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 = 12 };

  // 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) == 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&);
};


//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];
};


//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 - 1));
  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(0);
}

//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(0);
}

//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 = 4;
  int height = 1;
  while (height < kMaxHeight && ((rnd_.Next() % kBranching) == 0)) {
    height++;
  }
  assert(height > 0);
  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) < 0);
}


//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() - 1;
  //    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 == 0) {
        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() - 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;
    }
  }
}

//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() - 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;
    }
  }
}


//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(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);
  }
}

//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
", 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 = 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);
  }
}


//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。

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