meetup.exs

@spec meetup(pos_integer, pos_integer, weekday, schedule) :: :calendar.date()
  def meetup(year, month, weekday, schedule) do
    week = %{monday: 1, tuesday: 2, wednesday: 3, thursday: 4, friday: 5, saturday: 6, sunday: 7}
    week_multi = %{first: 0, second: 1, third: 2, fourth: 3, last: 4}

    {:ok, first_date} = Date.new(year, month, 1)
    beg_mo = Date.day_of_week(first_date)
    day = week[weekday]

    # if first day of month has surpassed target day, add 7 to get to nearest day
    date =
      case beg_mo > day do
        true ->
          7 + (day - beg_mo)

        false ->
          day - beg_mo
      end

    # multiples of 7 based on schedule
    date = date + 7 * week_multi[schedule]

    # hack for :last day of month overage
    date =
      case date > 30 do
        true -> date - 7
        false -> date
      end

    {year, month, date + 1}
  end