这里主要介绍levelDB中关于SkipList的实现，关于SkipList介绍请求参阅《SkipList》，这里不再引用了。
在levelDB中的使用：
levelDB中Memtable有一个核心的数据结构Skiplist，具体实现的代码稍有不同，但是基本原理是一致的。
levelDB中Skiplist定义为模板类：
// description:
// 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.
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); // 插入一个key到Skiplist中
// Returns true iff an entry that compares equal to key is in the list.
bool Contains(const Key& key) const; // Skiplist中key的节点是否存在
private:
enum { kMaxHeight = 12 }; // 最大level
// Immutable after construction
Comparator const compare_; // key值的比较函数，一旦初始化就不能变化了（当插入一些数据后，改变key，状态不可控）
Arena* const arena_; // Arena used for allocations of nodes // levelDB中使用的Arena内存池对象
Node* const head_; // Skiplist头结点
// Modified only by Insert(). Read racily by readers, but stale
// values are ok.
port::AtomicPointer max_height_; // Height of the entire list // Skiplist层数
inline int GetMaxHeight() const { // 返回Skiplist的层数
return reinterpret_cast<intptr_t>(max_height_.NoBarrier_Load());
}
// Read/written only by Insert().
Random rnd_; // 随机器，产生随机的level层数
Node* NewNode(const Key& key, int height); // 新建一个level=height，键位key的节点
int RandomHeight(); // 随机产生一个level层数
bool Equal(const Key& a, const Key& b) const { return (compare_(a, b) == 0); } // 比较2个key是否相等
// Return true if key is greater than the data stored in "n"
bool KeyIsAfterNode(const Key& key, Node* n) const; // 比较key与Node n中的key，是否key在后面
// 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; // 找到key对应的Node或是key后面紧邻的Node
// Return the latest node with a key < key, return head_ if there is no such node.
Node* FindLessThan(const Key& key) const; // 找到key前面紧邻的Node
// Return the last node in the list.
// Return head_ if list is empty.
Node* FindLast() const; // Skiplist最后一个Node
// No copying allowed
SkipList(const SkipList&); // 拷贝构造和赋值构造操作不允许
void operator=(const SkipList&);
};
// Implementation details follow
template<typename Key, class Comparator>
struct SkipList<Key,Comparator>::Node { // Skiplist节点Node定义
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]; // forward数组指针
};
template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::NewNode(const Key& key, int height) { // 新建一个Node节点（指定key及level层数）
char* mem = arena_->AllocateAligned(sizeof(Node) + sizeof(port::AtomicPointer) * (height - 1));
return new (mem) Node(key); // 显式调用new
}
template<typename Key, class Comparator>
SkipList<Key,Comparator>::SkipList(Comparator cmp, Arena* arena) // 构造函数
: compare_(cmp),
arena_(arena),
head_(NewNode(0 /* any key will do */, kMaxHeight)), // 头节点的key没有意义
max_height_(reinterpret_cast<void*>(1)),
rnd_(0xdeadbeef) {
for (int i = 0; i < kMaxHeight; i++) {
head_->SetNext(i, NULL); // 初始化头结点
}
}
template<typename Key, class Comparator>
int SkipList<Key,Comparator>::RandomHeight() { // 返回随机高度(Skiplist依赖于这个随机性)
// Increase height with probability 1 in kBranching
static const unsigned int kBranching = 4;
int height = 1;
while (height < kMaxHeight && ((rnd_.Next() % kBranching) == 0)) { //? 直接取一个随机数不行？为什么要循环几次？
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 { // Return true if key is greater than the key stored in "n"
// NULL n is considered infinite，NULL被视为无限大（这样就考虑了结尾的NIL）
return (n != NULL) && (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)) { // key在next节点后面，如果返回true，那么肯定next不为NULL
// Keep searching in this list
x = next;
} else {
if (prev != NULL) prev[level] = x; // 当前level上，x为高度>=key节点高度，且正好排在其前面，插入和删除时使用
if (level == 0) {
return next;
} else {
// Switch to next list( low level link list)
level--;
}
}
}
}
template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindLessThan(const Key& key) const { // Return the latest node with a key < key, return head_ if there is no such node.
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) { // 从最高level尽可能向后移动更远的距离
// 后面key>查找的key时，或next为空时，level--，直到level=0
if (level == 0) {
return x;
} else {
// Switch to next list
level--; // 从最高层往下后续查找
}
} else {
x = next;
}
}
}
template<typename Key, class Comparator>
typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindLast() const { // 先从最高level走到头，然后减少level继续走到头，一直到level=0
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;
}
}
}
template<typename Key, class Comparator>
void SkipList<Key,Comparator>::Insert(const Key& key) { // 插入key节点
// 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); // prev记录每个level上前一个节点
// Our data structure does not allow duplicate insertion
assert(x == NULL || !Equal(key, x->key));
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.
max_height_.NoBarrier_Store(reinterpret_cast<void*>(height));
}
x = NewNode(key, height); // 新建一个Node
for (int i = 0; i < height; i++) { // 根据当前节点的level层数，设置每个level的指针
// 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 { // Skiplist是否包含key
Node* x = FindGreaterOrEqual(key, NULL); // 查找大于或等于key的节点
if (x != NULL && Equal(key, x->key)) { // 非空，且相同，表示包含
return true;
} else {
return false;
}
}
// Iteration over the contents of a skiplist
template<typename Key, class Comparator>
class SkipList<Key,Comparator>::Iterator { // Skiplist迭代器
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 采用默认的copy构造函数，成员直接赋值
};
template<typename Key, class Comparator>
inline SkipList<Key,Comparator>::Iterator::Iterator(const SkipList* list) { // 构造函数，初始化iterator
list_ = list;
node_ = NULL;
}
template<typename Key, class Comparator>
inline bool SkipList<Key,Comparator>::Iterator::Valid() const { // Returns true iff the iterator is positioned at a valid node.
return node_ != NULL;
}
template<typename Key, class Comparator>
inline const Key& SkipList<Key,Comparator>::Iterator::key() const { // Returns the key at the current position.
assert(Valid());
return node_->key;
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Next() { // Advances to the next position.
assert(Valid());
node_ = node_->Next(0); // 从level 0后移指向下一个
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Prev() { // Advances to the previous position.
// Instead of using explicit "prev" links, we just search for the
// last node that falls before key.
assert(Valid());
node_ = list_->FindLessThan(node_->key); // 找到前一个节点，如果为head_，则设置为NULL
if (node_ == list_->head_) {
node_ = NULL;
}
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::Seek(const Key& target) { // Advance to the first entry with a key >= target
node_ = list_->FindGreaterOrEqual(target, NULL);
}
// 第一个节点
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::SeekToFirst() { // Position at the first entry in list.
node_ = list_->head_->Next(0);
}
template<typename Key, class Comparator>
inline void SkipList<Key,Comparator>::Iterator::SeekToLast() { // Position at the last entry in list.
node_ = list_->FindLast(); // 查找最后一个节点，如果链表为空时，设置为null
if (node_ == list_->head_) {
node_ = NULL;
}
}
结论
作为一种简单的数据结构，SkipList算法非常容易实现。在大多数应用中SkipList能够代替平衡树，SkipList和进行过优化的平衡树有着同样高的性能，性能远远超过未经优化的平衡二叉树。
代码编程技巧：可以采用内部类或者友元类定义迭代器iterator；