defmodule TodoEntry do
  defstruct id: 0, date: {}, title: ""
end

defmodule TodoList do
  defstruct next_auto_id: 1, entries: HashDict.new

  def new, do: %TodoList{}
  def add_entry(
    %TodoList{next_auto_id: new_entry_id, entries: entries} = todo_list,
    %TodoEntry{} = entry
    ) do
    entry = Map.put(entry, :id, new_entry_id)
    new_entries = HashDict.put(entries, new_entry_id, entry)
    %TodoList{todo_list | entries: new_entries, next_auto_id: new_entry_id + 1}
  end

  def entries(%TodoList{entries: entries}, date) do
    entries
    |> Stream.filter(fn({_, entry}) -> entry.date == date end)
    |> Enum.map(fn({_, entry}) -> entry end)
  end

  def update_entry(%TodoList{entries: entries} = todo_list, entry_id, update_fn) do
    case entries[entry_id] do
      nil -> todo_list
      old_entry ->
        old_entry_id = old_entry.id
      	new_entry = %TodoEntry{id: ^old_entry_id} = update_fn.(old_entry)
	new_entries = HashDict.put(entries, new_entry.id, new_entry)
	%TodoList{todo_list | entries: new_entries}
    end
  end
  def update_entry(todo_list, entry_id, %TodoEntry{} = new_entry) do
    update_entry(todo_list, entry_id, fn(_) -> new_entry end)
  end
end

todo_list = TodoList.new |> TodoList.add_entry(%TodoEntry{id: 0, date: {2013, 12, 19}, title: "Dentist"})