Merge #4746

4746: Fix hybrid search limit offset r=irevoire a=dureuill

# Pull Request

## Related issue
Fixes #4745

## What does this PR do?
- Apply offset and limit to the keyword search results when they are returned early.
- Add a test that is initially failing, and then passes


Co-authored-by: Louis Dureuil <louis@meilisearch.com>

meilisearch/tests/search/hybrid.rs

@@ -150,6 +150,35 @@ async fn simple_search() {
     snapshot!(response["semanticHitCount"], @"3");
 }
 
+#[actix_rt::test]
+async fn limit_offset() {
+    let server = Server::new().await;
+    let index = index_with_documents_user_provided(&server, &SIMPLE_SEARCH_DOCUMENTS_VEC).await;
+
+    let (response, code) = index
+        .search_post(
+            json!({"q": "Captain", "vector": [1.0, 1.0], "hybrid": {"semanticRatio": 0.2}, "retrieveVectors": true, "offset": 1, "limit": 1}),
+        )
+        .await;
+    snapshot!(code, @"200 OK");
+    snapshot!(response["hits"], @r###"[{"title":"Captain Marvel","desc":"a Shazam ersatz","id":"3","_vectors":{"default":{"embeddings":[[2.0,3.0]],"regenerate":false}}}]"###);
+    snapshot!(response["semanticHitCount"], @"0");
+    assert_eq!(response["hits"].as_array().unwrap().len(), 1);
+
+    let server = Server::new().await;
+    let index = index_with_documents_user_provided(&server, &SIMPLE_SEARCH_DOCUMENTS_VEC).await;
+
+    let (response, code) = index
+        .search_post(
+            json!({"q": "Captain", "vector": [1.0, 1.0], "hybrid": {"semanticRatio": 0.9}, "retrieveVectors": true, "offset": 1, "limit": 1}),
+        )
+        .await;
+    snapshot!(code, @"200 OK");
+    snapshot!(response["hits"], @r###"[{"title":"Captain Planet","desc":"He's not part of the Marvel Cinematic Universe","id":"2","_vectors":{"default":{"embeddings":[[1.0,2.0]],"regenerate":false}}}]"###);
+    snapshot!(response["semanticHitCount"], @"1");
+    assert_eq!(response["hits"].as_array().unwrap().len(), 1);
+}
+
 #[actix_rt::test]
 async fn simple_search_hf() {
     let server = Server::new().await;

milli/src/search/hybrid.rs

@@ -178,16 +178,16 @@ impl<'a> Search<'a> {
 
         // completely skip semantic search if the results of the keyword search are good enough
         if self.results_good_enough(&keyword_results, semantic_ratio) {
-            return Ok((keyword_results, Some(0)));
+            return Ok(return_keyword_results(self.limit, self.offset, keyword_results));
         }
 
         // no vector search against placeholder search
         let Some(query) = search.query.take() else {
-            return Ok((keyword_results, Some(0)));
+            return Ok(return_keyword_results(self.limit, self.offset, keyword_results));
         };
         // no embedder, no semantic search
         let Some(SemanticSearch { vector, embedder_name, embedder }) = semantic else {
-            return Ok((keyword_results, Some(0)));
+            return Ok(return_keyword_results(self.limit, self.offset, keyword_results));
         };
 
         let vector_query = match vector {
@@ -239,3 +239,44 @@ impl<'a> Search<'a> {
         true
     }
 }
+
+fn return_keyword_results(
+    limit: usize,
+    offset: usize,
+    SearchResult {
+        matching_words,
+        candidates,
+        mut documents_ids,
+        mut document_scores,
+        degraded,
+        used_negative_operator,
+    }: SearchResult,
+) -> (SearchResult, Option<u32>) {
+    let (documents_ids, document_scores) = if offset >= documents_ids.len() ||
+    // technically redudant because documents_ids.len() == document_scores.len(),
+    // defensive programming
+    offset >= document_scores.len()
+    {
+        (vec![], vec![])
+    } else {
+        // PANICS: offset < len
+        documents_ids.rotate_left(offset);
+        documents_ids.truncate(limit);
+
+        // PANICS: offset < len
+        document_scores.rotate_left(offset);
+        document_scores.truncate(limit);
+        (documents_ids, document_scores)
+    };
+    (
+        SearchResult {
+            matching_words,
+            candidates,
+            documents_ids,
+            document_scores,
+            degraded,
+            used_negative_operator,
+        },
+        Some(0),
+    )
+}