leveldb源代码分析4：SkipList

skiplist思想可以具体参考这：

Skip list

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或者是参考我的这篇博文：http://www.cnblogs.com/xuqiang/archive/2011/05/22/2053516.html， leveldb中的实现方式基本上和我的那篇博文中的实现方式类似。SkipList在db/skiplist.h中声明，向外界暴漏接口非常简单，如下：

  // 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;

private成员变量：

// 最大的level  enum { kMaxHeight = 12 };  // Immutable after construction  Comparator const compare_;  // 内存分配器  Arena* const arena_;    // Arena used for allocations of nodes    // 指向第一个节点，构造函数中初始化  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

我们下面来首先分析初始化操作，如下：

// 初始化:// 1. 初始化compare_// 2. 初始化arena_// 3. 初始化head_，指向指针数组// 4. 初始化max_height_// 5. 初始化rnd_随机数的seed// 6. 初始化head_指向的数组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) {          // 初始化head_指向的数组  for (int i = 0; i < kMaxHeight; i++) {    head_->SetNext(i, NULL);  }}

下面是一个插入操作的示意图：

leveldb中实现的插入代码就是按照上面的思路实现，首先查找到合适的位置，并记录查找过程中经过的路径，之后新生成一个节点，修改指针。

// 插入操作// 这里的key其实已经是经过处理的key，包含了用户指定的key和valuetemplate<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.  // prev记录的是查询路径，下面需要使用prev来修改新生成  // 节点的指针  Node* prev[kMaxHeight];  Node* x = FindGreaterOrEqual(key, prev);  // Our data structure does not allow duplicate insertion  // 不允许插入重复的值  assert(x == NULL || !Equal(key, x->key));    // 随即生成节点高度  int height = RandomHeight();  // 对prev数组中未赋值的元素进行赋值  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_ (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max_height变量    max_height_.NoBarrier_Store(reinterpret_cast<void*>(height));  }    // 新生成一个节点，之后插入数据  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].    // 修改两部分的指针，一部分是需要执行新插入节点的指针    // 另外的一部分是x节点的指针    x->NoBarrier_SetNext(i, prev[i]->NoBarrier_Next(i));    prev[i]->SetNext(i, x);  }}

函数FindGreaterOrEqual中完成查询操作，就是向下（level控制）和向右（x控制）移动过程，并不断经经过路径保存到参数prev中。

template<typename Key, class Comparator>typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindGreaterOrEqual(const Key& key,                                                                                      Node** prev)    const {        // 从最高层开始查找  Node* x = head_;  int level = GetMaxHeight() - 1;  while (true) {    Node* next = x->Next(level);    if (KeyIsAfterNode(key, next)) {    // 向右移动      // Keep searching in this list      x = next;    }    else    // 向下移动    {        // 记录查找路径      if (prev != NULL)        prev[level] = x;      if (level == 0) {        return next;      } else {        // Switch to next list下一层寻找        level--;      }    }  }}

查找操作基本上就是调用函数上面的函数FindGreaterOrEqual实现：

// 查询操作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;  }}

上面基本上就是skiplist在leveldb中实现，leveldb中没有使用复杂的红黑树等机制去保证数据的有序性，而是使用了轻快的skiplist实现。最后需要注意skiplist中每个节点存储key是用户传递keyvalue经过变幻（变幻方法参考http://blog.csdn.net/xuqianghit/article/details/6948164）得到的。