Librum

Librum is a Rust crate for cheaply serialising (encoding) and deserialising (decoding) data structures into binary streams

What separates this crate from others such as Bincode or Postcard is that this crate is extensively optimised for just binary encodings (whilst the mentioned crates specifically use Serde and build on a more abstract data model). The original goal of this project was specifically to guarantee size constraints for encodings on a per-type basis at compile-time. Therefore, this crate may be more suited for networking or other cases where many allocations are unwanted.

Keep in mind that this project is still work-in-progress. Until the interfaces are stabilised, different facilities may be replaced, removed, or altered in a breaking way.

This crate is compatible with no_std.

Performance

As Librum is optimised exclusively for a single, binary format, it may outperform other libraries that are more generic in nature.

The librum-benchmarks binary compares multiple scenarios using Librum and other, similar crates. According to my runs on an AMD Ryzen 7 3700X with default settings, these benchmarks indicate that Librum usually outperforms the other tested crates – as demonstrated in the following table:

All quantities are measured in seconds unless otherwise noted.

Currently, Librum's weakest point seems to be decoding. Please note that I myself find large (relatively speaking) inconsistencies between runs in these last two benchmarks. Do feel free to conduct your own tests of Librum.

Data model

Most primitives encode losslessly, with the main exceptions being usize and isize. These are instead first cast as u16 and i16, respectively, due to portability concerns (with respect to embedded systems).

Numerical primitives in general encode as little endian (and not "network order"). It is recommended for implementors to follow this convention as well.

See specific types' implementations for notes on their data models.

Note that the data model is currently not stabilised, and may not necessarily be in the near future (at least before specialisation). It may therefore be undesired to store encodings long-term.

Usage

This crate revolves around the Encode and Decode traits, both of which handle conversions to and from byte streams.

Many standard types come implemented with Librum, including most primitives as well as some standard library types such as Option and Result. Some features enable an extended set of implementations.

It is recommended in most cases to simply derive these two traits for user-defined types (although this is only supported with enumerations and structures -- not untagged unions). Here, each field is chained according to declaration order:

use librum::{Buf, Decode, Encode};

#[derive(Debug, Decode, Encode, PartialEq)]
struct Ints {
    value0: u8,
    value1: u16,
    value2: u32,
    value3: u64,
    value4: u128,
}

const VALUE: Ints = Ints {
    value0: 0x00,
    value1: 0x02_01,
    value2: 0x06_05_04_03,
    value3: 0x0E_0D_0C_0B_0A_09_08_07,
    value4: 0x1E_1D_1C_1B_1A_19_18_17_16_15_14_13_12_11_10_0F,
};

let mut buf = Buf::with_capacity(0x100);

buf.write(VALUE).unwrap();

assert_eq!(buf.len(), 0x1F);

assert_eq!(
    buf,
    [
        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
        0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
        0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
        0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E
    ].as_slice(),
);

assert_eq!(buf.read().unwrap(), VALUE);

Buffer types

The Encode and Decode traits both rely on streams for carrying the manipulated bytes.

These streams are separated into two type: O-streams (output streams) and i-streams (input streams). The Buf type can be used to handle these streams.

Encoding

To encode an object directly using the Encode trait, simply allocate a buffer for the encoding and wrap it in an OStream object:

use librum::{Encode, OStream, SizedEncode};

let mut buf = [0x00; char::MAX_ENCODED_SIZE];
let mut stream = OStream::new(&mut buf);

'Ж'.encode(&mut stream).unwrap();

assert_eq!(buf, [0x16, 0x04, 0x00, 0x00].as_slice());

Streams can also be used to chain multiple objects together:

use librum::{Encode, OStream, SizedEncode};

let mut buf = [0x0; char::MAX_ENCODED_SIZE * 0x5];
let mut stream = OStream::new(&mut buf);

// Note: For serialising multiple characters, the
// `String` and `SizedStr` types are usually
// preferred.

'ل'.encode(&mut stream).unwrap();
'ا'.encode(&mut stream).unwrap();
'م'.encode(&mut stream).unwrap();
'د'.encode(&mut stream).unwrap();
'ا'.encode(&mut stream).unwrap();

assert_eq!(buf, [
    0x44, 0x06, 0x00, 0x00, 0x27, 0x06, 0x00, 0x00,
    0x45, 0x06, 0x00, 0x00, 0x2F, 0x06, 0x00, 0x00,
    0x27, 0x06, 0x00, 0x00
]);

If the encoded type additionally implements SizedEncode, then the maximum size of any encoding is guaranteed with the MAX_ENCODED_SIZE constant.

Decoding

Decoding works with a similar syntax to encoding. To decode a byte array, simply call the decode method with an IStream object:

use librum::{Decode, IStream};

let data = [0x54, 0x45];
let mut stream = IStream::new(&data);

assert_eq!(u16::decode(&mut stream).unwrap(), 0x4554);

// Data can theoretically be reinterpretred:

stream = IStream::new(&data);

assert_eq!(u8::decode(&mut stream).unwrap(), 0x54);
assert_eq!(u8::decode(&mut stream).unwrap(), 0x45);

// Including as tuples:

stream = IStream::new(&data);

assert_eq!(<(u8, u8)>::decode(&mut stream).unwrap(), (0x54, 0x45));

Examples

A UDP server/client for geographic data:

use librum::{Buf, Encode, Decode, SizedEncode};
use std::io;
use std::net::{SocketAddr, ToSocketAddrs, UdpSocket};
use std::thread::spawn;

// City, region, etc.:
#[derive(Clone, Copy, Debug, Decode, Encode, Eq, PartialEq, SizedEncode)]
enum Area {
    AlQuds,
    Byzantion,
    Cusco,
    Tenochtitlan,
    // ...
}

// Client-to-server message:
#[derive(Debug, Decode, Encode, PartialEq, SizedEncode)]
enum Request {
    AtmosphericHumidity { area: Area },
    AtmosphericPressure { area: Area },
    AtmosphericTemperature { area: Area },
    // ...
}

// Server-to-client message:
#[derive(Debug, Decode, Encode, PartialEq, SizedEncode)]
enum Response {
    AtmosphericHumidity(f64),
    AtmosphericPressure(f64), // Pascal
    AtmosphericTemperature(f64), // Kelvin
    // ...
}

struct Party {
    pub socket: UdpSocket,

    pub request_buf:  Buf::<Request>,
    pub response_buf: Buf::<Response>,
}

impl Party {
    pub fn new<A: ToSocketAddrs>(addr: A) -> io::Result<Self> {
        let socket = UdpSocket::bind(addr)?;

        let this = Self {
            socket,

            request_buf:  Buf::new(),
            response_buf: Buf::new(),
        };

        Ok(this)
    }
}

let mut server = Party::new("127.0.0.1:27015").unwrap();

let mut client = Party::new("0.0.0.0:0").unwrap();

spawn(move || {
    let Party { socket, mut request_buf, mut response_buf } = server;

    // Recieve initial request from client.

    let (len, addr) = socket.recv_from(&mut request_buf).unwrap();
    request_buf.set_len(len);

    let request = request_buf.read().unwrap();
    assert_eq!(request, Request::AtmosphericTemperature { area: Area::AlQuds });

    // Handle request and respond back to client.

    let response = Response::AtmosphericTemperature(44.4); // For demonstration's sake.

    response_buf.write(response).unwrap();
    socket.send_to(&response_buf, addr).unwrap();
});

spawn(move || {
    let Party { socket, mut request_buf, mut response_buf } = client;

    // Send initial request to server.

    socket.connect("127.0.0.1:27015").unwrap();

    let request = Request::AtmosphericTemperature { area: Area::AlQuds };

    request_buf.write(request);
    socket.send(&request_buf).unwrap();

    // Recieve final response from server.

    socket.recv(&mut response_buf).unwrap();

    let response = response_buf.read().unwrap();
    assert_eq!(response, Response::AtmosphericTemperature(44.4));
});

Feature flags

Librum defines the following features:

• *alloc: Enables the Buf type and implementations for e.g. Box and Arc
• *proc-macro: Pulls the procedural macros from the librum-macros crate
• *std: Enables implementations for types such as Mutex and RwLock

Features marked with * are enabled by default.

Documentation

Librum has its documentation written in-source for use by rustdoc. See Docs.rs for an on-line, rendered instance.

Currently, these docs make use of some unstable features for the sake of readability. The nightly toolchain is therefore required when rendering them.

Contribution

Librum does not accept source code contributions at the moment. This is a personal choice by the maintainer and may be undone in the future.

Do however feel free to open up an issue on GitLab or (preferably) GitHub if you feel the need to express any concerns over the project.

Copyright & Licence

Copyright 2024 Gabriel Bjørnager Jensen.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program. If not, see https://www.gnu.org/licenses/.