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为什么去做这个？

在接触哈希表的实现时，大多数人习惯于使用数字来进行操作。但在实际工作中，尤其是在处理URL解析几乎是每天都会用到的场景时，我们更倾向于使用字符串。为什么选择使用哈希表来存储URL呢？

首先，在MQTT协议中，发布与订阅者之间的订阅树管理需要对URL进行解析并存储。这使得将解析后的URL存储到一个哈希表中变得非常必要。这一部分内容对理解mosquitto源码有非常大的帮助，同时也有很多实际的优化空间。

代码展示

头文件

#ifndef __HASH_H__#define __HASH_H__#include 
   
    #include 
    
     #include 
     
      #define HASH_TABLE_SIZE 676#define URL_SIZE          128#define URL_LEN           32#define ASCII_A           161#define FRIST_BIT         0#define TWO_BIT           1typedef char DataType[URL_SIZE];typedef struct node {    DataType Data;    struct node *pNext;} LinkNode;#endif
     
    
   

C文件

#include "hash.h"LinkNode *HashTable[HASH_TABLE_SIZE] = {0};char* getKeyValue(char* url) {    char* new_url = NULL;    new_url = strtok(url, "/");    new_url = strtok(NULL, "/");    return new_url;}int getIdxValue(char* key) {    char valuel = 0;    char value2 = 0;    char keykey[URL_SIZE] = {0};    int idxl = 0;    if (key != NULL) {        strcpy(keykey, key);        valuel = keykey[FRIST_BIT];        value2 = keykey[TWO_BIT];        idxl = (valuel - 'A' + ASCII_A) + (value2 - 'A' + ASCII_A);        return idxl;    }    return 0;}int InsertHashtable(char* url) {    int len = 0;    char old_url[URL_SIZE] = {0};    char* value = NULL;    strcpy(old_url, url);    char* key = getKeyValue(url);    if (key == NULL) {        return 0;    }    int idx = getIdxValue(key);    len = strlen(key);    value = strstr(old_url, key);    value++;    value += len;    LinkNode* pInsertNode = malloc(sizeof(LinkNode));    if (pInsertNode == NULL) {        perror("fail to malloc");        return -1;    }    strcpy(pInsertNode->Data, value);    pInsertNode->pNext = NULL;    LinkNode** ppTmp = &HashTable[idx];    while (ppTmp != NULL) {        ppTmp = &(*ppTmp)->pNext;    }    pInsertNode->pNext = *ppTmp;    *ppTmp = pInsertNode;}void ShowHashtable() {    int i = 0;    for (; i < HASH_TABLE_SIZE; i++) {        if (HashTable[i] != NULL) {            LinkNode* pTmpNode = HashTable[i];            while (pTmpNode != NULL) {                printf("%s ", pTmpNode->Data);                pTmpNode = pTmpNode->pNext;            }            printf("\n");        }        continue;    }}int FindHashtable(char* url) {    int len = 0;    char old_url[URL_SIZE] = {0};    char* value = NULL;    strcpy(old_url, url);    char* key = getKeyValue(url);    if (key == NULL) {        return 0;    }    int idx = getIdxValue(key);    len = strlen(key);    value = strstr(old_url, key);    value++;    value += len;    LinkNode* pTmpNode = HashTable[idx];    while (pTmpNode != NULL) {        if (!strcmp(pTmpNode->Data, value)) {            printf("Found URL: %s\n", pTmpNode->Data);            return 1;        }        pTmpNode = pTmpNode->pNext;    }    return 0;}void DestroyHashtable() {    int i = 0;    LinkNode* pFreeNode = NULL;    LinkNode* pTmpNode = NULL;    for (; i < HASH_TABLE_SIZE; i++) {        pTmpNode = HashTable[i];        while (pTmpNode != NULL) {            pFreeNode = pTmpNode;            pTmpNode = pTmpNode->pNext;            free(pFreeNode);        }    }}

主函数

#include "hash.h"char value[URL_SIZE] = {0};int i = 0;char url[URL_LEN][URL_SIZE] = {    "/ISA/Syste/device",    "/ISA/System/time",    "/ISA/Syste/time/timeZ",    "/ISA/System/upgradeFlag",    "/ISA/Syste/Hardwar/capabilities",    "/ISA/System/Software/capabilities",    "/ISA/Syste/Vid/inputs/OSDLanguage",    "/ISA/System/Video/inputs/1/overlays",    "/ISA/Event/channels/capabilities",    "/ISA/PTZCtrl/channels/1",    "/ISA/Ctrl/channe/1/maxele/capabil",    "/ISA/Image/proportionalpan",    "/ISA/Image/channels/1/i",    "/ISA/Security/sessionHear"};for (i = 0; i < URL_LEN; i++) {    InsertHashtable(url[i]);}ShowHashtable();printf("请输入查找URL:\n");scanf("%s", value);if (!FindHashtable(value)) {    printf("未找到该URL\n");}DestroyHashtable();

总结

这段代码是对新手或接触编程不多的开发者友好的实现，帮助他们更深入地理解哈希表的概念。在实现过程中，我对键值进行了双重处理，虽然达不到真正的哈希表，但仍包含了一些思想。

如果你对这部分内容感兴趣，可以继续跟我交流，这里没有首先、其次之类的句式，让我们一起讨论与提升。

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