1 files changed, 102 insertions, 0 deletions
diff --git a/third_party/rust/suggest/src/keyword.rs b/third_party/rust/suggest/src/keyword.rs
new file mode 100644
index 0000000000..d15688d016
--- /dev/null
+++ b/third_party/rust/suggest/src/keyword.rs
@@ -0,0 +1,102 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+/// Given a list of keywords for a suggestion, returns a phrase that best
+/// completes the user's query. This function uses two heuristics to pick the
+/// best match:
+///
+/// 1. Find the first keyword in the list that has at least one more word than
+///    the query, then trim the keyword up to the end of that word.
+/// 2. If there isn't a keyword with more words, pick the keyword that forms the
+///    longest suffix of the query. This might be the query itself.
+pub fn full_keyword(query: &str, keywords: &[impl AsRef<str>]) -> String {
+    let query_words_len = query.split_whitespace().count();
+    let min_phrase_words_len = if query.ends_with(char::is_whitespace) {
+        // If the query ends with a space, find a keyword with at least one more
+        // word, so that the completed phrase can show a word after the space.
+        query_words_len + 1
+    } else {
+        query_words_len
+    };
+    keywords
+        .iter()
+        .map(AsRef::as_ref)
+        .filter(|phrase| phrase.starts_with(query))
+        .map(|phrase| phrase.split_whitespace().collect::<Vec<_>>())
+        .find(|phrase_words| phrase_words.len() > min_phrase_words_len)
+        .map(|phrase_words| phrase_words[..min_phrase_words_len].join(" "))
+        .unwrap_or_else(|| {
+            keywords
+                .iter()
+                .map(AsRef::as_ref)
+                .filter(|phrase| phrase.starts_with(query) && query.len() < phrase.len())
+                .max_by_key(|phrase| phrase.trim().len())
+                .unwrap_or(query)
+                .to_owned()
+        })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn keywords_with_more_words() {
+        assert_eq!(
+            full_keyword(
+                "moz",
+                &[
+                    "moz",
+                    "mozi",
+                    "mozil",
+                    "mozill",
+                    "mozilla",
+                    "mozilla firefox"
+                ]
+            ),
+            "mozilla".to_owned(),
+        );
+        assert_eq!(
+            full_keyword(
+                "mozilla",
+                &[
+                    "moz",
+                    "mozi",
+                    "mozil",
+                    "mozill",
+                    "mozilla",
+                    "mozilla firefox"
+                ]
+            ),
+            "mozilla".to_owned(),
+        );
+    }
+
+    #[test]
+    fn keywords_with_longer_phrase() {
+        assert_eq!(
+            full_keyword("moz", &["moz", "mozi", "mozil", "mozill", "mozilla"]),
+            "mozilla".to_owned()
+        );
+        assert_eq!(
+            full_keyword(
+                "mozilla f",
+                &["moz", "mozi", "mozil", "mozill", "mozilla firefox"]
+            ),
+            "mozilla firefox".to_owned()
+        );
+    }
+
+    #[test]
+    fn query_ends_with_space() {
+        assert_eq!(
+            full_keyword(
+                "mozilla ",
+                &["moz", "mozi", "mozil", "mozill", "mozilla firefox"]
+            ),
+            "mozilla firefox".to_owned()
+        );
+    }
+}