# byteyarn

`byteyarn` - Space-efficient byte strings 🧶🐈‍⬛

A `Yarn` is a highly optimized string type that provides a number of
useful properties over `String`:

* Always two pointers wide, so it is always passed into and out of functions
  in registers.
* Small string optimization (SSO) up to 15 bytes on 64-bit architectures.
* Can be either an owned buffer or a borrowed buffer (like `Cow<str>`).
* Can be upcast to `'static` lifetime if it was constructed from a
  known-static string.
* `Option<Yarn>` has the same size and ABI as `Yarn`.

The main caveat is that `Yarn`s cannot be easily appended to, since they
do not track an internal capacity, and the slice returned by
`Yarn::as_slice()` does not have the same pointer stability properties as
`String` (these are rarely needed, though).

---

Yarns are useful for situations in which a copy-on-write string is necessary
and most of the strings are relatively small. Although `Yarn` itself is
not `Copy`, there is a separate `YarnRef` type that is. These types
have equivalent representations, and can be cheaply cast between each other.

The easiest way to create a yarn is with the `yarn!()`
macro, which is similar to `format!()`.

```rust
// Create a new yarn via `fmt`ing.
let yarn = yarn!("Answer: {}", 42);

// Convert that yarn into a reference.
let ry: YarnRef<str> = yarn.as_ref();

// Try up-casting the yarn into an "immortal yarn" without copying.
let copy: YarnRef<'static, str> = ry.immortalize().unwrap();

assert_eq!(yarn, copy);
```

Yarns are intended for storing text, either as UTF-8 or as
probably-UTF-8 bytes; `Yarn<str>` and `Yarn<u8>` serve these purposes,
and can be inter-converted with each other. The `Yarn::utf8_chunks()`
function can be used to iterate over definitely-valid-UTF-8 chunks within
a string.

Both kinds of yarns can be `Debug`ed and `Display`ed, and will print out as
strings would. In particular, invalid UTF-8 is converted into either `\xNN`
escapes or replacement characters (for `Debug` and `Display` respectively).

```rust
let invalid = ByteYarn::from_byte(0xff);
assert_eq!(format!("{invalid:?}"), r#""\xFF""#);
assert_eq!(format!("{invalid}"), "�");
```