kata.ex

defmodule Johnann do
  def john(day) when day >= 1 do
    get(day, :john)
  end

  def ann(day) when day >= 1 do
    get(day, :ann)
  end

  def sum_john(day) when day >= 1 do
    john(day)
    |> Enum.sum()
  end

  def sum_ann(day) when day >= 1 do
    ann(day)
    |> Enum.sum()
  end

  defp get(day, person) do
    day
    |> get_or_calculate()
    |> Map.get(person)
    |> Enum.sort()
    |> Enum.map(fn {_, v} -> v end)
  end

  defp get_or_calculate(day) do
    get_or_calculate(day, %{john: %{0 => 0}, ann: %{0 => 1}})
  end

  # The number of calculations have an upper-bound on the number of days
  # that have been calculated for John. When the number of entries in
  # John's cache are equal to or greater than the day being requested,
  # there is no need for further calculation
  defp get_or_calculate(day, %{john: john, ann: ann}) when map_size(john) < day do
    # The number of days calculated so far
    new_index = map_size(john)
    previous_index = new_index - 1

    johns_katas = calculate(john, ann, new_index, previous_index)
    john = Map.put(john, new_index, johns_katas)

    anns_katas = calculate(ann, john, new_index, previous_index)
    ann = Map.put(ann, new_index, anns_katas)

    get_or_calculate(day, %{ john: john, ann: ann })
  end

  defp get_or_calculate(_, cache) do
    cache
  end

  defp calculate(origin, other, new_index, previous_index) do
    old_index = Map.get(origin, previous_index)
    new_index - Map.get(other, old_index)
  end
end