redis zipmap (10)
5413 ワード
zipmapはredisのhashmapで、主に速度を保証しながらメモリの使用を減らし、メモリ構造は文字列であり、メモリの位置決めによってデータ構造を操作する.データ構造zmlen 1バイトlen 1または5バイト
"foo""bar""hello""world"
* String -> String Map data structure optimized for size.
* This file implements a data structure mapping strings to other strings
* implementing an O(n) lookup data structure designed to be very memory
* efficient.
*
* The Redis Hash type uses this data structure for hashes composed of a small
* number of elements, to switch to a hash table once a given number of
* elements is reached.
*
* Given that many times Redis Hashes are used to represent objects composed
* of few fields, this is a very big win in terms of used memory.
*
* Memory layout of a zipmap, for the map "foo" => "bar", "hello" => "world":
*
* "foo""bar""hello""world"
*
* zmlen , 254 !!
* is 1 byte length that holds the current size of the zipmap.
* When the zipmap length is greater than or equal to 254, this value
* is not used and the zipmap needs to be traversed to find out the length.
*
* key value , 1 5 , ziplist
* 252 , , 253,254 ,
* 5 。
* is the length of the following string (key or value).
* lengths are encoded in a single value or in a 5 bytes value.
* If the first byte value (as an unsigned 8 bit value) is between 0 and
* 253, it's a single-byte length. If it is 254 then a four bytes unsigned
* integer follows (in the host byte ordering). A value of 255 is used to
* signal the end of the hash.
*
* is the number of free unused bytes after the string, resulting
* from modification of values associated to a key. For instance if "foo"
* is set to "bar", and later "foo" will be set to "hi", it will have a
* free byte to use if the value will enlarge again later, or even in
* order to add a key/value pair if it fits.
* free , 8bit
* is always an unsigned 8 bit number, because if after an
* update operation there are more than a few free bytes, the zipmap will be
* reallocated to make sure it is as small as possible.
*
* The most compact representation of the above two elements hash is actually:
*
* "\x02\x03foo\x03\x00bar\x05hello\x05\x00world\xff"
*
* Note that because keys and values are prefixed length "objects",
* the lookup will take O(N) where N is the number of elements
* in the zipmap and *not* the number of bytes needed to represent the zipmap.
* This lowers the constant times considerably.
1.検索
* Search a key and retrieve the pointer and len of the associated value.
* If the key is found the function returns 1, otherwise 0. */
// key
int zipmapGet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char **value, unsigned int *vlen) {
unsigned char *p;
if ((p = zipmapLookupRaw(zm,key,klen,NULL)) == NULL) return 0;
p += zipmapRawKeyLength(p);
*vlen = zipmapDecodeLength(p);
*value = p + ZIPMAP_LEN_BYTES(*vlen) + 1;
return 1;
}
2.削除
/* Remove the specified key. If 'deleted' is not NULL the pointed integer is
* set to 0 if the key was not found, to 1 if it was found and deleted. */
unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted) {
unsigned int zmlen, freelen;
unsigned char *p = zipmapLookupRaw(zm,key,klen,&zmlen);
if (p) {
freelen = zipmapRawEntryLength(p);
memmove(p, p+freelen, zmlen-((p-zm)+freelen+1));
zm = zipmapResize(zm, zmlen-freelen);
/* Decrease zipmap length */
if (zm[0] < ZIPMAP_BIGLEN) zm[0]--;
if (deleted) *deleted = 1;
} else {
if (deleted) *deleted = 0;
}
return zm;
}
3.次
/* This function is used to iterate through all the zipmap elements.
* In the first call the first argument is the pointer to the zipmap + 1.
* In the next calls what zipmapNext returns is used as first argument.
* Example:
*
* unsigned char *i = zipmapRewind(my_zipmap);
* while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
* printf("%d bytes key at $p
", klen, key);
* printf("%d bytes value at $p
", vlen, value);
* }
*/
// (unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen ) is return value
// key
unsigned char *zipmapNext(unsigned char *zm, unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen) {
if (zm[0] == ZIPMAP_END) return NULL;
if (key) {
*key = zm;
*klen = zipmapDecodeLength(zm);
*key += ZIPMAP_LEN_BYTES(*klen);
}
zm += zipmapRawKeyLength(zm);
if (value) {
*value = zm+1;
*vlen = zipmapDecodeLength(zm);
*value += ZIPMAP_LEN_BYTES(*vlen);
}
zm += zipmapRawValueLength(zm);
return zm;
}