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:
* <zlbytes><zltail><zllen><entry><entry><zlend>
*
* <zlbytes> 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.
* <zipbytes>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
*
* <zltail> is the offset to the last entry in the list. This allows a pop
* operation on the far side of the list without the need for full traversal.
* <zltail>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
*
* <zllen> 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.
* <zllen>记录的是ziplist里面entry数据结点的总数
*
* <zlend> is a single byte special value, equal to 255, which indicates the
* end of the list.
* <zlend>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 header. 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;
/* <zlentry>的结构图线表示 <pre_node_len>(上一结点的长度信息)<node_encode>(本结点的编码方式和编码数据的长度信息)<node>(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
* make sure that the next entry can hold this entry's length in
* its prevlen field. */
nextdiff = (p[0] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : 0;
/* Store offset because a realloc may change the address of zl. */
//调整大小，为新结点的插入预留空间
offset = p-zl;
zl = ziplistResize(zl,curlen+reqlen+nextdiff);
p = zl+offset;
/* Apply memory move when necessary and update tail offset. */
if (p[0] != ZIP_END) {
/* Subtract one because of the ZIP_END bytes */
//如果插入的位置不是尾结点，则挪动位置
memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff);
/* Encode this entry's raw length in the next entry. */
zipPrevEncodeLength(p+reqlen,reqlen);
/* Update offset for tail */
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+reqlen);
/* When the tail contains more than one entry, we need to take
* "nextdiff" in account as well. Otherwise, a change in the
* size of prevlen doesn't have an effect on the *tail* offset. */
tail = zipEntry(p+reqlen);
if (p[reqlen+tail.headersize+tail.len] != ZIP_END) {
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
}
} else {
//如果是尾结点，直接设置新尾结点
/* This element will be the new tail. */
ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl);
}
/* When nextdiff != 0, the raw length of the next entry has changed, so
* we need to cascade the update throughout the ziplist */
if (nextdiff != 0) {
offset = p-zl;
zl = __ziplistCascadeUpdate(zl,p+reqlen);
p = zl+offset;
}
/* Write the entry */
//写入新的数据结点信息
p += zipPrevEncodeLength(p,prevlen);
p += zipEncodeLength(p,encoding,slen);
if (ZIP_IS_STR(encoding)) {
memcpy(p,s,slen);
} else {
zipSaveInteger(p,value,encoding);
}
//更新列表的长度加1
ZIPLIST_INCR_LENGTH(zl,1);
return zl;
}
下面是删除操作：
/* Delete "num" entries, starting at "p". Returns pointer to the ziplist. */
/* 删除方法涉及p指针的滑动，后面的地址内容都需要滑动 */
static unsigned char *__ziplistDelete(unsigned char *zl, unsigned char *p, unsigned int num) {
unsigned int i, totlen, deleted = 0;
size_t offset;
int nextdiff = 0;
zlentry first, tail;
first = zipEntry(p);
for (i = 0; p[0] != ZIP_END && i < num; i++) {
p += zipRawEntryLength(p);
deleted++;
}
totlen = p-first.p;
if (totlen > 0) {
if (p[0] != ZIP_END) {
/* Storing `prevrawlen` in this entry may increase or decrease the
* number of bytes required compare to the current `prevrawlen`.
* There always is room to store this, because it was previously
* stored by an entry that is now being deleted. */
nextdiff = zipPrevLenByteDiff(p,first.prevrawlen);
p -= nextdiff;
zipPrevEncodeLength(p,first.prevrawlen);
/* Update offset for tail */
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))-totlen);
/* When the tail contains more than one entry, we need to take
* "nextdiff" in account as well. Otherwise, a change in the
* size of prevlen doesn't have an effect on the *tail* offset. */
tail = zipEntry(p);
if (p[tail.headersize+tail.len] != ZIP_END) {
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
}
/* Move tail to the front of the ziplist */
memmove(first.p,p,
intrev32ifbe(ZIPLIST_BYTES(zl))-(p-zl)-1);
} else {
/* The entire tail was deleted. No need to move memory. */
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe((first.p-zl)-first.prevrawlen);
}
/* Resize and update length */
//调整列表大小
offset = first.p-zl;
zl = ziplistResize(zl, intrev32ifbe(ZIPLIST_BYTES(zl))-totlen+nextdiff);
ZIPLIST_INCR_LENGTH(zl,-deleted);
p = zl+offset;
/* When nextdiff != 0, the raw length of the next entry has changed, so
* we need to cascade the update throughout the ziplist */
if (nextdiff != 0)
zl = __ziplistCascadeUpdate(zl,p);
}
return zl;
}
该方法的意思是从index索引对应的结点开始算起，删除num个结点，这是删除的最原始的方法，其他方法都是对此方法的包装。
下面我们看看我们在redis命令行中输入的lpush或rpush调用的是什么方法呢？调用的形式：
zl = ziplistPush(zl, (unsigned char*)"foo", 3, ZIPLIST_TAIL);
zl = ziplistPush(zl, (unsigned char*)"quux", 4, ZIPLIST_TAIL);
zl = ziplistPush(zl, (unsigned char*)"hello", 5, ZIPLIST_HEAD);/* 在列表2边插入数据的方法 */
unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, unsigned int slen, int where) {
unsigned char *p;
//这里开始直接定位
p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_END(zl);
//组后调用插入数据的insert方法
return __ziplistInsert(zl,p,s,slen);
}
到最后还是调用了insert方法。在写之前看了一些别人分析的ziplist分析，感觉有些说的的都很粗略，还是自己仔细过一遍心里会清楚很多，建议大家多多阅读源码。每个人侧重点都是不一样的。最后给出头文件和比较关键的宏定义：
/* zip列表的末尾值 */
#define ZIP_END 255
/* zip列表的最大长度 */
#define ZIP_BIGLEN 254
/* Different encoding/length possibilities */
/* 不同的编码 */
#define ZIP_STR_MASK 0xc0
#define ZIP_INT_MASK 0x30
#define ZIP_STR_06B (0 << 6)
#define ZIP_STR_14B (1 << 6)
#define ZIP_STR_32B (2 << 6)
#define ZIP_INT_16B (0xc0 | 0<<4)
#define ZIP_INT_32B (0xc0 | 1<<4)
#define ZIP_INT_64B (0xc0 | 2<<4)
#define ZIP_INT_24B (0xc0 | 3<<4)
#define ZIP_INT_8B 0xfe
/* 4 bit integer immediate encoding */
#define ZIP_INT_IMM_MASK 0x0f //后续的好多运算都需要与掩码进行位运算
#define ZIP_INT_IMM_MIN 0xf1 /* 11110001 */
#define ZIP_INT_IMM_MAX 0xfd /* 11111101 */ //最大值不能为11111111，这跟最末尾的结点重复了
#define ZIP_INT_IMM_VAL(v) (v & ZIP_INT_IMM_MASK)
#define INT24_MAX 0x7fffff
#define INT24_MIN (-INT24_MAX - 1)
/* Macro to determine type */
#define ZIP_IS_STR(enc) (((enc) & ZIP_STR_MASK) < ZIP_STR_MASK)
/* Utility macros */
/* 下面是一些用来到时能够直接定位的数值偏移量 */
#define ZIPLIST_BYTES(zl) (*((uint32_t*)(zl)))
#define ZIPLIST_TAIL_OFFSET(zl) (*((uint32_t*)((zl)+sizeof(uint32_t))))
#define ZIPLIST_LENGTH(zl) (*((uint16_t*)((zl)+sizeof(uint32_t)*2)))
#define ZIPLIST_HEADER_SIZE (sizeof(uint32_t)*2+sizeof(uint16_t))
#define ZIPLIST_ENTRY_HEAD(zl) ((zl)+ZIPLIST_HEADER_SIZE)
#define ZIPLIST_ENTRY_TAIL(zl) ((zl)+intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl)))
#define ZIPLIST_ENTRY_END(zl) ((zl)+intrev32ifbe(ZIPLIST_BYTES(zl))-1)
/*
* Copyright (c) 2009-2012, Pieter Noordhuis <pcnoordhuis at gmail dot com>
* Copyright (c) 2009-2012, Salvatore Sanfilippo <antirez at gmail dot com>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* * Neither the name of Redis nor the names of its contributors may be used
* to endorse or promote products derived from this software without
* specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
/* 标记列表头节点和尾结点的标识 */
#define ZIPLIST_HEAD 0
#define ZIPLIST_TAIL 1
unsigned char *ziplistNew(void); //创建新列表
unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, unsigned int slen, int where); //像列表中推入数据
unsigned char *ziplistIndex(unsigned char *zl, int index); //索引定位到列表的某个位置
unsigned char *ziplistNext(unsigned char *zl, unsigned char *p); //获取当前列表位置的下一个值
unsigned char *ziplistPrev(unsigned char *zl, unsigned char *p); //获取当期列表位置的前一个值
unsigned int ziplistGet(unsigned char *p, unsigned char **sval, unsigned int *slen, long long *lval); //获取列表的信息
unsigned char *ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen); //向列表中插入数据
unsigned char *ziplistDelete(unsigned char *zl, unsigned char **p); //列表中删除某个结点
unsigned char *ziplistDeleteRange(unsigned char *zl, unsigned int index, unsigned int num); //从index索引对应的结点开始算起，删除num个结点
unsigned int ziplistCompare(unsigned char *p, unsigned char *s, unsigned int slen); //列表间的比较方法
unsigned char *ziplistFind(unsigned char *p, unsigned char *vstr, unsigned int vlen, unsigned int skip); //在列表中寻找某个结点
unsigned int ziplistLen(unsigned char *zl); //返回列表的长度
size_t ziplistBlobLen(unsigned char *zl); //返回列表的二进制长度，返回的是字节数