Fix search highlight for non-unicode chars

The `matching_bytes` function takes a `&Token` now and:
- gets the number of bytes to highlight (unchanged).
- uses `Token.num_graphemes_from_bytes` to get the number of grapheme
  clusters to highlight.

In essence, the `matching_bytes` function returns the number of matching
grapheme clusters instead of bytes. Should this function be renamed
then?

Added proper highlighting in the HTTP UI:
- requires dependency on `unicode-segmentation` to extract grapheme
  clusters from tokens
- `<mark>` tag is put around only the matched part
    - before this change, the entire word was highlighted even if only a
      part of it matched

http-ui/Cargo.toml

@@ -17,6 +17,7 @@ once_cell = "1.5.2"
 rayon = "1.5.0"
 structopt = { version = "0.3.21", default-features = false, features = ["wrap_help"] }
 tempfile = "3.2.0"
+unicode-segmentation = "1.6.0"
 
 # http server
 askama = "0.10.5"

http-ui/src/main.rs

@@ -34,6 +34,7 @@ use structopt::StructOpt;
 use tokio::fs::File as TFile;
 use tokio::io::AsyncWriteExt;
 use tokio::sync::broadcast;
+use unicode_segmentation::UnicodeSegmentation;
 use warp::filters::ws::Message;
 use warp::http::Response;
 use warp::Filter;
@@ -160,13 +161,21 @@ impl<'a, A: AsRef<[u8]>> Highlighter<'a, A> {
                 let analyzed = self.analyzer.analyze(&old_string);
                 for (word, token) in analyzed.reconstruct() {
                     if token.is_word() {
-                        let to_highlight = matching_words.matching_bytes(token.text()).is_some();
-                        if to_highlight {
-                            string.push_str("<mark>")
-                        }
-                        string.push_str(word);
-                        if to_highlight {
-                            string.push_str("</mark>")
+                        let chars_to_highlight = matching_words.matching_bytes(&token).unwrap_or(0);
+                        if chars_to_highlight > 0 {
+                            let graphemes = word.graphemes(true);
+                            let chars = graphemes.clone().into_iter();
+
+                            string.push_str("<mark>");
+                            string.push_str(
+                                chars.take(chars_to_highlight).collect::<String>().as_str(),
+                            );
+                            string.push_str("</mark>");
+
+                            let chars = graphemes.into_iter().skip(chars_to_highlight);
+                            string.push_str(chars.collect::<String>().as_str());
+                        } else {
+                            string.push_str(word);
                         }
                     } else {
                         string.push_str(word);

milli/src/search/matching_words.rs

@@ -3,6 +3,7 @@ use std::collections::{BTreeMap, HashSet};
 use std::ops::{Index, IndexMut};
 
 use levenshtein_automata::{Distance, DFA};
+use meilisearch_tokenizer::Token;
 
 use super::build_dfa;
 use crate::search::query_tree::{Operation, Query};
@@ -33,15 +34,18 @@ impl MatchingWords {
     }
 
     /// Returns the number of matching bytes if the word matches one of the query words.
-    pub fn matching_bytes(&self, word_to_highlight: &str) -> Option<usize> {
+    pub fn matching_bytes(&self, word_to_highlight: &Token) -> Option<usize> {
         self.dfas.iter().find_map(|(dfa, query_word, typo, is_prefix)| {
-            match dfa.eval(word_to_highlight) {
+            match dfa.eval(word_to_highlight.text()) {
                 Distance::Exact(t) if t <= *typo => {
                     if *is_prefix {
-                        let len = bytes_to_highlight(word_to_highlight, query_word);
-                        Some(len)
+                        let len = bytes_to_highlight(word_to_highlight.text(), query_word);
+                        Some(word_to_highlight.num_graphemes_from_bytes(len))
                     } else {
-                        Some(word_to_highlight.len())
+                        Some(
+                            word_to_highlight
+                                .num_graphemes_from_bytes(word_to_highlight.text().len()),
+                        )
                     }
                 }
                 _otherwise => None,