der. An overview of the different
* types and encodings is as follows:
* 头部信息中的另一个值记录着编码的方式，当编码的是字符串，头部的前2位为00,01,10共3种
* 如果编码的是整型数字的时候，则头部的前2位为11,代表的是整数编码，后面2位代表什么类型整型值将会在头部后面被编码
* 00-int16_t, 01-int32_t, 10-int64_t, 11-24 bit signed，还有比较特殊的2个，11111110-8 bit signed,
* 1111 0000 - 1111 1101,代表的是整型值0-12,头尾都已经存在，都不能使用，与传统的通过固定的指针表示长度，这么做的好处实现
* 可以更合理的分配内存
*
* String字符串编码的3种形式
* |00pppppp| - 1 byte
* String value with length less than or equal to 63 bytes (6 bits).
* |01pppppp|qqqqqqqq| - 2 bytes
* String value with length less than or equal to 16383 bytes (14 bits).
* |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
* String value with length greater than or equal to 16384 bytes.
* |11000000| - 1 byte
* Integer encoded as int16_t (2 bytes).
* |11010000| - 1 byte
* Integer encoded as int32_t (4 bytes).
* |11100000| - 1 byte
* Integer encoded as int64_t (8 bytes).
* |11110000| - 1 byte
* Integer encoded as 24 bit signed (3 bytes).
* |11111110| - 1 byte
* Integer encoded as 8 bit signed (1 byte).
* |1111xxxx| - (with xxxx between 0000 and 1101) immediate 4 bit integer.
* Unsigned integer from 0 to 12. The encoded value is actually from
* 1 to 13 because 0000 and 1111 can not be used, so 1 should be
* subtracted from the encoded 4 bit value to obtain the right value.
* |11111111| - End of ziplist.
*
* All the integers are represented in little endian byte order.
*
* ----------------------------------------------------------------------------
希望大家能仔细反复阅读，理解作者的设计思路，下面给出的他的实际结构体的定义：
/* 实际存放数据的数据结点 */
typedef struct zlentry {
//prevrawlen为上一个数据结点的长度，prevrawlensize为记录该长度数值所需要的字节数
unsigned int prevrawlensize, prevrawlen;
//len为当前数据结点的长度，lensize表示表示当前长度表示所需的字节数
unsigned int lensize, len;
//数据结点的头部信息长度的字节数
unsigned int headersize;
//编码的方式
unsigned char encoding;
//数据结点的数据(已包含头部等信息)，以字符串形式保存
unsigned char *p;
} zlentry;
/*
的结构图线表示 (上一结点的长度信息)(本结点的编码方式和编码数据的长度信息)(本结点的编码数据) */
我们看一下里面比较核心的操作，插入操作，里面涉及指针的各种来回移动，这些都是内存地址的调整：
/* Insert item at "p". */
/* 插入操作的实现 */
static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), reqlen;
unsigned int prevlensize, prevlen = 0;
size_t offset;
int nextdiff = 0;
unsigned char encoding = 0;
long long value = 123456789; /* initialized to avoid warning. Using a value
that is easy to see if for some reason
we use it uninitialized. */
zlentry tail;
/* Find out prevlen for the entry that is inserted. */
//寻找插入的位置
if (p[0] != ZIP_END) {
//定位到指定位置
ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
} else {
//如果插入的位置是尾结点，直接定位到尾结点，看第一个字节的就可以判断
unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);
if (ptail[0] != ZIP_END) {
prevlen = zipRawEntryLength(ptail);
}
}
/* See if the entry can be encoded */
if (zipTryEncoding(s,slen,&value,&encoding)) {
/* 'encoding' is set to the appropriate integer encoding */
reqlen = zipIntSize(encoding);
} else {
/* 'encoding' is untouched, however zipEncodeLength will use the
* string length to figure out how to encode it. */
reqlen = slen;
}
/* We need space for both the length of the previous entry and
* the length of the payload. */
reqlen += zipPrevEncodeLength(NULL,prevlen);
reqlen += zipEncodeLength(NULL,encoding,slen);
/* When the insert position is not equal to the tail, we need to
*