zipmap
Zipmap是为了实现保存Pair(String,String)数据的结构，是存储效率非常高的一种结构
zipmap结构
对于map :
"foo" => "bar", "hello" => "world":
"foo""bar""hello""world"
zmlen 1字节 键值对的个数
len 1字节表示key or value长度(0-253) 如果超过253 则使用5字节 第一个字节设为254 后面四个字节表示长度.
free 1字节 value长度中未使用的长度，主要考虑到value改变的情况.
end 0xFF 结束标志
此map的在内存中的结构为
0x02,0x03,foo,0x03,0x00,bar,0x05,hello,0x05,0x00,world,0xFF
2个键值对:len=3:foo:len=3:len=3:free=0:bar:len=5:hello:len=5:free=0:world:结束标志
主要api
unsigned char *zipmapNew(void);//创建一个空的map 只需要两个字节
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update);//插入或者跟新一个键值对
unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted);//删除一个键值对
unsigned char *zipmapRewind(unsigned char *zm);// 返回第一个元素的地址
unsigned char *zipmapNext(unsigned char *p, unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen);//得到p所指向的键值对的数据，并且返回下一个键值对的起始位置
int zipmapGet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char **value, unsigned int *vlen);//寻找key相等的键值对起始位置
int zipmapExists(unsigned char *zm, unsigned char *key, unsigned int klen);//key是否存在 调用了zipmapGet函数
unsigned int zipmapLen(unsigned char *zm);//如果长度小于254直接返回，否则需要遍历map得到其大小
size_t zipmapBlobLen(unsigned char *zm);//得到这个zipmap所占内存
zipmap插入or更新键值对
//不存在则插入否则更新
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) {
unsigned int zmlen, offset;
unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen);
unsigned int empty, vempty;
unsigned char *p;
freelen = reqlen;
if (update) *update = 0;
p = zipmapLookupRaw(zm,key,klen,&zmlen);//得到总长度 并且找到有没有这个key
if (p == NULL) {//没有找到 插入新的pair
zm = zipmapResize(zm, zmlen+reqlen);//重新申请内存
p = zm+zmlen-1;//要被填充键值对数据的起始位置
zmlen = zmlen+reqlen;//更新zipmap所占字节数
if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;//更新pair个数
} else {
//找到了key 更新 value
if (update) *update = 1;
freelen = zipmapRawEntryLength(p);//返回p所代表的键值对所占有的空间
if (freelen < reqlen) {//如果更新的这个键值对所需空间大于 此键值对目前所占有的空间 更新
offset = p-zm;//记录下p的偏移量
zm = zipmapResize(zm, zmlen-freelen+reqlen);//realloc
p = zm+offset;//重新得到p的地址
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));//后移数据，腾出空间放置更新的value
zmlen = zmlen-freelen+reqlen;//更新zipmap所占内存大小
freelen = reqlen;//freelen == reqlen
}
}
empty = freelen-reqlen;//如果此键值对目前所占有的空间 大于 更新的这个键值对所需空间
if (empty >= ZIPMAP_VALUE_MAX_FREE) {//empty>4的话就要重新分配内存
/* First, move the tail bytes to the front, then resize
* the zipmap to be bytes smaller. */
offset = p-zm;
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));//数据前移
zmlen -= empty;//减去需要需要减少的字节个数
zm = zipmapResize(zm, zmlen);//重新分配内存
p = zm+offset;
vempty = 0;//free 的值
} else {//设置free的值
vempty = empty;//free 的值
}
//把数据填充一下即可
p += zipmapEncodeLength(p,klen);
memcpy(p,key,klen);
p += klen;
p += zipmapEncodeLength(p,vlen);
*p++ = vempty;
memcpy(p,val,vlen);
return zm;
}
zipmap删除键
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);//有这个key吗
if (p) {//存在
freelen = zipmapRawEntryLength(p);//这个键值对所占内存字节数
memmove(p, p+freelen, zmlen-((p-zm)+freelen+1));//前移数据
zm = zipmapResize(zm, zmlen-freelen);//realloc内存
if (zm[0] < ZIPMAP_BIGLEN) zm[0]--;//如果可以 更新 键值对个数
if (deleted) *deleted = 1;//删除了
} else {
if (deleted) *deleted = 0;//没有这个键
}
return zm;
}
参考
- https://redis.io/
- https://blog.csdn.net/u012658346/article/details/51329360