typst/ecow

Compact, clone-on-write vector and string.

ecow

Compact, clone-on-write vector and string.

Types

• An EcoVec is a reference-counted clone-on-write vector. It takes up two words of space (= 2 usize) and has the same memory layout as a &[T] slice. Within its allocation, it stores a reference count, its capacity and its elements.

• An EcoString is a reference-counted clone-on-write string with inline storage. It takes up 16 bytes of space. It has 15 bytes of inline storage and starting from 16 bytes it becomes an EcoVec<u8>.

Example

// This is stored inline.
let small = ecow::EcoString::from("Welcome");

// This spills to the heap, but only once: `big` and `third` share the
// same underlying allocation. Vectors and spilled strings are only
// really cloned upon mutation.
let big = small + " to earth! 🌱";
let mut third = big.clone();

// This allocates again to mutate `third` without affecting `big`.
assert_eq!(third.pop(), Some('🌱'));
assert_eq!(third, "Welcome to earth! ");

Why should I use this instead of ...

Type	Details
Vec<T> / String	Normal vectors are a great general purpose data structure. But they have a quite big footprint (3 machine words) and are expensive to clone. The EcoVec has a bit of overhead for mutation, but is cheap to clone and only takes two words.
Arc<Vec<T>> / Arc<String>	These require two allocations instead of one and are less convenient to mutate.
Arc<[T]> / Arc<str>	While these require only one allocation, they aren't mutable.
Small vector	Different trade-off. Great when there are few, small Ts, but expensive to clone when spilled to the heap.
Small string	The EcoString combines different small string qualities into a very practical package: It has inline storage, a smaller footprint than a normal String, is efficient to clone even when spilled, and at the same time mutable.

License

This crate is dual-licensed under the MIT and Apache 2.0 licenses.