Redis源代码分析（三）---dict哈希结构

昨天分析完adlist的Redis代码。今天立即马不停蹄的继续学习Redis代码中的哈希部分的结构学习，只是在这里他不叫什么hashMap，而是叫dict。并且是一种全新设计的一种哈希结构，他仅仅是通过几个简单的结构体。再搭配上一些比較常见的哈希算法，就实现了类似高级语言中HashMap的作用了。也让我见识了一些哈希算法的实现。比方dbj hash的算法实现。俗称times33,算法，就是不停的*33,。这样的算是一种超级简单的哈希算法。

以下说说给我感觉Redis代码中哈希实现的不是那么简单。中间加了一些东西。比方说dictType定义了一些字典集合操作的公共方法。我把dict叫做字典总类，也能够说字典操作类，真正存放键值对的叫dictEntry。我叫做字典集合，字典集合存放在哈希表中，叫dictht，以下给出一张结构图来理理思路。

以下给出2个文件的代码解析：

dict.h:

<span style="font-size:14px;">/* 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... :)
 *
 * Copyright (c) 2006-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <stdint.h>

#ifndef __DICT_H
#define __DICT_H

/* 定义了成功与错误的值 */
#define DICT_OK 0
#define DICT_ERR 1

/* Unused arguments generate annoying warnings... */
/* dict没实用到时，用来提示警告的 */
#define DICT_NOTUSED(V) ((void) V)

/* 字典结构体，保存K-V值的结构体 */
typedef struct dictEntry {
	//字典key函数指针
    void *key;
    union {
        void *val;
        //无符号整型值
        uint64_t u64;
        //有符号整型值
        int64_t s64;
        double d;
    } v;
    //下一字典结点
    struct dictEntry *next;
} dictEntry;

/* 字典类型 */
typedef struct dictType {
	//哈希计算方法。返回整形变量
    unsigned int (*hashFunction)(const void *key);
    //复制key方法
    void *(*keyDup)(void *privdata, const void *key);
    //复制val方法
    void *(*valDup)(void *privdata, const void *obj);
    //key值比較方法
    int (*keyCompare)(void *privdata, const void *key1, const void *key2);
    //key的析构函数
    void (*keyDestructor)(void *privdata, void *key);
    //val的析构函数
    void (*valDestructor)(void *privdata, void *obj);
} 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. */
/* 哈希表结构体 */
typedef struct dictht {
	//字典实体
    dictEntry **table;
    //表格可容纳字典数量
    unsigned long size;
    unsigned long sizemask;
    //正在被使用的数量
    unsigned long used;
} dictht;

/* 字典主操作类 */
typedef struct dict {
	//字典类型
    dictType *type;
    //私有数据指针
    void *privdata;
    //字典哈希表，共2张。一张旧的，一张新的
    dictht ht[2];
    //重定位哈希时的下标
    long rehashidx; /* rehashing not in progress if rehashidx == -1 */
    //当前迭代器数量
    int iterators; /* number of iterators currently running */
} 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. */
/* 字典迭代器，假设是安全迭代器，这safe设置为1。能够调用dicAdd，dictFind */
/* 假设是不安全的，则仅仅能调用dicNext方法*/
typedef struct dictIterator {
	//当前字典
    dict *d;
    //下标
    long index;
    //表格。和安全值的表格代表的是旧的表格，还是新的表格
    int table, safe;
    //字典实体
    dictEntry *entry, *nextEntry;
    /* unsafe iterator fingerprint for misuse detection. */
    /* 指纹标记，避免不安全的迭代器滥用现象 */
    long long fingerprint;
} dictIterator;

/* 字典扫描方法 */
typedef void (dictScanFunction)(void *privdata, const dictEntry *de);

/* This is the initial size of every hash table */
/* 初始化哈希表的数目 */
#define DICT_HT_INITIAL_SIZE     4

/* ------------------------------- Macros ------------------------------------*/
/* 字典释放val函数时候调用，假设dict中的dictType定义了这个函数指针， */
#define dictFreeVal(d, entry) \
    if ((d)->type->valDestructor) \
        (d)->type->valDestructor((d)->privdata, (entry)->v.val)

/* 字典val函数复制时候调用，假设dict中的dictType定义了这个函数指针。 */
#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dictEntry中共用体v中有符号类型的值 */
#define dictSetSignedIntegerVal(entry, _val_) \
    do { entry->v.s64 = _val_; } while(0)

/* 设置dictEntry中共用体v中无符号类型的值 */
#define dictSetUnsignedIntegerVal(entry, _val_) \
    do { entry->v.u64 = _val_; } while(0)

/* 设置dictEntry中共用体v中double类型的值 */
#define dictSetDoubleVal(entry, _val_) \
    do { entry->v.d = _val_; } while(0)

/* 调用dictType定义的key析构函数 */
#define dictFreeKey(d, entry) \
    if ((d)->type->keyDestructor) \
        (d)->type->keyDestructor((d)->privdata, (entry)->key)

/* 调用dictType定义的key复制函数，未定义直接赋值 */
#define dictSetKey(d, entry, _key_) do { \
    if ((d)->type->keyDup) \
        entry->key = (d)->type->keyDup((d)->privdata, _key_); \
    else \
        entry->key = (_key_); \
} while(0)

/* 调用dictType定义的key比較函数，未定义直接key值直接比較 */
#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)   //哈希定位方法
#define dictGetKey(he) ((he)->key)    //获取dictEntry的key值
#define dictGetVal(he) ((he)->v.val)  //获取dicEntry中共用体v中定义的val值
#define dictGetSignedIntegerVal(he) ((he)->v.s64) //获取dicEntry中共用体v中定义的有符号值
#define dictGetUnsignedIntegerVal(he) ((he)->v.u64)  //获取dicEntry中共用体v中定义的无符号值
#define dictGetDoubleVal(he) ((he)->v.d)  //获取dicEntry中共用体v中定义的double类型值
#define dictSlots(d) ((d)->ht[0].size+(d)->ht[1].size)  //获取dict字典中总的表大小
#define dictSize(d) ((d)->ht[0].used+(d)->ht[1].used)   //获取dict字典中总的表的总正在被使用的数量
#define dictIsRehashing(d) ((d)->rehashidx != -1)   //字典有无被重定位过

/* API */
dict *dictCreate(dictType *type, void *privDataPtr);   //创建dict字典总类
int dictExpand(dict *d, unsigned long size);    //字典扩增方法
int dictAdd(dict *d, void *key, void *val);    //字典依据key, val加入一个字典集
dictEntry *dictAddRaw(dict *d, void *key);     //字典加入一个仅仅有key值的dicEntry
int dictReplace(dict *d, void *key, void *val); //替代dict中一个字典集
dictEntry *dictReplaceRaw(dict *d, void *key);  //替代dict中的一个字典。仅仅提供一个key值
int dictDelete(dict *d, const void *key);    //依据key删除一个字典集
int dictDeleteNoFree(dict *d, const void *key);  //字典集删除无、不调用free方法
void dictRelease(dict *d);   //释放整个dict
dictEntry * dictFind(dict *d, const void *key);  //依据key寻找字典集
void *dictFetchValue(dict *d, const void *key);  //依据key值寻找对应的val值
int dictResize(dict *d);  //又一次计算大小
dictIterator *dictGetIterator(dict *d); //获取字典迭代器
dictIterator *dictGetSafeIterator(dict *d);  //获取字典安全迭代器
dictEntry *dictNext(dictIterator *iter);   //依据字典迭代器获取字典集的下一字典集
void dictReleaseIterator(dictIterator *iter); //释放迭代器
dictEntry *dictGetRandomKey(dict *d);  //随机获取一个字典集
void dictPrintStats(dict *d);  //打印当前字典状态
unsigned int dictGenHashFunction(const void *key, int len); //输入的key值。目标长度，此方法帮你计算出索引值
unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len); //这里提供了一种比較简单的哈希算法
void dictEmpty(dict *d, void(callback)(void*)); //清空字典
void dictEnableResize(void);  //启用调整方法
void dictDisableResize(void); //禁用调整方法
int dictRehash(dict *d, int n); //hash重定位。主要从旧的表映射到新表中,分n轮定位
int dictRehashMilliseconds(dict *d, int ms);  //在给定时间内，循环运行哈希重定位
void dictSetHashFunctionSeed(unsigned int initval); //设置哈希方法种子
unsigned int dictGetHashFunctionSeed(void);  //获取哈希种子
unsigned long dictScan(dict *d, unsigned long v, dictScanFunction *fn, void *privdata); //字典扫描方法

/* Hash table types */
/* 哈希表类型  */
extern dictType dictTypeHeapStringCopyKey;
extern dictType dictTypeHeapStrings;
extern dictType dictTypeHeapStringCopyKeyValue;

#endif /* __DICT_H */
</span>

dict.c;

<span style="font-size:14px;">/* 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... :)
 *
 * Copyright (c) 2006-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include "fmacros.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <limits.h>
#include <sys/time.h>
#include <ctype.h>

#include "dict.h"
#include "zmalloc.h"
#include "redisassert.h"

/* 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()方法能够又一次调整哈希表的长度，
 *由于redis採用的是写时复制的算法，不会挪动太多的内存，仅仅有当调整数量大于一定比例才可能有效 */
static int dict_can_resize = 1;
static unsigned int dict_force_resize_ratio = 5;

/* -------------------------- private prototypes ---------------------------- */
/* 私有方法 */
static int _dictExpandIfNeeded(dict *ht);    //字典是否须要扩展
static unsigned long _dictNextPower(unsigned long size);
static int _dictKeyIndex(dict *ht, const void *key);
static int _dictInit(dict *ht, dictType *type, void *privDataPtr);  //字典初始化方法

/* -------------------------- hash functions -------------------------------- */
/* 哈希索引计算的方法 */

/* Thomas Wang's 32 bit Mix Function */
/* Thomas Wang's 32 bit Mix 的哈希算法直接输入key值，获取索引值，据说这样的冲突的概率非常低 */
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;
}

//哈希方法种子，跟产生随机数的种子作用应该是一样的
static uint32_t dict_hash_function_seed = 5381;

/* 重设哈希种子 */
void dictSetHashFunctionSeed(uint32_t seed) {
    dict_hash_function_seed = seed;
}

/* 获取哈希种子 */
uint32_t dictGetHashFunctionSeed(void) {
    return dict_hash_function_seed;
}

/* 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。
 *	不会由于机器之间高低位存储的不同而产生同样的结果 */
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种子。m。r的值都将会參与到计算中
    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) */
/* 这里提供了一种比較简单的哈希算法 */
unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len) {
	//以djb hash为基础，俗称“times33”就是不断的乘33
	//差点儿全部的流行的hash map都採用了DJB hash function
    unsigned int hash = (unsigned int)dict_hash_function_seed;

    while (len--)
        hash = ((hash << 5) + hash) + (tolower(*buf++)); /* hash * 33 + c */
    return hash;
}

/* ----------------------------- API implementation ------------------------- */

/* Reset a hash table already initialized with ht_init().
 * NOTE: This function should only be called by ht_destroy(). */
/* 重置哈希表方法，仅仅在ht_destroy时使用 */
static void _dictReset(dictht *ht)
{
	//清空相应的变量，ht->table的类型事实上是dictEntry，叫table名字太有歧义了
    ht->table = NULL;
    ht->size = 0;
    ht->sizemask = 0;
    ht->used = 0;
}

/* Create a new hash table */
/* 创建dict操作类 */
dict *dictCreate(dictType *type,
        void *privDataPtr)
{
    dict *d = zmalloc(sizeof(*d));

	//创建好空间之后调用初始化方法
    _dictInit(d,type,privDataPtr);
    return d;
}

/* Initialize the hash table */
/* 初始化dict类中的type，ht等变量 */
int _dictInit(dict *d, dictType *type,
        void *privDataPtr)
{
	//重置2个ht哈希表
    _dictReset(&d->ht[0]);
    _dictReset(&d->ht[1]);
    //赋值dictType
    d->type = type;
    d->privdata = privDataPtr;
    //-1代表还没有rehash过，
    d->rehashidx = -1;
    //当前使用中的迭代器为0
    d->iterators = 0;

    //返回DICT_OK，代表初始化成功
    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 */
/* 调整哈希表，用最少的值容纳全部的字典集合 */
int dictResize(dict *d)
{
    int minimal;

	//假设系统默认调整值不大于0或已经调rehash过的就提示出错，拒绝操作
    if (!dict_can_resize || dictIsRehashing(d)) return DICT_ERR;

    //最少数等于哈希标准鸿正在使用的数
    minimal = d->ht[0].used;
    if (minimal < DICT_HT_INITIAL_SIZE)
        minimal = DICT_HT_INITIAL_SIZE;

    //调用expand扩容
    return dictExpand(d, minimal);
}

/* Expand or create the hash table */
/* 哈希表扩增方法 */
int dictExpand(dict *d, unsigned long size)
{
    dictht n; /* the new hash table */
    //获取调整值，以2的幂次向上取
    unsigned long realsize = _dictNextPower(size);

    /* the size is invalid if it is smaller than the number of
     * elements already inside the hash table */
     //再次推断数量符合不符合
    if (dictIsRehashing(d) || d->ht[0].used > size)
        return DICT_ERR;

    /* Allocate the new hash table and initialize all pointers to NULL */
    //初始化大小
    n.size = realsize;
    n.sizemask = realsize-1;
    //为表格申请realsize个字典集的大小
    n.table = zcalloc(realsize*sizeof(dictEntry*));
    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. */
    if (d->ht[0].table == NULL) {
        d->ht[0] = n;
        return DICT_OK;
    }

    /* Prepare a second hash table for incremental rehashing */
   	//赋值给第二张表格
    d->ht[1] = n;
    d->rehashidx = 0;
    return DICT_OK;
}

/* 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. */
/* hash重定位，主要从旧的表映射到新表中
 * 假设返回1说明旧的表中还存在key迁移到新表中。0代表没有 */
int dictRehash(dict *d, int n) {
    if (!dictIsRehashing(d)) return 0;

	/* 依据參数分n步多次循环操作 */
    while(n--) {
        dictEntry *de, *nextde;

        /* Check if we already rehashed the whole table... */
        if (d->ht[0].used == 0) {
            zfree(d->ht[0].table);
            d->ht[0] = d->ht[1];
            _dictReset(&d->ht[1]);
            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);
        while(d->ht[0].table[d->rehashidx] == NULL) d->rehashidx++;
        de = d->ht[0].table[d->rehashidx];
        /* Move all the keys in this bucket from the old to the new hash HT */
        /* 移动的关键操作 */
        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;
            de->next = d->ht[1].table[h];
            d->ht[1].table[h] = de;
            d->ht[0].used--;
            d->ht[1].used++;
            de = nextde;
        }
        d->ht[0].table[d->rehashidx] = NULL;
        d->rehashidx++;
    }
    return 1;
}

/* 获取当前毫秒的时间 */
long long timeInMilliseconds(void) {
    struct timeval tv;

    gettimeofday(&tv,NULL);
    return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000);
}

/* Rehash for an amount of time between ms milliseconds and ms+1 milliseconds */
/* 在给定时间内。循环运行哈希重定位 */
int dictRehashMilliseconds(dict *d, int ms) {
    long long start = timeInMilliseconds();
    int rehashes = 0;

    while(dictRehash(d,100)) {
    	//重定位的次数累加
        rehashes += 100;
        //时间超出给定时间范围。则终止
        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. */
/* 当没有迭代器时候，进行重定位算法 */
static void _dictRehashStep(dict *d) {
    if (d->iterators == 0) dictRehash(d,1);
}

/* Add an element to the target hash table */
/* 加入一个dicEntry */
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.
 */
/* 加入一个指定key值的Entry */
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. */
    /* 假设指定的key已经存在，则直接返回NULL说明加入失败 */
    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];
    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. */
/* 替换一个子字典集，假设不存在直接加入，存在。覆盖val的值 */
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. */
    //不存在，这个key直接加入
    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. */
    //赋值方法
    auxentry = *entry;
    dictSetVal(d, entry, val);
    dictFreeVal(d, &auxentry);
    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. */
/* 加入字典，没有函数方法。假设存在，就不加入 */
dictEntry *dictReplaceRaw(dict *d, void *key) {
    dictEntry *entry = dictFind(d,key);

    return entry ? entry : dictAddRaw(d,key);
}

/* Search and remove an element */
/* 删除给定key的结点，可控制是否调用释放方法 */
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);
    //计算key相应的哈希索引
    h = dictHashKey(d, key);

    for (table = 0; table <= 1; table++) {
        idx = h & d->ht[table].sizemask;
        //找到详细的索引相应的结点
        he = d->ht[table].table[idx];
        prevHe = NULL;
        while(he) {
            if (dictCompareKeys(d, key, he->key)) {
                /* Unlink the element from the list */
                if (prevHe)
                    prevHe->next = he->next;
                else
                    d->ht[table].table[idx] = he->next;
                if (!nofree) {
                	//推断是否须要调用dict定义的free方法
                    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 */
}

/* 会调用free方法的删除方法 */
int dictDelete(dict *ht, const void *key) {
    return dictGenericDelete(ht,key,0);
}

/* 不会调用free方法的删除方法 */
int dictDeleteNoFree(dict *ht, const void *key) {
    return dictGenericDelete(ht,key,1);
}

/* Destroy an entire dictionary */
/* 清空整个哈希表 */
int _dictClear(dict *d, dictht *ht, void(callback)(void *)) {
    unsigned long i;

    /* Free all the elements */
    for (i = 0; i < ht->size && ht->used > 0; i++) {
        dictEntry *he, *nextHe;

		//每次情况会调用回调方法
        if (callback && (i & 65535) == 0) callback(d->privdata);

        if ((he = ht->table[i]) == NULL) continue;
        while(he) {
        	//依次释放结点
            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 */
/* 重置字典总类，清空2张表 */
void dictRelease(dict *d)
{
    _dictClear(d,&d->ht[0],NULL);
    _dictClear(d,&d->ht[1],NULL);
    zfree(d);
}

/* 依据key返回详细的字典集 */
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);
    for (table = 0; table <= 1; table++) {
        idx = h & d->ht[table].sizemask;
        he = d->ht[table].table[idx];
        while(he) {
            if (dictCompareKeys(d, key, he->key))
                return he;
            he = he->next;
        }
        if (!dictIsRehashing(d)) return NULL;
    }
    return NULL;
}

/* 获取目标字典集的方法 */
void *dictFetchValue(dict *d, const void *key) {
    dictEntry *he;

    he = dictFind(d,key);
    /* 获取字典集的方法 */
    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. */
/* 通过指纹来禁止每一个不安全的哈希迭代器的非法操作。每一个不安全迭代器仅仅能有一个指纹 */
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;
}

/* 获取哈希迭代器。默认不安全的 */
dictIterator *dictGetIterator(dict *d)
{
    dictIterator *iter = zmalloc(sizeof(*iter));

    iter->d = d;
    iter->table = 0;
    iter->index = -1;
    iter->safe = 0;
    iter->entry = NULL;
    iter->nextEntry = NULL;
    return iter;
}

/* 获取安全哈希迭代器 */
dictIterator *dictGetSafeIterator(dict *d) {
    dictIterator *i = dictGetIterator(d);

    i->safe = 1;
    return i;
}

/* 迭代器获取下一个集合点 */
dictEntry *dictNext(dictIterator *iter)
{
    while (1) {
        if (iter->entry == NULL) {
            dictht *ht = &iter->d->ht[iter->table];
            if (iter->index == -1 && iter->table == 0) {
            	//假设迭代器index下标为-1说明还没開始使用，设置迭代器的指纹或添加引用计数量
                if (iter->safe)
                    iter->d->iterators++;
                else
                    iter->fingerprint = dictFingerprint(iter->d);
            }
            //迭代器下标递增
            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;
                }
            }
            //依据下标选择集合点
            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--;
        else
        	//这时推断指纹是否还是之前定义的那个
            assert(iter->fingerprint == dictFingerprint(iter->d));
    }
    zfree(iter);
}

/* Return a random entry from the hash table. Useful to
 * implement randomized algorithms */
/* 随机获取一个集合点 */
dictEntry *dictGetRandomKey(dict *d)
{
    dictEntry *he, *orighe;
    unsigned int h;
    int listlen, listele;

    if (dictSize(d) == 0) return NULL;
    if (dictIsRehashing(d)) _dictRehashStep(d);
    if (dictIsRehashing(d)) {
        do {
        	//随机数向2个表格的总数求余运算
            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. */
    listlen = 0;
    orighe = he;
    while(he) {
        he = he->next;
        listlen++;
    }
    listele = random() % listlen;
    he = orighe;
    while(listele--) he = he->next;
    return he;
}

/* Function to reverse bits. Algorithm from:
 * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */
/* 非常奇妙的翻转位 */
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.
 */
/* 扫描方法 */
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 = 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 = t0->sizemask;
        m1 = 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 */
/* 推断是否须要扩容 */
static int _dictExpandIfNeeded(dict *d)
{
    /* Incremental rehashing already in progress. Return. */
    if (dictIsRehashing(d)) return DICT_OK;

    /* If the hash table is empty expand it to the initial 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. */
    /* 推断是否须要扩容 */
    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);
    }
    return DICT_OK;
}

/* Our hash table capability is a power of two */
/* 哈希表的容量以2的幂次方。所以数量以2的幂次向上取 */
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;
    }
}

/* 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. */
/* 获取key值相应的哈希索引值。假设已经存在此key则返回-1 */
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;
        /* 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;
    }
    return idx;
}

/* 清空整个字典，即清空里面的2张哈希表 */
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;
}

/*启用哈希表调整*/
void dictEnableResize(void) {
    dict_can_resize = 1;
}

/* 启用哈希表调整 */
void dictDisableResize(void) {
    dict_can_resize = 0;
}

#if 0

/* The following is code that we don't use for Redis currently, but that is part
of the library. */
/* redis中还存着调试的代码 */
/* ----------------------- Debugging ------------------------*/

#define DICT_STATS_VECTLEN 50
static 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);
}

/* 定义了3种类型的dictType，有些类型无val dup方法的定义 */
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>

哈希算法的索引计算事实上我还是有点不理解的地方的。比方他的索引计算，会从一张旧表映射到一个新表，作者出于什么目的，或许以后再看的时候才会明确吧。