public final class AppStateReducer implements Reducer<AppState> {
  private final Reducer<List<TodoItem>> todoItemsReducer;

  private final Reducer<TodoFilter> filterReducer;

  private AppStateReducer(Reducer<List<TodoItem>> todoItemsReducer, Reducer<TodoFilter> filterReducer) {
    this.todoItemsReducer = todoItemsReducer;
    this.filterReducer = filterReducer;
  }

  @Override
  public AppState reduce(AppState state, Action action) {
    List<TodoItem> todoItems = null;
    TodoFilter filter = null;

    if (state != null) {
      todoItems = state.todoItems();
      filter = state.filter();
    }

    List<TodoItem> todoItemsNext = todoItemsReducer.reduce(todoItems, action);
    TodoFilter filterNext = filterReducer.reduce(filter, action);

    //If all values are the same there is no need to create an object
    if (state != null
     && todoItems == todoItemsNext
     && filter == filterNext) {
      return state;
    } else {
      return new AppStateImpl(todoItemsNext, filterNext);
    }
  }

  public static Builder builder() {
    return new Builder();
  }

  public static class Builder {
    private Reducer<List<TodoItem>> todoItemsReducer;

    private Reducer<TodoFilter> filterReducer;

    private Builder() {
    }

    public Builder todoItemsReducer(Reducer<List<TodoItem>> todoItemsReducer) {
      this.todoItemsReducer = todoItemsReducer;
      return this;
    }

    public Builder filterReducer(Reducer<TodoFilter> filterReducer) {
      this.filterReducer = filterReducer;
      return this;
    }

    public AppStateReducer build() {
      if (todoItemsReducer == null) {
        throw new IllegalStateException("todoItemsReducer should not be null");
      }
      if (filterReducer == null) {
        throw new IllegalStateException("filterReducer should not be null");
      }
      return new AppStateReducer(todoItemsReducer, filterReducer);
    }
  }
}