orx-fixed-vec

An efficient fixed capacity vector with pinned elements.

A. Motivation

There might be various situations where pinned elements are helpful.

It is somehow required for async code, following blog could be useful for the interested.
It is crucial in representing self-referential types with thin references.

This crate focuses more on the latter. Particularly, it aims to make it safe and convenient to build performant self-referential collections such as linked lists, trees or graphs. See PinnedVec for complete documentation on the motivation.

FixedVec is one of the pinned vec implementations which can be wrapped by an ImpVec and allow building self referential collections.

B. Comparison with `SplitVec`

SplitVec is another PinnedVec implementation aiming the same goal but with different features. You may see the comparison in the table below.

`FixedVec`	`SplitVec`
Implements `PinnedVec` => can be wrapped by an `ImpVec`.	Implements `PinnedVec` => can be wrapped by an `ImpVec`.
Requires exact capacity to be known while creating.	Can be created with any level of prior information about required capacity.
Cannot grow beyond capacity; panics when `push` is called at capacity.	Can grow dynamically. Further, it provides control on how it must grow.
It is just a wrapper around `std::vec::Vec`; hence, has equivalent performance.	Performance-optimized built-in growth strategies also have `std::vec::Vec` equivalent performance.

After the performance optimizations on the SplitVec, it is now comparable to std::vec::Vec in terms of performance. This might make SplitVec a dominating choice over FixedVec.

C. Examples

C.1. Usage similar to `std::vec::Vec`

Most common std::vec::Vec operations are available in FixedVec with the same signature.

use orx_fixed_vec::prelude::*;

// capacity is not optional
let mut vec = FixedVec::new(4);

assert_eq!(4, vec.capacity());

vec.push(0);
assert!(!vec.is_full());
assert_eq!(3, vec.room());

vec.extend_from_slice(&[1, 2, 3]);
assert_eq!(vec, &[0, 1, 2, 3]);
assert!(vec.is_full());

// vec.push(42); // push would've panicked when vec.is_full()

vec[0] = 10;
assert_eq!(10, vec[0]);

vec.remove(0);
vec.insert(0, 0);

assert_eq!(6, vec.iter().sum());

assert_eq!(vec.clone(), vec);

let stdvec: Vec<_> = vec.into();
assert_eq!(&stdvec, &[0, 1, 2, 3]);

C.2. Pinned Elements

Unless elements are removed from the vector, the memory location of an element priorly pushed to the FixedVec never changes. This guarantee is utilized by ImpVec in enabling immutable growth to build self referential collections.

use orx_fixed_vec::prelude::*;

let mut vec = FixedVec::new(100);

// push the first element
vec.push(42usize);
assert_eq!(vec, &[42]);

// let's get a pointer to the first element
let addr42 = &vec[0] as *const usize;

// let's push 99 new elements
for i in 1..100 {
    vec.push(i);
}

for i in 0..100 {
    assert_eq!(if i == 0 { 42 } else { i }, vec[i]);
}

// the memory location of the first element remains intact
assert_eq!(addr42, &vec[0] as *const usize);

// we can safely dereference it and read the correct value
// the method is still unsafe for FixedVec
// but the undelrying guarantee will be used by ImpVec
assert_eq!(unsafe { *addr42 }, 42);

// the next push when `vec.is_full()` panics!
// vec.push(0);

E. Benchmarks

Recall that the motivation of using a split vector is to get benefit of the pinned elements, rather than to be used in place of the standard vector which is highly efficient. The aim of the benchmarks is to make sure that the performance gap is kept within acceptable and constant limits.

Since FixedVec is just a wrapper around the std::vec::Vec with additional guarantees on pinned elements; it is expected to have equivalent performance. This is verified by the benchmarks which can be found at the at benches folder.

License

This library is licensed under MIT license. See LICENSE for details.

orx-fixed-vec 0.4.6