SkipList的实现

当做笔记记录下来:)

相比于bTree，skiplist占用更多的空间。

而在并发的环境下skiplist会比bTree效率更高。

(bTree插入的时候需要rebalance，需要lock比较多的结点，而skiplist则只需要lock跟node相关的几个结点)

SkipList.h

#define MAX_LEVEL 16

typedef int KeyType;
typedef int ValueType;

typedef struct nodeStructure* Node;
struct nodeStructure
{
    KeyType key;
    ValueType value;
    Node forward[1];
};

class SkipList
{
public:
    SkipList();
    Node createNode(int level, const KeyType& key, const ValueType& value);
    void createList();
    bool insert(const KeyType& key, const ValueType& value);
    bool erase(const KeyType& key, ValueType& value);
    bool search(const KeyType& key, ValueType& value);
private:
    int randomLevel();
private:
    int _level;
    Node _header;
};

SkipList.cpp

#include "SkipList.h"
#include <iostream>

SkipList::SkipList()
{
    _level = 0;
    _header = createNode(MAX_LEVEL, 0, 0);
    Node tail = createNode(1, 0x7fffffff, 0);
    for (int i = 0; i < MAX_LEVEL; ++i) 
        _header->forward[i] = tail;
        
}

Node SkipList::createNode(int level, const KeyType& key, const ValueType& value)
{
    Node node = (Node)malloc(
        sizeof(nodeStructure) + sizeof(Node) * level);
    node->key = key;
    node->value = value;
    return node;
}

int SkipList::randomLevel()
{
    int k = 1;
    // 50% k++
    while (rand() % 2)
        k++;
    return (k < MAX_LEVEL) ? k : MAX_LEVEL - 1;
}

bool SkipList::insert(const KeyType& key, const ValueType& value)
{
    Node update[MAX_LEVEL];
    Node node = _header;
    // search
    for (int i = _level; i >= 0; --i)
    {
        while (node->forward[i]->key < key)
            node = node->forward[i];
        // record previous node
        update[i] = node;
    }

    // same key
    if (node->forward[0]->key == key)
        return false;

    int level = randomLevel();
    // update level
    if (level > _level)
    {
        for (int i = _level + 1; i <= level; ++i)
            update[i] = _header;
        _level = level;
    }

    // update pointer
    Node insNode = createNode(level, key, value);
    for (int i = level; i >= 0; --i)
    {
        node = update[i];
        insNode->forward[i] = node->forward[i];
        node->forward[i] = insNode;
    }
    return true;
}

// similar to insert
bool SkipList::erase(const KeyType& key, ValueType& value)
{
    Node update[MAX_LEVEL];
    Node node = _header;
    for (int i = _level; i >= 0; --i)
    {
        while (node->forward[i]->key < key)
            node = node->forward[i];
        update[i] = node;
    }

    node = node->forward[0];
    if (node->key != key)
        return false;

    for (int i = 0; i < _level; ++i)
    {
        if (update[i]->forward[i] != node)
            break;
        update[i]->forward[i] = node->forward[i];
    }
    free(node);
    return true;
}

bool SkipList::search(const KeyType& key, ValueType& value)
{
    Node node = _header;
    for (int i = _level; i >= 0; --i)
        // find the last node->key < key
        while (node->forward[i]->key < key)
            node = node->forward[i];
    // node->key >= key
    node = node->forward[0];
    if (node->key == key)
    {
        value = node->value;
        return true;
    }
    return false;
}