zipmap 是redis里的hashmap， 主要在保证速度的同时减少内存的使用，存储结构是一个字符串，通过内存定位来操作数据结构。
数据结构
zmlen 1 字节 len 1或者5字节

<zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"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":
 * 数据格式
 * <zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
 *
 * zmlen 表示长度，如果大于等于254则只能遍历了！！
 * <zmlen> 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.
 *
 * <len>代表了后面字符串key 或 value的值的长度，长度一般被编码1个字节或5个字节表示，这个和ziplist类似
 * 如果后面的字符串长度小于等于252个，可与用单字节表示，其他253,254等长度被用来表示其他作用了，当超过这个数时候
 * 则直接按5字节的方式存储长度。
 * <len> is the length of the following string (key or value).
 * <len> 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.
 *
 * <free> 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 用于表示现在未用的字节大小， 8bit大小
 * <free> 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.查找

* Search a key and retrieve the pointer and len of the associated value.
 * If the key is found the function returns 1, otherwise 0. */
// 根据key返回对应的地址
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.删除

/* 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.下一个

/* 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\n", klen, key);
 *     printf("%d bytes value at $p\n", vlen, value);
 * }
 */
// (unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen ) is return value
//迭代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;
}