Redis(版本4.0)日记20210801--压缩列表（ZIPLIST）

<zlbytes> <zltail> <zllen> <entry> <entry> ... <entry> <zlend>
压缩列表里包括以上结构：
uint32_t zlbytes:记录整个压缩列表占用的字节数：在对整个压缩列表进行内存
                重分配或者计算zlend时使用
uint32_t zltail:一个偏移量，记录压缩列表表尾节点距离压缩列表开始地址有
                多少字节：用于计算表尾节点地址
uint16_t zllen:当小于uint16_max(65535)时可以直接表示压缩列表节点数量，
               当大于uint16_max(65535)时，节点数量需要遍历整个压缩列
               表才能得出
若干entry: 根据存储的内容决定
uint8_t zlend:固定值0xFF（255），用于标记压缩列表末端

/* We use this function to receive information about a ziplist entry.
 * Note that this is not how the data is actually encoded, is just what we
 * get filled by a function in order to operate more easily. */
typedef struct zlentry {
    unsigned int prevrawlensize; /* Bytes used to encode the previos entry len*/
    unsigned int prevrawlen;     /* Previous entry len. */
    unsigned int lensize;        /* Bytes used to encode this entry type/len.
                                    For example strings have a 1, 2 or 5 bytes
                                    header. Integers always use a single byte.*/
    unsigned int len;            /* Bytes used to represent the actual entry.
                                    For strings this is just the string length
                                    while for integers it is 1, 2, 3, 4, 8 or
                                    0 (for 4 bit immediate) depending on the
                                    number range. */
    unsigned int headersize;     /* prevrawlensize + lensize. */
    unsigned char encoding;      /* Set to ZIP_STR_* or ZIP_INT_* depending on
                                    the entry encoding. However for 4 bits
                                    immediate integers this can assume a range
                                    of values and must be range-checked. */
    unsigned char *p;            /* Pointer to the very start of the entry, that
                                    is, this points to prev-entry-len field. */
} zlentry;