Bitmap of redis principle

Data structure model

Modern computers use binary systems ( position ) As the basic unit of information ,1 Bytes are equal to 8 position , for example ：big Strings are created by 3 Characters make up , But when stored in the computer, it is represented in binary ,big Respectively corresponding ASCII Here's the picture ：

• BitMap It's not a data structure in itself , It's actually a string , But it can operate on the bits of a string .
• BitMap A separate set of commands is provided , So in Redis Use in BitMap and The methods of using strings are different , You can put BitMap Think of it as an array of bits , Each unit of an array can only store 0 and 1, The subscript of the array is in BitMap It's called offset .

command

This section stores whether each individual user has visited the website in BitMap in , Record the user you visited as 1, Do not visit users remember to do 0, Use offset as user's ID.

1. Set up value ：set    key  offset    value

          Set the number of the key offset The value of a bit ( from 0 Count up ) , Suppose now by 20 Users ,userId= 2,6,12,16,20  's users visited the site ( user Id reduce 1 by The array subscript ), Then the current BitMap The initialization result is shown in Figure ：

The specific operation process is as follows ： unique:users:20210602 representative 2021-06-02 This independent access to the user's BitMap.

offset = userId -1

127.0.0.1:6379> setbit unique:users:20210602 1 1

(integer) 0

127.0.0.1:6379> setbit unique:users:20210602 5 1

(integer) 0

127.0.0.1:6379> setbit unique:users:20210602 11 1

(integer) 0

127.0.0.1:6379> setbit unique:users:20210602 15 1

(integer) 0

127.0.0.1:6379> setbit unique:users:20210602 19 1

(integer) 0

127.0.0.1:6379>

If there is a userId = 30 's users visited the website , that BitMap Structure becomes graph ：

2 obtain value ：getbit    key  offset 

userId= 2,6,12,16,20  's users visited the site ( user Id reduce 1 by The array subscript )

 

1. 127.0.0.1:6379> getbit unique:users:20210602 1

   (integer) 1

   127.0.0.1:6379> getbit unique:users:20210602 5

   (integer) 1

   127.0.0.1:6379> getbit unique:users:20210602 11

   (integer) 1

   127.0.0.1:6379> getbit unique:users:20210602 15

   (integer) 1

   127.0.0.1:6379> getbit unique:users:20210602 19

   (integer) 1

   127.0.0.1:6379>