Redis源码分析（三）——哈希结构dict

在这哈希结构即字典结构，在文件dict.h/dict.c中实现。 基本的K-V记录节点为dictEntry结构体； 哈希表的基本操作函数指针在dictType结构体中声明；  结构体dictht具体实现了哈希表，其中的成员dictEntry**  table为哈希表的节点指针数组（即一个桶节点）； 结构体dict 为整个哈希系统结构，包含两个哈希表，其中0号表为主要使用的哈希表，1号表在程序对0号进行rehash时才使用。   

哈希结构如下图所示：

  图1：由一个dictht和数个哈希节点dictEntry组成哈希表结构

图2：一个包含两个dictht哈希表的dict哈希系统

具体分析见注释。dict.h:

<<span style="font-size:18px;">span style="font-size:18px;color:#000099;">/* Hash Tables Implementation. * * This file implements in-memory hash tables with insert/del/replace/find/ * get-random-element operations. Hash tables will auto-resize if needed * tables of power of two in size are used, collisions are handled by * chaining. See the source code for more information... : */#include <stdint.h>#ifndef __DICT_H#define __DICT_H#define DICT_OK 0#define DICT_ERR 1/* Unused arguments generate annoying warnings... */#define DICT_NOTUSED(V) ((void) V)//哈希表节点 存储K-V值对typedef struct dictEntry {    void *key;    union {        void *val;        uint64_t u64;        int64_t s64;        double d;    } v;    struct dictEntry *next;} dictEntry;//字典类型  定义了相关的操作函数指针typedef struct dictType {    unsigned int (*hashFunction)(const void *key);//hash函数指针    void *(*keyDup)(void *privdata, const void *key);//复制key    void *(*valDup)(void *privdata, const void *obj);//复制value    int (*keyCompare)(void *privdata, const void *key1, const void *key2);//比较key    void (*keyDestructor)(void *privdata, void *key);//析构key    void (*valDestructor)(void *privdata, void *obj);//析构value} dictType;/* This is our hash table structure. Every dictionary has two of this as we * implement incremental rehashing, for the old to the new table. */#ifdef _WIN32//hash的具体实现结构体。用数组（桶）+开链的方式保存记录typedef struct dictht {    dictEntry **table;//哈希表的节点指针数组（table指向一个dictEntry *数组，数组中的每个dictEntry *元素指向一个桶），    size_t size; //每个table数组的大小（即桶的数目），为2的指数    size_t sizemask;//size-1，方便哈希值根据size取模（指针数组的长度掩码，用于计算索引值）    size_t used;//哈希表中现有节点数目} dictht;#endif//整个哈希系统 结构。包含两个哈希表，其中0号表为主要使用的哈希表，1号表在程序对0号进行rehash时才使用typedef struct dict {    dictType *type; //特定类型的处理函数    void *privdata;//类型处理函数的私有数据    dictht ht[2];   //两个哈希表实例。rehashidx指明下一个需要扩容的哈希表实例的编号，为-1时不扩容。    long rehashidx; /* rehashing not in progress if rehashidx == -1 */    int iterators; /* number of iterators currently running *///当前安全迭代器数量} dict;/* If safe is set to 1 this is a safe iterator, that means, you can call * dictAdd, dictFind, and other functions against the dictionary even while * iterating. Otherwise it is a non safe iterator, and only dictNext() * should be called while iterating. */ //safe设为1表示为安全迭代器，即即使在对哈希表进行迭代器时也能对其进行dictAdd, dictFind和其他操作； //若为非安全迭代器则只能dictNext()操作typedef struct dictIterator {    dict *d;  //字典实例    long index;//正在迭代的表的桶索引    int table, safe;//table表示当前操作的是哪张哈希表，safe设置是否为安全迭代器    dictEntry *entry, *nextEntry; //当前/next哈希节点    /* unsafe iterator fingerprint for misuse detection. *///指纹表识，防止非安全迭代器滥用    long long fingerprint;} dictIterator;//哈希表的遍历方法typedef void (dictScanFunction)(void *privdata, const dictEntry *de);/* This is the initial size of every hash table */#define DICT_HT_INITIAL_SIZE     4    //哈希表的初始长度/* ------------------------------- Macros ------------------------------------*///释放哈希节点。 如果定义dictType定义了valDestructor函数指针，则调用之#define dictFreeVal(d, entry) \    if ((d)->type->valDestructor) \        (d)->type->valDestructor((d)->privdata, (entry)->v.val)//设置哈希节点的value值。  如果dictType设置了valDup函数指针则使用其复制，否则直接复制#define dictSetVal(d, entry, _val_) do { \    if ((d)->type->valDup) \        entry->v.val = (d)->type->valDup((d)->privdata, _val_); \    else \        entry->v.val = (_val_); \} while(0)//设置节点的整型value值#define dictSetSignedIntegerVal(entry, _val_) \    do { entry->v.s64 = _val_; } while(0)//设置节点的非整型value值#define dictSetUnsignedIntegerVal(entry, _val_) \    do { entry->v.u64 = _val_; } while(0)//设置节点的double型value值#define dictSetDoubleVal(entry, _val_) \    do { entry->v.d = _val_; } while(0)//析构哈希表中节点的key#define dictFreeKey(d, entry) \    if ((d)->type->keyDestructor) \        (d)->type->keyDestructor((d)->privdata, (entry)->key)//设置哈希表中节点的key#define dictSetKey(d, entry, _key_) do { \    if ((d)->type->keyDup) \        entry->key = (d)->type->keyDup((d)->privdata, _key_); \    else \        entry->key = (_key_); \} while(0)//比较哈希表中的两个key#define dictCompareKeys(d, key1, key2) \    (((d)->type->keyCompare) ? \        (d)->type->keyCompare((d)->privdata, key1, key2) : \        (key1) == (key2))#define dictHashKey(d, key) (d)->type->hashFunction(key)//查找哈希表中key对应的节点#define dictGetKey(he) ((he)->key)            //获取dictEntry的key#define dictGetVal(he) ((he)->v.val)           //获取dictEntry的共用体中定义的value值#define dictGetSignedIntegerVal(he) ((he)->v.s64)//获取dictEntry的共用体中定义的有符号值#define dictGetUnsignedIntegerVal(he) ((he)->v.u64)//获取dictEntry的共用体中定义的无符号值#define dictGetDoubleVal(he) ((he)->v.d)  //获取dictEntry的共用体中定义的double值#define dictSlots(d) ((d)->ht[0].size+(d)->ht[1].size)//获取哈希结构系统中的两张哈希表的总的大小 ？#define dictSize(d) ((d)->ht[0].used+(d)->ht[1].used)//获取哈希结构系统中的两张哈希表总的现有节点数#define dictIsRehashing(d) ((d)->rehashidx != -1)  //哈希表是否正在被rehashing/* API *///创建一个新字典dict *dictCreate(dictType *type, void *privDataPtr);#ifdef _WIN32//字典扩容int dictExpand(dict *d, size_t size);#elseint dictExpand(dict *d, unsigned long size);#endif//添加新的K-V对到字典int dictAdd(dict *d, void *key, void *val);//添加一个原始的value的k-V到字典，并返回该k-v的指针dictEntry *dictAddRaw(dict *d, void *key);//添加或更新给定的key对应的valueint dictReplace(dict *d, void *key, void *val);//添加或更新key对应的value为原始value值。并返回该k-v的指针dictEntry *dictReplaceRaw(dict *d, void *key);//删除字典中的key对应的K-V值对int dictDelete(dict *d, const void *key);//删除字典中的key对应的K-V值对，但该k-v值对不被释放掉int dictDeleteNoFree(dict *d, const void *key);//清空并释放字典void dictRelease(dict *d);//在字典中查找key对应的节点dictEntry * dictFind(dict *d, const void *key);//在字典中查找key对应的节点的valuevoid *dictFetchValue(dict *d, const void *key);//调整字典大小int dictResize(dict *d);//获取字典的迭代器dictIterator *dictGetIterator(dict *d);//获取字典的安全迭代器dictIterator *dictGetSafeIterator(dict *d);//获取字典的迭代器的next节点dictEntry *dictNext(dictIterator *iter);//释放字典的迭代器void dictReleaseIterator(dictIterator *iter);//随机返回字典的一个节点dictEntry *dictGetRandomKey(dict *d);//打印字典的状态void dictPrintStats(dict *d);//输入key和目标长度，计算出对应的hash值unsigned int dictGenHashFunction(const void *key, int len);//对字符串计算出对应的hash值unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len);//清空并重置（但不释放）字典void dictEmpty(dict *d, void(callback)(void*));//启用调整方法void dictEnableResize(void);//禁用调整方法void dictDisableResize(void);//对字典进行给定步数的rehashint dictRehash(dict *d, int n);//在给定毫秒内对字典进行rehashint dictRehashMilliseconds(dict *d, int ms);//设置字典hash方法种子int dictSetHashFunctionSeed(unsigned int initval);//获取字典hash方法种子unsigned int dictGetHashFunctionSeed(void);//遍历字典unsigned long dictScan(dict *d, unsigned long v, dictScanFunction *fn, void *privdata);/* Hash table types *///声明几种dictTypeextern dictType dictTypeHeapStringCopyKey;extern dictType dictTypeHeapStrings;extern dictType dictTypeHeapStringCopyKeyValue;#endif /* __DICT_H */</span></span>

实现代码分析dict.c:

<span style="font-size:18px;">/* Using dictEnableResize() / dictDisableResize() we make possible to * enable/disable resizing of the hash table as needed. This is very important * for Redis, as we use copy-on-write and don't want to move too much memory * around when there is a child performing saving operations. * * Note that even when dict_can_resize is set to 0, not all resizes are * prevented: a hash table is still allowed to grow if the ratio between * the number of elements and the buckets > dict_force_resize_ratio. */ /* redis用了dictEnableResize() / dictDisableResize()方法可以重新调整哈希表的长度，  *因为redis采用的是写时复制的算法，不会挪动太多的内存。当把dict_can_resize is set to 0时， 当节点数目与桶的数目的比例大于dict_force_resize_ratio时，还是能 resizes  */  static int dict_can_resize = 1;//允许resizestatic unsigned int dict_force_resize_ratio = 5;//强制ersize的比例/* -------------------------- private prototypes ---------------------------- *///私有方法原型static int _dictExpandIfNeeded(dict *ht);//字典是否需要扩容#ifdef _WIN32static size_t _dictNextPower(size_t size);//#elsestatic unsigned long _dictNextPower(unsigned long size);#endifstatic int _dictKeyIndex(dict *ht, const void *key);//获取key在字典中的索引static int _dictInit(dict *ht, dictType *type, void *privDataPtr);//字典初始化方法/* -------------------------- hash functions -------------------------------- *///hash索引计算算法/* Thomas Wang's 32 bit Mix Function *///输入key获取索引值unsigned int dictIntHashFunction(unsigned int key){    key += ~(key << 15);    key ^=  (key >> 10);    key +=  (key << 3);    key ^=  (key >> 6);    key += ~(key << 11);    key ^=  (key >> 16);    return key;}//hash方法种子#define DICT_HASH_FUNCTION_SEED_UNITIALIZED 5381static uint32_t dict_hash_function_seed = DICT_HASH_FUNCTION_SEED_UNITIALIZED;//重设hash种子int dictSetHashFunctionSeed(uint32_t seed) {    if (dict_hash_function_seed == DICT_HASH_FUNCTION_SEED_UNITIALIZED) {        dict_hash_function_seed = seed;        return 0;    } else {        errno = E_FAIL;        return -1;    }}//获取hash种子uint32_t dictGetHashFunctionSeed(void) {    return dict_hash_function_seed;}//MurmurHash 是一种非加密型哈希函数，适用于一般的哈希检索操作。 由Austin Appleby在2008年发明， 并出现了多个变种，//与其它流行的哈希函数相比，对于规律性较强的key，MurmurHash的随机分布特征表现更良好。/* MurmurHash2, by Austin Appleby * Note - This code makes a few assumptions about how your machine behaves - * 1. We can read a 4-byte value from any address without crashing * 2. sizeof(int) == 4 * * And it has a few limitations - * * 1. It will not work incrementally. * 2. It will not produce the same results on little-endian and big-endian *    machines. */ /* 输入的key值，目标长度，此方法帮你计算出索引值，此方法特别表明，  *  不会因为机器之间高低位存储的不同而产生相同的结果 */ unsigned int dictGenHashFunction(const void *key, int len) {    /* 'm' and 'r' are mixing constants generated offline.     They're not really 'magic', they just happen to work well.  */ //seed  m   r都会参与到h的计算中    uint32_t seed = dict_hash_function_seed;    const uint32_t m = 0x5bd1e995;    const int r = 24;    /* Initialize the hash to a 'random' value */    uint32_t h = seed ^ len;    /* Mix 4 bytes at a time into the hash */    const unsigned char *data = (const unsigned char *)key;    while(len >= 4) {        uint32_t k = *(uint32_t*)data;        k *= m;        k ^= k >> r;        k *= m;        h *= m;        h ^= k;        data += 4;        len -= 4;    }    /* Handle the last few bytes of the input array  */    switch(len) {    case 3: h ^= data[2] << 16;    case 2: h ^= data[1] << 8;    case 1: h ^= data[0]; h *= m;    };    /* Do a few final mixes of the hash to ensure the last few     * bytes are well-incorporated. */    h ^= h >> 13;    h *= m;    h ^= h >> 15;    return (unsigned int)h;}/* And a case insensitive hash function (based on djb hash) *///hash 算法 djb2   将一个字符串hash为一个的整数  unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len) {    unsigned int hash = (unsigned int)dict_hash_function_seed;    while (len--)        hash = ((hash << 5) + hash) + (tolower(*buf++)); /* hash * 33 + c */  //tolower大写转换为小写    return hash;}/* ----------------------------- API implementation ------------------------- *//* Reset a hash table already initialized with ht_init(). * NOTE: This function should only be called by ht_destroy(). */ //对一个已经初始化的哈希表reset，注意：该方法只能被哈希表的析构函数调用static void _dictReset(dictht *ht){    ht->table = NULL;    ht->size = 0;    ht->sizemask = 0;    ht->used = 0;}/* Create a new hash table *///创建一个新的字典结构dict *dictCreate(dictType *type,        void *privDataPtr){    dict *d = zmalloc(sizeof(*d));//分配空间    _dictInit(d,type,privDataPtr);    return d;}/* Initialize the hash table *///初始化字典int _dictInit(dict *d, dictType *type,        void *privDataPtr){    _dictReset(&d->ht[0]);//重置字典中的哈希表0    _dictReset(&d->ht[1]);//重置字典中的哈希表1    d->type = type;    d->privdata = privDataPtr;    d->rehashidx = -1;    d->iterators = 0;    return DICT_OK;}/* Resize the table to the minimal size that contains all the elements, * but with the invariant of a USED/BUCKETS ratio near to <= 1 */ //重置字典中的哈希表0为最小size。最下长度为能容纳所有的节点，且满足不等式USED/BUCKETS ratio near to <= 1 int dictResize(dict *d){    int minimal;    if (!dict_can_resize || dictIsRehashing(d)) return DICT_ERR;    minimal = (int)d->ht[0].used;    if (minimal < DICT_HT_INITIAL_SIZE)        minimal = DICT_HT_INITIAL_SIZE;    return dictExpand(d, minimal);}#ifdef _WIN32/* Expand or create the hash table */int dictExpand(dict *d, size_t size){    dictht n; /* the new hash table */    size_t realsize = _dictNextPower(size);    /* the size is invalid if it is smaller than the number of     * elements already inside the hash table */    if (dictIsRehashing(d) || d->ht[0].used > size)        return DICT_ERR;    /* Allocate the new hash table and initialize all pointers to NULL */    n.size = realsize;    n.sizemask = realsize-1;    n.table = zcalloc(realsize*sizeof(dictEntry*));    n.used = (size_t) 0;    /* Is this the first initialization? If so it's not really a rehashing     * we just set the first hash table so that it can accept keys. */    if (d->ht[0].table == NULL) {        d->ht[0] = n;        return DICT_OK;    }    /* Prepare a second hash table for incremental rehashing */    d->ht[1] = n;    d->rehashidx = 0;/* Expand or create the hash table */    return DICT_OK;}#else/* Expand or create the hash table *///扩容或者创建一个表int dictExpand(dict *d, unsigned long size){    dictht n; /* the new hash table */    unsigned long realsize = _dictNextPower(size);    /* the size is invalid if it is smaller than the number of     * elements already inside the hash table */    if (dictIsRehashing(d) || d->ht[0].used > size)//如果表正在扩容或者size小于已有节点数，则无效        return DICT_ERR;    /* Allocate the new hash table and initialize all pointers to NULL */    n.size = realsize;//节点数目    n.sizemask = realsize-1;//掩码    n.table = zcalloc(realsize*sizeof(dictEntry*));//为桶数组指针table分配空间    n.used = 0;    /* Is this the first initialization? If so it's not really a rehashing     * we just set the first hash table so that it can accept keys. */ //如果表0为初次初始化，则设置表0为前面创建的表，使其可以接受key    if (d->ht[0].table == NULL) {        d->ht[0] = n;        return DICT_OK;    }    /* Prepare a second hash table for incremental rehashing *///准备好表1，以备rehahsh使用    d->ht[1] = n;    d->rehashidx = 0;    return DICT_OK;}#endif/* Performs N steps of incremental rehashing. Returns 1 if there are still * keys to move from the old to the new hash table, otherwise 0 is returned. * Note that a rehashing step consists in moving a bucket (that may have more * than one key as we use chaining) from the old to the new hash table. */ //操作N步，rehashing表0到表1.如果正在rehashing则返回1（即还没有完成全部通的迁移），否则已完成返回0.int dictRehash(dict *d, int n) {    if (!dictIsRehashing(d)) return 0;//没有正在rehash则返回0    while(n--) {//n步不能确保能把所有的桶迁移到新表        dictEntry *de, *nextde;        /* Check if we already rehashed the whole table... */        if (d->ht[0].used == 0) {//检测是否已完成rehash            zfree(d->ht[0].table);//释放旧表0            d->ht[0] = d->ht[1];//把新表1复制到表0            _dictReset(&d->ht[1]);//迁移完成后，表0被置为null            d->rehashidx = -1;            return 0;        }        /* Note that rehashidx can't overflow as we are sure there are more         * elements because ht[0].used != 0 */        assert(d->ht[0].size > (unsigned long)d->rehashidx);//还没有完成全部迁移，因此已经迁移的桶数目rehashidx应该小于旧表总的桶数目        while(d->ht[0].table[d->rehashidx] == NULL) d->rehashidx++;//已经迁移的桶数目，最终rehashidx为下一个即将迁移的桶索引        de = d->ht[0].table[d->rehashidx];//de指向即将迁移的旧桶        /* Move all the keys in this bucket from the old to the new hash HT */        while(de) {            unsigned int h;            nextde = de->next;            /* Get the index in the new hash table */            h = dictHashKey(d, de->key) & d->ht[1].sizemask;//获取新表中的索引            de->next = d->ht[1].table[h];//新迁移来的节点放在新表的对应桶的顶部（每个桶为一个单链表，next指针指向桶底）            d->ht[1].table[h] = de;            d->ht[0].used--;//旧表节点计数递减            d->ht[1].used++;//新表节点计数递增            de = nextde;        }        d->ht[0].table[d->rehashidx] = NULL;//旧表中的该桶已为空        d->rehashidx++;//桶索引递增    }    return 1;//表明还有桶没有迁移}//获取当前毫秒时间long long timeInMilliseconds(void) {#ifdef _WIN32    return GetTickCount();#else    struct timeval tv;    gettimeofday(&tv,NULL);    return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000);#endif}/* Rehash for an amount of time between ms milliseconds and ms+1 milliseconds *///在给定时间内rehash,返回rehash的步数int dictRehashMilliseconds(dict *d, int ms) {    long long start = timeInMilliseconds();    int rehashes = 0;    while(dictRehash(d,100)) {        rehashes += 100;        if (timeInMilliseconds()-start > ms) break;    }    return rehashes;}/* This function performs just a step of rehashing, and only if there are * no safe iterators bound to our hash table. When we have iterators in the * middle of a rehashing we can't mess with the two hash tables otherwise * some element can be missed or duplicated. * * This function is called by common lookup or update operations in the * dictionary so that the hash table automatically migrates from H1 to H2 * while it is actively used. */ //当没有安全迭代器时的一步rehash算法static void _dictRehashStep(dict *d) {    if (d->iterators == 0) dictRehash(d,1);}/* Add an element to the target hash table *///添加节点到目标表。int dictAdd(dict *d, void *key, void *val){    dictEntry *entry = dictAddRaw(d,key);    if (!entry) return DICT_ERR;    dictSetVal(d, entry, val);    return DICT_OK;}/* Low level add. This function adds the entry but instead of setting * a value returns the dictEntry structure to the user, that will make * sure to fill the value field as he wishes. * * This function is also directly exposed to user API to be called * mainly in order to store non-pointers inside the hash value, example: * * entry = dictAddRaw(dict,mykey); * if (entry != NULL) dictSetSignedIntegerVal(entry,1000); * * Return values: * * If key already exists NULL is returned. * If key was added, the hash entry is returned to be manipulated by the caller. */ //添加节点到表中。 如果key已经存在则返回null,否则返回添加的节点的指针（value设为原始值）dictEntry *dictAddRaw(dict *d, void *key){    int index;    dictEntry *entry;    dictht *ht;    if (dictIsRehashing(d)) _dictRehashStep(d);    /* Get the index of the new element, or -1 if     * the element already exists. */    if ((index = _dictKeyIndex(d, key)) == -1)        return NULL;    /* Allocate the memory and store the new entry */    ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0];    entry = zmalloc(sizeof(*entry));    entry->next = ht->table[index];//插入到对应的桶顶    ht->table[index] = entry;    ht->used++;    /* Set the hash entry fields. */    dictSetKey(d, entry, key);    return entry;}/* Add an element, discarding the old if the key already exists. * Return 1 if the key was added from scratch, 0 if there was already an * element with such key and dictReplace() just performed a value update * operation. */ //添加新k-v，如果已经存在则替换其value,并返回0,。如果没有对应的key则添加新节点，返回1.int dictReplace(dict *d, void *key, void *val){    dictEntry *entry, auxentry;    /* Try to add the element. If the key     * does not exists dictAdd will suceed. */    if (dictAdd(d, key, val) == DICT_OK)        return 1;    /* It already exists, get the entry */    entry = dictFind(d, key);    /* Set the new value and free the old one. Note that it is important     * to do that in this order, as the value may just be exactly the same     * as the previous one. In this context, think to reference counting,     * you want to increment (set), and then decrement (free), and not the     * reverse. */    auxentry = *entry;    dictSetVal(d, entry, val);    dictFreeVal(d, &auxentry);//释放旧value    return 0;}/* dictReplaceRaw() is simply a version of dictAddRaw() that always * returns the hash entry of the specified key, even if the key already * exists and can't be added (in that case the entry of the already * existing key is returned.) * * See dictAddRaw() for more information. *///添加一个key节点，返回节点指针，如果已经存在则返回原key对应节点指针dictEntry *dictReplaceRaw(dict *d, void *key) {    dictEntry *entry = dictFind(d,key);    return entry ? entry : dictAddRaw(d,key);}/* Search and remove an element *///查找两张表中所有key节点并删除该节点，nofee=0，则释放该节点static int dictGenericDelete(dict *d, const void *key, int nofree){    unsigned int h, idx;    dictEntry *he, *prevHe;    int table;    if (d->ht[0].size == 0) return DICT_ERR; /* d->ht[0].table is NULL */    if (dictIsRehashing(d)) _dictRehashStep(d);    h = dictHashKey(d, key);    for (table = 0; table <= 1; table++) {        idx = h & d->ht[table].sizemask;        he = d->ht[table].table[idx];        prevHe = NULL;        while(he) {            if (dictCompareKeys(d, key, he->key)) {//桶链中删除该节点                /* Unlink the element from the list */                if (prevHe)                    prevHe->next = he->next;                else                    d->ht[table].table[idx] = he->next;                if (!nofree) {                    dictFreeKey(d, he);                    dictFreeVal(d, he);                }                zfree(he);                d->ht[table].used--;                return DICT_OK;            }            prevHe = he;            he = he->next;        }        if (!dictIsRehashing(d)) break;    }    return DICT_ERR; /* not found */}//删除并释放所有key对应节点int dictDelete(dict *ht, const void *key) {    return dictGenericDelete(ht,key,0);}//删除但不释放 所有key节点int dictDeleteNoFree(dict *ht, const void *key) {    return dictGenericDelete(ht,key,1);}/* Destroy an entire dictionary *///析构字典中的整个表htint _dictClear(dict *d, dictht *ht, void(callback)(void *)) {#ifdef _WIN32    size_t i;#else    unsigned long i;#endif    /* Free all the elements */    for (i = 0; i < ht->size && ht->used > 0; i++) {//释放所有桶        dictEntry *he, *nextHe;        if (callback && (i & 65535) == 0) callback(d->privdata);        if ((he = ht->table[i]) == NULL) continue;        while(he) {//释放一个桶中的所有节点            nextHe = he->next;            dictFreeKey(d, he);            dictFreeVal(d, he);            zfree(he);            ht->used--;            he = nextHe;        }    }    /* Free the table and the allocated cache structure */    zfree(ht->table);    /* Re-initialize the table */    _dictReset(ht);    return DICT_OK; /* never fails */}/* Clear & Release the hash table *///清空并释放字典的所有表void dictRelease(dict *d){    _dictClear(d,&d->ht[0],NULL);    _dictClear(d,&d->ht[1],NULL);    zfree(d);}//在字典中查找key对应节点。从表0开始查找，找到即返回节点指针dictEntry *dictFind(dict *d, const void *key){    dictEntry *he;    unsigned int h, idx, table;    if (d->ht[0].size == 0) return NULL; /* We don't have a table at all */    if (dictIsRehashing(d)) _dictRehashStep(d);    h = dictHashKey(d, key);//桶索引    for (table = 0; table <= 1; table++) {        idx = h & d->ht[table].sizemask;        he = d->ht[table].table[idx];        while(he) {//在桶中逐个查找            if (dictCompareKeys(d, key, he->key))                return he;            he = he->next;        }        if (!dictIsRehashing(d)) return NULL;    }    return NULL;}//在字典中查找key对应的节点的valuevoid *dictFetchValue(dict *d, const void *key) {    dictEntry *he;    he = dictFind(d,key);    return he ? dictGetVal(he) : NULL;}/* A fingerprint is a 64 bit number that represents the state of the dictionary * at a given time, it's just a few dict properties xored together. * When an unsafe iterator is initialized, we get the dict fingerprint, and check * the fingerprint again when the iterator is released. * If the two fingerprints are different it means that the user of the iterator * performed forbidden operations against the dictionary while iterating. */ //指纹是表示字典当前时刻状态的一个64位数值，多个状态属性可通过位异或来表示。 //当一个非安全迭代器初始化时检测一次指纹，该迭代器释放以后再检测一次指纹， //如果两次指纹不一致，则说明通过该非安全迭代器执行了禁止的操作  //该指纹值是通过当前字典中两张表的相关参数经过hash计算得到的。不同的状态值计算得到不同的指纹值long long dictFingerprint(dict *d) {    long long integers[6], hash = 0;    int j;    integers[0] = (long) d->ht[0].table;    integers[1] = d->ht[0].size;    integers[2] = d->ht[0].used;    integers[3] = (long) d->ht[1].table;    integers[4] = d->ht[1].size;    integers[5] = d->ht[1].used;    /* We hash N integers by summing every successive integer with the integer     * hashing of the previous sum. Basically:     *     * Result = hash(hash(hash(int1)+int2)+int3) ...     *     * This way the same set of integers in a different order will (likely) hash     * to a different number. */    for (j = 0; j < 6; j++) {        hash += integers[j];        /* For the hashing step we use Tomas Wang's 64 bit integer hash. */        hash = (~hash) + (hash << 21); // hash = (hash << 21) - hash - 1;        hash = hash ^ (hash >> 24);        hash = (hash + (hash << 3)) + (hash << 8); // hash * 265        hash = hash ^ (hash >> 14);        hash = (hash + (hash << 2)) + (hash << 4); // hash * 21        hash = hash ^ (hash >> 28);        hash = hash + (hash << 31);    }    return hash;}//获取迭代器，默认不安全的 dictIterator *dictGetIterator(dict *d){    dictIterator *iter = zmalloc(sizeof(*iter));    iter->d = d;    iter->table = 0;//操作0号表    iter->index = -1;    iter->safe = 0;    iter->entry = NULL;    iter->nextEntry = NULL;    return iter;}//获取安全迭代器dictIterator *dictGetSafeIterator(dict *d) {    dictIterator *i = dictGetIterator(d);    i->safe = 1;    return i;}//返回原next节点，迭代器移到next节点，dictEntry *dictNext(dictIterator *iter){    while (1) {        if (iter->entry == NULL) {            dictht *ht = &iter->d->ht[iter->table];            if (iter->index == -1 && iter->table == 0) {                if (iter->safe)                    iter->d->iterators++;//为安全迭代器，则迭代器计数递增                else                    iter->fingerprint = dictFingerprint(iter->d);//非安全迭代器，则计算初始时刻的指纹            }            iter->index++;            if (iter->index >= (long) ht->size) {                if (dictIsRehashing(iter->d) && iter->table == 0) {                    iter->table++;                    iter->index = 0;                    ht = &iter->d->ht[1];                } else {                    break;                }            }            iter->entry = ht->table[iter->index];        } else {            iter->entry = iter->nextEntry;        }        if (iter->entry) {            /* We need to save the 'next' here, the iterator user             * may delete the entry we are returning. */            iter->nextEntry = iter->entry->next;            return iter->entry;        }    }    return NULL;}//释放迭代器void dictReleaseIterator(dictIterator *iter){    if (!(iter->index == -1 && iter->table == 0)) {        if (iter->safe)            iter->d->iterators--;//安全迭代器，计数递减        else            assert(iter->fingerprint == dictFingerprint(iter->d));//非安全迭代器，则判断前后指纹释放一致    }    zfree(iter);//释放迭代器}/* Return a random entry from the hash table. Useful to * implement randomized algorithms */ //返回一个随机key节点dictEntry *dictGetRandomKey(dict *d){    dictEntry *he, *orighe;    unsigned int h;    int listlen, listele;    if (dictSize(d) == 0) return NULL;    if (dictIsRehashing(d)) _dictRehashStep(d);//根据是否sehash过随机在表0/表1中找到一个非空的桶    if (dictIsRehashing(d)) {        do {            h = random() % (d->ht[0].size+d->ht[1].size);            he = (h >= d->ht[0].size) ? d->ht[1].table[h - d->ht[0].size] :                                      d->ht[0].table[h];        } while(he == NULL);    } else {        do {            h = random() & d->ht[0].sizemask;            he = d->ht[0].table[h];        } while(he == NULL);    }    /* Now we found a non empty bucket, but it is a linked     * list and we need to get a random element from the list.     * The only sane way to do so is counting the elements and     * select a random index. */ //在前面得到的桶中随机取得一个节点    listlen = 0;    orighe = he;    while(he) {//计算桶长度        he = he->next;        listlen++;    }    listele = random() % listlen;//获得一个随机索引    he = orighe;    while(listele--) he = he->next;//返回桶中该索引的节点指针    return he;}/* Function to reverse bits. Algorithm from: * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */ //位翻转算法static unsigned long rev(unsigned long v) {    unsigned long s = 8 * sizeof(v); // bit size; must be power of 2    unsigned long mask = ~0;    while ((s >>= 1) > 0) {        mask ^= (mask << s);        v = ((v >> s) & mask) | ((v << s) & ~mask);    }    return v;}/* dictScan() is used to iterate over the elements of a dictionary. * * Iterating works in the following way: * * 1) Initially you call the function using a cursor (v) value of 0. * 2) The function performs one step of the iteration, and returns the *    new cursor value that you must use in the next call. * 3) When the returned cursor is 0, the iteration is complete. * * The function guarantees that all the elements that are present in the * dictionary from the start to the end of the iteration are returned. * However it is possible that some element is returned multiple time. * * For every element returned, the callback 'fn' passed as argument is * called, with 'privdata' as first argument and the dictionar entry * 'de' as second argument. * * HOW IT WORKS. * * The algorithm used in the iteration was designed by Pieter Noordhuis. * The main idea is to increment a cursor starting from the higher order * bits, that is, instead of incrementing the cursor normally, the bits * of the cursor are reversed, then the cursor is incremented, and finally * the bits are reversed again. * * This strategy is needed because the hash table may be resized from one * call to the other call of the same iteration. * * dict.c hash tables are always power of two in size, and they * use chaining, so the position of an element in a given table is given * always by computing the bitwise AND between Hash(key) and SIZE-1 * (where SIZE-1 is always the mask that is equivalent to taking the rest *  of the division between the Hash of the key and SIZE). * * For example if the current hash table size is 16, the mask is * (in binary) 1111. The position of a key in the hash table will be always * the last four bits of the hash output, and so forth. * * WHAT HAPPENS IF THE TABLE CHANGES IN SIZE? * * If the hash table grows, elements can go anyway in one multiple of * the old bucket: for example let's say that we already iterated with * a 4 bit cursor 1100, since the mask is 1111 (hash table size = 16). * * If the hash table will be resized to 64 elements, and the new mask will * be 111111, the new buckets that you obtain substituting in ??1100 * either 0 or 1, can be targeted only by keys that we already visited * when scanning the bucket 1100 in the smaller hash table. * * By iterating the higher bits first, because of the inverted counter, the * cursor does not need to restart if the table size gets bigger, and will * just continue iterating with cursors that don't have '1100' at the end, * nor any other combination of final 4 bits already explored. * * Similarly when the table size shrinks over time, for example going from * 16 to 8, If a combination of the lower three bits (the mask for size 8 * is 111) was already completely explored, it will not be visited again * as we are sure that, we tried for example, both 0111 and 1111 (all the * variations of the higher bit) so we don't need to test it again. * * WAIT... YOU HAVE *TWO* TABLES DURING REHASHING! * * Yes, this is true, but we always iterate the smaller one of the tables, * testing also all the expansions of the current cursor into the larger * table. So for example if the current cursor is 101 and we also have a * larger table of size 16, we also test (0)101 and (1)101 inside the larger * table. This reduces the problem back to having only one table, where * the larger one, if exists, is just an expansion of the smaller one. * * LIMITATIONS * * This iterator is completely stateless, and this is a huge advantage, * including no additional memory used. * * The disadvantages resulting from this design are: * * 1) It is possible that we return duplicated elements. However this is usually *    easy to deal with in the application level. * 2) The iterator must return multiple elements per call, as it needs to always *    return all the keys chained in a given bucket, and all the expansions, so *    we are sure we don't miss keys moving. * 3) The reverse cursor is somewhat hard to understand at first, but this *    comment is supposed to help. */ //遍历字典unsigned long dictScan(dict *d,                       unsigned long v,                       dictScanFunction *fn,                       void *privdata){    dictht *t0, *t1;    const dictEntry *de;    unsigned long m0, m1;    if (dictSize(d) == 0) return 0;    if (!dictIsRehashing(d)) {        t0 = &(d->ht[0]);        m0 = (unsigned long)t0->sizemask;        /* Emit entries at cursor */        de = t0->table[v & m0];        while (de) {            fn(privdata, de);            de = de->next;        }    } else {        t0 = &d->ht[0];        t1 = &d->ht[1];        /* Make sure t0 is the smaller and t1 is the bigger table */        if (t0->size > t1->size) {            t0 = &d->ht[1];            t1 = &d->ht[0];        }        m0 = (unsigned long)t0->sizemask;        m1 = (unsigned long)t1->sizemask;        /* Emit entries at cursor */        de = t0->table[v & m0];        while (de) {            fn(privdata, de);            de = de->next;        }        /* Iterate over indices in larger table that are the expansion         * of the index pointed to by the cursor in the smaller table */        do {            /* Emit entries at cursor */            de = t1->table[v & m1];            while (de) {                fn(privdata, de);                de = de->next;            }            /* Increment bits not covered by the smaller mask */            v = (((v | m0) + 1) & ~m0) | (v & m0);            /* Continue while bits covered by mask difference is non-zero */        } while (v & (m0 ^ m1));    }    /* Set unmasked bits so incrementing the reversed cursor     * operates on the masked bits of the smaller table */    v |= ~m0;    /* Increment the reverse cursor */    v = rev(v);    v++;    v = rev(v);    return v;}/* ------------------------- private functions ------------------------------ *//* Expand the hash table if needed *///判断是否需要扩容static int _dictExpandIfNeeded(dict *d){    /* Incremental rehashing already in progress. Return. *///正在扩容，则返回    if (dictIsRehashing(d)) return DICT_OK;    /* If the hash table is empty expand it to the initial size. *///原来size为0，则扩容为初始size    if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE);    /* If we reached the 1:1 ratio, and we are allowed to resize the hash     * table (global setting) or we should avoid it but the ratio between     * elements/buckets is over the "safe" threshold, we resize doubling     * the number of buckets. */    if (d->ht[0].used >= d->ht[0].size &&        (dict_can_resize ||         d->ht[0].used/d->ht[0].size > dict_force_resize_ratio))    {        return dictExpand(d, d->ht[0].used*2);//扩容为原来的2倍    }    return DICT_OK;}#ifdef _WIN32/* Our hash table capability is a power of two *///哈希表的容量始终为2的指数static size_t _dictNextPower(size_t size){    size_t i = DICT_HT_INITIAL_SIZE;    if (size >= LONG_MAX) return LONG_MAX;    while(1) {        if (i >= size)            return i;        i *= 2;    }}#else/* Our hash table capability is a power of two */static unsigned long _dictNextPower(unsigned long size){    unsigned long i = DICT_HT_INITIAL_SIZE;    if (size >= LONG_MAX) return LONG_MAX;    while(1) {        if (i >= size)            return i;        i *= 2;    }}#endif/* Returns the index of a free slot that can be populated with * a hash entry for the given 'key'. * If the key already exists, -1 is returned. * * Note that if we are in the process of rehashing the hash table, the * index is always returned in the context of the second (new) hash table. */ //返回可用给定key填充的自由位置，如果该key已经存在，则返回-1. //如果正在rehash则返回在新表中的位置索引static int _dictKeyIndex(dict *d, const void *key){    unsigned int h, idx, table;    dictEntry *he;    /* Expand the hash table if needed */    if (_dictExpandIfNeeded(d) == DICT_ERR)        return -1;    /* Compute the key hash value */    h = dictHashKey(d, key);    for (table = 0; table <= 1; table++) {        idx = h & d->ht[table].sizemask;//桶索引        /* Search if this slot does not already contain the given key */        he = d->ht[table].table[idx];        while(he) {            if (dictCompareKeys(d, key, he->key))                return -1;            he = he->next;        }        if (!dictIsRehashing(d)) break;//如果没有rehash过，则只需要在表0中查找    }    return idx;}//清空字典void dictEmpty(dict *d, void(callback)(void*)) {    _dictClear(d,&d->ht[0],callback);    _dictClear(d,&d->ht[1],callback);    d->rehashidx = -1;    d->iterators = 0;}//启用resizevoid dictEnableResize(void) {    dict_can_resize = 1;}//禁用resizevoid dictDisableResize(void) {    dict_can_resize = 0;}#if 0/* The following is code that we don't use for Redis currently, but that is partof the library. *//* ----------------------- Debugging ------------------------*/#define DICT_STATS_VECTLEN 50static void _dictPrintStatsHt(dictht *ht) {    unsigned long i, slots = 0, chainlen, maxchainlen = 0;    unsigned long totchainlen = 0;    unsigned long clvector[DICT_STATS_VECTLEN];    if (ht->used == 0) {        printf("No stats available for empty dictionaries\n");        return;    }    for (i = 0; i < DICT_STATS_VECTLEN; i++) clvector[i] = 0;    for (i = 0; i < ht->size; i++) {        dictEntry *he;        if (ht->table[i] == NULL) {            clvector[0]++;            continue;        }        slots++;        /* For each hash entry on this slot... */        chainlen = 0;        he = ht->table[i];        while(he) {            chainlen++;            he = he->next;        }        clvector[(chainlen < DICT_STATS_VECTLEN) ? chainlen : (DICT_STATS_VECTLEN-1)]++;        if (chainlen > maxchainlen) maxchainlen = chainlen;        totchainlen += chainlen;    }    printf("Hash table stats:\n");    printf(" table size: %ld\n", ht->size);    printf(" number of elements: %ld\n", ht->used);    printf(" different slots: %ld\n", slots);    printf(" max chain length: %ld\n", maxchainlen);    printf(" avg chain length (counted): %.02f\n", (float)totchainlen/slots);    printf(" avg chain length (computed): %.02f\n", (float)ht->used/slots);    printf(" Chain length distribution:\n");    for (i = 0; i < DICT_STATS_VECTLEN-1; i++) {        if (clvector[i] == 0) continue;        printf("   %s%ld: %ld (%.02f%%)\n",(i == DICT_STATS_VECTLEN-1)?">= ":"", i, clvector[i], ((float)clvector[i]/ht->size)*100);    }}void dictPrintStats(dict *d) {    _dictPrintStatsHt(&d->ht[0]);    if (dictIsRehashing(d)) {        printf("-- Rehashing into ht[1]:\n");        _dictPrintStatsHt(&d->ht[1]);    }}/* ----------------------- StringCopy Hash Table Type ------------------------*/static unsigned int _dictStringCopyHTHashFunction(const void *key){    return dictGenHashFunction(key, strlen(key));}static void *_dictStringDup(void *privdata, const void *key){    int len = strlen(key);    char *copy = zmalloc(len+1);    DICT_NOTUSED(privdata);    memcpy(copy, key, len);    copy[len] = '\0';    return copy;}static int _dictStringCopyHTKeyCompare(void *privdata, const void *key1,        const void *key2){    DICT_NOTUSED(privdata);    return strcmp(key1, key2) == 0;}static void _dictStringDestructor(void *privdata, void *key){    DICT_NOTUSED(privdata);    zfree(key);}dictType dictTypeHeapStringCopyKey = {    _dictStringCopyHTHashFunction, /* hash function */    _dictStringDup,                /* key dup */    NULL,                          /* val dup */    _dictStringCopyHTKeyCompare,   /* key compare */    _dictStringDestructor,         /* key destructor */    NULL                           /* val destructor */};/* This is like StringCopy but does not auto-duplicate the key. * It's used for intepreter's shared strings. */dictType dictTypeHeapStrings = {    _dictStringCopyHTHashFunction, /* hash function */    NULL,                          /* key dup */    NULL,                          /* val dup */    _dictStringCopyHTKeyCompare,   /* key compare */    _dictStringDestructor,         /* key destructor */    NULL                           /* val destructor */};/* This is like StringCopy but also automatically handle dynamic * allocated C strings as values. */dictType dictTypeHeapStringCopyKeyValue = {    _dictStringCopyHTHashFunction, /* hash function */    _dictStringDup,                /* key dup */    _dictStringDup,                /* val dup */    _dictStringCopyHTKeyCompare,   /* key compare */    _dictStringDestructor,         /* key destructor */    _dictStringDestructor,         /* val destructor */};#endif</span>

小结：

# Redis中的数据库和哈希键都基于字典来实现

# Redis字典的底层实现为哈希表，每个字典使用两张哈希表，一般只是用表0，只有在进行rehash时，才会同时使用表0和表1.其中表1用作迁移过渡表。

# 哈希表使用链地址法（桶节点）来解决键索引冲突的问题。

# 对哈希表的rehash是分多次，渐进式的进行的。