#multiset #stack-allocated #simd #stack #collection


Stack allocated uint multiset, with optional SIMD implementations

8 releases (5 breaking)

0.6.1 Sep 29, 2023
0.6.0 May 9, 2021
0.5.0 May 5, 2021
0.3.5 Mar 18, 2021

#152 in Data structures

Download history 87/week @ 2024-03-30 66/week @ 2024-04-13

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docs.rs Crates.io

High performance, stack allocated uint multiset implementation on rust stable, with optional simd implementations available using rust nightly.

minimum supported rust version: 1.51



use utote::Multiset;

// A multiset of 5 elements, which can be counted up to u8::MAX
let mut multiset = Multiset::from([0u8, 3, 4, 0, 5]);
assert_eq!(multiset.total(), 12);

let equivalent_multiset = Multiset::<u8, 5>::from([0, 3, 4, 0, 5]);
assert_eq!(multiset, equivalent_multiset);

multiset.insert(2, 6);
assert_eq!(multiset, Multiset::from([0, 3, 6, 0, 5]));

for elem in multiset.iter() {
    println!("{}", elem);

assert_eq!(multiset.contains(0), false);
assert_eq!(multiset.contains(1), true);

Some common set-like operations:

use utote::Multiset;

let ms_sub: Multiset<u32, 3> = Multiset::from([0, 1, 1]);
let ms_super = Multiset::from([1, 1, 2]);

assert_eq!(ms_sub.is_subset(&ms_super), true);

assert_eq!(ms_sub.union(&ms_super), Multiset::from([1, 1, 2]));

assert_eq!(ms_super.is_proper_superset(&ms_sub), true);

// Any multiset where all counters are zero is equivalent to
// the empty multiset.
let empty: Multiset<u64, 2> = Multiset::from([0, 0]);
assert_eq!(empty, Multiset::empty());

Implementation Notes

The Utote Multiset has a single generic API but multiple equivalent scalar and simd implementations of various functions where the use of simd can enhance performance. The simd functionality is nightly only, while the scalar versions can be used on stable.

The nightly only simd implementation uses packed_simd and the unstable features: const_generics and const_evaluatable_checked (all behind the feature flag "simd"). packed_simd was chosen over alternatives due to its simplicity and based on the assumption that when std::simd is stabilised it will look similar in API structure to packed_simd as it is now.

Once const generics and portable simd support hit stable this crate will also become fully stable. Until these features are stabilised the version of Utote will stay below 1.0.0.

Since multisets are essentially collections of counters + some useful methods on those counters, and to keep things simple, implementations are only provided for uint types. The current Multiset is thus quite low level, perhaps better serving as a backend for a higher level multiset that works for any given type.

Although it would be simple to implement Deref<[T]> for Multiset I decided against this for two reasons. Firstly to avoid suggestively exposing methods in the API for Multiset which could sort the counts. Since the order of the counts is intrinsic to the implementation working I wanted to avoid any confusion that this would be appropriate. Second, as most of the functional methods for Multiset will eventually be implemented on slice and then used from different multiset varieties implmenting deref to slice could cause confusion in the code.

Future Development

  • Provide a heap allocated MultisetVec type which uses a Vec for storage rather than an array.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


The implementations in this crate are inspired by generic-array, nalgebra and simba.


0.6.0 (Breaking)

  • API changes
    • Rename Multiset::argmax => Multiset::elem_count_max
    • Rename Multiset::argmin => Multiset::elem_count_min
    • Rename Multiset::imax => Multiset::elem_max
    • Rename Multiset::imin => Multiset::elem_min
    • Rename Multiset::max => Multiset::count_max
    • Rename Multiset::min => Multiset::count_min
  • Cleanup & expand documentation
  • Ensure PartialOrd impl uses most efficient method
  • Add From mut Multiset ref
  • Fix simd impls of is_any_lesser & is_any_greater
  • Remove unnecessary SmallRng uses

0.5.0 (Breaking)

  • Provide uniform generic interface
  • Re-implement scalar and simd backends
  • Remove all type aliases
  • Remove all simd types / considerations from the API
  • Remove some const constructors to enable stable generic interface
  • Improve documentation
  • Add Rem ops
  • Add broadcast arithmetic ops
  • Add From implementations
  • Complete FromIterator and IntoIterator impl coverage
  • Add Index and IndexMut implementations
  • Simplify multiple functions
  • Add functions:
    • difference
    • symmetric_difference
    • from_elements
    • is_disjoint
    • get_mut
    • get_unchecked_mut
  • Add dynamic dispatch on detected cpu features for simd backends, currently supporting:
    • AVX2
    • AVX
    • SSE4.2


  • Minor performance improvements
  • make empty & repeat constructors const

0.4.0 (Breaking)

  • Minimum rust version: 1.51
  • Deprecate direct simd implementation
  • Utilise const generics (removing generic-array & typenum)


  • fix choose_random implementations


  • impl Send & Sync for Multiset
  • re-export typenum


  • impl FromIterator of refs for Multisets
  • re-export simd types and generic-array traits


  • Move to manual implementations of common traits on Multiset
  • Manually define type aliases


  • Make rng generic in choose_random


  • Made rand dependency optional
  • Switched from StdRng to SmallRng