Crate gen_value

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GenValue is a library for using values and indexes with a generation in a Vec.

Generational indexes give a Vec an identifier which can always refer to the same value (unless the value is removed) while re-using memory efficiently.

Generational indexes are commonly used in entity component systems.

This library is a simple, naive, and general purpose implementation of the idea. In most systems, an optimized and performant version of the idea (which depends on the use case) can be built.



gen_value = "0.7.0"


Using a Vec with generations associated with values and indexes.

use gen_value::{vec::GenVec, Error};

#[derive(Debug, PartialEq)]
struct Value<T>(T);

// * The element type is `Value<u32>`.
// * The generation type by default is a `usize`.
// * The index type by default is a `usize`.
// * The generational index type (index type, generation type)
//   by default is a tuple `(usize, usize)`.
let mut gen_vec = GenVec::<Value<u32>>::default();

// Insert entities
let first_entity_index: (usize, usize) = gen_vec.insert(Value(42))?;
let other_entity_index = gen_vec.insert(Value(9))?;
assert_eq!(gen_vec.get(first_entity_index).ok(), Some(&Value(42)));
assert_eq!(*gen_vec.get(other_entity_index)?, Value(9));
assert_eq!(gen_vec.len(), 2);

// Remove first entity's value

// Other entity can still be retrieved with same index 
assert_eq!(*gen_vec.get(other_entity_index)?, Value(9));
// Cannot get first entity's value
assert_eq!(gen_vec.get(first_entity_index).ok(), None);
// `Vec` length is still 2
assert_eq!(gen_vec.len(), 2);

// Insert last entity
let last_entity_index = gen_vec.insert(Value(3))?;
assert_eq!(*gen_vec.get(other_entity_index)?, Value(9));
assert_eq!(*gen_vec.get(last_entity_index)?, Value(3));
// First entity index still cannot access a value
assert_eq!(gen_vec.len(), 2);

// The indexes and generations used
assert_eq!(first_entity_index, (0, 0));
assert_eq!(other_entity_index, (1, 0));
// The last entity re-used index 0 but with a newer generation
assert_eq!(last_entity_index, (0, 1));

Ok::<_, Error>(())


It is relatively fast to access an element in a Vec. Given an index, a relatively simple offset calculation is made from the beginning of the vector and an element can be read. Even better, because a vector’s elements are located adjacent in memory, vectors are ideal for CPU cache prefetching.

In comparision, a HashMap requires hashing a key and then possibly following multiple pointers to find the element. A BTreeMap may have to do several comparisions before finding the entry with the desired key. Both should be relatively slower than accessing an element by an index in a Vec.

On the other hand, a Vec index is not usually a stable identifier for an element. By inserting a new element or removing an element, the positions for the elements usually change, so an index may access different elements over time.

For a HashMap and a BTreeMap, keys are stable identifiers for values (as long as the keys follow the documented invariants). If a key is used to store a value in a map, an equal key can be used to retrieve the value later (assuming the value was not removed). The keys can be copied to various parts of the program and still be used later.

In many use cases, mapping stable identifiers to values is desirable.

How can we have stable identifiers and yet try to keep the performance properties for a Vec?

Never Move Elements

One way for a Vec to have stable identifiers is to never move elements. Once a value is pushed to the end of the Vec, the value will always have the same index. All operations which move elements can be disallowed (e.g. inserting a new element at a position).

Still, removing an element from a collection is important functionality. A possible solution is to add a “removed” tombstone value by using a Vec<Option<T>>. Values are pushed as Some(T) and when removed will be set to None. Indexes become stable identifiers for elements.

Unfortunately, there is an increasing amount of unused memory as elements are set to None when being removed.

Indexes and Values with Generations

Generational indexes are a possible solution to maximizing the usage of memory while keeping elements in the same position. Every element in the Vec is associated with a generation value, which is usually an integer. A generational index is an index with a generation value. Depending on if the generation values match or if one generation value is greater than another, an operation will succeed.

A Vec<T> becomes Vec<(G, T)> and an individual element index I becomes (I, G) where G is the generation type.

So in a Vec<(usize, Person)>, the element at index 9 could have 2 for the generation and a Person value. If the generational index (9, 2) is used, the element at the 9th offset is read. Then, the generation associated with the Person value is compared with the generation associated with the index. Since both are 2, the operation succeeds. If the generational index was (9, 1), then the operation would fail because the generations would not match.

Suppose there is a generational index ((3,1)) and the Vec at index 3 has a value with generation 1. When the element is “removed”, the generation associated with the value in the Vec is increased. So the Vec would have generation 2 at index 3. The generational index ((3,1)) could no longer be used to access the Vec value at index 3 because the generations are no longer equal. While the value is not technically dropped (in Rust’s terminology), the value is effectively removed from access.

The index with the latest generation is available to store a new value, so memory is reused instead of left unused. An entity using the previous generation of an index will no longer be able to access the removed value while an entity using the latest generation of the index will retrieve the value at the index.

The allocation of new indexes and increases in generations must be logically correct for program correctness.

Similar Projects

Both crates are inspired by Catherine West’s excellent closing keynote for RustConf 2018.

The major difference between the other crates is this crate’s direct control of the index types and generation types through type parameters. If a use case only needs the generation to be u8, this crate can use a u8. The generational index type itself can be specified (instead of using a single Index type or (I, G) tuple) for some type safety use cases. Each crate chose different APIs to expose.


Licensed under either of Apache License, Version 2.0 or MIT License 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.



  • Generational indexes.
  • Vec indexed with externally managed generational indexes.
  • Vec indexed with internally managed generational indexes.



  • A type which can attempt to return the smallest value which is still bigger than the current value.