# ecow
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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
```rust
// 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>`][vec] / [`String`][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] / [`Arc<String>`][arc] | These require two allocations instead of one and are less convenient to mutate. |
| [`Arc<[T]>`][arc] / [`Arc<str>`][arc]       | While these require only one allocation, they aren't mutable. |
| Small vector                                | Different trade-off. Great when there are few, small `T`s, 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. |

[vec]: https://doc.rust-lang.org/std/vec/struct.Vec.html
[string]: https://doc.rust-lang.org/std/string/struct.String.html
[arc]: https://doc.rust-lang.org/std/sync/struct.Arc.html

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