# StackCollections v0.2: Fixed-size bit vectors and bit sets

**URL:** <https://discourse.julialang.org/t/stackcollections-v0-2-fixed-size-bit-vectors-and-bit-sets/139798>\
**Category:** Package Announcements\
**Created:** [October 2, 2026, 7:15pm UTC](https://discourse.julialang.org/t/stackcollections-v0-2-fixed-size-bit-vectors-and-bit-sets/139798 "2026-10-02T19:15:32Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![jakobnissen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jakobnissen/32/13477_2.png) [@jakobnissen](https://discourse.julialang.org/u/jakobnissen)\
**Post date:** [October 2, 2026, 7:15pm UTC](https://discourse.julialang.org/t/stackcollections-v0-2-fixed-size-bit-vectors-and-bit-sets/139798/1 "2026-10-02T19:15:32Z")

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I present StackCollections v0.2, in which I have completely rewritten the package from v0.1.

StackCollections.jl provides small, immutable isbits collections. Currently, it provides:

- `USet{U <: Unsigned} <: AbstractSet{UInt32}`, an isbits bit set similar to `Base.BitSet`, but stored in a single `U`. Basically a normal integer-as-bitset with an `AbstractSet` interface.
- `UVec{U <: Unsigned} <: AbstractVector{Bool}`, an isbits bit vector similar to `Base.BitVector`, but stored in a single `U`

### Why use StackCollections.jl?

The types in StackCollections.jl are mostly useful as performance optimized implementations the Base equivalent of `USet` and `UVec` (which is `BitSet` or `BitVector`, respectively). If performance is not a concern, you can use the `Base` types and the StackCollections type don’t provide much extra.

However, if you do need extra performance, then the `USet` and `UVec` types are generally faster. They are small isbits types and so typically not heap allocated, and are stored inline in arrays. A `USet{UInt8}` is stored in a single byte. Most operations on the types are register-only ops and do not touch memory.

### Right, but why use a package and not just write bit sets myself?

Integers as bit sets / bit vectors is a common trick and most programmers have implemented them many times. So why would you want to reach for a package for this basic functionality when you can implement it yourself easily?

- **I already dealt with annoying edge cases** : Did you think about how floats should be handled, such as `1.0`? What about `1e100` which is not convertible to `Integer`? What about `-0.0` which is zero, but not `isequal` to `0`? What about `missing`? Or `1+0im`? [Logical indexing when it conflicts with masking](https://github.com/JuliaLang/julia/issues/63325)? Etc etc.
- **Lots of non-obvious methods are already implemented**. A small bit vector implementation is easy, but there is a ton of methods you _could_ add specializations for. You probably won’t need specialized vararg `symdiff` or specialized three-argument `reverse` - but it’s nice it’s there.
- **StackCollections.jl is microoptimized**. I did my best to avoid branches, outlined errors, and pulled all the bit tricks I (and Claude) could think of.
- **The implementation is already well tested** - so you don’t have to write tests for your ad hoc implementation.

### History

StackCollections.jl was one of my first published Julia packages from 2019. When I wrote it I was a much worse programmer than I am now, hence the subpar v0.1 release and unfortunate package name - it probably should have been called BitsCollections.  
Since making StackCollections.jl, I have used bit sets and bit vectors a few times, where I have ad hoc implemented them, always thinking that I should just use StackCollections.jl, but that I couldn’t be bothered to clean up the package.

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**Author:** ![matthias314](https://avatars.discourse-cdn.com/v4/letter/m/a88e4f/32.png) [@matthias314](https://discourse.julialang.org/u/matthias314)\
**Post date:** [October 2, 2026, 9:42pm UTC](https://discourse.julialang.org/t/stackcollections-v0-2-fixed-size-bit-vectors-and-bit-sets/139798/2 "2026-10-02T21:42:34Z")

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`USet` looks very much like [`SmallBitSet`](https://matthias314.github.io/SmallCollections.jl/stable/smallbitset/#SmallCollections.SmallBitSet) from SmallCollections.jl. `UVec{U}` seems to be somewhat similar to [`PackedVector{U,1,Bool}`](https://matthias314.github.io/SmallCollections.jl/stable/packedvector/#SmallCollections.PackedVector) from the same package. The difference is that the length of a `PackedVector` is stored separately. I think it’s a good idea to store the length in the bit mask, see [this post](https://discourse.julialang.org/t/ann-emulatedbitintegers-jl/137830/17).

(Disclaimer: I’m the author of SmallCollections.jl. StackCollections.jl is older; I was not aware of it.)