LevelDB源码之SkipList原理

 

LevelDB源码之SkipList原理

分类： LevelDB源码系列 2013-08-15 22:01  315人阅读  评论(0)  收藏  举报

LevelDB源码系列

感觉SkipList只要搞清楚高度就好了.下面是随机生成高度的函数RandomHeight()

 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;
 }

插入操作：

 template<typename Key, class Comparator>
 void SkipList<Key,Comparator>::Insert(const Key& key) {
   // TODO(opt): We can use a barrier-free（(屏蔽，障碍/无障碍） variant(变种) of FindGreaterOrEqual()
   // here since Insert() is externally synchronized.
   //prev为各层的前置节点
   Node* prev[kMaxHeight];
   //查找插入的节点
   Node* x = FindGreaterOrEqual(key, prev);

   // Our data structure does not allow duplicate insertion
   assert(x == NULL || !Equal(key, x->key));
   //随机生成节点高度
   int height = RandomHeight();
   if (height > GetMaxHeight()) {
     for (int i = GetMaxHeight(); i < height; i++) {
       prev[i] = head_;
     }
     //fprintf(stderr, "Change height from %d to %d\n", max_height_, height);

     // 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_ (NULL), or a new value set in
     // the loop below.  In the former case the reader will
     // immediately drop to the next level since NULL sorts after all
     // keys.  In the latter case the reader will use the new node.
     //设置最大高度
     max_height_.NoBarrier_Store(reinterpret_cast<void*>(height));
   }
   //生成一个新的Node
   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);
   }
 }

查找操作：

 template<typename Key, class Comparator>
 typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindGreaterOrEqual(const Key& key, Node** prev)
     const {
   Node* x = head_;
   //从最顶层开始查找
   int level = GetMaxHeight() - 1;
   while (true) {
     Node* next = x->Next(level);
     //向右查找
     if (KeyIsAfterNode(key, next)) {
       // Keep searching in this list
       x = next;
     } else {
         //向下查找
       if (prev != NULL) prev[level] = x;
       if (level == 0) {
         return next;
       } else {
         // Switch to next list
         level--;
       }
     }
   }
 }

整个SkipList.h源码：

 // 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.
 //
 // 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 <assert.h>
 #include <stdlib.h>
 #include "port/port.h"
 #include "util/arena.h"
 #include "util/random.h"
 /**
  * SkipList应用概率来保证平衡，平衡树采用严格的旋转来保证平衡
  * 使用SkipList是的查找特定值得时间复杂度是O(logN)，与平衡树有着相同的复杂度，节省空间，每个检点平均1.33个指针
  * leveldb的最高层数为12，只允许插入和修改，Record的key不允许重复，添加一个Sequene Num
  * 规范：参数使用引用，返回值使用指针
  *
  */
 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);

   // 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
   };

   //跳表的最高level数
  private:
   enum { kMaxHeight = 12 };

   // Immutable(不可变) after construction
   Comparator const compare_;
   //舞台，竞技场，场所（只负责申请空间的内存池）
   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.
   port::AtomicPointer max_height_;   // Height of the entire list

   inline int GetMaxHeight() const {
     return static_cast<int>(
         reinterpret_cast<intptr_t>(max_height_.NoBarrier_Load()));
   }

   // Read/written only by Insert().
   Random rnd_;

   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 NULL 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;

   // No copying allowed
   SkipList(const SkipList&);
   void operator=(const SkipList&);
 };

 // Implementation details follow
 template<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 reinterpret_cast<Node*>(next_[n].Acquire_Load());
   }
   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].Release_Store(x);
   }

   // No-barrier variants that can be safely used in a few locations.
   Node* NoBarrier_Next(int n) {
     assert(n >= 0);
     return reinterpret_cast<Node*>(next_[n].NoBarrier_Load());
   }
   void NoBarrier_SetNext(int n, Node* x) {
     assert(n >= 0);
     next_[n].NoBarrier_Store(x);
   }

  private:
   // Array of length equal to the node height.  next_[0] is lowest level link.
   port::AtomicPointer next_[1];
 };

 template<typename Key, class Comparator>
 typename SkipList<Key,Comparator>::Node*
 SkipList<Key,Comparator>::NewNode(const Key& key, int height) {
   char* mem = arena_->AllocateAligned(
       sizeof(Node) + sizeof(port::AtomicPointer) * (height - 1));
   return new (mem) Node(key);
 }

 template<typename Key, class Comparator>
 inline SkipList<Key,Comparator>::Iterator::Iterator(const SkipList* list) {
   list_ = list;
   node_ = NULL;
 }

 template<typename Key, class Comparator>
 inline bool SkipList<Key,Comparator>::Iterator::Valid() const {
   return node_ != NULL;
 }

 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_ = NULL;
   }
 }

 template<typename Key, class Comparator>
 inline void SkipList<Key,Comparator>::Iterator::Seek(const Key& target) {
   node_ = list_->FindGreaterOrEqual(target, NULL);
 }

 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_ = NULL;
   }
 }

 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 != NULL) && (compare_(n->key, key) < 0);
 }
 /**
  * 查找，从左向右，从上向下查找
  */
 template<typename Key, class Comparator>
 typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindGreaterOrEqual(const Key& key, Node** prev)
     const {
   Node* x = head_;
   //从最顶层开始查找
   int level = GetMaxHeight() - 1;
   while (true) {
     Node* next = x->Next(level);
     //向右查找
     if (KeyIsAfterNode(key, next)) {
       // Keep searching in this list
       x = next;
     } else {
         //向下查找
       if (prev != NULL) 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 == NULL || 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 == NULL) {
       if (level == 0) {
         return x;
       } else {
         // Switch to next list
         level--;
       }
     } else {
       x = next;
     }
   }
 }
 /**
  * SkipList初始化
  *
  */
 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_(reinterpret_cast<void*>(1)),
       rnd_(0xdeadbeef) {
   for (int i = 0; i < kMaxHeight; i++) {
     head_->SetNext(i, NULL);
   }
 }
 /**
  * 插入一个新的key
  *
  */
 template<typename Key, class Comparator>
 void SkipList<Key,Comparator>::Insert(const Key& key) {
   // TODO(opt): We can use a barrier-free（(屏蔽，障碍/无障碍） variant(变种) of FindGreaterOrEqual()
   // here since Insert() is externally synchronized.
   //prev为各层的前置节点
   Node* prev[kMaxHeight];
   //查找插入的节点
   Node* x = FindGreaterOrEqual(key, prev);

   // Our data structure does not allow duplicate insertion
   assert(x == NULL || !Equal(key, x->key));
   //随机生成节点高度
   int height = RandomHeight();
   if (height > GetMaxHeight()) {
     for (int i = GetMaxHeight(); i < height; i++) {
       prev[i] = head_;
     }
     //fprintf(stderr, "Change height from %d to %d\n", max_height_, height);

     // 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_ (NULL), or a new value set in
     // the loop below.  In the former case the reader will
     // immediately drop to the next level since NULL sorts after all
     // keys.  In the latter case the reader will use the new node.
     //设置最大高度
     max_height_.NoBarrier_Store(reinterpret_cast<void*>(height));
   }
   //生成一个新的Node
   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);
   }
 }
 /**
  * 包含
  */
 template<typename Key, class Comparator>
 bool SkipList<Key,Comparator>::Contains(const Key& key) const {
   Node* x = FindGreaterOrEqual(key, NULL);
   if (x != NULL && Equal(key, x->key)) {
     return true;
   } else {
     return false;
   }
 }

 }  // namespace leveldb