Redis从入门到放弃系列(四) Set

本文例子基于：5.0.4 Set是Redis中一种比较常见的数据结构,当存储的member为十进制64位有符号整数范围内的整数的字符串的时候其实现为intset,其他为hashtable

首先让我们来看一下该如何在redis里面使用Set类型

1//设置key的集合中的值为member
2sadd key member [member ...]
3

代码示例：

1> sadd books java python c
2(integer) 3
3//当我们重复添加相同的数据的时候，redis返回为0
4> sadd books java python c
5(integer) 0
6----------------------------------
7//返回books集合的所有元素
8>smembers books 
91) "c"
102) "python"
113) "java"
12----------------------------------
13//判断某个元素是否在集合里面
14>sismember books c
15(integer) 1
16>sismember books 99
17(integer) 0
18----------------------------------
19//两个集合的交集
20> sadd new_books java c++ R
21(integer) 3
22> SINTER books new_books
231) "java"
24----------------------------------
25//两个集合的并集
26> SUNION books new_books
271) "java"
282) "python"
293) "c"
304) "c++"
315) "R"
32//两个集合的差集
33> SMEMBERS books
341) "c"
352) "python"
363) "java"
37> SMEMBERS new_books
381) "R"
392) "c++"
403) "java"
41>  SDIFF books new_books
421) "python"
432) "c"
44>  SDIFF new_books books
451) "R"
462) "c++"
47

至此,redis set的用法先告一段落.

源码解析

本文开头的时候讲set实现分为intset跟hashtable,hashtable这块讲解的话可以去回头看一下Redis从入门到放弃系列(二) Hash 本节重点来讲一下intset~当存储元素为整数的时候,redis为了节省空间,采用了intset这种数据结构来做存储,我们知道set结构存储字符串的时候都是无序的,可当采用intset来存储的整数的时候, set是有序的，内部采用了二分法方便快速查询让我们先来看一下intset内部结构

1typedef struct intset {
2    uint32_t encoding;
3    uint32_t length;
4    int8_t contents[];
5} intset;
6

我们发现intset里面其实是由一个变量类型，一个长度表示的,也就是说要计算当前intset占据的字节:encoding * length; redis在使用intset的时候,首先会判断当前插入的value的大小，然后返回不同字节的类型

1/* Note that these encodings are ordered, so:
2 * INTSET_ENC_INT16 < INTSET_ENC_INT32 < INTSET_ENC_INT64. */
3#define INTSET_ENC_INT16 (sizeof(int16_t))
4#define INTSET_ENC_INT32 (sizeof(int32_t))
5#define INTSET_ENC_INT64 (sizeof(int64_t))
6
7/* Return the required encoding for the provided value. */
8static uint8_t _intsetValueEncoding(int64_t v) {
9    if (v < INT32_MIN || v > INT32_MAX)
10        return INTSET_ENC_INT64;
11    else if (v < INT16_MIN || v > INT16_MAX)
12        return INTSET_ENC_INT32;
13    else
14        return INTSET_ENC_INT16;
15}
16

当每次插入的value的值大于当前类型的话,redis会将intset升级为更大的编码

1/* Upgrades the intset to a larger encoding and inserts the given integer. */
2static intset *intsetUpgradeAndAdd(intset *is, int64_t value) {
3    uint8_t curenc = intrev32ifbe(is->encoding);
4    uint8_t newenc = _intsetValueEncoding(value);
5    int length = intrev32ifbe(is->length);
6    int prepend = value < 0 ? 1 : 0;
7
8    /* First set new encoding and resize */
9    is->encoding = intrev32ifbe(newenc);
10    is = intsetResize(is,intrev32ifbe(is->length)+1);
11
12    /* Upgrade back-to-front so we don't overwrite values.
13     * Note that the "prepend" variable is used to make sure we have an empty
14     * space at either the beginning or the end of the intset. */
15    while(length--)
16        _intsetSet(is,length+prepend,_intsetGetEncoded(is,length,curenc));
17
18    /* Set the value at the beginning or the end. */
19    if (prepend)
20        _intsetSet(is,0,value);
21    else
22        _intsetSet(is,intrev32ifbe(is->length),value);
23    is->length = intrev32ifbe(intrev32ifbe(is->length)+1);
24    return is;
25}
26

前面我们说过,intset是一个有序的,然后查找的时候采用了二分法来查找元素,那么其内部是如何实现的呢?

1/* Search for the position of "value". Return 1 when the value was found and
2 * sets "pos" to the position of the value within the intset. Return 0 when
3 * the value is not present in the intset and sets "pos" to the position
4 * where "value" can be inserted. */
5static uint8_t intsetSearch(intset *is, int64_t value, uint32_t *pos) {
6    int min = 0, max = intrev32ifbe(is->length)-1, mid = -1;
7    int64_t cur = -1;
8
9    /* The value can never be found when the set is empty */
10    if (intrev32ifbe(is->length) == 0) {
11        if (pos) *pos = 0;
12        return 0;
13    } else {
14        /* Check for the case where we know we cannot find the value,
15         * but do know the insert position. */
16        if (value > _intsetGet(is,max)) {
17            if (pos) *pos = intrev32ifbe(is->length);
18            return 0;
19        } else if (value < _intsetGet(is,0)) {
20            if (pos) *pos = 0;
21            return 0;
22        }
23    }
24
25    while(max >= min) {
26        mid = ((unsigned int)min + (unsigned int)max) >> 1;
27        cur = _intsetGet(is,mid);
28        if (value > cur) {
29            min = mid+1;
30        } else if (value < cur) {
31            max = mid-1;
32        } else {
33            break;
34        }
35    }
36
37    if (value == cur) {
38        if (pos) *pos = mid;
39        return 1;
40    } else {
41        if (pos) *pos = min;
42        return 0;
43    }
44}
45/* Insert an integer in the intset */
46intset *intsetAdd(intset *is, int64_t value, uint8_t *success) {
47    uint8_t valenc = _intsetValueEncoding(value);
48    uint32_t pos;
49    if (success) *success = 1;
50
51    /* Upgrade encoding if necessary. If we need to upgrade, we know that
52     * this value should be either appended (if > 0) or prepended (if < 0),
53     * because it lies outside the range of existing values. */
54    if (valenc > intrev32ifbe(is->encoding)) {
55        /* This always succeeds, so we don't need to curry *success. */
56        return intsetUpgradeAndAdd(is,value);
57    } else {
58        /* Abort if the value is already present in the set.
59         * This call will populate "pos" with the right position to insert
60         * the value when it cannot be found. */
61        if (intsetSearch(is,value,&pos)) {
62            if (success) *success = 0;
63            return is;
64        }
65
66        is = intsetResize(is,intrev32ifbe(is->length)+1);
67        if (pos < intrev32ifbe(is->length)) intsetMoveTail(is,pos,pos+1);
68    }
69
70    _intsetSet(is,pos,value);
71    is->length = intrev32ifbe(intrev32ifbe(is->length)+1);
72    return is;
73}
74

重点在while操作那一段~我们可以看到,其查找采用了二分法,那么如何让其有序呢?

1if (intsetSearch(is,value,&pos)) {
2    if (success) *success = 0;
3    return is;
4}
5

看到这一段了吧？判断查找的时候，将pos的位置查找了出来,给下面_intsetSet操作做前奏~

应用场景

1.去重~ 2.查看两个人的共同爱好

写在最后

祝大家520快乐~

Redis从入门到放弃系列(四) Set