fix: when computing the len of the map, do not count elements that are None (#402)

crates/loro-internal/src/handler.rs

@@ -3615,7 +3615,14 @@ impl MapHandler {
     pub fn len(&self) -> usize {
         match &self.inner {
             MaybeDetached::Detached(m) => m.try_lock().unwrap().value.len(),
-            MaybeDetached::Attached(a) => a.with_state(|state| state.as_map_state().unwrap().len()),
+            MaybeDetached::Attached(a) => a.with_state(|state| {
+                state
+                    .as_map_state()
+                    .unwrap()
+                    .iter()
+                    .filter(|&(_, v)| v.value.is_some())
+                    .count()
+            }),
         }
     }
 

crates/loro/tests/loro_rust_test.rs

@@ -854,3 +854,20 @@ fn awareness() {
     );
     assert_eq!(b.get_all_states().get(&2).map(|x| x.state.clone()), None);
 }
+
+#[test]
+// https://github.com/loro-dev/loro/issues/397
+fn len_and_is_empty_inconsistency() {
+    let doc = LoroDoc::new();
+    let map = doc.get_map("map");
+    println!("{:#?}", map);
+    assert!(map.is_empty());
+    map.insert("leaf", 42i64).unwrap();
+    println!("{:#?}", map.get("leaf"));
+
+    assert_eq!(map.len(), 1);
+    map.delete("leaf").unwrap();
+    println!("{:#?}", map.get("leaf"));
+    assert_eq!(map.len(), 0);
+    assert!(map.is_empty());
+}