sqlite variable-length integers 实现方式

网站地址：http://sqlite.org/src4/doc/trunk/www/varint.wiki

这种存储方式的好处在于，

1 可以用比较少的位数来存储比较小的数字（在实际应用中，存储的比较多），而用比较多的位数来存储比较大的数字（在实际应用中，存储的比较少）。

2 长度可以在第一个字节就可以知道

3 编码之后的字节串的排序没改变。一个已经排序好的数组，编码之后，还是排好序的

编码：

• If V<=240 then output a single by A0 equal to V.

• If V<=2287 then output A0 as (V-240)/256 + 241 and A1 as (V-240)%256.

• If V<=67823 then output A0 as 249, A1 as (V-2288)/256, and A2 as (V-2288)%256.

• If V<=16777215 then output A0 as 250 and A1 through A3 as a big-endian 3-byte integer.

• If V<=4294967295 then output A0 as 251 and A1..A4 as a big-ending 4-byte integer.

• If V<=1099511627775 then output A0 as 252 and A1..A5 as a big-ending 5-byte integer.

• If V<=281474976710655 then output A0 as 253 and A1..A6 as a big-ending 6-byte integer.

• If V<=72057594037927935 then output A0 as 254 and A1..A7 as a big-ending 7-byte integer.

• Otherwise then output A0 as 255 and A1..A8 as a big-ending 8-byte integer.

解码

• If A0 is between 0 and 240 inclusive, then the result is the value of A0.

• If A0 is between 241 and 248 inclusive, then the result is 240+256*(A0-241)+A1.

• If A0 is 249 then the result is 2288+256*A1+A2.

• If A0 is 250 then the result is A1..A3 as a 3-byte big-ending integer.

• If A0 is 251 then the result is A1..A4 as a 4-byte big-ending integer.

• If A0 is 252 then the result is A1..A5 as a 5-byte big-ending integer.

• If A0 is 253 then the result is A1..A6 as a 6-byte big-ending integer.

• If A0 is 254 then the result is A1..A7 as a 7-byte big-ending integer.

• If A0 is 255 then the result is A1..A8 as a 8-byte big-ending integer.

位数 最大数字

1240

22287

367823

42²⁴-1

52³²-1

62⁴⁰-1

72⁴⁸-1

82⁵⁶-1

92⁶⁴-1