# `smallbox`

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`Small Box` optimization: store small item on stack and fallback to heap for large item. Requires Rust 1.56+.

## [**Documentation**](https://andylokandy.github.io/smallbox)

# Usage

First, add the following to your `Cargo.toml`:

```toml
[dependencies]
smallbox = "0.8"
```

Next, add this to your crate root:

```rust
extern crate smallbox;
```

If you want this crate to work with dynamic-sized type, you can request it via:

```toml
[dependencies]
smallbox = { version = "0.8", features = ["coerce"] }
```

Currently `smallbox` by default links to the standard library, but if you would
instead like to use this crate in a `#![no_std]` situation or crate, you can request this via:

```toml
[dependencies.smallbox]
version = "0.8"
features = ["coerce"]
default-features = false
```


# Feature Flags

This crate has the following cargo feature flags:

- `std`
  - Optional, enabled by default
  - Use libstd
  - If `std` feature flag is opted out, `alloc` crate
    will be linked, which requires nightly rust.

- `coerce`
  - Optional
  - Require nightly rust
  - Allow automatic coersion from sized `SmallBox` to unsized `SmallBox`.


# Unsized Type

There are two ways to have an unsized `SmallBox`: Using `smallbox!()` macro or coercing from a sized `SmallBox` instance(requires nightly compiler).

Using the `smallbox!()` macro is the only option on stable rust. This macro will check the type of given value and
the target type `T`. For any invalid type coersions, this macro will invoke a compile-time error.

Once the feature `coerce` is enabled, sized `SmallBox<T>` will be automatically coerced into `SmallBox<T: ?Sized>` if necessary.

# Example

Eliminate heap alloction for small items by `SmallBox`:

```rust
use smallbox::SmallBox;
use smallbox::space::S4;

let small: SmallBox<_, S4> = SmallBox::new([0; 2]);
let large: SmallBox<_, S4> = SmallBox::new([0; 32]);

assert_eq!(small.len(), 2);
assert_eq!(large.len(), 32);

assert_eq!(*small, [0; 2]);
assert_eq!(*large, [0; 32]);

assert!(small.is_heap() == false);
assert!(large.is_heap() == true);
```

## Unsized type

Construct with `smallbox!()` macro:

```rust
#[macro_use]
extern crate smallbox;

use smallbox::SmallBox;
use smallbox::space::*;

let array: SmallBox<[usize], S2> = smallbox!([0usize, 1]);

assert_eq!(array.len(), 2);
assert_eq!(*array, [0, 1]);
```

With `coerce` feature:

```rust
use smallbox::SmallBox;
use smallbox::space::*;
 
let array: SmallBox<[usize], S2> = SmallBox::new([0usize, 1]);

assert_eq!(array.len(), 2);
assert_eq!(*array, [0, 1]);
```

`Any` downcasting:

```rust
#[macro_use]
extern crate smallbox;

use std::any::Any;
use smallbox::SmallBox;
use smallbox::space::S2;

let num: SmallBox<Any, S2> = smallbox!(1234u32);

if let Some(num) = num.downcast_ref::<u32>() {
    assert_eq!(*num, 1234);
} else {
    unreachable!();
}
```


# Capacity

The capacity is expressed by the size of type parameter `Space`,
regardless of what actually the `Space` is.

The crate provides some spaces in module `smallbox::space`,
from `S1`, `S2`, `S4` to `S64`, representing `"n * usize"` spaces.

Anyway, you can defind your own space type
such as byte array `[u8; 64]`.
Please note that the space alignment is also important. If the alignment
of the space is smaller than the alignment of the value, the value
will be stored in the heap.


# Benchmark

The test platform is Windows 10 on Intel E3 v1230 v3.

```
running 6 tests
test box_large_item                  ... bench:         104 ns/iter (+/- 14)
test box_small_item                  ... bench:          49 ns/iter (+/- 5)
test smallbox_large_item_large_space ... bench:          52 ns/iter (+/- 6)
test smallbox_large_item_small_space ... bench:         106 ns/iter (+/- 25)
test smallbox_small_item_large_space ... bench:          18 ns/iter (+/- 1)
test smallbox_small_item_small_space ... bench:           2 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 0 ignored; 6 measured; 0 filtered out
```


# Contribution

All kinds of contribution are welcome.

- **Issue** Feel free to open an issue when you find typos, bugs, or have any question.
- **Pull requests**. Better implementation, more tests, more documents and typo fixes are all welcome.


# License

Licensed under either of

 * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.