Merge #483

483: Enhance matching words r=Kerollmops a=ManyTheFish

# Summary

Enhance milli word-matcher making it handle match computing and cropping.

# Implementation

## Computing best matches for cropping

Before we were considering that the first match of the attribute was the best one, this was accurate when only one word was searched but was missing the target when more than one word was searched.

Now we are searching for the best matches interval to crop around, the chosen interval is the one:
1) that have the highest count of unique matches
> for example, if we have a query `split the world`, then the interval `the split the split the` has 5 matches but only 2 unique matches (1 for `split` and 1 for `the`) where the interval `split of the world` has 3 matches and 3 unique matches. So the interval `split of the world` is considered better.
2) that have the minimum distance between matches
> for example, if we have a query `split the world`, then the interval `split of the world` has a distance of 3 (2 between `split` and `the`, and 1 between `the` and `world`) where the interval `split the world` has a distance of 2. So the interval `split the world` is considered better.
3) that have the highest count of ordered matches
> for example, if we have a query `split the world`, then the interval `the world split` has 2 ordered words where the interval `split the world` has 3. So the interval `split the world` is considered better.

## Cropping around the best matches interval

Before we were cropping around the interval without checking the context.

Now we are cropping around words in the same context as matching words.
This means that we will keep words that are farther from the matching words but are in the same phrase, than words that are nearer but separated by a dot.

> For instance, for the matching word `Split` the text:
`Natalie risk her future. Split The World is a book written by Emily Henry. I never read it.`
will be cropped like:
`…. Split The World is a book written by Emily Henry. …`
and  not like:
`Natalie risk her future. Split The World is a book …`


Co-authored-by: ManyTheFish <many@meilisearch.com>

milli/src/search/mod.rs

@@ -17,7 +17,9 @@ use roaring::bitmap::RoaringBitmap;
 
 pub use self::facet::{FacetDistribution, FacetNumberIter, Filter};
 use self::fst_utils::{Complement, Intersection, StartsWith, Union};
-pub use self::matching_words::MatchingWords;
+pub use self::matches::{
+    FormatOptions, MatchBounds, Matcher, MatcherBuilder, MatchingWord, MatchingWords,
+};
 use self::query_tree::QueryTreeBuilder;
 use crate::error::UserError;
 use crate::search::criteria::r#final::{Final, FinalResult};
@@ -32,7 +34,7 @@ mod criteria;
 mod distinct;
 mod facet;
 mod fst_utils;
-mod matching_words;
+mod matches;
 mod query_tree;
 
 pub struct Search<'a> {
@@ -114,7 +116,7 @@ impl<'a> Search<'a> {
     pub fn execute(&self) -> Result<SearchResult> {
         // We create the query tree by spliting the query into tokens.
         let before = Instant::now();
-        let (query_tree, primitive_query) = match self.query.as_ref() {
+        let (query_tree, primitive_query, matching_words) = match self.query.as_ref() {
             Some(query) => {
                 let mut builder = QueryTreeBuilder::new(self.rtxn, self.index);
                 builder.optional_words(self.optional_words);
@@ -132,9 +134,11 @@ impl<'a> Search<'a> {
                 let analyzer = Analyzer::new(config);
                 let result = analyzer.analyze(query);
                 let tokens = result.tokens();
-                builder.build(tokens)?.map_or((None, None), |(qt, pq)| (Some(qt), Some(pq)))
+                builder
+                    .build(tokens)?
+                    .map_or((None, None, None), |(qt, pq, mw)| (Some(qt), Some(pq), Some(mw)))
             }
-            None => (None, None),
+            None => (None, None, None),
         };
 
         debug!("query tree: {:?} took {:.02?}", query_tree, before.elapsed());
@@ -148,11 +152,6 @@ impl<'a> Search<'a> {
 
         debug!("facet candidates: {:?} took {:.02?}", filtered_candidates, before.elapsed());
 
-        let matching_words = match query_tree.as_ref() {
-            Some(query_tree) => MatchingWords::from_query_tree(&query_tree),
-            None => MatchingWords::default(),
-        };
-
         // We check that we are allowed to use the sort criteria, we check
         // that they are declared in the sortable fields.
         if let Some(sort_criteria) = &self.sort_criteria {
@@ -193,13 +192,13 @@ impl<'a> Search<'a> {
         )?;
 
         match self.index.distinct_field(self.rtxn)? {
-            None => self.perform_sort(NoopDistinct, matching_words, criteria),
+            None => self.perform_sort(NoopDistinct, matching_words.unwrap_or_default(), criteria),
             Some(name) => {
                 let field_ids_map = self.index.fields_ids_map(self.rtxn)?;
                 match field_ids_map.id(name) {
                     Some(fid) => {
                         let distinct = FacetDistinct::new(fid, self.index, self.rtxn);
-                        self.perform_sort(distinct, matching_words, criteria)
+                        self.perform_sort(distinct, matching_words.unwrap_or_default(), criteria)
                     }
                     None => Ok(SearchResult::default()),
                 }