Redis Bitmaps

Redis位图：
位图并不是一种实际的数据类型，而是在字符串数据类型上定义的按位操作集合（对普通的字符串键也可以进行位图操作）。由于字符数据是二进制安全的数据块并且最大长度可以达到512MB，它们适用于2的32次方个不同的位。

位操作可以分成两种：一种是常量时间单个位操作，例如单个位赋值或取值，一种是对多个位组成的位组进行操作，例如计算某个范围中有赋值位的数量等。

位图最大的优势之一就是使用它存储信息时可极大节约存储空间。

Bitmaps are not an actual data type, but a set of bit-oriented operations defined on the String type. Since strings are binary safe blobs and their maximum length is 512 MB, they are suitable to set up to 2³² different bits.

Bit operations are divided into two groups: constant-time single bit operations, like setting a bit to 1 or 0, or getting its value, and operations on groups of bits, for example counting the number of set bits in a given range of bits (e.g., population counting).

One of the biggest advantages of bitmaps is that they often provide extreme space savings when storing information. For example in a system where different users are represented by incremental user IDs, it is possible to remember a single bit information (for example, knowing whether a user wants to receive a newsletter) of 4 billion of users using just 512 MB of memory.

Bits are set and retrieved using the SETBIT and GETBIT commands

使用bitpos key bit_value（0或者1）来获取最近的0/1值位的位置
使用bitcount key [start end] 来获取指定的键中位值=1的个数，或者指定键的位范围中
还有bitop operation dest_key key [key1 .. ] 使用指定键值对目标键值按位操作，支持与、或、异或和非操作

位图的缺点：
只支持连续自然数值范围，非连续自然数值范围时需要额外信息来协助使用。