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 = "1.1"

Description

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

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:

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

Or to build this table you can run python tools/collect_benchmark_table.py and then find the results in bench_table.md

License

Licensed under the MIT license: http://opensource.org/licenses/MIT