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目录&索引

• 功能与不足简述
• 程序代码
• • 实现可配置哈希函数的哈希表类 code c++
  • 运行结果
• 结论

功能与不足简述

如 HashTable 类定义所示，自定义实现 insert、erase、find、中括号访问及扩容等方法。

不足——通过源码阅读扩容，其扩容发生包括如下三个方面：初始化、处理哈希冲突满足拉链长度且不满足 capacity、size 占到 capacity 一定比例。诸如此类，详见下文代码，尚待优化。

扩容性能分析——rehash、创建新的数组、遍历放到新的数组。思考：其一，从避免扩容角度出发？其二，rehash 如何设计？结合个人设计再阅读源码。

程序代码

实现可配置哈希函数的哈希表类 code c++

#include <iostream>
#include <functional>
#include <algorithm>
#include <vector>
using namespace std;

class Node {
public :
    Node() = default;
    Node(string, int, Node *);
    string key();
    int value;
    Node *next();
    void set_next(Node *);
    void insert(Node *);
    void erase_next();
private:
    string __key;
    Node *__next;
};

class HashTable {
public :
    typedef function<int(string)> HASH_FUNC_T;
    HashTable(HASH_FUNC_T hash_func, int size);
    bool insert(string, int);
    bool erase(string);
    bool find(string);
    int capacity();
    int &operator[](string);
private:
    Node *__insert(string, int);
    Node *__find(string);
    void __expand();
    int __size, data_cnt;
    vector<Node> data;
    HASH_FUNC_T hash_func;
};

Node::Node(string key, int value = 0, Node *next = nullptr) : __key(key), value(value), __next(next) {}
string Node::key() { return this->__key; }
Node *Node::next() { return this->__next; }
void Node::set_next(Node *next) { this->__next = next; }
void Node::insert(Node *p) { p->__next = this->__next; this->__next = p; }
void Node::erase_next() {
    if (this->__next == nullptr) return ;
    Node *q = this->__next;
    this->__next = q->__next;
    delete q;
    return ;
}

HashTable::HashTable(HASH_FUNC_T hash_func, int size = 2) : __size(size), data(__size), hash_func(hash_func), data_cnt(0) {}
bool HashTable::insert(string key, int value = 0) {
    if (find(key)) return false;
    return __insert(key, value);
}
Node *HashTable::__insert(string key, int value = 0) {
    if (data_cnt >= __size) __expand(); // >= size * 10 改为 * 1
    int h = hash_func(key) % __size;
    data[h].insert(new Node(key, value));
    data_cnt += 1;
    return data[h].next();
}
void HashTable::__expand() {
    int new_size = __size * 2;
    HashTable temp_h(hash_func, new_size);
    for (int i = 0; i < __size; ++i) {
        for (Node *p = data[i].next(); p; p = p->next()) {
            temp_h[p->key()] = p->value;
        }
    }
    swap(temp_h, *this);
    return ;
}
bool HashTable::erase(string s) {
    int h = hash_func(s) % __size;
    for (Node *p = &data[h]; p->next(); p = p->next()) {
        if (p->next()->key() != s) continue;
        p->erase_next();
        data_cnt -= 1;
        return true;
    }
    return false;
}
bool HashTable::find(string s) {
    return __find(s);
}
Node *HashTable::__find(string key) {
    int h = hash_func(key) % __size;
    for (Node *p = data[h].next(); p; p = p->next()) {
        if (p->key() == key) return p;
    }
    return nullptr;
}
int HashTable::capacity() { return this->__size; }
int &HashTable::operator[](string key) {
    Node *p;
    if (!(p = __find(key))) return (__insert(key)->value);
    return p->value;
}

int BKDRHash(string s) {
    int seed = 31;
    int h = 0;
    for (int i = 0; s[i]; ++i) {
        h = h * seed + s[i];
    }
    return h & 0x7fffffff;
}

class APHash_Class {
public :
    int operator()(string s) {
        int h = 0;
        for (int i = 0; s[i]; ++i) {
            if (i % 2) {
                h = (h << 3) ^ s[i] ^ (h >> 5);
            } else {
                h = ~((h << 7)) ^ s[i] ^ (h >> 11);
            }
        }
        return h & 0x7fffffff;
    }
};

int main() { // 可以配置哈希函数的哈希表类, 增加 map 形式, 增加扩容
    APHash_Class APHash;
    HashTable h1(BKDRHash);
    HashTable h2(APHash);
    int op;
    string s;
    cout << h1.capacity() << endl;
    cout << h2.capacity() << endl;
    h1["hello"] = 123;
    h1["world"] = 456;
    h1["haizei"] = 789;
    cout << h1.capacity() << endl;
    cout << h2.capacity() << endl;
    cout << h1["hello"] << " " << h1["world"] << " " << h1["hahaha"] << endl;
    while (cin >> op >> s) {
        switch (op) {
            case 0: {
                cout << "insert" << s << " to hash table 1 = ";
                cout << h1.insert(s) << endl;
                cout << "insert" << s << " to hash table 2 = ";
                cout << h2.insert(s) << endl;
            } break; // insert
            case 1: {
                cout << "erase" << s << " from hash table 1 = ";
                cout << h1.erase(s) << endl;
                cout << "erase" << s << " from hash table 2 = ";
                cout << h2.erase(s) << endl;
            } break; // erase
            case 2: {
                cout << "find" << s << " at hash table 1 = ";
                cout << h1.find(s) << endl;
                cout << "find" << s << " at hash table 2 = ";
                cout << h2.find(s) << endl;
            } break; // find
        }
    }
    return 0;
}

运行结果

2
2
4
2
123 456 0
0
zhangxueyou
insertzhangxueyou to hash table 1 = 1
insertzhangxueyou to hash table 2 = 1
0
lizongsheng
insertlizongsheng to hash table 1 = 1
insertlizongsheng to hash table 2 = 1
1
lizongsheng
eraselizongsheng from hash table 1 = 1
eraselizongsheng from hash table 2 = 1
2
lizongsheng
findlizongsheng at hash table 1 = 0
findlizongsheng at hash table 2 = 0
2 
zhangxueyou
findzhangxueyou at hash table 1 = 1
findzhangxueyou at hash table 2 = 1
2
huge
findhuge at hash table 1 = 0
findhuge at hash table 2 = 0

结论

代码示例，有问题留言。

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