bitstream-io

A Rust library for reading or writing binary values to or from streams which may not be aligned at a whole byte.

This library is intended to be flexible enough to wrap around any stream which implements the Read or Write traits. It also supports a wide array of integer data types as containers for those binary values.

Minimum Compiler Version

Beginning with version 2.4, the minimum compiler version has been updated to Rust 1.79 in order to support compile-time assertion in const blocks, which can be used to check for a class of errors at compile-time rather than runtime.

lib.rs:

Traits and helpers for bitstream handling functionality

Bitstream readers are for reading signed and unsigned integer values from a stream whose sizes may not be whole bytes. Bitstream writers are for writing signed and unsigned integer values to a stream, also potentially un-aligned at a whole byte.

Both big-endian and little-endian streams are supported.

The only requirement for wrapped reader streams is that they must implement the Read trait, and the only requirement for writer streams is that they must implement the Write trait.

In addition, reader streams do not consume any more bytes from the underlying reader than necessary, buffering only a single partial byte as needed. Writer streams also write out all whole bytes as they are accumulated.

Readers and writers are also designed to work with integer types of any possible size. Many of Rust's built-in integer types are supported by default.

Minimum Compiler Version

Beginning with version 2.4, the minimum compiler version has been updated to Rust 1.79.

The issue is that reading an excessive number of bits to a type which is too small to hold them, or writing an excessive number of bits from too small of a type, are always errors:

use std::io::{Read, Cursor};
use bitstream_io::{BigEndian, BitReader, BitRead};
let data = [0; 10];
let mut r = BitReader::endian(Cursor::new(&data), BigEndian);
let x: Result<u32, _> = r.read(64);  // reading 64 bits to u32 always fails at runtime
assert!(x.is_err());

but those errors will not be caught until the program runs, which is less than ideal for the common case in which the number of bits is already known at compile-time.

But starting with Rust 1.79, we can now have read and write methods which take a constant number of bits and can validate the number of bits are small enough for the type being read/written at compile-time:

use std::io::{Read, Cursor};
use bitstream_io::{BigEndian, BitReader, BitRead};
let data = [0; 10];
let mut r = BitReader::endian(Cursor::new(&data), BigEndian);
let x: Result<u32, _> = r.read_in::<64, _>();  // doesn't compile at all

Since catching potential bugs at compile-time is preferable to encountering errors at runtime, this will hopefully be an improvement in the long run.

Migrating From Pre 1.0.0

There are now BitRead and BitWrite traits for bitstream reading and writing (analogous to the standard library's Read and Write traits) which you will also need to import. The upside to this approach is that library consumers can now make functions and methods generic over any sort of bit reader or bit writer, regardless of the underlying stream byte source or endianness.