exclusive_date_range.exs

defmodule ExclusiveDateRange do

  defstruct first: nil, last: nil

  def new(first, last) do
    %ExclusiveDateRange{first: first, last: last}
  end

  defimpl Enumerable, for: ExclusiveDateRange do
    def reduce _dr, {:halt, acc}, _fun do
      {:halted, acc}
    end

    def reduce(dr, {:suspend, acc}, fun) do
      {:suspended, acc, &reduce(dr, &1, fun)}
    end

    def reduce(%ExclusiveDateRange{first: d, last: d}, {:cont, acc}, _fun) do
      {:done, acc}
    end

    def reduce(%ExclusiveDateRange{first: d1, last: d2}, {:cont, acc}, fun) do
      reduce(%ExclusiveDateRange{first: next(d1), last: d2}, fun.(d1, acc), fun)
    end

    defp next %Date{year: y, month: m, day: d} do
      case Date.new(y, m, d + 1) do
        {:ok, nd} -> nd
        {:error, :invalid_date} ->
          case Date.new(y, m + 1, 1) do
            {:ok, nd} -> nd
            {:error, :invalid_date} ->
              {:ok, nd} = Date.new(y + 1, 1, 1)
              nd
          end
      end
    end

    def member?(%ExclusiveDateRange{first: d1, last: d2}, value) do
      days1 = date_to_gregorian_days d1
      days2 = date_to_gregorian_days(d2) - 1
      days3 = date_to_gregorian_days value
      {:ok, Enum.member?(days1..days2, days3)}
    end

    def count(%ExclusiveDateRange{first: d1, last: d2}) do
      days1 = date_to_gregorian_days d1
      days2 = date_to_gregorian_days d2
      {:ok, days2 - days1}
    end

    defp date_to_gregorian_days date do
      date |> Date.to_erl |> :calendar.date_to_gregorian_days
    end

  end
end