Implement nick normalization and mentioning

src/nick.rs

@@ -1,5 +1,8 @@
 //! Nick-related utility functions.
 
+use caseless::Caseless;
+use unicode_normalization::UnicodeNormalization;
+
 use crate::emoji::Emoji;
 
 /// Does not remove emoji.
@@ -45,7 +48,60 @@ pub fn hue_without_removing_emoji(nick: &str) -> u8 {
 /// This is a reimplementation of [euphoria's nick hue hashing algorithm][0]. It
 /// should always return the same value as the official client's implementation.
 ///
-/// [0]: https://github.com/euphoria-io/heim/blob/master/client/lib/hueHash.js
+/// [0]: https://github.com/euphoria-io/heim/blob/978c921063e6b06012fc8d16d9fbf1b3a0be1191/client/lib/hueHash.js
 pub fn hue(emoji: &Emoji, nick: &str) -> u8 {
     hue_without_removing_emoji(&emoji.remove(nick))
 }
+
+fn delimits_mention(c: char) -> bool {
+    match c {
+        ',' | '.' | '!' | '?' | ';' | '&' | '<' | '>' | '\'' | '"' => true,
+        _ => c.is_whitespace(),
+    }
+}
+
+/// Normalize a nick to a form that can be compared against other nicks.
+///
+/// This normalization is less aggressive than the nick hue normalization. It is
+/// also less aggressive than the normalization used by the euphoria client to
+/// determine who is pinged by a mention. This means that it will not compute
+/// the same normal form for all pairs of nicks that ping each other in the
+/// euphoria client.
+///
+/// A nick and its mention form calculated via [`mention`] will always evaluate
+/// to the same normal form.
+///
+/// The steps performed are as follows:
+///
+/// 1. Remove all whitespace characters as well as the characters
+///    `,`, `.`, `!`, `?`, `;`, `&`, `<`, `>`, `'`, `"`.
+///    All of these are used by the [frontend to delimit mentions][0].
+///    The `>` character was confirmed to delimit mentions via experimentation.
+///
+/// 2. Convert to NFKC
+/// 3. Case fold
+///
+/// Steps 2 and 3 are meant to be an alternative to the NKFC_Casefold derived
+/// property that's easier to implement, even though it may be incorrect in some
+/// edge cases.
+///
+/// [0]: https://github.com/euphoria-io/heim/blob/978c921063e6b06012fc8d16d9fbf1b3a0be1191/client/lib/stores/chat.js#L14
+pub fn normalize(nick: &str) -> String {
+    nick.chars()
+        .filter(|c| !delimits_mention(*c)) // Step 1
+        .nfkc() // Step 2
+        .default_case_fold() // Step 3
+        .collect()
+}
+
+/// Compute a mentionable version of a nick while remaining as close to the
+/// original as possible.
+///
+/// The return value of this function appended to an `@` character will
+/// highlight as a mention in the official euphoria client. It should ping any
+/// people using the original nick. It might also ping other people.
+///
+/// This function performs step 1 of [`normalize`].
+pub fn mention(nick: &str) -> String {
+    nick.replace(delimits_mention, "")
+}