defmodule Todo.List do
  defstruct entries: %{}, next_id: 1

  def new(entries \\ []) do
    Enum.reduce(entries, %__MODULE__{}, &add(&2, &1))
  end

  def add(
        %__MODULE__{entries: entries, next_id: next_id} = todo_list,
        %{date: _, title: _} = entry
      ) do
    new_entry = Map.put(entry, :id, next_id)

    new_entries = Map.put(entries, next_id, new_entry)

    %__MODULE__{todo_list | entries: new_entries, next_id: next_id + 1}
  end

  def entries(%__MODULE__{} = todo_list, date) do
    todo_list.entries
    |> Map.filter(fn {_, entry} -> entry.date == date end)
    |> Map.values()
  end

  def update(%__MODULE__{entries: entries} = todo_list, id, update_fun)
      when is_function(update_fun, 1) do
    case Map.fetch(entries, id) do
      :error ->
        todo_list

      {:ok, entry} ->
        new_entry = update_fun.(entry)
        new_entries = Map.put(entries, id, new_entry)
        %__MODULE__{todo_list | entries: new_entries}
    end
  end

  def delete(%__MODULE__{entries: entries} = todo_list, id) when is_number(id) do
    new_entries = Map.delete(entries, id)

    %__MODULE__{todo_list | entries: new_entries}
  end
end