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// decide(): Handle command, query state, return events
export const decide = (db, command) => {
  const todo = db.viewState("single-todo", { todoId: command.todoId });

  if (!todo) throw StateChangeError.notFound("Todo not found");

  const eventType = todo.completed ? "TodoUncompleted" : "TodoCompleted";

  return {
    events: [{ type: eventType, subject: command.todoId }],
    conditions: [unchangedSince(command.todoId, todo.revision)]
  };
};

// evolve(): Apply events to state
export const evolve = (state, event) => {
  switch (event.type) {
    case "TodoCreated":
      return { ...state, todos: [...state.todos, event.data] };
    case "TodoCompleted":
      return updateTodo(state, event.todoId, { completed: true });
    case "TodoDeleted":
      return { ...state, todos: state.todos.filter(t => t.id !== event.todoId) };
  }
};