Crate david_set [] [src]

david-set contains a few collections that are optimized to scale in size well for small numbers of elements, while still scaling well in time (and size) for numbers of elements. We have two set types:

  1. Set is basically interchangeable with HashSet, although it does require that its elements implement the Copy trait, since otherwise I would have to learn to write correct unsafe code, which would be scary.

  2. CastSet is places a stronger requirement on its elements, which must have trait Cast. This is intended for elements that are Copy, can be cheaply converted to usize, and are sufficiently evenly distributed that they do not require real hashing. Basically, this is suitable if you want to store a set of indices into an array. All the basic integer types should satisfy trait Cast. Oh, and this set also requires that one value of your type is "invalid". For the unsigned integer types, we take their maximum value to mean invalid. This constraint allows us to save a bit more space.

Both of these set types will do no heap allocation for small sets of small elements. CastSet will store up to 16 bytes of elements before doing any heap allocation, while Set stores sets up to size 8 without allocation. Both sets are typically faster than HashSet by a factor of around two, although for sets with more than 8 elements Set is in fact identical to HashSet in performance.


use david_set::Set;
let mut s: Set<usize> = Set::new();
use david_set::CastSet;
let mut s: CastSet<usize> = CastSet::new();



A set implemented for types that can be cast to usize


A set that is a HashSet when it has many elements, but is just an array for small set sizes.


A set that is stored in a Vec



The number of elements stored in an array before moving up to the HashSet implementation.



Trait for any type that can be converted to a usize. This could actually be a hash function, but we will assume that it is fast, so I'm not calling it Hash.