Merge #3942

3942: Normalize for the search the facets values r=ManyTheFish a=Kerollmops

This PR improves and fixes the search for facet values feature. Searching for _bre_ wasn't returning facet values like _brévent_ or _brô_.

The issue was related to the fact that facets are normalized but not in the same way as the `searchableAttributes` are. We decided to normalize them further and add another intermediate database where the key is the normalized facet value, and the value is a set of the non-normalized facets. We then use these non-normalized ones to get the correct counts by fetching the associated databases.

### What's missing in this PR?
 - [x] Apply the change to the whole set of `SearchForFacetValue::execute` conditions.
 - [x] Factorize the code that does an intermediate normalized value fetch in a function.
 - [x] Add or modify the search for facet value test.

Co-authored-by: Clément Renault <clement@meilisearch.com>
Co-authored-by: Kerollmops <clement@meilisearch.com>

milli/src/update/index_documents/helpers/merge_functions.rs

@@ -1,4 +1,5 @@
 use std::borrow::Cow;
+use std::collections::BTreeSet;
 use std::io;
 use std::result::Result as StdResult;
 
@@ -44,6 +45,27 @@ pub fn merge_roaring_bitmaps<'a>(_key: &[u8], values: &[Cow<'a, [u8]>]) -> Resul
     }
 }
 
+pub fn merge_btreeset_string<'a>(_key: &[u8], values: &[Cow<'a, [u8]>]) -> Result<Cow<'a, [u8]>> {
+    if values.len() == 1 {
+        Ok(values[0].clone())
+    } else {
+        // TODO improve the perf by using a `#[borrow] Cow<str>`.
+        let strings: BTreeSet<String> = values
+            .iter()
+            .map(AsRef::as_ref)
+            .map(serde_json::from_slice::<BTreeSet<String>>)
+            .map(StdResult::unwrap)
+            .reduce(|mut current, new| {
+                for x in new {
+                    current.insert(x);
+                }
+                current
+            })
+            .unwrap();
+        Ok(Cow::Owned(serde_json::to_vec(&strings).unwrap()))
+    }
+}
+
 pub fn keep_first<'a>(_key: &[u8], values: &[Cow<'a, [u8]>]) -> Result<Cow<'a, [u8]>> {
     Ok(values[0].clone())
 }

milli/src/update/index_documents/helpers/mod.rs

@@ -13,9 +13,9 @@ pub use grenad_helpers::{
     GrenadParameters, MergeableReader,
 };
 pub use merge_functions::{
-    concat_u32s_array, keep_first, keep_latest_obkv, merge_cbo_roaring_bitmaps,
-    merge_obkvs_and_operations, merge_roaring_bitmaps, merge_two_obkvs, serialize_roaring_bitmap,
-    MergeFn,
+    concat_u32s_array, keep_first, keep_latest_obkv, merge_btreeset_string,
+    merge_cbo_roaring_bitmaps, merge_obkvs_and_operations, merge_roaring_bitmaps, merge_two_obkvs,
+    serialize_roaring_bitmap, MergeFn,
 };
 
 use crate::MAX_WORD_LENGTH;

milli/src/update/index_documents/mod.rs

@@ -26,7 +26,7 @@ pub use self::enrich::{
 };
 pub use self::helpers::{
     as_cloneable_grenad, create_sorter, create_writer, fst_stream_into_hashset,
-    fst_stream_into_vec, merge_cbo_roaring_bitmaps, merge_roaring_bitmaps,
+    fst_stream_into_vec, merge_btreeset_string, merge_cbo_roaring_bitmaps, merge_roaring_bitmaps,
     sorter_into_lmdb_database, valid_lmdb_key, writer_into_reader, ClonableMmap, MergeFn,
 };
 use self::helpers::{grenad_obkv_into_chunks, GrenadParameters};