redis专属链表ziplist使用代码示例
作者：袖梨
2022-06-29
本篇文章小编给大家分享一下redis专属链表ziplist使用代码示例，文章代码介绍的很详细，小编觉得挺不错的，现在分享给大家供大家参考，有需要的小伙伴们可以来看看。
问题抛出
用过 Python 的列表吗？就是那种可以存储任意类型数据的，支持随机读取的数据结构。
没有用过的话那就没办法了。
本质上这种列表可以使用数组、链表作为其底层结构，不知道Python中的列表是以什么作为底层结构的。
但是redis的列表既不是用链表，也不是用数组作为其底层实现的，原因也显而易见：数组不方便，弄个二维的？柔性的？怎么写？链表可以实现，通用链表嘛，数据域放 void* 就可以实现列表功能。但是，链表的缺点也很明显，容易造成内存碎片。
在这个大环境下，秉承着“能省就省”的指导思想，请你设计一款数据结构。
结构设计
这个图里要注意，右侧是没有记录“当前元素的大小”的
这个图挺详细哈，都省得我对每一个字段释义了，整挺好。
其他话，文件开头的注释也讲的很清楚了。（ziplist.c）
/* 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 OVERALL LAYOUT * ====================== * * The general layout of the ziplist is as follows: * *... * * NOTE: all fields are stored in little endian, if not specified otherwise. * * is an unsigned integer to hold the number of bytes that * the ziplist occupies, including the four bytes of the zlbytes field itself. * This value needs to be stored to be able to resize the entire structure * without the need to traverse it first. * * 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. * * is the number of entries. When there are more than * 2^16-2 entries, this value is set to 2^16-1 and we need to traverse the * entire list to know how many items it holds. * * is a special entry representing the end of the ziplist. * Is encoded as a single byte equal to 255. No other normal entry starts * with a byte set to the value of 255. * * ZIPLIST ENTRIES * =============== * * Every entry in the ziplist is prefixed by metadata 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 entry encoding is * provided. It represents the entry type, integer or string, and in the case * of strings it also represents the length of the string payload. * So a complete entry is stored like this: * * * * Sometimes the encoding represents the entry itself, like for small integers * as we'll see later. In such a case the part is missing, and we * could have just: * * * * 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 representing the length as an unsinged 8 bit integer. When the length * is greater than or equal to 254, it will consume 5 bytes. The first byte is * set to 254 (FE) to indicate a larger value is following. The remaining 4 * bytes take the length of the previous entry as value. * * So practically an entry is encoded in the following way: * * * * Or alternatively if the previous entry length is greater than 253 bytes * the following encoding is used: * * 0xFE <4 bytes unsigned little endian prevlen> * * The encoding field of the entry depends on the content of the * entry. When the entry is a string, the first 2 bits of the encoding first * byte 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. The first byte is always enough * to determine the kind of entry. * * |00pppppp| - 1 byte * String value with length less than or equal to 63 bytes (6 bits). * "pppppp" represents the unsigned 6 bit length. * |01pppppp|qqqqqqqq| - 2 bytes * String value with length less than or equal to 16383 bytes (14 bits). * IMPORTANT: The 14 bit number is stored in big endian. * |10000000|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes * String value with length greater than or equal to 16384 bytes. * Only the 4 bytes following the first byte represents the length * up to 2^32-1. The 6 lower bits of the first byte are not used and * are set to zero. * IMPORTANT: The 32 bit number is stored in big endian. * |11000000| - 3 bytes * Integer encoded as int16_t (2 bytes). * |11010000| - 5 bytes * Integer encoded as int32_t (4 bytes). * |11100000| - 9 bytes * Integer encoded as int64_t (8 bytes). * |11110000| - 4 bytes * Integer encoded as 24 bit signed (3 bytes). * |11111110| - 2 bytes * 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 special entry. * * Like for the ziplist header, all the integers are represented in little * endian byte order, even when this code is compiled in big endian systems. * * EXAMPLES OF ACTUAL ZIPLISTS * =========================== * * The following is a ziplist containing the two elements representing * the strings "2" and "5". It is composed of 15 bytes, that we visually * split into sections: * * [0f 00 00 00] [0c 00 00 00] [02 00] [00 f3] [02 f6] [ff] * | | | | | | * zlbytes zltail entries "2" "5" end * * The first 4 bytes represent the number 15, that is the number of bytes * the whole ziplist is composed of. The second 4 bytes are the offset * at which the last ziplist entry is found, that is 12, in fact the * last entry, that is "5", is at offset 12 inside the ziplist. * The next 16 bit integer represents the number of elements inside the * ziplist, its value is 2 since there are just two elements inside. * Finally "00 f3" is the first entry representing the number 2. It is * composed of the previous entry length, which is zero because this is * our first entry, and the byte F3 which corresponds to the encoding * |1111xxxx| with xxxx between 0001 and 1101. We need to remove the "F" * higher order bits 1111, and subtract 1 from the "3", so the entry value * is "2". The next entry has a prevlen of 02, since the first entry is * composed of exactly two bytes. The entry itself, F6, is encoded exactly * like the first entry, and 6-1 = 5, so the value of the entry is 5. * Finally the special entry FF signals the end of the ziplist. * * Adding another element to the above string with the value "Hello World" * allows us to show how the ziplist encodes small strings. We'll just show * the hex dump of the entry itself. Imagine the bytes as following the * entry that stores "5" in the ziplist above: * * [02] [0b] [48 65 6c 6c 6f 20 57 6f 72 6c 64] * * The first byte, 02, is the length of the previous entry. The next * byte represents the encoding in the pattern |00pppppp| that means * that the entry is a string of length , so 0B means that * an 11 bytes string follows. From the third byte (48) to the last (64) * there are just the ASCII characters for "Hello World". * * ---------------------------------------------------------------------------- * * Copyright (c) 2009-2012, Pieter Noordhuis * Copyright (c) 2009-2017, Salvatore Sanfilippo * All rights reserved. */
看完了么？接下来就是基操阶段了，对于任何一种数据结构，基操无非增删查改。
实际节点
typedef struct zlentry {
unsigned int prevrawlensize; /* Bytes used to encode the previous 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;
基本操作
我觉得这张图还是要再摆一下：
这个图里要注意，右侧是没有记录“当前元素的大小”的
增
真实插入的是这个函数：
讲真，头皮有点发麻。那么我们等下还是用老套路，按步骤拆开来看。
/* Insert item at "p". */
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. */
if (p[0] != ZIP_END) {
ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
} else {
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 zipStoreEntryEncoding 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 += zipStorePrevEntryLength(NULL,prevlen);
reqlen += zipStoreEntryEncoding(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. */
int forcelarge = 0;
nextdiff = (p[0] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : 0;
if (nextdiff == -4 && reqlen < 4) {
nextdiff = 0;
forcelarge = 1;
}
/* Store offset because a realloc may change the address of zl. */
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 */
memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff);
/* Encode this entry's raw length in the next entry. */
if (forcelarge)
zipStorePrevEntryLengthLarge(p+reqlen,reqlen);
else
zipStorePrevEntryLength(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. */
zipEntry(p+reqlen, &tail);
if (p[reqlen+tail.headersize+tail.len] != ZIP_END) {
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
}
} else {
/* 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 */
p += zipStorePrevEntryLength(p,prevlen);
p += zipStoreEntryEncoding(p,encoding,slen);
if (ZIP_IS_STR(encoding)) {
memcpy(p,s,slen);
} else {
zipSaveInteger(p,value,encoding);
}
ZIPLIST_INCR_LENGTH(zl,1);
return zl;
}
对“链表”插入数据有几个步骤？
1、偏移
2、插进去
3、缝合
那这个“列表”，比较特殊一点，特殊在哪里？特殊在它比较紧凑，而且数据类型，其实也就两种，要么integer，要么string。所以它的步骤是？
1、数据重新编码
2、解析数据并分配空间
3、接入数据
重新编码
什么是重新编码？插入一个元素，是不是需要对：“前一个元素的大小、本身大小、当前元素编码” 这些数据进行一个统计，然后一并插入。就编这个。
插入位置无非三个，头中尾。
头：前一个元素大小为0，因为前面没有元素。
中：待插入位置后一个元素记录的“前一个元素大小”，当然，之后本身大小就成为了后一个元素眼中的“前一个元素大小”。
尾：那就要把三个字段加起来了。
具体怎么重新编码就不看了吧，这篇本来就已经很长了。
解析数据
再往下就是解析数据了。
首先尝试将数据解析为整数，如果可以解析，就按照压缩列表整数类型编码存储；如果解析失败，就按照压缩列表字节数组类型编码存储。
解析之后，数值存储在 value 中，编码格式存储在 encoding中。如果解析成功，还要计算整数所占字节数。变量 reqlen 存储当前元素所需空间大小，再累加其他两个字段的空间大小，就是本节点所需空间大小了。
重新分配空间
看注释这架势，咋滴，还存在没地方给它塞？
来我们看看。
这里的分配空间不是简单的就新插进来的数据多少空间就分配多少，如果没有仔细阅读上面那段英文的话，嗯，可以选择绕回去仔细阅读一下那个节点组成。特别是那个：
/* * 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 representing the length as an unsinged 8 bit integer. When the length * is greater than or equal to 254, it will consume 5 bytes. The first byte is * set to 254 (FE) to indicate a larger value is following. The remaining 4 * bytes take the length of the previous entry as value. */
所以这个 previous 就是个不确定因素。有可能人家本来是 1 1 排列的，中间插进来一个之后变成 1 1 5 排列了；也有可能人家是1 5 排列的、5 1 排列的，总之就是不确定。
所以，在 entryX 的位置插入一个数据之后，entryX+1 的 previous 可能不变，可能加四，也可能减四，谁也说不准。说不准那不就得测一下嘛。所以就测一下，仅此而已。
接入数据
数据怎么接入？鉴于这里真心不是链表，是列表。
所以，按数组那一套来。对。
很麻烦吧。其实不麻烦，你在redis里见过它给你中间插入的机会了吗？更不要说头插了，你见过它给你头插的机会了吗？
插个题外话：大数据插入时，数组不一定输给链表。在尾插的时候，数组的优势是远超链表的（当然，仅限于尾插）。在我两个月前的博客里有做过这一系列的实验。
删就不写了吧，增的逆操作，从系列开始就没写过删。不过这里删就不可避免的大量数据进行复制了（如果不真删，只是做个删除标志呢？这样会省时间，但是时候会造成内存碎片化。不过可以设计一个定期调整内存的函数，比方说重用三分之一的块之后紧凑一下？内存不够用的时候紧凑一下？STL就是这么干的）。
查也没啥好讲的了吧，这个数据结构的应用场景一般就是对键进行检索，这里就是个值，不一样的是这个值是一串的。
所以除了提供原有的前后向遍历之外，还提供了 range 查询，不难的。
