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# Size Balanced Tree

Size Balanced Tree (SBT) 是由中国 OI 选手陈启峰在 2007 年提出的一种自平衡二叉搜索树 (Self-Balanced Binary Search Tree, SBBST), 通过检查子树的节点数量进行自身的平衡维护．相比于红黑树，AVL 等主流自平衡二叉搜索树而言，Size Balanced Tree 支持在 𝑂(l o g⁡𝑛)O(log⁡n) 的时间复杂度内查询某个键值在树中的排名 (rank).

## 节点定义

相比与普通二叉搜索树，SBT 的每个节点 𝑁 N 仅需要多维护一个整数字段 `size`, 用于储存以 𝑁 N 为根的子树中节点的个数．节点类型 `Node` 的具体定义如下：

Identifier | Type | Description
--- | --- | ---
`left` | `Node*` | 左子节点引用
`right` | `Node*` | 右子节点引用
`size` | `int` | 以该节点为根的子树中节点的个数

## 性质

Size Balanced Tree 中任意节点 𝑁 N 满足如下几条性质：

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    size(N.left) >= size(N.right.left)
    size(N.left) >= size(N.right.right)
    size(N.right) >= size(N.left.left)
    size(N.right) >= size(N.left.right)

使用自然语言可描述为：任意节点的 `size` 不小于其兄弟节点（Sibling）的所有子节点（Nephew）的 `size`.

## 平衡维护

### 旋转

SBT 主要通过旋转操作改变自身高度从而进行平衡维护．其旋转操作与绝大部分自平衡二叉搜索树类似，唯一区别在于在完成旋转之后需要对旋转过程中左右子节点发生改变的节点更新 `size`. 示例代码如下：

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    void updateSize() {
      USize leftSize = this->left != nullptr ? this->left->size : 0;
      USize rightSize = this->right != nullptr ? this->right->size : 0;
      this->size = leftSize + rightSize + 1;
    }

    static void rotateLeft(NodePtr& node) {
      assert(node != nullptr);
      // clang-format off
      //     |                       |
      //     N                       S
      //    / \     l-rotate(N)     / \
      //   L   S    ==========>    N   R
      //      / \                 / \
      //     M   R               L   M
      // clang-format on
      NodePtr successor = node->right;
      node->right = successor->left;
      successor->left = node;

      node->updateSize();
      successor->updateSize();

      node = successor;
    }

    static void rotateRight(NodePtr& node) {
      assert(node != nullptr);
      // clang-format off
      //       |                   |
      //       N                   S
      //      / \   r-rotate(N)   / \
      //     S   R  ==========>  L   N
      //    / \                     / \
      //   L   M                   M   R
      // clang-format on
      NodePtr successor = node->left;
      node->left = successor->right;
      successor->right = node;

      node->updateSize();
      successor->updateSize();

      node = successor;
    }

### 维护

#### Case 1

`size(N.left) < size(N.right.left)`

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    if (size(node->right->left) > size(node->left)) {
      // clang-format off
      //     |                     |                      |
      //     N                     N                     [M]
      //    / \    r-rotate(R)    / \     l-rotate(N)    / \
      //  <L>  R   ==========>  <L> [M]   ==========>   N   R
      //      /                       \                /
      //    [M]                        R             <L>
      // clang-format on
      rotateRight(node->right);
      rotateLeft(node);
      fixBalance(node->left);
      fixBalance(node->right);
      fixBalance(node);
      return;
    }

#### Case 2

`size(N.left) < size(N.right.right)`

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    if (size(node->right->right) > size(node->left)) {
      // clang-format off
      //     |                       |
      //     N                       R
      //    / \     l-rotate(N)     / \
      //  <L>  R    ==========>    N  [M]
      //        \                 /
      //        [M]             <L>
      // clang-format on
      rotateLeft(node);
      fixBalance(node->left);
      fixBalance(node);
      return;
    }

#### Case 3

`size(N.right) < size(N.left.left)`

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    if (size(node->left->left) > size(node->right)) {
      // clang-format off
      //       |                       |
      //       N                       L
      //      / \     r-rotate(N)     / \
      //     L  <R>   ==========>   [M]  N
      //    /                             \
      //  [M]                             <R>
      // clang-format on
      rotateRight(node);
      fixBalance(node->right);
      fixBalance(node);
      return;
    }

#### Case 4

`size(N.right) < size(N.left.right)`

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    if (size(node->left->right) > size(node->right)) {
      // clang-format off
      //     |                     |                      |
      //     N                     N                     [M]
      //    / \    l-rotate(L)    / \     r-rotate(N)    / \
      //   L  <R>  ==========>  [M] <R>   ==========>   L   N
      //    \                   /                            \
      //    [M]                L                             <R>
      // clang-format on
      rotateLeft(node->left);
      rotateRight(node);
      fixBalance(node->left);
      fixBalance(node->right);
      fixBalance(node);
      return;
    }

## 操作

### 插入

SBT 的插入操作需要在完成普通二叉搜索树的插入操作的基础上递归地进行节点 `size` 字段的更新及平衡维护．示例代码如下：

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    if (compare(key, node->key)) {
      /* key < node->key */
      if (node->left == nullptr) {
        node->left = Node::from(key, value);
        node->updateSize();
      } else {
        insert(node->left, key, value, replace);
        node->updateSize();
        fixBalance(node);
      }
    } else {
      /* key > node->key */
      if (node->right == nullptr) {
        node->right = Node::from(key, value);
        node->updateSize();
      } else {
        insert(node->right, key, value, replace);
        node->updateSize();
        fixBalance(node);
      }
    }

### 删除

根据 Size Balanced Tree 的提出者陈启峰在其论文中对于删除操作的描述：

> It can result in a destroyed SBT. But with the insertion above, a BST is still kept at the height of 𝑂(l o g⁡𝑛)O(log⁡n) where 𝑛 n is the total number of insertions, not the current size.

删除操作虽然有可能使得 SBT 的性质被打破，但并不会使树的高度增高，因此不会影响后续操作的效率．但在实际情况下，如果在一次批量插入操作后只进行大量的删除和查询操作，依然有可能由于树的失衡影响整体效率，因此本文在实现 SBT 的删除操作时依然选择加入平衡维护．参考代码如下：

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    bool remove(NodePtr& node, K key, NodeConsumer action) {
      assert(node != nullptr);

      if (key != node->key) {
        if (compare(key, node->key)) {
          /* key < node->key */
          NodePtr& left = node->left;
          if (left != nullptr && remove(left, key, action)) {
            node->updateSize();
            fixBalance(node);
            return true;
          } else {
            return false;
          }
        } else {
          /* key > node->key */
          NodePtr& right = node->right;
          if (right != nullptr && remove(right, key, action)) {
            node->updateSize();
            fixBalance(node);
            return true;
          } else {
            return false;
          }
        }
      }

      assert(key == node->key);
      action(node);

      if (node->isLeaf()) {
        // Case 1: no child
        node = nullptr;
      } else if (node->right == nullptr) {
        // Case 2: left child only
        // clang-format off
        //     P
        //     |  remove(N)  P
        //     N  ========>  |
        //    /              L
        //   L
        // clang-format on
        node = node->left;
      } else if (node->left == nullptr) {
        // Case 3: right child only
        // clang-format off
        //   P
        //   |    remove(N)  P
        //   N    ========>  |
        //    \              R
        //     R
        // clang-format on
        node = node->right;
      } else if (node->right->left == nullptr) {
        // Case 4: both left and right child, right child has no left child
        // clang-format off
        //    |                 |
        //    N    remove(N)    R
        //   / \   ========>   /
        //  L   R             L
        // clang-format on
        NodePtr right = node->right;
        swapNode(node, right);
        right->right = node->right;
        node = right;
        node->updateSize();
        fixBalance(node);
      } else {
        // Case 5: both left and right child, right child is not a leaf
        // clang-format off
        //   Step 1. find the node N with the smallest key
        //           and its parent P on the right subtree
        //   Step 2. swap S and N
        //   Step 3. remove node N like Case 1 or Case 3
        //   Step 4. update size for all nodes on the path
        //           from S to P
        //     |                  |
        //     N                  S                 |
        //    / \                / \                S
        //   L  ..  swap(N, S)  L  ..  remove(N)   / \
        //       |  =========>      |  ========>  L  ..
        //       P                  P                 |
        //      / \                / \                P
        //     S  ..              N  ..              / \
        //      \                  \                R  ..
        //       R                  R
        //
        // clang-format on

        std::stack<NodePtr> path;

        // Step 1
        NodePtr successor = node->right;
        NodePtr parent = node;
        path.push(node);

        while (successor->left != nullptr) {
          path.push(successor);
          parent = successor;
          successor = parent->left;
        }

        // Step 2
        swapNode(node, successor);

        // Step 3
        parent->left = node->right;
        // Restore node
        node = successor;

        // Step 4
        while (!path.empty()) {
          path.top()->updateSize();
          path.pop();
        }
      }

      return true;
    }

值得注意的是，在上述代码的 Case 5 中使用后继节点 𝑆 S（也可以选择前驱节点）替换待删除节点 𝑁 N 并删除替换后的 𝑁 N 以后，需要更新替换前 𝑆 S 节点的父节点 𝑃 P 到替换后的 𝑆 S 节点这条路径（如代码中注释所示）上的所有节点的 `size` 字段．本文的实现选择使用栈依次记录路径上的节点，最后再按遍历的相反顺序出栈进行更新．

### 查询排名

由于 SBT 节点中储存了子树节点个数的信息，因此可以在 𝑂(l o g⁡𝑛)O(log⁡n) 的时间复杂度下查询某个 `key` 的排名（或者大于/小于某个 `key` 的节点个数）．示例代码如下：

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    USize countLess(ConstNodePtr node, K key, bool countEqual = false) const {
      if (node == nullptr) {
        return 0;
      } else if (key < node->key) {
        return countLess(node->left, key, countEqual);
      } else if (key > node->key) {
        return size(node->left) + 1 + countLess(node->right, key, countEqual);
      } else {
        return size(node->left) + (countEqual ? 1 : 0);
      }
    }

    USize countGreater(ConstNodePtr node, K key, bool countEqual = false) const {
      if (node == nullptr) {
        return 0;
      } else if (key < node->key) {
        return size(node->right) + 1 + countGreater(node->left, key, countEqual);
      } else if (key > node->key) {
        return countGreater(node->right, key, countEqual);
      } else {
        return size(node->right) + (countEqual ? 1 : 0);
      }
    }

## 参考代码

下面的代码是用 SBT 实现的 `Map`，即有序不可重映射：

完整代码

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    /**
     * @file SizeBalancedTreeMap.hpp
     * @brief An SizeBalancedTree-based map implementation
     * @details The map is sorted according to the natural ordering of its
     *  keys or by a {@code Compare} function provided; This implementation
     *  provides guaranteed log(n) time cost for the contains, get, insert
     *  and remove operations.
     * @author [r.ivance](https://github.com/RIvance)
     */

    #ifndef SIZE_BALANCED_TREE_MAP_HPP
    #define SIZE_BALANCED_TREE_MAP_HPP

    #include <cassert>
    #include <cstddef>
    #include <cstdint>
    #include <functional>
    #include <memory>
    #include <stack>
    #include <utility>
    #include <vector>

    /**
     * An SizeBalancedTree-based map implementation
     * http://wcipeg.com/wiki/Size_Balanced_Tree
     * @tparam Key the type of keys maintained by this map
     * @tparam Value the type of mapped values
     * @tparam Compare the compare function
     */
    template <typename Key, typename Value, typename Compare = std::less<Key> >
    class SizeBalancedTreeMap {
     private:
      using USize = size_t;

      Compare compare = Compare();

     public:
      struct Entry {
        Key key;
        Value value;

        bool operator==(const Entry &rhs) const noexcept {
          return this->key == rhs.key && this->value == rhs.value;
        }

        bool operator!=(const Entry &rhs) const noexcept {
          return this->key != rhs.key || this->value != rhs.value;
        }
      };

     private:
      struct Node {
        using Ptr = std::shared_ptr<Node>;
        using Provider = const std::function<Ptr(void)> &;
        using Consumer = const std::function<void(const Ptr &)> &;

        Key key;
        Value value{};

        Ptr left = nullptr;
        Ptr right = nullptr;

        USize size = 1;

        explicit Node(Key k) : key(std::move(k)) {}

        explicit Node(Key k, Value v) : key(std::move(k)), value(std::move(v)) {}

        ~Node() = default;

        inline bool isLeaf() const noexcept {
          return this->left == nullptr && this->right == nullptr;
        }

        inline void updateSize() {
          USize leftSize = this->left != nullptr ? this->left->size : 0;
          USize rightSize = this->right != nullptr ? this->right->size : 0;
          this->size = leftSize + rightSize + 1;
        }

        inline Entry entry() const { return Entry{key, value}; }

        static Ptr from(const Key &k) { return std::make_shared<Node>(Node(k)); }

        static Ptr from(const Key &k, const Value &v) {
          return std::make_shared<Node>(Node(k, v));
        }
      };

      using NodePtr = typename Node::Ptr;
      using ConstNodePtr = const NodePtr &;
      using NodeProvider = typename Node::Provider;
      using NodeConsumer = typename Node::Consumer;

      NodePtr root = nullptr;

      using K = const Key &;
      using V = const Value &;

     public:
      using EntryList = std::vector<Entry>;
      using KeyValueConsumer = const std::function<void(K, V)> &;
      using MutKeyValueConsumer = const std::function<void(K, Value &)> &;
      using KeyValueFilter = const std::function<bool(K, V)> &;

      class NoSuchMappingException : protected std::exception {
       private:
        const char *message;

       public:
        explicit NoSuchMappingException(const char *msg) : message(msg) {}

        const char *what() const noexcept override { return message; }
      };

      SizeBalancedTreeMap() noexcept = default;

      /**
       * Returns the number of entries in this map.
       * @return size_t
       */
      inline USize size() const noexcept {
        if (this->root != nullptr) {
          return this->root->size;
        } else {
          return 0;
        }
      }

      /**
       * Returns true if this collection contains no elements.
       * @return bool
       */
      inline bool empty() const noexcept { return this->root == nullptr; }

      /**
       * Removes all of the elements from this map.
       */
      void clear() noexcept { this->root = nullptr; }

      /**
       * Returns the value to which the specified key is mapped; If this map
       * contains no mapping for the key, a {@code NoSuchMappingException} will
       * be thrown.
       * @param key
       * @return SizeBalancedTreeMap<Key, Value>::Value
       * @throws NoSuchMappingException
       */
      Value get(K key) const {
        if (this->root == nullptr) {
          throw NoSuchMappingException("Invalid key");
        } else {
          NodePtr node = this->getNode(this->root, key);
          if (node != nullptr) {
            return node->value;
          } else {
            throw NoSuchMappingException("Invalid key");
          }
        }
      }

      /**
       * Returns the value to which the specified key is mapped; If this map
       * contains no mapping for the key, a new mapping with a default value
       * will be inserted.
       * @param key
       * @return SizeBalancedTreeMap<Key, Value>::Value &
       */
      Value &getOrDefault(K key) {
        if (this->root == nullptr) {
          this->root = Node::from(key);
          return this->root->value;
        } else {
          return this
              ->getNodeOrProvide(this->root, key,
                                 [&key]() { return Node::from(key); })
              ->value;
        }
      }

      /**
       * Returns true if this map contains a mapping for the specified key.
       * @param key
       * @return bool
       */
      bool contains(K key) const {
        return this->getNode(this->root, key) != nullptr;
      }

      /**
       * Associates the specified value with the specified key in this map.
       * @param key
       * @param value
       */
      void insert(K key, V value) {
        if (this->root == nullptr) {
          this->root = Node::from(key, value);
        } else {
          this->insert(this->root, key, value);
        }
      }

      /**
       * If the specified key is not already associated with a value, associates
       * it with the given value and returns true, else returns false.
       * @param key
       * @param value
       * @return bool
       */
      bool insertIfAbsent(K key, V value) {
        USize sizeBeforeInsertion = this->size();
        if (this->root == nullptr) {
          this->root = Node::from(key, value);
        } else {
          this->insert(this->root, key, value, false);
        }
        return this->size() > sizeBeforeInsertion;
      }

      /**
       * If the specified key is not already associated with a value, associates
       * it with the given value and returns the value, else returns the associated
       * value.
       * @param key
       * @param value
       * @return SizeBalancedTreeMap<Key, Value>::Value &
       */
      Value &getOrInsert(K key, V value) {
        if (this->root == nullptr) {
          this->root = Node::from(key, value);
          return root->value;
        } else {
          NodePtr node = getNodeOrProvide(this->root, key,
                                          [&]() { return Node::from(key, value); });
          return node->value;
        }
      }

      Value operator[](K key) const { return this->get(key); }

      Value &operator[](K key) { return this->getOrDefault(key); }

      /**
       * Removes the mapping for a key from this map if it is present;
       * Returns true if the mapping is present else returns false
       * @param key the key of the mapping
       * @return bool
       */
      bool remove(K key) {
        if (this->root == nullptr) {
          return false;
        } else {
          return this->remove(this->root, key, [](ConstNodePtr) {});
        }
      }

      /**
       * Removes the mapping for a key from this map if it is present and returns
       * the value which is mapped to the key; If this map contains no mapping for
       * the key, a {@code NoSuchMappingException} will be thrown.
       * @param key
       * @return SizeBalancedTreeMap<Key, Value>::Value
       * @throws NoSuchMappingException
       */
      Value getAndRemove(K key) {
        Value result;
        NodeConsumer action = [&](ConstNodePtr node) { result = node->value; };

        if (root == nullptr) {
          throw NoSuchMappingException("Invalid key");
        } else {
          if (remove(this->root, key, action)) {
            return result;
          } else {
            throw NoSuchMappingException("Invalid key");
          }
        }
      }

      /**
       * Gets the entry corresponding to the specified key; if no such entry
       * exists, returns the entry for the least key greater than the specified
       * key; if no such entry exists (i.e., the greatest key in the Tree is less
       * than the specified key), a {@code NoSuchMappingException} will be thrown.
       * @param key
       * @return SizeBalancedTreeMap<Key, Value>::Entry
       * @throws NoSuchMappingException
       */
      Entry getCeilingEntry(K key) const {
        if (this->root == nullptr) {
          throw NoSuchMappingException("No ceiling entry in this map");
        }

        NodePtr node = this->root;
        std::stack<NodePtr> ancestors;

        while (node != nullptr) {
          if (key == node->key) {
            return node->entry();
          }

          if (compare(key, node->key)) {
            /* key < node->key */
            if (node->left != nullptr) {
              ancestors.push(node);
              node = node->left;
            } else {
              return node->entry();
            }
          } else {
            /* key > node->key */
            if (node->right != nullptr) {
              ancestors.push(node);
              node = node->right;
            } else {
              if (ancestors.empty()) {
                throw NoSuchMappingException("No ceiling entry in this map");
              }

              NodePtr parent = ancestors.top();
              ancestors.pop();

              while (node == parent->right) {
                node = parent;
                if (!ancestors.empty()) {
                  parent = ancestors.top();
                  ancestors.pop();
                } else {
                  throw NoSuchMappingException("No ceiling entry in this map");
                }
              }

              return parent->entry();
            }
          }
        }

        throw NoSuchMappingException("No ceiling entry in this map");
      }

      /**
       * Gets the entry corresponding to the specified key; if no such entry exists,
       * returns the entry for the greatest key less than the specified key;
       * if no such entry exists, a {@code NoSuchMappingException} will be thrown.
       * @param key
       * @return SizeBalancedTreeMap<Key, Value>::Entry
       * @throws NoSuchMappingException
       */
      Entry getFloorEntry(K key) const {
        if (this->root == nullptr) {
          throw NoSuchMappingException("No floor entry exists in this map");
        }

        NodePtr node = this->root;
        std::stack<NodePtr> ancestors;

        while (node != nullptr) {
          if (key == node->key) {
            return node->entry();
          }

          if (compare(key, node->key)) {
            /* key < node->key */
            if (node->left != nullptr) {
              ancestors.push(node);
              node = node->left;
            } else {
              if (ancestors.empty()) {
                throw NoSuchMappingException("No floor entry exists in this map");
              }

              NodePtr parent = ancestors.top();
              ancestors.pop();

              while (node == parent->left) {
                node = parent;
                if (!ancestors.empty()) {
                  parent = ancestors.top();
                  ancestors.pop();
                } else {
                  throw NoSuchMappingException("No floor entry exists in this map");
                }
              }

              return parent->entry();
            }
          } else {
            /* key > node->key */
            if (node->right != nullptr) {
              ancestors.push(node);
              node = node->right;
            } else {
              return node->entry();
            }
          }
        }

        throw NoSuchMappingException("No floor entry exists in this map");
      }

      /**
       * Gets the entry for the least key greater than the specified
       * key; if no such entry exists, returns the entry for the least
       * key greater than the specified key; if no such entry exists,
       * a {@code NoSuchMappingException} will be thrown.
       * @param key
       * @return SizeBalancedTreeMap<Key, Value>::Entry
       * @throws NoSuchMappingException
       */
      Entry getHigherEntry(K key) {
        if (this->root == nullptr) {
          throw NoSuchMappingException("No higher entry exists in this map");
        }

        NodePtr node = this->root;
        std::stack<NodePtr> ancestors;

        while (node != nullptr) {
          if (compare(key, node->key)) {
            /* key < node->key */
            if (node->left != nullptr) {
              ancestors.push(node);
              node = node->left;
            } else {
              return node->entry();
            }
          } else {
            /* key >= node->key */
            if (node->right != nullptr) {
              ancestors.push(node);
              node = node->right;
            } else {
              if (ancestors.empty()) {
                throw NoSuchMappingException("No higher entry exists in this map");
              }

              NodePtr parent = ancestors.top();
              ancestors.pop();

              while (node == parent->right) {
                node = parent;
                if (!ancestors.empty()) {
                  parent = ancestors.top();
                  ancestors.pop();
                } else {
                  throw NoSuchMappingException(
                      "No higher entry exists in this map");
                }
              }

              return parent->entry();
            }
          }
        }

        throw NoSuchMappingException("No higher entry exists in this map");
      }

      /**
       * Returns the entry for the greatest key less than the specified key; if
       * no such entry exists (i.e., the least key in the Tree is greater than
       * the specified key), a {@code NoSuchMappingException} will be thrown.
       * @param key
       * @return SizeBalancedTreeMap<Key, Value>::Entry
       * @throws NoSuchMappingException
       */
      Entry getLowerEntry(K key) const {
        if (this->root == nullptr) {
          throw NoSuchMappingException("No lower entry exists in this map");
        }

        NodePtr node = this->root;
        std::stack<NodePtr> ancestors;

        while (node != nullptr) {
          if (compare(key, node->key) || key == node->key) {
            /* key <= node->key */
            if (node->left != nullptr) {
              ancestors.push(node);
              node = node->left;
            } else {
              if (ancestors.empty()) {
                throw NoSuchMappingException("No lower entry exists in this map");
              }

              NodePtr parent = ancestors.top();
              ancestors.pop();

              while (node == parent->left) {
                node = parent;
                if (!ancestors.empty()) {
                  parent = ancestors.top();
                  ancestors.pop();
                } else {
                  throw NoSuchMappingException("No lower entry exists in this map");
                }
              }

              return parent->entry();
            }
          } else {
            /* key > node->key */
            if (node->right != nullptr) {
              ancestors.push(node);
              node = node->right;
            } else {
              return node->entry();
            }
          }
        }

        throw NoSuchMappingException("No lower entry exists in this map");
      }

      /**
       * Count the number of entries that are less than the given key
       * @param key
       * @return USize
       */
      USize countLessThan(K key) { return this->countLess(this->root, key); }

      /**
       * Count the number of entries that are less or equal to the given key
       * @param key
       * @return USize
       */
      USize countLessOrEqualTo(K key) {
        return this->countLess(this->root, key, true);
      }

      /**
       * Count the number of entries that are greater than the given key
       * @param key
       * @return USize
       */
      USize countGreaterThan(K key) { return this->countGreater(this->root, key); }

      /**
       * Count the number of entries that are greater or equal to the given key
       * @param key
       * @return USize
       */
      USize countGreaterOrEqualTo(K key) {
        return this->countGreater(this->root, key, true);
      }

      /**
       * Remove all entries that satisfy the filter condition.
       * @param filter
       */
      void removeAll(KeyValueFilter filter) {
        std::vector<Key> keys;
        this->inorderTraversal([&](ConstNodePtr node) {
          if (filter(node->key, node->value)) {
            keys.push_back(node->key);
          }
        });
        for (const Key &key : keys) {
          this->remove(key);
        }
      }

      /**
       * Performs the given action for each key and value entry in this map.
       * The value is immutable for the action.
       * @param action
       */
      void forEach(KeyValueConsumer action) const {
        this->inorderTraversal(
            [&](ConstNodePtr node) { action(node->key, node->value); });
      }

      /**
       * Performs the given action for each key and value entry in this map.
       * The value is mutable for the action.
       * @param action
       */
      void forEachMut(MutKeyValueConsumer action) {
        this->inorderTraversal(
            [&](ConstNodePtr node) { action(node->key, node->value); });
      }

      /**
       * Returns a list containing all of the entries in this map.
       * @return SizeBalancedTreeMap<Key, Value>::EntryList
       */
      EntryList toEntryList() const {
        EntryList entryList;
        this->inorderTraversal(
            [&](ConstNodePtr node) { entryList.push_back(node->entry()); });
        return entryList;
      }

     private:
      static USize size(ConstNodePtr node) {
        return node != nullptr ? node->size : 0;
      }

      static void rotateLeft(NodePtr &node) {
        assert(node != nullptr);
        // clang-format off
        //     |                       |
        //     N                       S
        //    / \     l-rotate(N)     / \
        //   L   S    ==========>    N   R
        //      / \                 / \
        //     M   R               L   M
        // clang-format on
        NodePtr successor = node->right;
        node->right = successor->left;
        successor->left = node;

        node->updateSize();
        successor->updateSize();

        node = successor;
      }

      static void rotateRight(NodePtr &node) {
        assert(node != nullptr);
        // clang-format off
        //       |                   |
        //       N                   S
        //      / \   r-rotate(N)   / \
        //     S   R  ==========>  L   N
        //    / \                     / \
        //   L   M                   M   R
        // clang-format on
        NodePtr successor = node->left;
        node->left = successor->right;
        successor->right = node;

        node->updateSize();
        successor->updateSize();

        node = successor;
      }

      static void swapNode(NodePtr &lhs, NodePtr &rhs) {
        std::swap(lhs->key, rhs->key);
        std::swap(lhs->value, rhs->value);
        std::swap(lhs, rhs);
      }

      static void fixBalance(NodePtr &node) {
        if (node == nullptr) {
          return;
        }

        if (node->left != nullptr) {
          if (size(node->left->left) > size(node->right)) {
            // clang-format off
            //       |                       |
            //       N                       L
            //      / \     r-rotate(N)     / \
            //     L  <R>   ==========>   [M]  N
            //    /                             \
            //  [M]                             <R>
            // clang-format on
            rotateRight(node);
            fixBalance(node->right);
            fixBalance(node);
            return;
          } else if (size(node->left->right) > size(node->right)) {
            // clang-format off
            //     |                     |                      |
            //     N                     N                     [M]
            //    / \    l-rotate(L)    / \     r-rotate(N)    / \
            //   L  <R>  ==========>  [M] <R>   ==========>   L   N
            //    \                   /                            \
            //    [M]                L                             <R>
            // clang-format on
            rotateLeft(node->left);
            rotateRight(node);
            fixBalance(node->left);
            fixBalance(node->right);
            fixBalance(node);
            return;
          }
        }

        if (node->right != nullptr) {
          if (size(node->right->right) > size(node->left)) {
            // clang-format off
            //     |                       |
            //     N                       R
            //    / \     l-rotate(N)     / \
            //  <L>  R    ==========>    N  [M]
            //        \                 /
            //        [M]             <L>
            // clang-format on
            rotateLeft(node);
            fixBalance(node->left);
            fixBalance(node);
            return;
          } else if (size(node->right->left) > size(node->left)) {
            // clang-format off
            //     |                     |                      |
            //     N                     N                     [M]
            //    / \    r-rotate(R)    / \     l-rotate(N)    / \
            //  <L>  R   ==========>  <L> [M]   ==========>   N   R
            //      /                       \                /
            //    [M]                        R             <L>
            // clang-format on
            rotateRight(node->right);
            rotateLeft(node);
            fixBalance(node->left);
            fixBalance(node->right);
            fixBalance(node);
            return;
          }
        }
      }

      NodePtr getNodeOrProvide(NodePtr &node, K key, NodeProvider provide) {
        assert(node != nullptr);

        if (key == node->key) {
          return node;
        }

        assert(key != node->key);

        NodePtr result;

        if (compare(key, node->key)) {
          /* key < node->key */
          if (node->left == nullptr) {
            result = node->left = provide();
            node->updateSize();
          } else {
            result = getNodeOrProvide(node->left, key, provide);
            node->updateSize();
            fixBalance(node);
          }
        } else {
          /* key > node->key */
          if (node->right == nullptr) {
            result = node->right = provide();
            node->updateSize();
          } else {
            result = getNodeOrProvide(node->right, key, provide);
            node->updateSize();
            fixBalance(node);
          }
        }

        return result;
      }

      NodePtr getNode(ConstNodePtr node, K key) const {
        assert(node != nullptr);

        if (key == node->key) {
          return node;
        }

        if (compare(key, node->key)) {
          /* key < node->key */
          return node->left == nullptr ? nullptr : getNode(node->left, key);
        } else {
          /* key > node->key */
          return node->right == nullptr ? nullptr : getNode(node->right, key);
        }
      }

      void insert(NodePtr &node, K key, V value, bool replace = true) {
        assert(node != nullptr);

        if (key == node->key) {
          if (replace) {
            node->value = value;
          }
          return;
        }

        assert(key != node->key);

        if (compare(key, node->key)) {
          /* key < node->key */
          if (node->left == nullptr) {
            node->left = Node::from(key, value);
            node->updateSize();
          } else {
            insert(node->left, key, value, replace);
            node->updateSize();
            fixBalance(node);
          }
        } else {
          /* key > node->key */
          if (node->right == nullptr) {
            node->right = Node::from(key, value);
            node->updateSize();
          } else {
            insert(node->right, key, value, replace);
            node->updateSize();
            fixBalance(node);
          }
        }
      }

      bool remove(NodePtr &node, K key, NodeConsumer action) {
        assert(node != nullptr);

        if (key != node->key) {
          if (compare(key, node->key)) {
            /* key < node->key */
            NodePtr &left = node->left;
            if (left != nullptr && remove(left, key, action)) {
              node->updateSize();
              fixBalance(node);
              return true;
            } else {
              return false;
            }
          } else {
            /* key > node->key */
            NodePtr &right = node->right;
            if (right != nullptr && remove(right, key, action)) {
              node->updateSize();
              fixBalance(node);
              return true;
            } else {
              return false;
            }
          }
        }

        assert(key == node->key);
        action(node);

        if (node->isLeaf()) {
          // Case 1: no child
          node = nullptr;
        } else if (node->right == nullptr) {
          // clang-format off
          // Case 2: left child only
          //     P
          //     |  remove(N)  P
          //     N  ========>  |
          //    /              L
          //   L
          // clang-format on
          node = node->left;
        } else if (node->left == nullptr) {
          // clang-format off
          // Case 3: right child only
          //   P
          //   |    remove(N)  P
          //   N    ========>  |
          //    \              R
          //     R
          // clang-format on
          node = node->right;
        } else if (node->right->left == nullptr) {
          // clang-format off
          // Case 4: both left and right child, right child has no left child
          //    |                 |
          //    N    remove(N)    R
          //   / \   ========>   /
          //  L   R             L
          // clang-format on
          NodePtr right = node->right;
          swapNode(node, right);
          right->right = node->right;
          node = right;
          node->updateSize();
          fixBalance(node);
        } else {
          // clang-format off
          // Case 5: both left and right child, right child is not a leaf
          //   Step 1. find the node N with the smallest key
          //           and its parent P on the right subtree
          //   Step 2. swap S and N
          //   Step 3. remove node N like Case 1 or Case 3
          //   Step 4. update size for all nodes on the path
          //           from S to P
          //     |                  |
          //     N                  S                 |
          //    / \                / \                S
          //   L  ..  swap(N, S)  L  ..  remove(N)   / \
          //       |  =========>      |  ========>  L  ..
          //       P                  P                 |
          //      / \                / \                P
          //     S  ..              N  ..              / \
          //      \                  \                R  ..
          //       R                  R
          // clang-format on

          std::stack<NodePtr> path;

          // Step 1
          NodePtr successor = node->right;
          NodePtr parent = node;
          path.push(node);

          while (successor->left != nullptr) {
            path.push(successor);
            parent = successor;
            successor = parent->left;
          }

          // Step 2
          swapNode(node, successor);

          // Step 3
          parent->left = node->right;
          // Restore node
          node = successor;

          // Step 4
          while (!path.empty()) {
            path.top()->updateSize();
            path.pop();
          }
        }

        return true;
      }

      USize countLess(ConstNodePtr node, K key, bool countEqual = false) const {
        if (node == nullptr) {
          return 0;
        } else if (key < node->key) {
          return countLess(node->left, key, countEqual);
        } else if (key > node->key) {
          return size(node->left) + 1 + countLess(node->right, key, countEqual);
        } else {
          return size(node->left) + (countEqual ? 1 : 0);
        }
      }

      USize countGreater(ConstNodePtr node, K key, bool countEqual = false) const {
        if (node == nullptr) {
          return 0;
        } else if (key < node->key) {
          return size(node->right) + 1 + countGreater(node->left, key, countEqual);
        } else if (key > node->key) {
          return countGreater(node->right, key, countEqual);
        } else {
          return size(node->right) + (countEqual ? 1 : 0);
        }
      }

      void inorderTraversal(NodeConsumer action) const {
        if (this->root == nullptr) {
          return;
        }

        std::stack<NodePtr> stack;
        NodePtr node = this->root;

        while (node != nullptr || !stack.empty()) {
          while (node != nullptr) {
            stack.push(node);
            node = node->left;
          }
          if (!stack.empty()) {
            node = stack.top();
            stack.pop();
            action(node);
            node = node->right;
          }
        }
      }
    };

    #endif  // SIZE_BALANCED_TREE_MAP_HPP

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