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
Redis从入门到放弃系列(三) List
首先让我们来看一下该如何在redis里面使用Set类型
//设置key的集合中的值为member sadd key member [member ...] 复制代码
> sadd books java python c (integer) 3 //当我们重复添加相同的数据的时候,redis返回为0 > sadd books java python c (integer) 0 ---------------------------------- //返回books集合的所有元素 >smembers books 1) "c" 2) "python" 3) "java" ---------------------------------- //判断某个元素是否在集合里面 >sismember books c (integer) 1 >sismember books 99 (integer) 0 ---------------------------------- //两个集合的交集 > sadd new_books java c++ R (integer) 3 > SINTER books new_books 1) "java" ---------------------------------- //两个集合的并集 > SUNION books new_books 1) "java" 2) "python" 3) "c" 4) "c++" 5) "R" //两个集合的差集 > SMEMBERS books 1) "c" 2) "python" 3) "java" > SMEMBERS new_books 1) "R" 2) "c++" 3) "java" > SDIFF books new_books 1) "python" 2) "c" > SDIFF new_books books 1) "R" 2) "c++" 复制代码
至此,redis set的用法先告一段落.
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
typedef struct intset { uint32_t encoding; uint32_t length; int8_t contents[]; } intset; 复制代码
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
/* Note that these encodings are ordered, so: * INTSET_ENC_INT16 < INTSET_ENC_INT32 < INTSET_ENC_INT64. */ #define INTSET_ENC_INT16 (sizeof(int16_t)) #define INTSET_ENC_INT32 (sizeof(int32_t)) #define INTSET_ENC_INT64 (sizeof(int64_t)) /* Return the required encoding for the provided value. */ static uint8_t _intsetValueEncoding(int64_t v) { if (v < INT32_MIN || v > INT32_MAX) return INTSET_ENC_INT64; else if (v < INT16_MIN || v > INT16_MAX) return INTSET_ENC_INT32; else return INTSET_ENC_INT16; } 复制代码
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
/* Upgrades the intset to a larger encoding and inserts the given integer. */ static intset *intsetUpgradeAndAdd(intset *is, int64_t value) { uint8_t curenc = intrev32ifbe(is->encoding); uint8_t newenc = _intsetValueEncoding(value); int length = intrev32ifbe(is->length); int prepend = value < 0 ? 1 : 0; /* First set new encoding and resize */ is->encoding = intrev32ifbe(newenc); is = intsetResize(is,intrev32ifbe(is->length)+1); /* Upgrade back-to-front so we don't overwrite values. * Note that the "prepend" variable is used to make sure we have an empty * space at either the beginning or the end of the intset. */ while(length--) _intsetSet(is,length+prepend,_intsetGetEncoded(is,length,curenc)); /* Set the value at the beginning or the end. */ if (prepend) _intsetSet(is,0,value); else _intsetSet(is,intrev32ifbe(is->length),value); is->length = intrev32ifbe(intrev32ifbe(is->length)+1); return is; } 复制代码
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?
/* Search for the position of "value". Return 1 when the value was found and * sets "pos" to the position of the value within the intset. Return 0 when * the value is not present in the intset and sets "pos" to the position * where "value" can be inserted. */ static uint8_t intsetSearch(intset *is, int64_t value, uint32_t *pos) { int min = 0, max = intrev32ifbe(is->length)-1, mid = -1; int64_t cur = -1; /* The value can never be found when the set is empty */ if (intrev32ifbe(is->length) == 0) { if (pos) *pos = 0; return 0; } else { /* Check for the case where we know we cannot find the value, * but do know the insert position. */ if (value > _intsetGet(is,max)) { if (pos) *pos = intrev32ifbe(is->length); return 0; } else if (value < _intsetGet(is,0)) { if (pos) *pos = 0; return 0; } } while(max >= min) { mid = ((unsigned int)min + (unsigned int)max) >> 1; cur = _intsetGet(is,mid); if (value > cur) { min = mid+1; } else if (value < cur) { max = mid-1; } else { break; } } if (value == cur) { if (pos) *pos = mid; return 1; } else { if (pos) *pos = min; return 0; } } /* Insert an integer in the intset */ intset *intsetAdd(intset *is, int64_t value, uint8_t *success) { uint8_t valenc = _intsetValueEncoding(value); uint32_t pos; if (success) *success = 1; /* Upgrade encoding if necessary. If we need to upgrade, we know that * this value should be either appended (if > 0) or prepended (if < 0), * because it lies outside the range of existing values. */ if (valenc > intrev32ifbe(is->encoding)) { /* This always succeeds, so we don't need to curry *success. */ return intsetUpgradeAndAdd(is,value); } else { /* Abort if the value is already present in the set. * This call will populate "pos" with the right position to insert * the value when it cannot be found. */ if (intsetSearch(is,value,&pos)) { if (success) *success = 0; return is; } is = intsetResize(is,intrev32ifbe(is->length)+1); if (pos < intrev32ifbe(is->length)) intsetMoveTail(is,pos,pos+1); } _intsetSet(is,pos,value); is->length = intrev32ifbe(intrev32ifbe(is->length)+1); return is; } 复制代码
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?
if (intsetSearch(is,value,&pos)) { if (success) *success = 0; return is; } 复制代码
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~