ziplist和之前我解析过的adlist列表名字看上去的很像，但是作用却完全不同。之前的adlist主要针对的是普通的数据链表操作。而今天的ziplist指的是压缩链表，为什么叫压缩链表呢，因为链表中我们一般常用pre，next来指明当前的结点的前一个指针或当前的结点的下一个指针，这其实是在一定程度上占据了比较多的内存空间，ziplist采用了长度的表示方法，整个ziplist其实是超级长的字符串，通过里面各个结点的长度，上一个结点的长度等信息，通过快速定位实现相关操作，而且编写者，在长度上也做了动态分配字节的方法，表示长度，避免了一定的内存耗费，比如一个结点的字符串长度每个都很短，而你使用好几个字节表示字符串的长度，显然造成大量浪费，所以在长度表示方面，ziplist 就做到了压缩，也体现了压缩的性能。ziplist 用在什么地方呢，ziplist 就是用在我们平常最常用的一个命令rpush,lpush等这些往链表添加数据的方法，这些数据就是存在ziplist 中的。之后我们会看到相应的实现方法。
在学习ziplist的开始，一定要理解他的结构，关于这一点，必须花一定时间想想，要不然不太容易明白人家的设计。下面是我的理解，帮助大家理解：
/* The ziplist is a specially encoded dually linked list that is designed * to be very memory efficient. It stores both strings and integer values, * where integers are encoded as actual integers instead of a series of * characters. It allows push and pop operations on either side of the list * in O(1) time. However, because every operation requires a reallocation of * the memory used by the ziplist, the actual complexity is related to the * amount of memory used by the ziplist. * * ziplist是一个编码后的列表，特殊的设计使得内存操作非常有效率，此列表可以同时存放 * 字符串和整数类型，列表可以在头尾各边支持推加和弹出操作在O(1)常量时间,但是，因为每次 * 操作设计到内存的重新分配释放，所以加大了操作的复杂性 * ---------------------------------------------------------------------------- * * ziplist的结构组成： * ZIPLIST OVERALL LAYOUT: * The general layout of the ziplist is as follows: ** * is an unsigned integer to hold the number of bytes that the * ziplist occupies. This value needs to be stored to be able to resize the * entire structure without the need to traverse it first. * 代表着ziplist占有的字节数，这方便当重新调整大小的时候不需要重新从头遍历 * * 记录了最后一个entry的位置在列表中，可以方便快速在列表末尾弹出操作 * * is the number of entries.When this value is larger than 2**16-2, * we need to traverse the entire list to know how many items it holds. * 记录的是ziplist里面entry数据结点的总数 * * is a single byte special value, equal to 255, which indicates the * end of the list. * 代表的是结束标识别，用单字节表示，值是255，就是11111111 * * ZIPLIST ENTRIES: * Every entry in the ziplist is prefixed by a header that contains two pieces * of information. First, the length of the previous entry is stored to be * able to traverse the list from back to front. Second, the encoding with an * optional string length of the entry itself is stored. * 每个entry数据结点主要包含2部分信息，第一个，上一个结点的长度，主要就可以可以从任意结点从后往前遍历整个列表 * 第二个，编码字符串的方式的类型保存 * * The length of the previous entry is encoded in the following way: * If this length is smaller than 254 bytes, it will only consume a single * byte that takes the length as value. When the length is greater than or * equal to 254, it will consume 5 bytes. The first byte is set to 254 to * indicate a larger value is following. The remaining 4 bytes take the * length of the previous entry as value. * 之前的数据结点的字符串长度的长度少于254个字节，他将消耗单个字节，一个字节8位，最大可表示长度为2的8次方 * 当字符串的长度大于254个字节，则用5个字节表示，第一个字节被设置成254,其余的4个字节占据的长度为之前的数据结点的长度 * * The other header field of the entry itself depends on the contents of the * entry. When the entry is a string, the first 2 bits of this header will hold * the type of encoding used to store the length of the string, followed by the * actual length of the string. When the entry is an integer the first 2 bits * are both set to 1. The following 2 bits are used to specify what kind of * integer will be stored after this hea