git / moonbite-core / src/leveldb/db/skiplist.h · raw

// Copyright (c) 2011 The LevelDB Authors. All rights reserved.// Use of this source code is governed by a BSD-style license that can be// found in the LICENSE file. See the AUTHORS file for names of contributors.#ifndef STORAGE_LEVELDB_DB_SKIPLIST_H_#define STORAGE_LEVELDB_DB_SKIPLIST_H_// Thread safety// -------------//// Writes require external synchronization, most likely a mutex.// Reads require a guarantee that the SkipList will not be destroyed// while the read is in progress.  Apart from that, reads progress// without any internal locking or synchronization.//// Invariants://// (1) Allocated nodes are never deleted until the SkipList is// destroyed.  This is trivially guaranteed by the code since we// never delete any skip list nodes.//// (2) The contents of a Node except for the next/prev pointers are// immutable after the Node has been linked into the SkipList.// Only Insert() modifies the list, and it is careful to initialize// a node and use release-stores to publish the nodes in one or// more lists.//// ... prev vs. next pointer ordering ...#include <atomic>#include <cassert>#include <cstdlib>#include "util/arena.h"#include "util/random.h"namespace leveldb {class Arena;template <typename Key, class Comparator>class SkipList { private:  struct Node; public:  // Create a new SkipList object that will use "cmp" for comparing keys,  // and will allocate memory using "*arena".  Objects allocated in the arena  // must remain allocated for the lifetime of the skiplist object.  explicit SkipList(Comparator cmp, Arena* arena);  SkipList(const SkipList&) = delete;  SkipList& operator=(const SkipList&) = delete;  // Insert key into the list.  // REQUIRES: nothing that compares equal to key is currently in the list.  void Insert(const Key& key);  // Returns true iff an entry that compares equal to key is in the list.  bool Contains(const Key& key) const;  // Iteration over the contents of a skip list  class Iterator {   public:    // Initialize an iterator over the specified list.    // The returned iterator is not valid.    explicit Iterator(const SkipList* list);    // Returns true iff the iterator is positioned at a valid node.    bool Valid() const;    // Returns the key at the current position.    // REQUIRES: Valid()    const Key& key() const;    // Advances to the next position.    // REQUIRES: Valid()    void Next();    // Advances to the previous position.    // REQUIRES: Valid()    void Prev();    // Advance to the first entry with a key >= target    void Seek(const Key& target);    // Position at the first entry in list.    // Final state of iterator is Valid() iff list is not empty.    void SeekToFirst();    // Position at the last entry in list.    // Final state of iterator is Valid() iff list is not empty.    void SeekToLast();   private:    const SkipList* list_;    Node* node_;    // Intentionally copyable  }; private:  enum { kMaxHeight = 12 };  inline int GetMaxHeight() const {    return max_height_.load(std::memory_order_relaxed);  }  Node* NewNode(const Key& key, int height);  int RandomHeight();  bool Equal(const Key& a, const Key& b) const { return (compare_(a, b) == 0); }  // Return true if key is greater than the data stored in "n"  bool KeyIsAfterNode(const Key& key, Node* n) const;  // Return the earliest node that comes at or after key.  // Return nullptr if there is no such node.  //  // If prev is non-null, fills prev[level] with pointer to previous  // node at "level" for every level in [0..max_height_-1].  Node* FindGreaterOrEqual(const Key& key, Node** prev) const;  // Return the latest node with a key < key.  // Return head_ if there is no such node.  Node* FindLessThan(const Key& key) const;  // Return the last node in the list.  // Return head_ if list is empty.  Node* FindLast() const;  // Immutable after construction  Comparator const compare_;  Arena* const arena_;  // Arena used for allocations of nodes  Node* const head_;  // Modified only by Insert().  Read racily by readers, but stale  // values are ok.  std::atomic<int> max_height_;  // Height of the entire list  // Read/written only by Insert().  Random rnd_;};// Implementation details followtemplate <typename Key, class Comparator>struct SkipList<Key, Comparator>::Node {  explicit Node(const Key& k) : key(k) {}  Key const key;  // Accessors/mutators for links.  Wrapped in methods so we can  // add the appropriate barriers as necessary.  Node* Next(int n) {    assert(n >= 0);    // Use an 'acquire load' so that we observe a fully initialized    // version of the returned Node.    return next_[n].load(std::memory_order_acquire);  }  void SetNext(int n, Node* x) {    assert(n >= 0);    // Use a 'release store' so that anybody who reads through this    // pointer observes a fully initialized version of the inserted node.    next_[n].store(x, std::memory_order_release);  }  // No-barrier variants that can be safely used in a few locations.  Node* NoBarrier_Next(int n) {    assert(n >= 0);    return next_[n].load(std::memory_order_relaxed);  }  void NoBarrier_SetNext(int n, Node* x) {    assert(n >= 0);    next_[n].store(x, std::memory_order_relaxed);  } private:  // Array of length equal to the node height.  next_[0] is lowest level link.  std::atomic<Node*> next_[1];};template <typename Key, class Comparator>typename SkipList<Key, Comparator>::Node* SkipList<Key, Comparator>::NewNode(    const Key& key, int height) {  char* const node_memory = arena_->AllocateAligned(      sizeof(Node) + sizeof(std::atomic<Node*>) * (height - 1));  return new (node_memory) Node(key);}template <typename Key, class Comparator>inline SkipList<Key, Comparator>::Iterator::Iterator(const SkipList* list) {  list_ = list;  node_ = nullptr;}template <typename Key, class Comparator>inline bool SkipList<Key, Comparator>::Iterator::Valid() const {  return node_ != nullptr;}template <typename Key, class Comparator>inline const Key& SkipList<Key, Comparator>::Iterator::key() const {  assert(Valid());  return node_->key;}template <typename Key, class Comparator>inline void SkipList<Key, Comparator>::Iterator::Next() {  assert(Valid());  node_ = node_->Next(0);}template <typename Key, class Comparator>inline void SkipList<Key, Comparator>::Iterator::Prev() {  // Instead of using explicit "prev" links, we just search for the  // last node that falls before key.  assert(Valid());  node_ = list_->FindLessThan(node_->key);  if (node_ == list_->head_) {    node_ = nullptr;  }}template <typename Key, class Comparator>inline void SkipList<Key, Comparator>::Iterator::Seek(const Key& target) {  node_ = list_->FindGreaterOrEqual(target, nullptr);}template <typename Key, class Comparator>inline void SkipList<Key, Comparator>::Iterator::SeekToFirst() {  node_ = list_->head_->Next(0);}template <typename Key, class Comparator>inline void SkipList<Key, Comparator>::Iterator::SeekToLast() {  node_ = list_->FindLast();  if (node_ == list_->head_) {    node_ = nullptr;  }}template <typename Key, class Comparator>int SkipList<Key, Comparator>::RandomHeight() {  // Increase height with probability 1 in kBranching  static const unsigned int kBranching = 4;  int height = 1;  while (height < kMaxHeight && ((rnd_.Next() % kBranching) == 0)) {    height++;  }  assert(height > 0);  assert(height <= kMaxHeight);  return height;}template <typename Key, class Comparator>bool SkipList<Key, Comparator>::KeyIsAfterNode(const Key& key, Node* n) const {  // null n is considered infinite  return (n != nullptr) && (compare_(n->key, key) < 0);}template <typename Key, class Comparator>typename SkipList<Key, Comparator>::Node*SkipList<Key, Comparator>::FindGreaterOrEqual(const Key& key,                                              Node** prev) const {  Node* x = head_;  int level = GetMaxHeight() - 1;  while (true) {    Node* next = x->Next(level);    if (KeyIsAfterNode(key, next)) {      // Keep searching in this list      x = next;    } else {      if (prev != nullptr) prev[level] = x;      if (level == 0) {        return next;      } else {        // Switch to next list        level--;      }    }  }}template <typename Key, class Comparator>typename SkipList<Key, Comparator>::Node*SkipList<Key, Comparator>::FindLessThan(const Key& key) const {  Node* x = head_;  int level = GetMaxHeight() - 1;  while (true) {    assert(x == head_ || compare_(x->key, key) < 0);    Node* next = x->Next(level);    if (next == nullptr || compare_(next->key, key) >= 0) {      if (level == 0) {        return x;      } else {        // Switch to next list        level--;      }    } else {      x = next;    }  }}template <typename Key, class Comparator>typename SkipList<Key, Comparator>::Node* SkipList<Key, Comparator>::FindLast()    const {  Node* x = head_;  int level = GetMaxHeight() - 1;  while (true) {    Node* next = x->Next(level);    if (next == nullptr) {      if (level == 0) {        return x;      } else {        // Switch to next list        level--;      }    } else {      x = next;    }  }}template <typename Key, class Comparator>SkipList<Key, Comparator>::SkipList(Comparator cmp, Arena* arena)    : compare_(cmp),      arena_(arena),      head_(NewNode(0 /* any key will do */, kMaxHeight)),      max_height_(1),      rnd_(0xdeadbeef) {  for (int i = 0; i < kMaxHeight; i++) {    head_->SetNext(i, nullptr);  }}template <typename Key, class Comparator>void SkipList<Key, Comparator>::Insert(const Key& key) {  // TODO(opt): We can use a barrier-free variant of FindGreaterOrEqual()  // here since Insert() is externally synchronized.  Node* prev[kMaxHeight];  Node* x = FindGreaterOrEqual(key, prev);  // Our data structure does not allow duplicate insertion  assert(x == nullptr || !Equal(key, x->key));  int height = RandomHeight();  if (height > GetMaxHeight()) {    for (int i = GetMaxHeight(); i < height; i++) {      prev[i] = head_;    }    // It is ok to mutate max_height_ without any synchronization    // with concurrent readers.  A concurrent reader that observes    // the new value of max_height_ will see either the old value of    // new level pointers from head_ (nullptr), or a new value set in    // the loop below.  In the former case the reader will    // immediately drop to the next level since nullptr sorts after all    // keys.  In the latter case the reader will use the new node.    max_height_.store(height, std::memory_order_relaxed);  }  x = NewNode(key, height);  for (int i = 0; i < height; i++) {    // NoBarrier_SetNext() suffices since we will add a barrier when    // we publish a pointer to "x" in prev[i].    x->NoBarrier_SetNext(i, prev[i]->NoBarrier_Next(i));    prev[i]->SetNext(i, x);  }}template <typename Key, class Comparator>bool SkipList<Key, Comparator>::Contains(const Key& key) const {  Node* x = FindGreaterOrEqual(key, nullptr);  if (x != nullptr && Equal(key, x->key)) {    return true;  } else {    return false;  }}}  // namespace leveldb#endif  // STORAGE_LEVELDB_DB_SKIPLIST_H_