cbitmap

A conventional, compact and core (no_std) bitmap.

Use cases

You are recommended to use this crate when you want to maintain a bitmap containing a large but fixed number of bits. Especially, when you are caring memory usage/alignment, for cbitmap wastes almost no places.

For example, you may want to manage a set of resources, which can be described by two states, and a bitmap is fit for you.

If you want to maintain a small set of flags, like 2 or 3, we recommend flagset instead.

The most extensive and mature implementation of bitmap might be bitvec. You are recommended to use it if you are caring about maturity.

Also, bitset-core is another powerful crate that implemented compact bitset trait. However, the implementation between bitset-core and cbitmap is quiet different.

What's more, the performance of cbitmap has not been tested since it's on alpha version. If you care most about performance, please make a careful consideration before choice.

Features

We have provided a crate::bitmap::Bitmap type:

pub struct Bitmap<const BYTES: usize> {
    bits: [u8; BYTES],
}

You are recommended to use macros to create new bitmaps:

use cbitmap::bitmap::*;

let map = newmap!(0b_01; 2);

See also crate::he_lang.

The bitmap can be manipulated in conventional ways, like Bitmap::test(), Bitmap::set(), Bitmap::reset(), Bitmap::flip() and Bitmap::at(). Please see the documentation for detailed examples.

The bitmap is actually a wrapper of u8 array [u8; BYTES]. It can be put on diverse places on memory. For example, if the map is relatively small like 8 or 16 bits, you can put it on stack safely. If it is larger like 256 or 1024 bits, you may want to put it on heap.

Examples

Here is a simple example:

use cbitmap::bitmap::*;
 
// A macro are provided to create a bitmap.
let mut map = newmap!(;16);

// There is a set of methods to manipulate the bitmap:
map.set(10);
map.reset(10);
map.flip(10);

// Some C++ like methods are provided:
assert_eq!(map.test(10), true);
assert_eq!(map.any(), true);
assert_eq!(map.none(), false);

// Also provide other useful methods:
assert_eq!(map.find_first_one(), 10);

// You can access a single bit using wrappers:
let mut bit = map.at_mut(10);
assert_eq!(*bit, true);
bit.flip();
assert_eq!(*map.at(10), false);

Please see the documentation of Bitmap and the examples dir for detailed examples.

You can use cargo run --example <name> to run the examples we provide. A simple example is bitmap-base, another extensive example about practical usage is bitmap-usecase, where bitmap is used to manage raw memory resources.

Current constraints

Generic const expr

The bitmap is specified with its size-in-bytes by BYTES. This is slightly different from conventional bitset<N> in C++, where N indicating the size-in-bits. We implemented bitmap in this way to stay on rust-stable, where the #![feature(generic_const_exprs)] is not supported yet, thus, it is not allowed to do like this:

// requiring #![feature(generic_const_exprs)]
pub struct Bitmap<const N: usize> {
    bits: [u8; (N + 7) / 8],
}

We have provided an alternative way to let you specify the size in bits. The macro crate::newmap achieves this:

const BITS: usize = 16;
let map = newmap!(;BITS);
let another = newmap!(;BITS * 2);

In principle, it is nevertheless possible to use constexpr when instantiating a struct:

// allowed:
let map = Bitmap::<{64 / 8}>::new();

Index

A bitset<N> in C++ can be indexed by Index op []. We have met some problems when implementing this feature. Specifically, implementing core::ops::IndexMut for a struct is like this:

impl IndexMut for T {
    type Output = U;
    fn index(&mut self, index: usize) -> &mut Self::Output { ... }
}

The ref in &mut Self::Output requires self to own the indexed output.

In Bitmap, Output is required to be "bits". It is necessary to use a wrapper type to provide interfaces to access a single bits. We have provided BitRef and BitRefMut as the wrappers.

However, the bitmap is not expected to hold a large set of wrappers, in order to save memories.

Due to this issue, we only provide Bitmap::at_mut() as methods to multably index into the bitmap.

It is noteworthy that, we provide Bitmap::at() to get BitRef, and we also provide immutable Index. However, immutable Index only returns a bool value, not BitRef due to a similar issue.

Updates

• 0.3.2

• • Add Index.
• • Add as_ref(), as_mut(), as_ptr(), as_mut_ptr().
• • Wrap general methods like set() into a trait.

• 0.3.1

• • Add basic benchmarks.
• • Add new methods compatible with C++: Bitmap::any(), Bitmap::none(), Bitmap::all(), Bitmap::count() and Bitmap::test().
• • Add useful methods: [Bitmap::find_first_one()], [Bitmap::find_first_zero()].
• • Add an example bitmap-usecase, showing a use case of managing raw memory resources with bitmap.

• 0.3.0

• • Optimize memory usage by removing the Option (the size of bitmap is 1 byte bigger than the generic, which is not friendly to memory align).
• • Add new method: FillPrefix.
• • Improve creating macros: now length argument receive const exprs.

• 0.2.0

• • Add building macros.

• 0.1.1

• • Update docs.

• 0.1.0

• • First publish.