void delete_internal_node(node_type *node) {
+    node->destroy();
+    assert(node != root());
+    mutable_internal_allocator()->deallocate(
+        reinterpret_cast<char*>(node), sizeof(internal_fields));
+  }
+  void delete_internal_root_node() {
+    root()->destroy();
+    mutable_internal_allocator()->deallocate(
+        reinterpret_cast<char*>(root()), sizeof(root_fields));
+  }
+  void delete_leaf_node(node_type *node) {
+    node->destroy();
+    mutable_internal_allocator()->deallocate(
+        reinterpret_cast<char*>(node),
+        sizeof(base_fields) + node->max_count() * sizeof(value_type));
+  }
+
+  // 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 ? iter : end();
+  }
+  const_iterator internal_end(const_iterator iter) const {
+    return iter.node ? iter : end();
+  }
+
+  // Inserts a value into the btree immediately before iter. Requires that
+  // key(v) <= iter.key() and (--iter).key() <= key(v).
+  iterator internal_insert(iterator iter, const value_type &v);
+
+  // Returns an iterator pointing to the first value >= the value "iter" is
+  // pointing at. Note that "iter" might be pointing to an invalid location as
+  // iter.position == iter.node->count(). This routine simply moves iter up in
+  // the tree to a valid location.
+  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. We provide 2 versions of internal_locate. The first
+  // version (internal_locate_plain_compare) always returns 0 for the second
+  // field of the pair. The second version (internal_locate_compare_to) is for
+  // the key-compare-to specialization and returns either kExactMatch (if the
+  // key was found in the tree) or -kExactMatch (if it wasn't) in the second
+  // field of the pair. The compare_to specialization allows the caller to
+  // avoid a subsequent comparison to determine if an exact match was made,
+  // speeding up string keys.
+  template <typename IterType>
+  std::pair<IterType, int> internal_locate(
+      const key_type &key, IterType iter) const;
+  template <typename IterType>
+  std::pair<IterType, int> internal_locate_plain_compare(
+      const key_type &key, IterType iter) const;
+  template <typename IterType>
+  std::pair<IterType, int> internal_locate_compare_to(
+      const key_type &key, IterType iter) const;
+
+  // Internal routine which implements lower_bound().
+  template <typename IterType>
+  IterType internal_lower_bound(
+      const key_type &key, IterType iter) const;
+
+  // Internal routine which implements upper_bound().
+  template <typename IterType>
+  IterType internal_upper_bound(
+      const key_type &key, IterType iter) const;
+
+  // Internal routine which implements find_unique().
+  template <typename IterType>
+  IterType internal_find_unique(
+      const key_type &key, IterType iter) const;
+
+  // Internal routine which implements find_multi().
+  template <typename IterType>
+  IterType internal_find_multi(
+      const key_type &key, IterType iter) const;
+
+  // Deletes a node and all of its children.
+  void internal_clear(node_type *node);
+
+  // Dumps a node and all of its children to the specified ostream.
+  void internal_dump(std::ostream &os, const node_type *node, int level) 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 {
+    if (!node) {
+      return node_stats(0, 0);
+    }
+    if (node->leaf()) {
+      return node_stats(1, 0);
+    }
+    node_stats res(0, 1);
+    for (int i = 0; i <= node->count(); ++i) {
+      res += internal_stats(node->child(i));
+    }
+    return res;
+  }
+
+ private:
+  empty_base_handle<internal_allocator_type, node_type*> root_;
+
+ private:
+  // A never instantiated helper function that returns big_ if we have a
+  // key-compare-to functor or if R is bool and small_ otherwise.
+  template <typename R>
+  static typename if_<
+   if_<is_key_compare_to::value,
+             std::is_same<R, int>,
+             std::is_same<R, bool> >::type::value,
+   big_, small_>::type key_compare_checker(R);
+
+  // A never instantiated helper function that returns the key comparison
+  // functor.
+  static key_compare key_compare_helper();
+
+  // Verify that key_compare returns a bool. This is similar to the way
+  // is_convertible in base/type_traits.h works. Note that key_compare_checker
+  // is never actually invoked. The compiler will select which
+  // key_compare_checker() to instantiate and then figure out the size of the
+  // return type of key_compare_checker() at compile time which we then check
+  // against the sizeof of big_.
+  COMPILE_ASSERT(
+      sizeof(key_compare_checker(key_compare_helper()(key_type(), key_type()))) ==
+      sizeof(big_),
+      key_comparison_function_must_return_bool);
+
+  // Note: We insist on kTargetValues, which is computed from
+  // Params::kTargetNodeSize, must fit the base_fields::field_type.
+  COMPILE_ASSERT(kNodeValues <
+                 (1 << (8 * sizeof(typename base_fields::field_type))),
+                 target_node_size_too_large);
+
+  // Test the assumption made in setting kNodeValueSpace.
+  COMPILE_ASSERT(sizeof(base_fields) >= 2 * sizeof(void*),
+                 node_space_assumption_incorrect);
+};
+
+////
+// btree_node methods
+template <typename P>
+inline void btree_node<P>::insert_value(int i, const value_type &x) {
+  assert(i <= count());
+  value_init(count(), x);
+  for (int j = count(); j > i; --j) {
+    value_swap(j, this, j - 1);
+  }
+  set_count(count() + 1);
+
+  if (!leaf()) {
+    ++i;
+    for (int j = count(); j > i; --j) {
+      *mutable_child(j) = child(j - 1);
+      child(j)->set_position(j);
+    }
+    *mutable_child(i) = NULL;
+  }
+}
+
+template <typename P>
+inline void btree_node<P>::remove_value(int i) {
+  if (!leaf()) {
+    assert(child(i + 1)->count() == 0);
+    for (int j = i + 1; j < count(); ++j) {
+      *mutable_child(j) = child(j + 1);
+      child(j)->set_position(j);
+    }
+    *mutable_child(count()) = NULL;
+  }
+
+  set_count(count() - 1);
+  for (; i < count(); ++i) {
+    value_swap(i, this, i + 1);
+  }
+  value_destroy(i);
+}
+
+template <typename P>
+void btree_node<P>::rebalance_right_to_left(btree_node *src, int to_move) {
+  assert(parent() == src->parent());
+  assert(position() + 1 == src->position());
+  assert(src->count() >= count());
+  assert(to_move >= 1);
+  assert(to_move <= src->count());
+
+  // Make room in the left node for the new values.
+  for (int i = 0; i < to_move; ++i) {
+    value_init(i + count());
+  }
+
+  // Move the delimiting value to the left node and the new delimiting value
+  // from the right node.
+  value_swap(count(), parent(), position());
+  parent()->value_swap(position(), src, to_move - 1);
+
+  // Move the values from the right to the left node.
+  for (int i = 1; i < to_move; ++i) {
+    value_swap(count() + i, src, i - 1);
+  }
+  // Shift the values in the right node to their correct position.
+  for (int i = to_move; i < src->count(); ++i) {
+    src->value_swap(i - to_move, src, i);
+  }
+  for (int i = 1; i <= to_move; ++i) {
+    src->value_destroy(src->count() - i);
+  }
+
+  if (!leaf()) {
+    // Move the child pointers from the right to the left node.
+    for (int i = 0; i < to_move; ++i) {
+      set_child(1 + count() + i, src->child(i));
+    }
+    for (int i = 0; i <= src->count() - to_move; ++i) {
+      assert(i + to_move <= src->max_count());
+      src->set_child(i, src->child(i + to_move));
+      *src->mutable_child(i + to_move) = NULL;
+    }
+  }
+
+  // Fixup the counts on the src and dest nodes.
+  set_count(count() + to_move);
+  src->set_count(src->count() - to_move);
+}
+
+template <typename P>
+void btree_node<P>::rebalance_left_to_right(btree_node *dest, int to_move) {
+  assert(parent() == dest->parent());
+  assert(position() + 1 == dest->position());
+  assert(count() >= dest->count());
+  assert(to_move >= 1);
+  assert(to_move <= count());
+
+  // Make room in the right node for the new values.
+  for (int i = 0; i < to_move; ++i) {
+    dest->value_init(i + dest->count());
+  }
+  for (int i = dest->count() - 1; i >= 0; --i) {
+    dest->value_swap(i, dest, i + to_move);
+  }
+
+  // Move the delimiting value to the right node and the new delimiting value
+  // from the left node.
+  dest->value_swap(to_move - 1, parent(), position());
+  parent()->value_swap(position(), this, count() - to_move);
+  value_destroy(count() - to_move);
+
+  // Move the values from the left to the right node.
+  for (int i = 1; i < to_move; ++i) {
+    value_swap(count() - to_move + i, dest, i - 1);
+    value_destroy(count() - to_move + i);
+  }
+
+  if (!leaf()) {
+    // Move the child pointers from the left to the right node.
+    for (int i = dest->count(); i >= 0; --i) {
+      dest->set_child(i + to_move, dest->child(i));
+      *dest->mutable_child(i) = NULL;
+    }
+    for (int i = 1; i <= to_move; ++i) {
+      dest->set_child(i - 1, child(count() - to_move + i));
+      *mutable_child(count() - to_move + i) = NULL;
+    }
+  }
+
+  // Fixup the counts on the src and dest nodes.
+  set_count(count() - to_move);
+  dest->set_count(dest->count() + to_move);
+}
+
+template <typename P>
+void btree_node<P>::split(btree_node *dest, int insert_position) {
+  assert(dest->count() == 0);
+
+  // 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 == 0) {
+    dest->set_count(count() - 1);
+  } else if (insert_position == max_count()) {
+    dest->set_count(0);
+  } else {
+    dest->set_count(count() / 2);
+  }
+  set_count(count() - dest->count());
+  assert(count() >= 1);
+
+  // Move values from the left sibling to the right sibling.
+  for (int i = 0; i < dest->count(); ++i) {
+    dest->value_init(i);
+    value_swap(count() + i, dest, i);
+    value_destroy(count() + i);
+  }
+
+  // The split key is the largest value in the left sibling.
+  set_count(count() - 1);
+  parent()->insert_value(position(), value_type());
+  value_swap(count(), parent(), position());
+  value_destroy(count());
+  parent()->set_child(position() + 1, dest);
+
+  if (!leaf()) {
+    for (int i = 0; i <= dest->count(); ++i) {
+      assert(child(count() + i + 1) != NULL);
+      dest->set_child(i, child(count() + i + 1));
+      *mutable_child(count() + i + 1) = NULL;
+    }
+  }
+}
+
+template <typename P>
+void btree_node<P>::merge(btree_node *src) {
+  assert(parent() == src->parent());
+  assert(position() + 1 == src->position());
+
+  // Move the delimiting value to the left node.
+  value_init(count());
+  value_swap(count(), parent(), position());
+
+  // Move the values from the right to the left node.
+  for (int i = 0; i < src->count(); ++i) {
+    value_init(1 + count() + i);
+    value_swap(1 + count() + i, src, i);
+    src->value_destroy(i);
+  }
+
+  if (!leaf()) {
+    // Move the child pointers from the right to the left node.
+    for (int i = 0; i <= src->count(); ++i) {
+      set_child(1 + count() + i, src->child(i));
+      *src->mutable_child(i) = NULL;
+    }
+  }
+
+  // Fixup the counts on the src and dest nodes.
+  set_count(1 + count() + src->count());
+  src->set_count(0);
+
+  // Remove the value on the parent node.
+  parent()->remove_value(position());
+}
+
+template <typename P>
+void btree_node<P>::swap(btree_node *x) {
+  assert(leaf() == x->leaf());
+
+  // Swap the values.
+  for (int i = count(); i < x->count(); ++i) {
+    value_init(i);
+  }
+  for (int i = x->count(); i < count(); ++i) {
+    x->value_init(i);
+  }
+  int n = std::max(count(), x->count());
+  for (int i = 0; i < n; ++i) {
+    value_swap(i, x, i);
+  }
+  for (int i = count(); i < x->count(); ++i) {
+    x->value_destroy(i);
+  }
+  for (int i = x->count(); i < count(); ++i) {
+    value_destroy(i);
+  }
+
+  if (!leaf()) {
+    // Swap the child pointers.
+    for (int i = 0; i <= n; ++i) {
+      btree_swap_helper(*mutable_child(i), *x->mutable_child(i));
+    }
+    for (int i = 0; i <= count(); ++i) {
+      x->child(i)->fields_.parent = x;
+    }
+    for (int i = 0; i <= x->count(); ++i) {
+      child(i)->fields_.parent = this;
+    }
+  }
+
+  // Swap the counts.
+  btree_swap_helper(fields_.count, x->fields_.count);
+}
+
+////
+// btree_iterator methods
+template <typename N, typename R, typename P>
+void btree_iterator<N, R, P>::increment_slow() {
+  if (node->leaf()) {
+    assert(position >= node->count());
+    self_type save(*this);
+    while (position == node->count() && !node->is_root()) {
+      assert(node->parent()->child(node->position()) == node);
+      position = node->position();
+      node = node->parent();
+    }
+    if (position == node->count()) {
+      *this = save;
+    }
+  } else {
+    assert(position < node->count());
+    node = node->child(position + 1);
+    while (!node->leaf()) {
+      node = node->child(0);
+    }
+    position = 0;
+  }
+}
+
+template <typename N, typename R, typename P>
+void btree_iterator<N, R, P>::increment_by(int count) {
+  while (count > 0) {
+    if (node->leaf()) {
+      int rest = node->count() - position;
+      position += std::min(rest, count);
+      count = count - rest;
+      if (position < node->count()) {
+        return;
+      }
+    } else {
+      --count;
+    }
+    increment_slow();
+  }
+}
+
+template <typename N, typename R, typename P>
+void btree_iterator<N, R, P>::decrement_slow() {
+  if (node->leaf()) {
+    assert(position <= -1);
+    self_type save(*this);
+    while (position < 0 && !node->is_root()) {
+      assert(node->parent()->child(node->position()) == node);
+      position = node->position() - 1;
+      node = node->parent();
+    }
+    if (position < 0) {
+      *this = save;
+    }
+  } else {
+    assert(position >= 0);
+    node = node->child(position);
+    while (!node->leaf()) {
+      node = node->child(node->count());
+    }
+    position = node->count() - 1;
+  }
+}
+
+////
+// btree methods
+template <typename P>
+btree<P>::btree(const key_compare &comp, const allocator_type &alloc)
+    : key_compare(comp),
+      root_(alloc, NULL) {
+}
+
+template <typename P>
+btree<P>::btree(const self_type &x)
+    : key_compare(x.key_comp()),
+      root_(x.internal_allocator(), NULL) {
+  assign(x);
+}
+
+template <typename P> template <typename ValuePointer>
+std::pair<typename btree<P>::iterator, bool>
+btree<P>::insert_unique(const key_type &key, ValuePointer value) {
+  if (empty()) {
+    *mutable_root() = new_leaf_root_node(1);
+  }
+
+  std::pair<iterator, int> res = internal_locate(key, iterator(root(), 0));
+  iterator &iter = res.first;
+  if (res.second == kExactMatch) {
+    // The key already exists in the tree, do nothing.
+    return std::make_pair(internal_last(iter), false);
+  } else if (!res.second) {
+    iterator last = internal_last(iter);
+    if (last.node && !compare_keys(key, last.key())) {
+      // The key already exists in the tree, do nothing.
+      return std::make_pair(last, false);
+    }
+  }
+
+  return std::make_pair(internal_insert(iter, *value), true);
+}
+
+template <typename P>
+inline typename btree<P>::iterator
+btree<P>::insert_unique(iterator position, const value_type &v) {
+  if (!empty()) {
+    const key_type &key = params_type::key(v);
+    if (position == end() || compare_keys(key, position.key())) {
+      iterator prev = position;
+      if (position == begin() || compare_keys((--prev).key(), key)) {
+        // prev.key() < key < position.key()
+        return internal_insert(position, v);
+      }
+    } else if (compare_keys(position.key(), key)) {
+      iterator next = position;
+      ++next;
+      if (next == end() || compare_keys(key, next.key())) {
+        // position.key() < key < next.key()
+        return internal_insert(next, v);
+      }
+    } else {
+      // position.key() == key
+      return position;
+    }
+  }
+  return insert_unique(v).first;
+}
+
+template <typename P> template <typename InputIterator>
+void btree<P>::insert_unique(InputIterator b, InputIterator e) {
+  for (; b != e; ++b) {
+    insert_unique(end(), *b);
+  }
+}
+
+template <typename P> template <typename ValuePointer>
+typename btree<P>::iterator
+btree<P>::insert_multi(const key_type &key, ValuePointer value) {
+  if (empty()) {
+    *mutable_root() = new_leaf_root_node(1);
+  }
+
+  iterator iter = internal_upper_bound(key, iterator(root(), 0));
+  if (!iter.node) {
+    iter = end();
+  }
+  return internal_insert(iter, *value);
+}
+
+template <typename P>
+typename btree<P>::iterator
+btree<P>::insert_multi(iterator position, const value_type &v) {
+  if (!empty()) {
+    const key_type &key = params_type::key(v);
+    if (position == end() || !compare_keys(position.key(), key)) {
+      iterator prev = position;
+      if (position == begin() || !compare_keys(key, (--prev).key())) {
+        // prev.key() <= key <= position.key()
+        return internal_insert(position, v);
+      }
+    } else {
+      iterator next = position;
+      ++next;
+      if (next == end() || !compare_keys(next.key(), key)) {
+        // position.key() < key <= next.key()
+        return internal_insert(next, v);
+      }
+    }
+  }
+  return insert_multi(v);
+}
+
+template <typename P> template <typename InputIterator>
+void btree<P>::insert_multi(InputIterator b, InputIterator e) {
+  for (; b != e; ++b) {
+    insert_multi(end(), *b);
+  }
+}
+
+template <typename P>
+void btree<P>::assign(const self_type &x) {
+  clear();
+
+  *mutable_key_comp() = x.key_comp();
+  *mutable_internal_allocator() = x.internal_allocator();
+
+  // Assignment can avoid key comparisons because we know the order of the
+  // values is the same order we'll store them in.
+  for (const_iterator iter = x.begin(); iter != x.end(); ++iter) {
+    if (empty()) {
+      insert_multi(*iter);
+    } else {
+      // If the btree is not empty, we can just insert the new value at the end
+      // of the tree!
+      internal_insert(end(), *iter);
+    }
+  }
+}
+
+template <typename P>
+typename btree<P>::iterator btree<P>::erase(iterator iter) {
+  bool internal_delete = false;
+  if (!iter.node->leaf()) {
+    // Deletion of a value on an internal node. Swap the key with the largest
+    // value of our left child. This is easy, we just decrement iter.
+    iterator tmp_iter(iter--);
+    assert(iter.node->leaf());
+    assert(!compare_keys(tmp_iter.key(), iter.key()));
+    iter.node->value_swap(iter.position, tmp_iter.node, tmp_iter.position);
+    internal_delete = true;
+    --*mutable_size();
+  } else if (!root()->leaf()) {
+    --*mutable_size();
+  }
+
+  // Delete the key from the leaf.
+  iter.node->remove_value(iter.position);
+
+  // 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.
+
+  // Merge/rebalance as we walk back up the tree.
+  iterator res(iter);
+  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);
+    if (iter.node->leaf()) {
+      res = iter;
+    }
+    if (!merged) {
+      break;
+    }
+    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->count()) {
+    res.position = res.node->count() - 1;
+    ++res;
+  }
+  // If we erased from an internal node, advance the iterator.
+  if (internal_delete) {
+    ++res;
+  }
+  return res;
+}
+
+template <typename P>
+int btree<P>::erase(iterator b, iterator e) {
+  int count = distance(b, e);
+  for (int i = 0; i < count; i++) {
+    b = erase(b);
+  }
+  return count;
+}
+
+template <typename P>
+int btree<P>::erase_unique(const key_type &key) {
+  iterator iter = internal_find_unique(key, iterator(root(), 0));
+  if (!iter.node) {
+    // The key doesn't exist in the tree, return nothing done.
+    return 0;
+  }
+  erase(iter);
+  return 1;
+}
+
+template <typename P>
+int btree<P>::erase_multi(const key_type &key) {
+  iterator b = internal_lower_bound(key, iterator(root(), 0));
+  if (!b.node) {
+    // The key doesn't exist in the tree, return nothing done.
+    return 0;
+  }
+  // Delete all of the keys between begin and upper_bound(key).
+  iterator e = internal_end(
+      internal_upper_bound(key, iterator(root(), 0)));
+  return erase(b, e);
+}
+
+template <typename P>
+void btree<P>::clear() {
+  if (root() != NULL) {
+    internal_clear(root());
+  }
+  *mutable_root() = NULL;
+}
+
+template <typename P>
+void btree<P>::swap(self_type &x) {
+  std::swap(static_cast<key_compare&>(*this), static_cast<key_compare&>(x));
+  std::swap(root_, x.root_);
+}
+
+template <typename P>
+void btree<P>::verify() const {
+  if (root() != NULL) {
+    assert(size() == internal_verify(root(), NULL, NULL));
+    assert(leftmost() == (++const_iterator(root(), -1)).node);
+    assert(rightmost() == (--const_iterator(root(), root()->count())).node);
+    assert(leftmost()->leaf());
+    assert(rightmost()->leaf());
+  } else {
+    assert(size() == 0);
+    assert(leftmost() == NULL);
+    assert(rightmost() == NULL);
+  }
+}
+
+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());
+
+  // First try to make room on the node by rebalancing.
+  node_type *parent = node->parent();
+  if (node != root()) {
+    if (node->position() > 0) {
+      // Try rebalancing with our left sibling.
+      node_type *left = parent->child(node->position() - 1);
+      if (left->count() < left->max_count()) {
+        // 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 = (left->max_count() - left->count()) /
+            (1 + (insert_position < left->max_count()));
+        to_move = std::max(1, to_move);
+
+        if (((insert_position - to_move) >= 0) ||
+            ((left->count() + to_move) < left->max_count())) {
+          left->rebalance_right_to_left(node, to_move);
+
+          assert(node->max_count() - node->count() == to_move);
+          insert_position = insert_position - to_move;
+          if (insert_position < 0) {
+            insert_position = insert_position + left->count() + 1;
+            node = left;
+          }
+
+          assert(node->count() < node->max_count());
+          return;
+        }
+      }
+    }
+
+    if (node->position() < parent->count()) {
+      // Try rebalancing with our right sibling.
+      node_type *right = parent->child(node->position() + 1);
+      if (right->count() < right->max_count()) {
+        // 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 = (right->max_count() - right->count()) /
+            (1 + (insert_position > 0));
+        to_move = std::max(1, to_move);
+
+        if ((insert_position <= (node->count() - to_move)) ||
+            ((right->count() + to_move) < right->max_count())) {
+          node->rebalance_left_to_right(right, to_move);
+
+          if (insert_position > node->count()) {
+            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.
+    if (parent->count() == parent->max_count()) {
+      iterator parent_iter(node->parent(), node->position());
+      rebalance_or_split(&parent_iter);
+    }
+  } else {
+    // Rebalancing not possible because this is the root node.
+    if (root()->leaf()) {
+      // The root node is currently a leaf node: create a new root node and set
+      // the current root node as the child of the new root.
+      parent = new_internal_root_node();
+      parent->set_child(0, root());
+      *mutable_root() = parent;
+      assert(*mutable_rightmost() == parent->child(0));
+    } else {
+      // The root node is an internal node. We do not want to create a new root
+      // node because the root node is special and holds the size of the tree
+      // and a pointer to the rightmost node. So we create a new internal node
+      // and move all of the items on the current root into the new node.
+      parent = new_internal_node(parent);
+      parent->set_child(0, parent);
+      parent->swap(root());
+      node = parent;
+    }
+  }
+
+  // Split the node.
+  node_type *split_node;
+  if (node->leaf()) {
+    split_node = new_leaf_node(parent);
+    node->split(split_node, insert_position);
+    if (rightmost() == node) {
+      *mutable_rightmost() = split_node;
+    }
+  } else {
+    split_node = new_internal_node(parent);
+    node->split(split_node, insert_position);
+  }
+
+  if (insert_position > node->count()) {
+    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);
+  if (right->leaf()) {
+    if (rightmost() == right) {
+      *mutable_rightmost() = left;
+    }
+    delete_leaf_node(right);
+  } else {
+    delete_internal_node(right);
+  }
+}
+
+template <typename P>
+bool btree<P>::try_merge_or_rebalance(iterator *iter) {
+  node_type *parent = iter->node->parent();
+  if (iter->node->position() > 0) {
+    // Try merging with our left sibling.
+    node_type *left = parent->child(iter->node->position() - 1);
+    if ((1 + left->count() + iter->node->count()) <= left->max_count()) {
+      iter->position += 1 + left->count();
+      merge_nodes(left, iter->node);
+      iter->node = left;
+      return true;
+    }
+  }
+  if (iter->node->position() < parent->count()) {
+    // Try merging with our right sibling.
+    node_type *right = parent->child(iter->node->position() + 1);
+    if ((1 + iter->node->count() + right->count()) <= right->max_count()) {
+      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 > 0))) {
+      int to_move = (right->count() - iter->node->count()) / 2;
+      to_move = std::min(to_move, right->count() - 1);
+      iter->node->rebalance_right_to_left(right, to_move);
+      return false;
+    }
+  }
+  if (iter->node->position() > 0) {
+    // 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->count()))) {
+      int to_move = (left->count() - iter->node->count()) / 2;
+      to_move = std::min(to_move, left->count() - 1);
+      left->rebalance_left_to_right(iter->node, to_move);
+      iter->position += to_move;
+      return false;
+    }
+  }
+  return false;
+}
+
+template <typename P>
+void btree<P>::try_shrink() {
+  if (root()->count() > 0) {
+    return;
+  }
+  // Deleted the last item on the root node, shrink the height of the tree.
+  if (root()->leaf()) {
+    assert(size() == 0);
+    delete_leaf_node(root());
+    *mutable_root() = NULL;
+  } else {
+    node_type *child = root()->child(0);
+    if (child->leaf()) {
+      // The child is a leaf node so simply make it the root node in the tree.
+      child->make_root();
+      delete_internal_root_node();
+      *mutable_root() = child;
+    } else {
+      // The child is an internal node. We want to keep the existing root node
+      // so we move all of the values from the child node into the existing
+      // (empty) root node.
+      child->swap(root());
+      delete_internal_node(child);
+    }
+  }
+}
+
+template <typename P> template <typename IterType>
+inline IterType btree<P>::internal_last(IterType iter) {
+  while (iter.node && iter.position == iter.node->count()) {
+    iter.position = iter.node->position();
+    iter.node = iter.node->parent();
+    if (iter.node->leaf()) {
+      iter.node = NULL;
+    }
+  }
+  return iter;
+}
+
+template <typename P>
+inline typename btree<P>::iterator
+btree<P>::internal_insert(iterator iter, const value_type &v) {
+  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;
+  }
+  if (iter.node->count() == iter.node->max_count()) {
+    // Make room in the leaf for the new item.
+    if (iter.node->max_count() < kNodeValues) {
+      // Insertion into the root where the root is smaller that 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 * iter.node->max_count()));
+      iter.node->swap(root());
+      delete_leaf_node(root());
+      *mutable_root() = iter.node;
+    } else {
+      rebalance_or_split(&iter);
+      ++*mutable_size();
+    }
+  } else if (!root()->leaf()) {
+    ++*mutable_size();
+  }
+  iter.node->insert_value(iter.position, v);
+  return iter;
+}
+
+template <typename P> template <typename IterType>
+inline std::pair<IterType, int> btree<P>::internal_locate(
+    const key_type &key, IterType iter) const {
+  return internal_locate_type::dispatch(key, *this, iter);
+}
+
+template <typename P> template <typename IterType>
+inline std::pair<IterType, int> btree<P>::internal_locate_plain_compare(
+    const key_type &key, IterType iter) const {
+  for (;;) {
+    iter.position = iter.node->lower_bound(key, key_comp());
+    if (iter.node->leaf()) {
+      break;
+    }
+    iter.node = iter.node->child(iter.position);
+  }
+  return std::make_pair(iter, 0);
+}
+
+template <typename P> template <typename IterType>
+inline std::pair<IterType, int> btree<P>::internal_locate_compare_to(
+    const key_type &key, IterType iter) const {
+  for (;;) {
+    int res = iter.node->lower_bound(key, key_comp());
+    iter.position = res & kMatchMask;
+    if (res & kExactMatch) {
+      return std::make_pair(iter, static_cast<int>(kExactMatch));
+    }
+    if (iter.node->leaf()) {
+      break;
+    }
+    iter.node = iter.node->child(iter.position);
+  }
+  return std::make_pair(iter, -kExactMatch);
+}
+
+template <typename P> template <typename IterType>
+IterType btree<P>::internal_lower_bound(
+    const key_type &key, IterType iter) const {
+  if (iter.node) {
+    for (;;) {
+      iter.position =
+          iter.node->lower_bound(key, key_comp()) & kMatchMask;
+      if (iter.node->leaf()) {
+        break;
+      }
+      iter.node = iter.node->child(iter.position);
+    }
+    iter = internal_last(iter);
+  }
+  return iter;
+}
+
+template <typename P> template <typename IterType>
+IterType btree<P>::internal_upper_bound(
+    const key_type &key, IterType iter) const {
+  if (iter.node) {
+    for (;;) {
+      iter.position = iter.node->upper_bound(key, key_comp());
+      if (iter.node->leaf()) {
+        break;
+      }
+      iter.node = iter.node->child(iter.position);
+    }
+    iter = internal_last(iter);
+  }
+  return iter;
+}
+
+template <typename P> template <typename IterType>
+IterType btree<P>::internal_find_unique(
+    const key_type &key, IterType iter) const {
+  if (iter.node) {
+    std::pair<IterType, int> res = internal_locate(key, iter);
+    if (res.second == kExactMatch) {
+      return res.first;
+    }
+    if (!res.second) {
+      iter = internal_last(res.first);
+      if (iter.node && !compare_keys(key, iter.key())) {
+        return iter;
+      }
+    }
+  }
+  return IterType(NULL, 0);
+}
+
+template <typename P> template <typename IterType>
+IterType btree<P>::internal_find_multi(
+    const key_type &key, IterType iter) const {
+  if (iter.node) {
+    iter = internal_lower_bound(key, iter);
+    if (iter.node) {
+      iter = internal_last(iter);
+      if (iter.node && !compare_keys(key, iter.key())) {
+        return iter;
+      }
+    }
+  }
+  return IterType(NULL, 0);
+}
+
+template <typename P>
+void btree<P>::internal_clear(node_type *node) {
+  if (!node->leaf()) {
+    for (int i = 0; i <= node->count(); ++i) {
+      internal_clear(node->child(i));
+    }
+    if (node == root()) {
+      delete_internal_root_node();
+    } else {
+      delete_internal_node(node);
+    }
+  } else {
+    delete_leaf_node(node);
+  }
+}
+
+template <typename P>
+void btree<P>::internal_dump(
+    std::ostream &os, const node_type *node, int level) const {
+  for (int i = 0; i < node->count(); ++i) {
+    if (!node->leaf()) {
+      internal_dump(os, node->child(i), level + 1);
+    }
+    for (int j = 0; j < level; ++j) {
+      os << "  ";
+    }
+    os << node->key(i) << " [" << level << "]\n";
+  }
+  if (!node->leaf()) {
+    internal_dump(os, node->child(node->count()), level + 1);
+  }
+}
+
+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(0), *lo));
+  }
+  if (hi) {
+    assert(!compare_keys(*hi, node->key(node->count() - 1)));
+  }
+  for (int i = 1; i < node->count(); ++i) {
+    assert(!compare_keys(node->key(i), node->key(i - 1)));
+  }
+  int count = node->count();
+  if (!node->leaf()) {
+    for (int i = 0; i <= node->count(); ++i) {
+      assert(node->child(i) != NULL);
+      assert(node->child(i)->parent() == node);
+      assert(node->child(i)->position() == i);
+      count += internal_verify(
+          node->child(i),
+          (i == 0) ? lo : &node->key(i - 1),
+          (i == node->count()) ? hi : &node->key(i));
+    }
+  }
+  return count;
+}
+
+} // namespace btree
+
+#endif  // UTIL_BTREE_BTREE_H__