Crate varuint

Variable length signed and unsigned integer types. Types support up to 128-bit integers, both are encoded to 1-17 bytes.

Encoding rules are based on SQLite 4 Varuint type with modifications for support of 128-bit long integers. Varint is encoded using the Protobuf ZigZag approach and reuses Varuint as a storage.

Unlike the Protobuf encoding rules Varuint needs the first byte only to find out the length of the whole value. Microbenchmarks say that it is a lot faster.

How to use

Add dependency to your Cargo.toml:

[dependencies]
varuint = "0.5"

Add imports to your code:

use varuint::{Varint, Varuint, Serializable, Deserializable};

Use it:

use std::mem;

use varuint::*;

fn test_varint(v: i128, size: usize) {
    let v = Varint(v);
    assert_eq!(size, v.size_hint());
    let mut arr: [u8; 17] = unsafe { mem::uninitialized() };
    assert_eq!(size, v.serialize(&mut (&mut arr as &mut [u8])).unwrap());
    assert_eq!(v, Varint::deserialize(&mut (&arr as &[u8])).unwrap());
}

fn main() {
    test_varint(0, 1);
    test_varint(1, 1);
    test_varint(-1, 1);
}

Encoding rules

Encoding rules for Varuint are (assuming value is V):

• If V<=240 then output a single byte A0 equal to V.
• If V<=2031 then output A0 as (V-240)/256 + 241 and A1 as (V-240)%256.
• If V<=67567 then output A0 as 248, A1 as (V-2032)/256, and A2 as (V-2032)%256.
• If V<=16777215 then output A0 as 249 and A1 through A3 as a little-endian 3-byte integer.
• If V<=4294967295 then output A0 as 250 and A1..A4 as a little-endian 4-byte integer.
• If V<=1099511627775 then output A0 as 251 and A1..A5 as a little-endian 5-byte integer.
• If V<=281474976710655 then output A0 as 252 and A1..A6 as a little-endian 6-byte integer.
• If V<=72057594037927935 then output A0 as 253 and A1..A7 as a little-endian 7-byte integer.
• If V<=9223372036854775807 then output A0 as 254 and A1..A8 as a little-endian 8-byte integer.
• Otherwise output A0 as 255 and A1..A16 as a little-endian 16-byte integer.

Varint converted to the Varuint in the first place and then encoded as an unsigned integer. Conversion method makes values closer to 0 take less space. See Protobuf docs for details.

Structs

Varint	Variable length signed integer.
Varuint	Variable length unsigned integer.

Traits

Deserializable	Trait for deserializable types
Serializable	Trait for serializable types