feruca – Unicode collation in Rust

feruca is a basic implementation of the Unicode Collation Algorithm in 100% safe Rust (outside of the tests module). It is current with Unicode version 14.0. The name of the library is a portmanteau of Ferris 🦀 and UCA.

I mean a few things by "basic implementation." First, I don't expect that this is highly performant. My rough attempts at benchmarking suggest that feruca is on the order of 20–40x 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 at present only one public function: collate, which accepts two string references (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.

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

} ```