Ensure Julian.* calendars round trip

Author: kipcole9

lib/cldr/calendar/calendars/julian_compiler.ex

@@ -1,4 +1,5 @@
 defmodule Cldr.Calendar.Julian.Compiler do
+  # See https://stevemorse.org/jcal/julian.html
 
   defmacro __before_compile__(env) do
     opts =
@@ -21,6 +22,13 @@ defmodule Cldr.Calendar.Julian.Compiler do
 
       defguard year_rollover(month, day) when month <= @new_year_starting_month and day < @new_year_starting_day
 
+      # Adjust the year to be a Jan 1st starting year and carry
+      # on
+
+      def date_to_iso_days(year, month, day) when year_rollover(month, day) do
+        Cldr.Calendar.Julian.date_to_iso_days(year + 1, month, day)
+      end
+
       def date_to_iso_days(year, month, day) do
         Cldr.Calendar.Julian.date_to_iso_days(year, month, day)
       end
@@ -34,12 +42,17 @@ defmodule Cldr.Calendar.Julian.Compiler do
         {year, month, day} = Cldr.Calendar.Julian.date_from_iso_days(iso_days)
 
         if month <= @new_year_starting_month and day < @new_year_starting_day do
-          {year + 1, month, day}
+          {year - 1, month, day}
         else
           {year, month, day}
         end
       end
 
+      def naive_datetime_from_iso_days({iso_days, _}) do
+        {year, month, day} = date_from_iso_days(iso_days)
+        {year, month, day, 0, 0, 0, {0, 0}}
+      end
+
       def days_in_year(year) do
         first_day = date_to_iso_days(year, @new_year_starting_month, @new_year_starting_day)
         first_day_next_year = date_to_iso_days(year + 1, @new_year_starting_month, @new_year_starting_day)
@@ -195,7 +208,6 @@ defmodule Cldr.Calendar.Julian.Compiler do
       defdelegate shift_naive_datetime(year, month, day, hour, minute, second, millisecond, duration), to: Cldr.Calendar.Julian
       defdelegate iso_days_to_end_of_day(iso_days), to: Cldr.Calendar.Julian
       defdelegate iso_days_to_beginning_of_day(iso_days), to: Cldr.Calendar.Julian
-      defdelegate naive_datetime_from_iso_days(iso_days), to: Cldr.Calendar.Julian
       defdelegate parse_utc_datetime(string), to: Cldr.Calendar.Julian
       defdelegate parse_time(string), to: Cldr.Calendar.Julian
       defdelegate parse_naive_datetime(string), to: Cldr.Calendar.Julian

mix/julian_calendars/julian_dec_25.ex

@@ -0,0 +1,4 @@
+defmodule Cldr.Calendar.Julian.Dec25 do
+  use Cldr.Calendar.Julian, new_year_starting_month_and_day: {12, 25}
+
+end

mix/julian_calendars/julian_jan_1.ex

@@ -0,0 +1,4 @@
+defmodule Cldr.Calendar.Julian.Jan1 do
+  use Cldr.Calendar.Julian, new_year_starting_month_and_day: {1, 1}
+
+end

mix/julian_calendars/julian_march_1.ex

@@ -0,0 +1,4 @@
+defmodule Cldr.Calendar.Julian.March1 do
+  use Cldr.Calendar.Julian, new_year_starting_month_and_day: {3, 1}
+
+end

mix/julian_calendars/julian_march_25.ex

@@ -0,0 +1,4 @@
+defmodule Cldr.Calendar.Julian.March25 do
+  use Cldr.Calendar.Julian, new_year_starting_month_and_day: {3, 25}
+
+end

mix/julian_calendars/julian_sep_1.ex

@@ -0,0 +1,4 @@
+defmodule Cldr.Calendar.Julian.Sept1 do
+  use Cldr.Calendar.Julian, new_year_starting_month_and_day: {9, 1}
+
+end

test/julian_calendar_test.exs

@@ -9,4 +9,24 @@ defmodule Cldr.Calendar.Julian.Test do
   test "plus years in a Julian date" do
     assert Cldr.Calendar.Julian.plus(1, 1, 1, :years, 1)
   end
+
+  test "Calendar conversion from Julian starting March 25 for dates before new year" do
+    assert {:ok, ~D[1751-01-12 Cldr.Calendar.Gregorian]} ==
+      Date.convert(~D[1750-01-01 Cldr.Calendar.Julian.March25], Cldr.Calendar.Gregorian)
+  end
+
+  test "Calendar conversion from Julian starting March 25 for dates after new year" do
+    assert {:ok, ~D[1751-04-05 Cldr.Calendar.Gregorian]} ==
+      Date.convert(~D[1751-03-25 Cldr.Calendar.Julian.March25], Cldr.Calendar.Gregorian)
+  end
+
+  test "Calendar conversion to Julian starting March 25 for dates before new year" do
+    assert {:ok, ~D[1750-01-01 Cldr.Calendar.Julian.March25]}  ==
+      Date.convert(~D[1751-01-12 Cldr.Calendar.Gregorian], Cldr.Calendar.Julian.March25)
+  end
+
+  test "Calendar conversion to Julian starting March 25 for dates after new year" do
+    assert {:ok, ~D[1751-03-25 Cldr.Calendar.Julian.March25]} ==
+      Date.convert(~D[1751-04-05 Cldr.Calendar.Gregorian], Cldr.Calendar.Julian.March25)
+  end
 end

test/round_trip_test.exs

@@ -87,6 +87,17 @@ defmodule Cldr.Calendar.RoundTrip.Test do
     end
   end
 
+  test "that Cldr.Calendar.Julian.March25 dates all round trip" do
+    for year <- 0001..1800,
+        month <- 1..12,
+        day <- 1..28 do
+      {:ok, julian} = Date.new(year, month, day, Cldr.Calendar.Julian.March25)
+      {:ok, iso} = Date.convert(julian, Calendar.ISO)
+      {:ok, converted} = Date.convert(iso, Cldr.Calendar.Julian.March25)
+      assert Date.compare(julian, converted) == :eq
+    end
+  end
+
   if function_exported?(Code, :fetch_docs, 1) do
     test "that no module docs are generated for a backend" do
       assert {:docs_v1, _, :elixir, _, :hidden, %{}, _} = Code.fetch_docs(NoDocs.Cldr.Calendar)