fix a panic at search time when the index is empty

Author: irevoire

src/reader.rs

@@ -203,6 +203,9 @@ impl<'t, D: Distance> Reader<'t, D> {
         search_k: Option<NonZeroUsize>,
         candidates: Option<&RoaringBitmap>,
     ) -> Result<Vec<(ItemId, f32)>> {
+        if self.items.is_empty() {
+            return Ok(Vec::new());
+        }
         // Since the datastructure describes a kind of btree, the capacity is something in the order of:
         // The number of root nodes + log2 of the total number of vectors.
         let mut queue =

src/tests/reader.rs

@@ -203,3 +203,19 @@ fn filtering() {
     id(99): distance(99)
     "###);
 }
+
+#[test]
+fn search_in_empty_database() {
+    // See https://github.com/meilisearch/arroy/issues/74
+    let handle = create_database::<Euclidean>();
+
+    let mut wtxn = handle.env.write_txn().unwrap();
+    let writer = Writer::new(handle.database, 0, 2);
+    writer.build(&mut wtxn, &mut rng(), None).unwrap();
+    wtxn.commit().unwrap();
+
+    let rtxn = handle.env.read_txn().unwrap();
+    let reader = Reader::open(&rtxn, 0, handle.database).unwrap();
+    let ret = reader.nns_by_vector(&rtxn, &[0., 0.], 10, None, None).unwrap();
+    insta::assert_debug_snapshot!(ret, @"[]");
+}