Implemented RoundRobinPicker

app/src/main/java/ru/dragonestia/picker/repository/impl/picker/RoundRobinPicker.java

@@ -0,0 +1,48 @@
+package ru.dragonestia.picker.repository.impl.picker;
+
+import ru.dragonestia.picker.model.Room;
+import ru.dragonestia.picker.model.User;
+import ru.dragonestia.picker.repository.UserRepository;
+import ru.dragonestia.picker.repository.impl.collection.QueuedLinkedList;
+
+import java.util.Collection;
+
+public class RoundRobinPicker implements Picker<Room, User> {
+
+    private final UserRepository userRepository;
+    private final QueuedLinkedList<RoomWrapper> list = new QueuedLinkedList<>();
+
+    public RoundRobinPicker(UserRepository userRepository) {
+        this.userRepository = userRepository;
+    }
+
+    @Override
+    public void add(Room room) {
+        synchronized (list) {
+            list.add(new RoomWrapper(room, () -> userRepository.usersOf(room).size()));
+        }
+    }
+
+    @Override
+    public void remove(Room room) {
+        synchronized (list) {
+            list.removeById(room.getId());
+        }
+    }
+
+    @Override
+    public Room pick(Collection<User> users) {
+        int amount = users.size();
+        RoomWrapper wrapper;
+
+        synchronized (list) {
+            try {
+                while (!(wrapper = list.pick()).canAddUnits(amount));
+            } catch (RuntimeException ex) {
+                throw new RuntimeException("There are no rooms available");
+            }
+        }
+
+        return wrapper.getItem();
+    }
+}