index-ext

A crate for flexible indexing.

Automatic index type conversion

This crate makes it ergonomic to use arbitrary integer types as indices. This is especially important for libraries where indices are dictated by an external standard. Another reason could be platform or performance requirements due to which usize is the wrong choice. With the types and trait provided here, this just works for smaller and larger integer types than usize.

use index_ext::Int;
let buffer = [0; 256];
assert_eq!(buffer[Int(255_u8)], 0);
assert_eq!(buffer[Int(255_i32)], 0);
assert_eq!(buffer.get_int(-1_i8), None);
assert_eq!(buffer.get_int(u128::max_value()), None);

Statically checked indices

The concept of tags, a type identifying a unique slice length, allows one to prove through the type system that some integer is a valid index for a slice. There are two ways to use it safely, by borrowing the original slice and generative lifetimes or by using compile time constants, and one way to unsafely use arbitrary types.

use index_ext::tag;

tag::with_ref(&[0, 1, 2, 3][..], |slice, len| {
  // Index construction is checked/fallible..
  let idx = len.into_len().index(2).unwrap();

  // But use is NOT. The method get_safe does no runtime checks.
  assert_eq!(*slice.get_safe(idx), 2);
});

This looks less impressive than it is because any short example is also caught by the value range analysis of the optimizer. However, you can safely store these indices in structures and pass them across function boundaries without fail and you get a guarantee that the access check is elided. See the Huffman example for a use case with real differences.

Nightly dependent features

Const generics promise to provide even more possibilities. Currently, when one wants to reference a statically sized array within a dynamic slice then best choices are not the most ergonomic ones. On very recent nightly Rust we can leverage parameter deduction and const generics to design an index type that combines the best aspects.

use index_ext::array::RangeTo;
let rgba = [0; 4];
let rgb: [u8; 3] = rgba[RangeTo];
let [r, g, b] = &rgba[RangeTo];

License

Triple licensed under any of Apache-2.0, MIT, or zlib terms.