fix: missing occurrences when there is a dst transition

lib/cocktail/validation/shift.ex

@@ -20,6 +20,7 @@
       time
       |> shift_time("#{type}": amount)
       |> apply_option(option)
+      |> maybe_dst_change(time)
 
     {:change, new_time}
   end
@@ -29,4 +30,27 @@
   defp apply_option(time, :beginning_of_day), do: time |> beginning_of_day()
   defp apply_option(time, :beginning_of_hour), do: %{time | minute: 0, second: 0, microsecond: {0, 0}}
   defp apply_option(time, :beginning_of_minute), do: %{time | second: 0, microsecond: {0, 0}}
+
+  defp maybe_dst_change(%DateTime{} = new_time, %DateTime{} = time) do
+    dst_diff = new_time.std_offset - time.std_offset
+
+    case dst_diff do
+      0 ->
+        new_time
+
+      diff ->
+        maybe_shift_time(new_time, time, diff)
+    end
+  end
+
+  defp maybe_dst_change(new_time, _time), do: new_time
+
+  defp maybe_shift_time(new_time, time, dst_diff) do
+    shifted_time = shift_time(new_time, seconds: -dst_diff)
+
+    case DateTime.compare(shifted_time, time) do
+      :eq -> new_time
+      _ -> shifted_time
+    end
+  end
 end

test/cocktail/daily_test.exs

@@ -163,4 +163,34 @@ defmodule Cocktail.DailyTest do
            |> Cocktail.Schedule.occurrences(~N[2015-01-24 18:30:00])
            |> Enum.take(100) == [~N[2015-01-24 18:30:00]]
   end
+
+  test "generating occurrences with spring forward transition" do
+    schedule =
+      ~Y[2022-03-12 18:00:00 America/Los_Angeles]
+      |> Schedule.new()
+      |> Schedule.add_recurrence_rule(:daily)
+
+    times = schedule |> Schedule.occurrences() |> Enum.take(3)
+
+    assert times == [
+             ~Y[2022-03-12 18:00:00 America/Los_Angeles],
+             ~Y[2022-03-13 18:00:00 America/Los_Angeles],
+             ~Y[2022-03-14 18:00:00 America/Los_Angeles]
+           ]
+  end
+
+  test "generating occurrences with fall back transition" do
+    schedule =
+      ~Y[2022-11-05 18:00:00 America/Los_Angeles]
+      |> Schedule.new()
+      |> Schedule.add_recurrence_rule(:daily)
+
+    times = schedule |> Schedule.occurrences() |> Enum.take(3)
+
+    assert times == [
+             ~Y[2022-11-05 18:00:00 America/Los_Angeles],
+             ~Y[2022-11-06 18:00:00 America/Los_Angeles],
+             ~Y[2022-11-07 18:00:00 America/Los_Angeles]
+           ]
+  end
 end

test/cocktail/weekly_test.exs

@@ -154,4 +154,19 @@ defmodule Cocktail.WeeklyTest do
              ~Y[2017-01-09 10:00:00 America/Los_Angeles]
            ]
   end
+
+  test "Weekly with dst transition" do
+    times =
+      ~Y[2022-03-12 06:00:00 America/Los_Angeles]
+      |> Cocktail.schedule()
+      |> Schedule.add_recurrence_rule(:weekly)
+      |> Cocktail.Schedule.occurrences()
+      |> Enum.take(3)
+
+    assert times == [
+             ~Y[2022-03-12 06:00:00 America/Los_Angeles],
+             ~Y[2022-03-19 06:00:00 America/Los_Angeles],
+             ~Y[2022-03-26 06:00:00 America/Los_Angeles]
+           ]
+  end
 end