# orx-fixed-vec

[![orx-fixed-vec crate](https://img.shields.io/crates/v/orx-fixed-vec.svg)](https://crates.io/crates/orx-fixed-vec)
[![orx-fixed-vec documentation](https://docs.rs/orx-fixed-vec/badge.svg)](https://docs.rs/orx-fixed-vec)

An efficient constant access time vector with fixed capacity and pinned elements.

## A. Motivation

There are various situations where pinned elements are critical.

* It is critical in enabling **efficient, convenient and safe self-referential collections** with thin references, see [`SelfRefCol`](https://crates.io/crates/orx-selfref-col) for details, and its special cases such as [`LinkedList`](https://crates.io/crates/orx-linked-list).
* It is important for **concurrent** programs as it eliminates safety concerns related with elements implicitly carried to different memory locations. This helps reducing and dealing with the complexity of concurrency, and leads to efficient concurrent data structures. See [`ConcurrentIter`](https://crates.io/crates/orx-concurrent-iter), [`ConcurrentBag`](https://crates.io/crates/orx-concurrent-bag) or [`ConcurrentOrderedBag`](https://crates.io/crates/orx-concurrent-ordered-bag) for such concurrent data structures which are conveniently built on the pinned element guarantees of pinned vectors.
* It is essential in allowing an **immutable push** vector; i.e., [`ImpVec`](https://crates.io/crates/orx-imp-vec). This is a very useful operation when the desired collection is a bag or a container of things, rather than having a collective meaning. In such cases, `ImpVec` allows avoiding certain borrow checker complexities, heap allocations and wide pointers such as `Box` or `Rc` or etc.

## B. Comparison with `SplitVec`

[`SplitVec`](https://crates.io/crates/orx-split-vec) is another [`PinnedVec`](https://crates.io/crates/orx-pinned-vec) 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` or `SelfRefCol` or `ConcurrentBag`, etc. | Implements `PinnedVec` => can as well be wrapped by them.         |
| 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.

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

### C.2. Pinned Elements

Unless elements are removed from the vector, the memory location of an element already pushed to the `SplitVec` <ins>never</ins> changes unless explicitly changed.

```rust
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
// dereferencing is still unsafe for FixedVec,
// but the underlying guarantee will be used by wrappers such as ImpVec or SelfRefCol
assert_eq!(unsafe { *addr42 }, 42);

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

## D. Benchmarks

Since `FixedVec` is just a wrapper around the `std::vec::Vec` with additional pinned element guarantee; it is expected to have equivalent performance. This is tested and confirmed by benchmarks that can be found at the at [benches](https://github.com/orxfun/orx-fixed-vec/blob/main/benches) folder.


## License

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