Redis原始碼閱讀三：雜湊表
摘要：
直接上程式碼：
typedef struct dictEntry {
void *key;
union {
void *val;
uint64_t u64;
int64_t s64;
double d;
...
直接上程式碼：
typedef struct dictEntry {
void *key;
union {
void *val;
uint64_t u64;
int64_t s64;
double d;
} v;
struct dictEntry *next;
} dictEntry;
typedef struct dictType {
uint64_t (*hashFunction)(const void *key);
void *(*keyDup)(void *privdata, const void *key);
void *(*valDup)(void *privdata, const void *obj);
int (*keyCompare)(void *privdata, const void *key1, const void *key2);
void (*keyDestructor)(void *privdata, void *key);
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. */
typedef struct dictht {
dictEntry **table;
unsigned long size;
unsigned long sizemask;
unsigned long used;
} dictht;
typedef struct dict {
dictType *type;
void *privdata;
dictht ht[2];
long rehashidx; /* rehashing not in progress if rehashidx == -1 */
unsigned long iterators; /* number of iterators currently running */
} dict;
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採用面向介面程式設計，把函式定義放到
struct dictType裡，這裡是對字典中所有 元素進行操作的函式 -
struct dictEntry是字典中K-V真正存放的地方，void *key是K，而union ... v是V -
每個字典有兩張雜湊表，在
dictht ht[2]這裡定義。rehashidx是一個標記位，代表是否處於 重新雜湊。其中rehash採用的是漸進式rehash，可以看到程式碼中有好幾個地方有這樣的程式碼：
if (dictIsRehashing(d)) _dictRehashStep(d);
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dict使用的雜湊函式見：ofollow,noindex" target="_blank">https://en.wikipedia.org/wiki/SipHash
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解決衝突是用鏈式
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迭代，字典在有安全跌帶器的時候不會進行rehash，見程式碼：
/* 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. */
static void _dictRehashStep(dict *d) {
if (d->iterators == 0) dictRehash(d,1);
}