bitvec

Managing Memory Bit by Bit

Table of Contents

1. Introduction
   1. Unique Capabilities
   2. Competitors
2. Usage
   1. Code Sample
3. API
   1. Data Structures
      1. BitSlice
      2. BitVec
      3. BitBox
   2. Traits
      1. BitOrder
      2. BitStore
      3. BitField
      4. Bits
   3. Bit Orderings
   4. Macros
4. Feature Flags
   1. alloc
   2. atomic
   3. serde
   4. std
5. Warnings
6. Planned Features

Introduction

bitvec enables Rust projects to have complete, bit-level, control of memory, with types that fit in with existing Rust idioms and patterns. The bit-precision pointers in this crate allow creation of more powerful bit-masks, set arithmetic, compact data storage, and I/O packet processing.

The core export of this crate is the type BitSlice. This type describes a region of memory with individually-addressable bits. It is accessed by standard Rust references: &BitSlice<_, _> and &mut BitSlice<_, _>. These references are able to describe and operate on regions that start and end on any arbitrary bit address, without requiring alignment to a memory element boundary.

Additionally, this crate exports a BitVec resizable type for dynamic construction of BitSlice buffers.

Unique Capabilities

bitvec has three features which distinguish it from the plethora of other bit vector and bitstream I/O crates available: the &BitSlice reference handle, proxy reference types which allow mutable references to individual bits, and user-provided bit ordering.

1. A specialized pointer type enables bitvec to describe any region of memory with bit-level precision using only two machine words of space. This compacted pointer type is not found in any other bit-vector library, and enables the construction of &BitSlice<O, T> and &mut BitSlice<O, T> reference types. These types can then be used in trait APIs that expect references, and can be correctly traced by the borrow checker for aliasing and lifetime rules.

   The region handles &/mut BitSlice, BitBox, and BitVec are all the same size as their standard library counterparts &/mut [bool], Box<[bool]>, and Vec<bool>.

2. Additionally, BitSlice has a child type, BitMut. Restrictions in the Rust language do not allow this to be used as a referent type (there is no &mut Bit). However, this is a subtype of &mut BitSlice which implements Deref and DerefMut to a local bool, and uses Drop to write its bool into a parent BitSlice. This allows for inelegant, but working, mutable borrowing of single bits.

3. Other bit-vector crates allow users to specify the type of memory element (one of the unsigned integer fundamental types) used by a bit region, but only bitvec also allows users to specify the ordering of bits within those elements. This parametricity enables bitvec to provide the fastest behavior as the default condition, while also allowing users to choose the ordering that matches their specific requirements.

   Least-significant-bit-first ordering is typically faster than most-significant-bit-first, as it is a single instruction 1 << idx to produce a mask. However, many I/O protocols require most-significant-bit-first ordering. bitvec serves both use cases with ease, whereas other crates are specialized to one scenario and penalized in others.

Competitors

There are many published Rust crates in this area of work. Most of them focus on one particular use case for bitstreams, and cannot be extended to others. Of the crates that provide a bit-level view of memory, bitvec is the most general, most capable, and provides the highest degree of fidelity to the standard library.

bitvec does not provide a small-vector optimization, memory compression, or discontiguous memory structures. There are crates that provide these, or you can build these from bitvec structures, but they are currently outside the scope of the bitvec project.

Usage

Minimum Supported Rust Version: 1.36.0

The 1.36 release stablized the alloc crate, allowing the allocation types (BitVec, BitBox) to be used in #![no_std] environments with the stable compiler series.

Disabling the default features and not enabling features = ["alloc"] lowers the minimum stable Rust version to 1.34.0, which stabilized the atomic types used internally. Disabling features = ["atomic"] does not lower the minimum Rust version further, as the radium dependency is unconditionally floored at 1.34.

To use bitvec, depend on it in your Cargo manifest:

# Cargo.toml

[dependencies]
bitvec = "0.18"

and import its prelude anywhere you need to use its types:

//  src/lib.rs

use bitvec::prelude::*;

The prelude imports the data types, traits, macros, and type aliases needed to make the crate usable in dependent code.

You can also bring the prelude module into scope without importing its symbols directly, to avoid contaminating the local scope:

//  src/lib.rs

use bitvec::prelude as bv;

which will require a bv:: prefix to use any of the prelude symbols.

The symbols imported by the prelude are:

• BitField: provides an analogue of C++ bitfields
• BitOrder: translates semantic indices to bit positions within memory
• BitSlice: specialization of [bool]
• BitStore: describes the unsigned integer types used to store bits
• Bits: conversion traits from Rust types to &/mut BitSlice
• Lsb0: a bit ordering, from least significant up to most significant
• Local: the local C bit ordering
• Msb0: a bit ordering, from most significant down to least significant
• Word: the local CPU word size

Additionally, when targeting a system where alloc is present, the prelude adds the following:

• BitBox: specialization of Box<[bool]>
• BitVec: specialization of Vec<bool>
• bitbox!: a vec!-like macro which constructs BitBoxes
• bitvec!: a vec!-like macro which constructs BitVecs

Code Sample

This snippet highlights a selection of library functionality. It is deliberately not representative of likely usage.

use bitvec::prelude::*;

use std::iter::repeat;

fn main() {
  //  macro constructor
  let mut bv = bitvec![Msb0, u8; 0, 1, 0, 1];

  //  `BitVec` has the `Vec` inherent API
  bv.reserve(8);

  //  and can be filled from any source of `bool`
  bv.extend(repeat(false).take(4));
  bv.extend(repeat(true).take(4));

  //  `BitSlice` can yield its raw memory
  assert_eq!(
    bv.as_slice(),
    &[0b0101_0000, 0b1111_0000],
    //  ^ index 0       ^ index 11
  );
  assert_eq!(bv.len(), 12);
  assert!(bv.capacity() >= 16);

  //  `BitVec`, like `Vec`, is a LIFO stack
  bv.push(true);
  bv.push(false);
  bv.push(true);

  //  `BitSlice` has indexing
  assert!(bv[12]);
  assert!(!bv[13]);
  assert!(bv[14]);
  assert!(bv.get(15).is_none());

  //  … and range indexing
  let last = &bv[12 ..];
  assert_eq!(last.len(), 3);
  assert!(last.any());

  (0 .. 3).for_each(|_| {
    bv.pop();
  });

  //  `BitSlice` provides set arithmetic against any `bool` source
  bv &= repeat(true);
  bv |= repeat(false);
  //  Invert all live bits
  bv ^= repeat(true);
  //  And again
  bv = !bv;

  //  It can iterate
  let mut count = 0;
  for bit in bv.iter() {
    if *bit {
      count += 1;
    }
  }
  assert_eq!(count, 6);

  //  … mutably
  for (idx, mut bit) in bv.iter_mut().enumerate() {
    *bit ^= idx % 2 == 0;
  }

  //  `BitSlice` can store variables
  bv[1 .. 7].store(0x3Fu8);
  //  and fetch them
  assert_eq!(bv[1 .. 7].load::<u8>(), 0x3F);
}

All bitvec types are designed to be removable wrappers over raw memory elements. BitSlice<O, T> can unwrap into a slice [T], BitVec<O, T> into a vector Vec<T>, and BitBox<O, T> into a boxed slice Box<[T]>. They also offer zero-copy methods which convert from their standard library counterpart into themselves. This allows users to prepare memory however they wish, wrap it in bit-resolution views, and then unwrap it when they are finished.

API

The crate has a large API surface, as it provides a specialization of core’s slice fundamental and alloc’s boxed slice and vector types. These three types implement the entirety (as far as is possible) of the standard library’s API surface for these types. In addition, bitvec exposes many of the support types required to make it work.

The prelude contains all the types needed to use the crate. In addition, all exported types are available through their module path for direct import.

Data Structures

bitvec provides three data structures: BitSlice, BitVec, and BitBox. These structures all require two type parameters: a memory type used as the backing storage (BitStore), and an ordering of bits within that memory (BitOrder).

BitSlice

The BitSlice type is a specialization of the standard library’s [] slice fundamental. Like [bool], BitSlice<O, T> is a dynamically-sized type that describes any region of contiguous memory. As a !Sized type, it can never be used directly, but only held by reference: &BitSlice<O, T> and &mut BitSlice<O, T>.

Note that, unlike the standard library’s [T] type, BitSlice<O, T> can not be placed in any pointer types. Box, Rc, and Arc are all incapable of holding a BitSlice<O, T> region!

BitSlice<O, T> presents an API that behaves exactly like [bool], with the specialization that the contained data is tightly packed in memory. The only major missing component that [bool] can do that BitSlice<O, T> cannot is provide mutable indexing: IndexMut is required to produce and &mut bool, and BitSlice is incapable of manifesting this type.

As with standard-library slices, BitSlice<O, T> has no restrictions on where in memory it can begin and end. It correctly manages any sequence of contiguous bits, starting and ending at any bit in memory.

Its maximum length is usize::max_value() / 8, due to implementation constraints. This is 64 MiB on a 32-bit platform, and 256 PiB on a 64-bit platform.

In addition to mirroring the standard library slice APIs, BitSlice<O, T> also provides APIs specific to its role as a compact sequence of bits. It provides query methods for set properties (all, any) as well as set-arithmetic operations in the bitwise operators &, |, ^, and !.

let mut raw = [0u8; 16];
let bits: BitSlice<Lsb0, u8> = BitSlice::from_slice(&raw[..]);

BitVec

The BitVec type specializes the standard library’s Vec dynamic array. Like Vec<bool>, BitVec<O, T> is a dynamically-resizable type that owns a buffer of heap memory.

BitVec<O, T> reproduces the Vec<bool> API, with the same specialization property and IndexMut restriction as BitSlice<O, T>. It has the same relationship to BitSlice<O, T> as Vec<bool> does to [bool].

let mut bv: BitVec<Local, Word> = BitVec::with_capacity(64);

BitBox

The BitBox type specializes Box<[bool]>. It is a frozen BitVec<O, T>, and has all the same relationships with BitSlice<O, T> that Box<[bool]> does with [bool].

let mut boxed: BitBox<Msb0, u64> = BitBox::from_slice(&[0, 1]);

Traits

bitvec uses traits to compose the behavior of its data structures, and to provide bridges between normal Rust objects and bitvec structures.

BitOrder

This trait maps from semantic indices to bit positions in memory. It is responsible for creating the bit-masks and shift amounts used to interact with the backing storage of a BitSlice, and controls how abstract indices relate to concrete values.

Users are free to implement this trait themselves. The trait documents the rules it requires implementors to uphold in more detail, but the summary is:

1. a BitOrder implementor will never receive an index not less than the bit width of a storage type T,
2. it must never produce a position value not less than the width of that storage type,
3. it must provide a unique, 1:1 mapping between index values and bit positions,
4. its mapping between index and position must be pure, with no stateful or varying behavior, and
5. it must never produce a bitmask with zero or multiple bits set true.

BitOrder requires users to implement a function which translates index values to position values, and provides a default function which constructs bitmasks for positions. Users may override the mask function for speed, as long as they uphold its 1-hot requirement.

BitStore

This trait unifies the memory types used as base units of storage, and allows users to select which they want to use. It is implemented for u8, u16, and u32 on all systems; it is implemented for u64 only on 64-bit systems. It is forbidden for users to implement, only to observe.

The type alias Word redirects to the local target’s usize equivalent: u32 on 32-bit systems, and u64 on 64-bit. Implementation details prohibit usize from being a storage type at this time.

BitField

This is an extension trait which enables BitSlice<O, T> to provide load/store behavior of values. It is currently only implemented on BitSlice<Lsb0, T> and BitSlice<Msb0, T>, but may be extended in the future.

This trait permits a BitSlice over any BitStore type to load and store values of any other BitStore type, at any bit width not greater than the width of the type being stored/loaded.

This trait, to my knowledge, is the only fully generalized implementation of C/C++ struct bitfields or Erlang bitstreams in Rust.

let bits: &mut BitSlice<Msb0, u8> = get_bitslice();
bits[3 .. 14].store::<u16>(0x1234);
assert_eq!(bits[3 .. 14].load::<u16>(), Some(0x1234));

Bits

This trait provides conversion methods from Rust data to BitSlice handles. It is implemented on T, [T], and [T; 0 ..= 32] for all BitStore types.

let immut: u32 = 0;
let bits = immut.bits::<Lsb0>();

let mut mutable: u16 = 0;
let bits_mut = mutable.bits_mut::<Msb0>();

Bit Orderings

Two implementations of BitOrder are provided. Msb0 traverses a memory element from its most significant bit monotonically down to its least, and Lsb0 traverses a memory element from its least significant bit monotonically up to its most.

The type alias Local is provided as a default ordering. It aliases to Lsb0 on little-endian byte-ordered targets, and Msb0 on big-endian byte-ordered systems, to match the C behavior.

Macros

When targeting a system that has an allocator, the bitbox and bitvec macros provide construction of BitBox<O, T> and BitVec<O, T> from literals, just as vec! produces Vec<T> objects. These macros can take BitOrder and BitStore type names, and either a sequence of 1 and 0 or a bit and a repetition counter.

bitbox![Msb0, u32; 1, 0, 1, 0];
bitvec![0; 64];

Feature Flags

bitvec uses feature flags to control the presence or absence of crate-global behaviors. The uncommented features below (alloc, atomic, std) are provided by default and require explicit opt-out; the commented features (serde) require explicit opt-in.

# Cargo.toml

[dependencies.bitvec]
version = "0.18"
default-features = false
features = [
  "alloc",
  "atomic",
  # "serde",
  "std",
]

alloc

The BitBox and BitVec types require the distribution’s alloc crate in order to allocate their memory. If users are writing for a target that does not have a dynamic allocator, they may disable this feature to remove those types and only retain the BitSlice region type.

Users targeting an environment that has an allocator, but does not have a standard library, may reënable this feature.

atomic

BitSlice requires safe shared mutability in order to correctly handle the case of multiple &/mut BitSlice handles covering the same memory element. As it is undefined behavior to ever produced aliased references to a mutating location, bitvec must route access through either the core::sync::atomic module or the core::cell::Cell<T> type.

The atomic feature permits BitSlice to implement Send and Sync so that slice handles may be dispatched across threads for parallelization. Disabling this feature causes the shared-mutability logic to fall back to Cell, which is not thread-safe, and removes those markers.

BitBox and BitVec are theoretically thread-safe regardless of this feature, as the borrow checker’s rules combine with the allocator regime to forbid shared-mutation faults with these types. However, as a conservative rule and to maintain consistency, these types also remove their thread-safety markers when atomic is disabled.

serde

Enables support for Serde de/serialization.

BitSlice implements Serialize but not Deserialize, as it cannot allocate and serde does not permit deserializing into preällocated memory.

BitBox and BitVec implement both Serialize and Deserialize.

These implementations de/serialize the memory behind a handle into some protocol. No implementations are provided to de/serialize other types against a BitSlice or BitVec. If you need to read typed data out of a BitSlice, or write it into a BitVec, you will need to do this yourself. The BitField trait may be of help for this purpose.

std

Links against the standard library.

Currently, this feature does not enable anything not already covered by alloc. This will be removed before the 1.0 release unless std-only functionality is added.

Warnings

The BitSlice type causes memory aliasing. Consider this example:

let mut data = 0u8;
let bits = data.bits_mut::<Local>();
let (l, r) = bits.split_at_mut(4);

The l and r slices both refer to the data element, and both may mutate it. The implementation of BitSlice’s mutating functions, combined with the atomic feature flag, ensure that the crate will never cause undefined behavior, lost writes, or writes out of bounds. The BitOrder implementation in use must uphold the trait requirements in order to ensure that this behavior is correct.

The atomic feature prevents cross-thread race conditions by making all read or write access to the memory atomic. When it is disabled, BitSlice loses its ability to cross between threads.

Planned Features

Contributions of items in this list are absolutely welcome! Contributions of other work are also welcome, but I reserve the right to not merge them into the crate.

• Creation of specialized pointers corresponding to Rc<BitSlice> and Arc<BitSlice>
• Procedural macros for bitvec! and bitbox!
• An FFI module, and bindings for other languages