Sparse Set Container

A container based on a sparse set.

It is useful if you want a container with performance close to Vec but you also want to safely store the indexes to the elements (so that they are not invalidated on removals).
E.g. you have a list of elements in UI that the user can add and remove, but you want to refer to the elements of that list from somewhere else.

Usage

Add this to your Cargo.toml:

[dependencies]
sparse_set_container = "1.0"

Description

An array-like container based on sparse set implementation that allows O(1) access to elements without hashing and allows cache-friendly iterations.

Operation	SparseSet	Vec
push	O(1)	O(1)
lookup	O(1)	O(1)
size/len	O(1)	O(1)
remove	O(n)	O(n)
swap_remove	O(1)	O(1)

For iterating over the elements SparseSet exposes an iterator over an internal Vec with values, which is as efficient as iterating over a Vec directly.

Differences to Vec:

Instead of using indexes, when adding an element, it returns a lightweight key structure that can be used to access the element later
- The key is not invalidated when elements are removed from the container
- If the pointed-at element was removed, the key will not be pointing to any other elements, even if new elements are inserted
There is a slight overhead in insertion/lookup/removal operations compared to Vec
Consumes more memory:
- for each value 4*sizeof(usize) bytes on top of the size of the element itself
  - (e.g. 32 bytes per element on 64-bit systems)
- per each 2^(sizeof(usize)*8) removals the memory consumption will also grow by 2*sizeof(usize)
  - (e.g. 16 bytes per 18446744073709551616 elements removed on 64-bit systems)
Many Vec operations are not supported (create an issue on github if you want to request one)

Examples

extern crate sparse_set_container;
use sparse_set_container::SparseSet;

fn main() {
    let mut elements = SparseSet::new();
    elements.push("1");
    let key2 = elements.push("2");
    elements.push("3");

    elements.remove(key2);
    elements.push("4");

    if !elements.contains(key2) {
        println!("Value 2 is not in the container");
    }

    // Prints 1 3 4
    for v in elements.values() {
        print!("{} ", v);
    }

    // Prints 1 3 4
    for k in elements.keys() {
        print!("{} ", elements.get(k).unwrap());
    }
}

Benchmarks

The values captured to illustrate the difference between this SparseSet container implementation, Vec, and standard HashMap:

Benchmark	`SparseSet<String>`	`Vec<String>`	`HashMap<i32, String>`
Create empty	0 ns ±0	0 ns ±0	1 ns ±1
Create with capacity	54 ns ±0	16 ns ±0	33 ns ±1
Push 100 elements	3,842 ns ±95	2,659 ns ±101	5,636 ns ±191
With capacity push 100	3,343 ns ±25	3,257 ns ±75	4,426 ns ±122
Lookup 100 elements	88 ns ±2	39 ns ±23	464 ns ±12
Iterate over 100 elements	30 ns ±0	30 ns ±0	41 ns ±1
Clone with 100 elements	2,388 ns ±22	2,247 ns ±12	1,527 ns ±60
Clone 100 and remove 10	3,174 ns ±161	2,367 ns ±140	1,704 ns ±159
Clone 100 and swap_remove 10	2,575 ns ±134	2,181 ns ±68	N/A

To run the benchmark on your machine, execute cargo run --example bench --release

License