Add next_period/1

lib/tz/compiler.ex

@@ -131,6 +131,36 @@ defmodule Tz.Compiler do
         def iana_version() do
           unquote(tzdata_version)
         end
+
+        def compiled_at() do
+          unquote(Macro.escape(DateTime.utc_now()))
+        end
+
+        def next_period(%DateTime{} = datetime) do
+          {gregorian_secs, _} = DateTime.to_gregorian_seconds(datetime)
+
+          {:ok, periods} = periods(datetime.time_zone)
+          reversed_periods = Enum.reverse(periods)
+
+          period = Enum.find(reversed_periods, fn {from, _, _, _} -> gregorian_secs < from end)
+
+          if period do
+            period
+          else
+            {utc_secs, {utc_to_std_offset, _, _}, _, rules_and_template} = hd(periods)
+
+            periods =
+              Tz.TimeZoneDatabase.generate_dynamic_periods(
+                utc_secs,
+                utc_to_std_offset,
+                rules_and_template
+              )
+
+            reversed_periods = Enum.reverse(periods)
+
+            Enum.find(reversed_periods, fn {from, _, _, _} -> gregorian_secs < from end)
+          end
+        end
       end,
       for period_or_link <- periods_and_links do
         case period_or_link do

lib/tz/time_zone_database.ex

@@ -104,7 +104,8 @@ defmodule Tz.TimeZoneDatabase do
     }
   end
 
-  defp generate_dynamic_periods(secs, utc_offset, {rule_name, format_time_zone_abbr}) do
+  @doc false
+  def generate_dynamic_periods(secs, utc_offset, {rule_name, format_time_zone_abbr}) do
     %{year: year} = gregorian_seconds_to_naive_datetime(secs)
 
     [rule1, rule2] = Tz.OngoingChangingRulesProvider.rules(rule_name)
@@ -114,7 +115,9 @@ defmodule Tz.TimeZoneDatabase do
         Tz.IanaFileParser.change_rule_year(rule2, year - 1),
         Tz.IanaFileParser.change_rule_year(rule1, year - 1),
         Tz.IanaFileParser.change_rule_year(rule2, year),
-        Tz.IanaFileParser.change_rule_year(rule1, year)
+        Tz.IanaFileParser.change_rule_year(rule1, year),
+        Tz.IanaFileParser.change_rule_year(rule2, year + 1),
+        Tz.IanaFileParser.change_rule_year(rule1, year + 1)
       ])
 
     zone_line = %{

test/time_zone_database_test.exs

@@ -170,4 +170,42 @@ defmodule TimeZoneDatabaseTest do
 
     assert DateTime.to_iso8601(datetime) == "2030-01-01T00:00:00+00:00"
   end
+
+  test "next_period/1 and previous_period/1" do
+    {:ok, dt} =
+      DateTime.new(~D[2030-09-01], ~T[10:00:00], "Europe/Copenhagen", Tz.TimeZoneDatabase)
+
+    {from, _, _, _} = Tz.PeriodsProvider.next_period(dt)
+
+    datetime_next_period =
+      DateTime.from_gregorian_seconds(from)
+      |> DateTime.shift_zone!(dt.time_zone, Tz.TimeZoneDatabase)
+
+    {:ambiguous, first_dt, second_dt} =
+      DateTime.new(~D[2030-10-27], ~T[02:00:00], "Europe/Copenhagen", Tz.TimeZoneDatabase)
+
+    assert DateTime.compare(datetime_next_period, second_dt) == :eq
+
+    {from, _, _, _} = Tz.PeriodsProvider.next_period(second_dt)
+
+    datetime_next_period =
+      DateTime.from_gregorian_seconds(from)
+      |> DateTime.shift_zone!(dt.time_zone, Tz.TimeZoneDatabase)
+
+    {:gap, _dt_just_before, dt_just_after} =
+      DateTime.new(~D[2031-03-30], ~T[02:30:00], "Europe/Copenhagen", Tz.TimeZoneDatabase)
+
+    assert DateTime.compare(datetime_next_period, dt_just_after) == :eq
+
+    {from, _, _, _} = Tz.PeriodsProvider.next_period(first_dt)
+
+    datetime_next_period =
+      DateTime.from_gregorian_seconds(from)
+      |> DateTime.shift_zone!(dt.time_zone, Tz.TimeZoneDatabase)
+
+    {:ambiguous, _first_dt, second_dt} =
+      DateTime.new(~D[2030-10-27], ~T[02:00:00], "Europe/Copenhagen", Tz.TimeZoneDatabase)
+
+    assert DateTime.compare(datetime_next_period, second_dt) == :eq
+  end
 end