Sparse Set Container

A container based on a sparse set.

It is useful if you want a container with performance close to Vec but also 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 = "1.2"

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 a tightly packed slice with values, which is as efficient as iterating over a Vec.

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 illustrate the difference between this SparseSet container implementation, Vec, and standard HashMap, as well as comparing to other libraries with similar functionality.

Benchmark	`SparseSet<String>`	`Vec<String>`	`HashMap<i32, String>`	`thunderdome::Arena<String>`	`generational_arena::Arena<String>`	`slotmap::SlotMap<_, String>`	`slotmap::DenseSlotMap<_, String>`
Create empty	0 ns ±0	0 ns ±0	1 ns ±0	0 ns ±0	14 ns ±0	7 ns ±0	7 ns ±1
Create with capacity (1000)	19 ns ±0	18 ns ±0	35 ns ±3	18 ns ±0	652 ns ±26	18 ns ±0	51 ns ±3
Push 100 elements	3,520 ns ±96	3,322 ns ±212	5,307 ns ±341	3,457 ns ±220	3,501 ns ±226	3,352 ns ±106	3,974 ns ±201
With capacity push 100	3,385 ns ±136	3,234 ns ±210	4,281 ns ±74	3,309 ns ±98	3,210 ns ±70	3,212 ns ±90	3,377 ns ±102
Lookup 100 elements	89 ns ±2	42 ns ±7	447 ns ±35	78 ns ±2	78 ns ±2	64 ns ±1	86 ns ±3
Iterate over 100 elements	30 ns ±1	32 ns ±2	42 ns ±1	93 ns ±2	69 ns ±2	36 ns ±1	32 ns ±1
Clone with 100 elements	2,476 ns ±76	2,411 ns ±73	1,538 ns ±58	2,449 ns ±86	2,505 ns ±81	2,472 ns ±74	2,496 ns ±42
Clone 100 and remove 10	3,215 ns ±117	2,454 ns ±52	1,678 ns ±112	2,539 ns ±85	2,618 ns ±125	2,585 ns ±86	2,556 ns ±83
Clone 100 and swap_remove 10	2,546 ns ±74	2,262 ns ±86	N/A	N/A	N/A	N/A	N/A

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