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昨天分析完adlist的Redis代码，今天马上马不停蹄的继续学习Redis代码中的哈希部分的结构学习，不过在这里他不叫什么hashMap，而是叫dict，而且是一种全新设计的一种哈希结构，他只是通过几个简单的结构体，再搭配上一些比较常见的哈希算法，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，比如dbj hash的算法实现，俗称times33,算法，就是不停的*33,。这种算是一种超级简单的哈希算法。
下面说说给我感觉Redis代码中哈希实现的不是那么简单，中间加了一些东西，比如说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，也可以说字典操作类，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，下面给出一张结构图来理理思路。
下面给出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没有用到时，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值比较方法
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，如果是安全迭代器，这safe设置为1，可以调用dicAdd，dictFind */
/* 如果是不安全的，则只能调用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，如果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，如果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，没有定义直接赋值 */
#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比较函数，没有定义直接key值直接比较 */
#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); //字典根据key, val添加一个字典集
dictEntry *dictAddRaw(dict *d, void *key); //字典添加一个只有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，只提供一个key值
int dictDelete(dict *d, const void *key); //根据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); //根据key寻找字典集
void *dictFetchValue(dict *d, const void *key); //根据key值寻找相应的val值
int dictResize(dict *d); //重新计算大小
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); //根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); //这里提供了一种比较简单的哈希算法
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，循环执行哈希重定位
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，只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); //字典是否需要扩展
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，据说这种冲突的概率很低 */
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种子，m，r的值都将会参与到计算中
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) */
/* 这里提供了一种比较简单的哈希算法 */
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
//几乎所有的流行的hash map都采用了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，只在ht_destroy时使用 */
static void _dictReset(dictht *ht)
{
//清空相应的变量，ht->table的类型其实是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，用最少的值容纳所有的字典集合 */
int dictResize(dict *d)
{
int minimal;
//如果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 */
//再次判断数量符合不符合
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
* 如果返回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;
/* 根据参数分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，循环执行哈希重定位 */
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 */
/* 添加一个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.
*/
/* 添加一个指定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. */
/* 如果指定的key已经存在，则直接返回NULL说明添加失败 */
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，如果不存在直接添加，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直接添加
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. */
/* 添加字典，没有函数方法，如果存在，就不添加 */
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对应的哈希索引
h = dictHashKey(d, key);
for (table = 0; table <= 1; table++) {
idx = h & d->ht[table].sizemask;
//找到具体的索引对应的结点
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) {
//判断是否需要调用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) == 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);
}
/* 根据key返回具体的字典集 */
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. */
/* 通过指纹来禁止每个不安全的哈希迭代器的非法操作，每个不安全迭代器只能有一个指纹 */
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) {
//如果迭代器index下标为-1说明还没开始使用，设置迭代器的指纹或增加引用计数量
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;
}
}
//根据下标选择集合点
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
//这时判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/~se ... tml#ReverseParallel */
/* 很神奇的翻转位 */
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 */
/* 判断是否需要扩容 */
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. */
/* 判断是否需要扩容 */
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值对应的哈希索引值，如果已经存在此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 = 0;
for (i = 0; i < ht->size; i++) {
dictEntry *he;
if (ht->table == NULL) {
clvector[0]++;
continue;
}
slots++;
/* For each hash entry on this slot... */
chainlen = 0;
he = ht->table;
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 == 0) continue;
printf(" %s%ld: %ld (%.02f%%)\n",(i == DICT_STATS_VECTLEN-1)?">= ":"", i, clvector, ((float)clvector/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其实我还是有点不理解的地方的，比如他的索引计算，会从一张旧表映射到一个新表，作者出于什么目的，也许以后再看的时候才会明白吧。
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发表于 2014-10-11 08:36:52
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