leveldb 学习记录(四) skiplist补与变长数字

在leveldb 学习记录(一) skiplist 已经将skiplist的插入 查找等操作流程用图示说明

这里在介绍 下skiplist的代码

里面有几个模块

template<typename Key, class Comparator>
class SkipList {......}

class Arena;(内存池模块 暂时不介绍)

struct Node;(节点 存储key 和指向其他Node的指针)

//Node 构造函数  KEY赋值
// 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.
  // 访问修改器使用 包装在方法中 可以使用内存屏障
  // Node指向的另一层?
  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());
  }
  //设置下层Node指向
  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.
  // 读取本Node在N层的NODE指向  非内存屏蔽操作
  Node* NoBarrier_Next(int n) {
    assert(n >= 0);
    return reinterpret_cast<Node*>(next_[n].NoBarrier_Load());
  }

  //设置Node 在N层的Node指向  非内存屏蔽操作
  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.
  // 原子指针数组  指向其他Node  创建时候动态确认数组长度
  port::AtomicPointer next_[1];
};

　　

结构示意图如下

其他操作可参见系列文章一

最后附上我注释的代码

 #include <assert.h>
 #include <stdlib.h>
 #include "port/port.h"
 #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);

   // 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 =  };

   // 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.
   port::AtomicPointer max_height_;   // Height of the entire list

   inline int GetMaxHeight() const {
     return 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) == ); }

   // 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&);
 };

 //Node 构造函数  KEY赋值
 // 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.
   // 访问修改器使用 包装在方法中 可以使用内存屏障
   // Node指向的另一层?
   Node* Next(int n) {
     assert(n >= );
     // Use an 'acquire load' so that we observe a fully initialized
     // version of the returned Node.
     return reinterpret_cast<Node*>(next_[n].Acquire_Load());
   }
   //设置下层Node指向
   void SetNext(int n, Node* x) {
     assert(n >= );
     // 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.
   // 读取本Node在N层的NODE指向  非内存屏蔽操作
   Node* NoBarrier_Next(int n) {
     assert(n >= );
     return reinterpret_cast<Node*>(next_[n].NoBarrier_Load());
   }

   //设置Node 在N层的Node指向  非内存屏蔽操作
   void NoBarrier_SetNext(int n, Node* x) {
     assert(n >= );
     next_[n].NoBarrier_Store(x);
   }

  private:
   // Array of length equal to the node height.  next_[0] is lowest level link.
   // 原子指针数组  指向其他Node  创建时候动态确认数组长度
   port::AtomicPointer next_[];
 };

 //创建一个NODE 内存池arena_分配内存  height确认该NODE的Node指向数量
 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 - ));
   return new (mem) Node(key);
 }

 //根据输入的SkipList指针构造一个  iterator
 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;
 }

 //返回iterator指向的Node的key
 template<typename Key, class Comparator>
 inline const Key& SkipList<Key,Comparator>::Iterator::key() const {
   assert(Valid());
   return node_->key;
 }

 //获取iterator指向的下一个node  在第0层获取(Node第0层均有记录)
 template<typename Key, class Comparator>
 inline void SkipList<Key,Comparator>::Iterator::Next() {
   assert(Valid());
   node_ = node_->Next();
 }

 //寻找该node上一个node
 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;
   }
 }

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

 //重置到head  Node 第0层 第一个Node
 template<typename Key, class Comparator>
 inline void SkipList<Key,Comparator>::Iterator::SeekToFirst() {
   node_ = list_->head_->Next();
 }

 //重置到last Node
 template<typename Key, class Comparator>
 inline void SkipList<Key,Comparator>::Iterator::SeekToLast() {
   node_ = list_->FindLast();
   if (node_ == list_->head_) {
     node_ = NULL;
   }
 }

 //返回height  随机增加1
 template<typename Key, class Comparator>
 int SkipList<Key,Comparator>::RandomHeight() {
   // Increase height with probability 1 in kBranching
   static const unsigned int kBranching = ;
   int height = ;
   while (height < kMaxHeight && ((rnd_.Next() % kBranching) == )) {
     height++;
   }
   assert(height > );
   assert(height <= kMaxHeight);
   return height;
 }

 //调用 cmp 比较是大小
 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) < );
 }

 //寻找比key大或者等于的Node
 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() - ;
   //    k在Next后面 则在本level寻找 否则 在下一层level寻找
   while (true) {
     Node* next = x->(level);
     if (KeyIsAfterNode(key, next)) {
       // Keep searching in this list
       x = next;
     } else {
       if (prev != NULL) prev[level] = x;
       if (level == ) {
         return next;
       } else {
         // Switch to next list
         level--;
       }
     }
   }
 }

 //寻找小于等于KEY的第一个NODE
 template<typename Key, class Comparator>
 typename SkipList<Key,Comparator>::Node*
 SkipList<Key,Comparator>::FindLessThan(const Key& key) const {
   Node* x = head_;
   int level = GetMaxHeight() - ;
   while (true) {
     assert(x == head_ || compare_(x->key, key) < );
     Node* next = x->Next(level);
     if (next == NULL || compare_(next->key, key) >= ) {
       if (level == ) {
         return x;
       } else {
         // Switch to next list
         level--;
       }
     } else {
       x = next;
     }
   }
 }

 //在最高层 寻找最后一个Node
 template<typename Key, class Comparator>
 typename SkipList<Key,Comparator>::Node* SkipList<Key,Comparator>::FindLast()
     const {
   Node* x = head_;
   int level = GetMaxHeight() - ;
   while (true) {
     Node* next = x->Next(level);
     if (next == NULL) {
       if (level == ) {
         return x;
       } else {
         // Switch to next list
         level--;
       }
     } else {
       x = next;
     }
   }
 }

 //构建函数 第一个head Node key是0 后继Node指针最大kMaxHeight
 template<typename Key, class Comparator>
 SkipList<Key,Comparator>::SkipList(Comparator cmp, Arena* arena)
     : compare_(cmp),
       arena_(arena),
       head_(NewNode( /* any key will do */, kMaxHeight)),
       max_height_(reinterpret_cast<void*>()),
       rnd_(0xdeadbeef) {
   for (int i = ; i < kMaxHeight; i++) {
     head_->SetNext(i, NULL);
   }
 }

 //insert算法
 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 == 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);
   for (int i = ; 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);
   }
 }

 //寻找等于KEY 即可确认是否包含与KEY相等的Node
 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;
   }
 }

 }

varint变长数

leveldb使用了Protocol Buffers的变长字节整形编码 每个字节中使用7位来表示数字 最高位用作他处

300的编码，编码后是两个字节：

1010 1100 0000 0010

　　它这个例子是每个字节左边是高位，可以看到每个字节的最高位是一个标识位，从左到右第一个字节是10101100，最高位是1，说明后面还有字节需要解码，然后第二个字节是00000010，最高位是0，后面没字节了。所以这两个字节就需要解码成一个整数，再往下看。

代码

1010 1100 0000 0010
→  010 1100  000 0010 //每个字节去掉最高位
→  000 0010  010 1100 //字节序转换，两个字节互换位置
→  000 0010 ++ 010 1100 //两个字节进行链接操作（不是相加）
→  100101100 //合并后的结果，高位位0的部分截取掉
→  256 + 32 + 8 + 4 = 300 //每个值为1的bit位乘以以2为底的幂，得出编码后的值

主要函数在

coding.h

extern void PutFixed32(std::string* dst, uint32_t value);
extern void PutFixed64(std::string* dst, uint64_t value);
extern void PutVarint32(std::string* dst, uint32_t value);
extern void PutVarint64(std::string* dst, uint64_t value);
extern void PutLengthPrefixedSlice(std::string* dst, const Slice& value);

extern bool GetVarint32(Slice* input, uint32_t* value);
extern bool GetVarint64(Slice* input, uint64_t* value);
extern bool GetLengthPrefixedSlice(Slice* input, Slice* result);

extern const char* GetVarint32Ptr(const char* p,const char* limit, uint32_t* v);
extern const char* GetVarint64Ptr(const char* p,const char* limit, uint64_t* v);

extern int VarintLength(uint64_t v);

extern void EncodeFixed32(char* dst, uint32_t value);
extern void EncodeFixed64(char* dst, uint64_t value);

extern char* EncodeVarint32(char* dst, uint32_t value);
extern char* EncodeVarint64(char* dst, uint64_t value);

参考

https://www.cnblogs.com/onlytiancai/archive/2010/10/16/1853003.html