feruca – Unicode collation in Rust

feruca is a basic implementation of the Unicode Collation Algorithm in Rust. It's current with Unicode version 14.0. The name of the library is a portmanteau of Ferris 🦀 and UCA.

Outside of the tests module, no unsafe is used directly in this library. It relies on the well-vetted bstr to accept input (in the form of either &str or &[u8]), to perform UTF-8 validation, and to generate a list of Unicode scalar values, which can then be processed for collation. The idea is to be tolerant of input that may not be entirely kosher UTF-8.

In describing feruca as a "basic implementation," I have a few things in mind. First, I don't expect that it will win any awards for performance. My rough attempts at benchmarking suggest that this is on the order of 10–20x slower than ucol from icu4c. But my initial priority was to pass the official conformance tests. feruca also passes the conformance tests for the CLDR root collation order (more on this below).

Second, there is not yet support for tailoring, beyond being able to choose between the Default Unicode Collation Element Table (DUCET) and the default variation from CLDR. (You can additionally choose between the "non-ignorable" and "shifted" strategies for handling variable-weight characters.) Adding further support for tailoring is a near-term priority.

Third, the library has effectively[0] just one public function: collate, which accepts two string references or byte slices (plus a CollationOptions struct), and returns an Ordering. That is, you can pass collate to the standard library function sort_by (see "Example usage").

For many people and use cases, UCA sorting will not work properly without being able to specify a certain locale. That being said, the CLDR root collation order is already quite useful. When calling the collate function, you can pass default options (see below), which specify the use of the CLDR table with the "shifted" strategy. I think this is a good starting point.

[0]: There is also a variant form, collate_no_tiebreak, which will return Ordering::Equal for any two strings that produce the same UCA sort key. (The normal version will fall back on byte-value comparison in such cases.)

Example usage

```rust use feruca::{collate, CollationOptions};

fn main() { let mut uca = [ "چنگیز", "Éloi", "Ötzi", "Melissa", "صدام", "Mélissa", "Overton", "Elrond", ];

let mut naive = uca;

uca.sort_by(|a, b| collate(a, b, CollationOptions::default()));
naive.sort();

for item in uca {
    println!("{}", item);
}
// Éloi
// Elrond
// Melissa
// Mélissa
// Ötzi
// Overton
// چنگیز
// صدام

// Add a line of space (in case you run this verbatim)
println!();

for item in naive {
    println!("{}", item);
}
// Elrond
// Melissa
// Mélissa
// Overton
// Éloi
// Ötzi
// صدام
// چنگیز

} ```