BitVec – Managing memory bit by bit

This crate provides packed bit-level analogues to [T] and Vec<T>. The slice type BitSlice and the vector type BitVec allow bitwise access to a region of memory in any endian ordering or underlying primitive type. This permits construction of space-efficient sets or fine-grained control over the values in a region of memory.

BitVec is a strict expansion of BitSlice to include allocation management. Since BitVec is shorter to type, the rest of this document will use it by default, and mark out sections that apply only to the vector type and not to the slice type. Unless marked, assume that the text applies to both.

BitVec is generic over an ordering cursor, using the trait Endian, and the primitive type, using the trait Bits. This means that BitVec structures can be built with a great deal of flexibility over how they manage their memory and translate between the in-memory representation and their semantic contents.

BitVec acts as closely to a standard Vec as possible, and can be assumed by default to be what a Vec<u1> would be if such a type were possible to express in Rust. It has stack semantics, in that push and pop operations take place only on one end of the BitVec’s buffer. It supports iteration, bitwise operations, and rendering for Display and Debug.

Usage

I wrote this crate because I was unhappy with the other bit-vector crates available. I specifically need to manage raw memory in bit-level precision, and this is not a behavior pattern the other bit-vector crates made easily available to me. This served as the guiding star for my development process on this crate, and remains the crate’s primary goal.

To this end, the default type parameters for the BitVec type use u8 as the storage primitive and use big-endian ordering of bits: the forwards direction is from MSb to LSb, and the backwards direction is from LSb to MSb.

To use this crate, you need to depend on it in Cargo.toml:

[dependencies]
bitvec = "0.5"

and include it in your crate root src/main.rs or src/lib.rs:

#[macro_use]
extern crate bitvec;

use bitvec::*;

This gives you access to the bitvec! macro for building BitVec types similarly to the vec! macro, and imports the following symbols:

• BitSlice<E: Endian, T: Bits> – the actual bit-slice reference type It is generic over a cursor type (E) and storage type (T). Note that BitSlice is unsized, and can never be held directly; it must always be behind a reference such as &BitSlice or &mut BitSlice.

  Furthermore, it is impossible to put BitSlice into any kind of intelligent pointer such as a Box or Rc! Any work that involves managing the memory behind a bitwise type must go through BitVec instead. This may change in the future as I learn how to better manage this library, but for now this limitation stands.

• BitVec<E: Endian, T: Bits> – the actual bit-vector structure type. It is generic over a cursor type (E) and storage type (T).

• Endian – an open trait that defines an ordering schema for BitVec to use. Little and big endian orderings are provided by default. If you wish to implement other ordering types, the Endian trait requires one function:

  • fn curr<T: Bits>(index: u8) -> u8 takes a semantic index and computes a bit offset into the primitive T for it.

  and provides default implementations for two functions that you may need to override:

  • fn next<T: Bits>(index: u8) -> (u8, bool)
  • fn prev<T: Bits>(index: u8) -> (u8, bool)

  These two functions compute the next and previous, respectively, semantic indices and overflow markers from a given semantic index. The boolean flag indicates that moving the index forward or backward would cross into a new primitive storage element.

• BigEndian – a zero-sized struct that implements Endian by defining the forward direction as towards LSb and the backward direction as towards MSb.

• LittleEndian – a zero-sized struct that implements Endian by defining the forward direction as towards MSb and the backward direction as towards LSb.

• Bits – a sealed trait that provides generic access to the four Rust primitives usable as storage types: u8, u16, u32, and u64. usize and the signed integers do not implement Bits and cannot be used as the storage type. u128 also does not implement Bits, as I am not confident in its memory representation, and dropping support for it allowed me to support older compilers.

BitVec has largely the same API as Vec, and should be easy to use.

The bitvec! macro requires type information as its first two arguments. Because macros do not have access to the type checker, this currently only accepts the literal tokens BigEndian or LittleEndian as the first argument, one of the four unsigned integer primitives as the second argument, and then as many values as you wish to insert into the BitVec. It accepts any integer value, and maps them to bits by comparing against 0. 0 becomes 0 and any other integer, whether it is odd or not, becomes 1. While the syntax is loose, you should only use 0 and 1 to fill the macro, for readability and lack of surprise.

Example

#[macro_use]
extern crate bitvec;

use bitvec::*;

use std::iter::repeat;

fn main() {
    let mut bv = bitvec![BigEndian, u8; 0, 1, 0, 1];
    bv.reserve(8);
    for bit in repeat(false).take(4).chain(repeat(true).take(4)) {
        bv.push(bit);
    }

    //  Memory access
    assert_eq!(bv.as_ref(), &[0b0101_0000, 0b1111_0000]);
    //                 index 0 -^               ^- index 11
    assert_eq!(bv.len(), 12);
    assert!(bv.capacity() >= 16);

    //  Arithmetic operations
    bv &= repeat(true);
    bv = bv | repeat(false);
    bv ^= repeat(false);
    bv = !bv;

    //  Borrowing iteration
    let mut iter = bv.iter();
    //  index 0
    if let Some(false) = iter.next() {} else { panic!() };
    //  index 11
    if let Some(true) = iter.next_back() {} else { panic!() };
    assert_eq!(iter.len(), 10);
}

Immutable and mutable access to the underlying memory is provided by the AsRef and AsMut implementations, so the BitVec can be readily passed to transport functions.

BitVec implements Borrow down to BitSlice, and BitSlice implements ToOwned up to BitVec, so they can be used in a Cow or wherever this API is desired. Any case where a Vec/[T] pair cannot be replaced with a BitVec/BitSlice pair is a bug in this library, and a bug report is appropriate.

BitVec can relinquish its owned memory as a Box<[T]> via the .into_boxed_slice() method, and BitSlice can relinquish access to its memory simply by going out of scope.

Planned Features

#![no_std] support that uses core libraries for allocation, and #![no_core] support that strips the vector type entirely and only provides the slice type.