Cldr.Calendar.previous(range, :month) sometimes is in error

[kipcole9]

In a week-based calendar, the idea of a "month" is notional and is represented by the :weeks_in_month configuration key. In some cases the Cldr.Calendar.previous/2 function is returning a result that is 2 "months" back, not one.

For example:

  defmodule C454 do
    use Cldr.Calendar.Base.Week,
      day_of_week: 7,
      first_or_last: :first,
      min_days_in_first_week: 7,
      month_of_year: 2,
      weeks_in_month: [4, 5, 4]
  end

  test "a descending sequence of months in a week calendar follow each other" do
    {:ok, d} = Date.new(2030, 1, 1, C454)
    m = Cldr.Calendar.Interval.month(d)
    Enum.reduce 1..1000, m, fn _i, m ->
      m2 = Cldr.Calendar.previous(m, :month)
      assert Cldr.Calendar.date_to_iso_days(m2.last) + 1 == Cldr.Calendar.date_to_iso_days(m.first)
      m2
    end
  end

Will error with:

code:  assert Cldr.Calendar.date_to_iso_days(m2.last()) + 1 == Cldr.Calendar.date_to_iso_days(m.first())
left:  741448
right: 741476

Decoded, this means:

Month: #DateRange<~D[2030-W01-1 Cldr.Calendar.PeriodSequenceTest.C454], ~D[2030-W04-7 Cldr.Calendar.PeriodSequenceTest.C454]>; 
Previous: #DateRange<~D[2029-W44-1 Cldr.Calendar.PeriodSequenceTest.C454], ~D[2029-W48-7 Cldr.Calendar.PeriodSequenceTest.C454]>

Which is 2 months back, not one.

[bglusman]

Thanks Kip! Note for anyone else looking at this, it can also manifest as "previous" month being equal to current month dates, suspect this is the same bug but I was unsure whether to open one issue or two so spoke to Kip and he thinks he has the source/is probably one bug for both.

[kipcole9]

I have push a commit which I believe fixes this problem. I added a series of test to confirm that next and previous periods align properly to each other. If possible please test on master branch before I push to hex if you can.