base: fix empty room name calculation

Step 3.iii of the naming algorithm[0] covers name calculation for empty
rooms; however, it isn't an else condition for steps 3.i and 3.ii. It's a
final conditional in the algorithm before returning the name. The
current implementation treats it as an else condition.

To fix this, use step 3.i and 3.ii to calculate a room name then check
if the user is alone. If alone, return "Empty room (was...)" containing
any former member names. If alone and there aren't any former members,
return "Empty room"

Fixes #133

[0] https://matrix.org/docs/spec/client_server/latest#calculating-the-display-name-for-a-room

matrix_sdk_base/src/rooms/mod.rs

@@ -160,7 +160,7 @@ fn calculate_room_name(
     let invited_joined = invited_member_count + joined_member_count;
     let invited_joined_minus_one = invited_joined.saturating_sub(1);
 
-    if heroes_count >= invited_joined_minus_one {
+    let names = if heroes_count >= invited_joined_minus_one {
         let mut names = heroes;
         // stabilize ordering
         names.sort_unstable();
@@ -173,7 +173,18 @@ fn calculate_room_name(
         // the `else`?
         format!("{}, and {} others", names.join(", "), (invited_joined - heroes_count))
     } else {
-        "Empty room".to_string()
+        "".to_string()
+    };
+
+    // User is alone.
+    if invited_joined <= 1 {
+        if names.is_empty() {
+            "Empty room".to_string()
+        } else {
+            format!("Empty room (was {})", names)
+        }
+    } else {
+        names
     }
 }
 
@@ -194,5 +205,23 @@ mod tests {
 
         actual = calculate_room_name(5, 0, vec!["a", "b", "c"]);
         assert_eq!("a, b, c, and 2 others", actual);
+
+        actual = calculate_room_name(0, 0, vec![]);
+        assert_eq!("Empty room", actual);
+
+        actual = calculate_room_name(1, 0, vec![]);
+        assert_eq!("Empty room", actual);
+
+        actual = calculate_room_name(0, 1, vec![]);
+        assert_eq!("Empty room", actual);
+
+        actual = calculate_room_name(1, 0, vec!["a"]);
+        assert_eq!("Empty room (was a)", actual);
+
+        actual = calculate_room_name(1, 0, vec!["a", "b"]);
+        assert_eq!("Empty room (was a, b)", actual);
+
+        actual = calculate_room_name(1, 0, vec!["a", "b", "c"]);
+        assert_eq!("Empty room (was a, b, c)", actual);
     }
 }