heaparray

This crate aims to give people better control of how they allocate memory, by providing a customizable way to allocate blocks of memory, that optionally contains metadata about the block itself. This makes it much easier to implement Dynamically-Sized Types (DSTs), and also reduces the number of pointer indirections necessary to share data between threads.

It has two main features that provide the foundation for the rest:

• Storing data next to an array: From the Rust documentation on exotically sized types, at the end of the section on dynamically-sized types:

  Currently the only properly supported way to create a custom DST is by making your type generic and performing an unsizing coercion ... (Yes, custom DSTs are a largely half-baked feature for now.)

  This crate aims to provide some of that functionality; the code that the docs give is the following:

  struct MySuperSliceable<T: ?Sized> {
      info: u32,
      data: T
  }
  
  fn main() {
      let sized: MySuperSliceable<[u8; 8]> = MySuperSliceable {
          info: 17,
          data: [0; 8],
      };
  
      let dynamic: &MySuperSliceable<[u8]> = &sized;
  
      // prints: "17 [0, 0, 0, 0, 0, 0, 0, 0]"
      println!("{} {:?}", dynamic.info, &dynamic.data);
  }
  

  using this crate, the MySuperSliceable<[u8]> type would be implemented like this:

  use heaparray::*;
  
  fn main() {
      let dynamic = HeapArray::<u8,u32>::with_label(17, 8, |_,_| 0);
      println!("{:?}", dynamic);
  }
  

• Thin pointer arrays: in Rust, unsized structs are referenced with pointers that are stored with an associated length. This behavior isn't always desired, so this crate provides both thin and fat pointer-referenced arrays, where the length is stored with the data instead of with the pointer in the thin pointer variant.

Features

• Arrays are allocated on the heap, with optional extra space allocated for metadata
• Support for 1-word and 2-word pointers
• Atomically reference-counted memory blocks of arbitrary size without using a Vec; this means you can access reference-counted memory with only a single pointer indirection.
• Swap owned objects in and out with array.insert()
• Arbitrarily sized objects using label and an array of bytes (u8)

Examples

Creating an array:

use heaparray::*;
let len = 10;
let array = HeapArray::new(len, |idx| idx + 3);
assert!(array[1] == 4);

Indexing works as you would expect:

use heaparray::*;
let mut array = HeapArray::new(10, |_| 0);
array[3] = 2;
assert!(array[3] == 2);

You can take ownership of objects back from the container:

let mut array = HeapArray::new(10, |_| Vec::<u8>::new());
let replacement_object = Vec::new();
let owned_object = array.insert(0, replacement_object);

but you need to give the array a replacement object to fill its slot with.

Additionally, you can customize what information should be stored alongside the elements in the array using the HeapArray::with_label function:

struct MyLabel {
    pub even: usize,
    pub odd: usize,
}

let mut array = HeapArray::with_label(
    MyLabel { even: 0, odd: 0 },
    100,
    |label, index| {
        if index % 2 == 0 {
            label.even += 1;
            index
        } else {
            label.odd += 1;
            index
        }
    });

Use of unsafe Keyword

This library relies heavily on the use of the unsafe keyword to do both reference counting and atomic operations; there are 40 instances total, not including tests.

Customizability

All of the implementation details of this crate are public and documented; if you'd like to implement your own version of the tools available through this crate, note that you don't need to reinvent the wheel; many of the types in this crate are generic over certain traits, so you might not need to do that much.

License: MIT