feat: Implement OffsetDateTime.with_offset

pyoda_time/_offset_date_time.py

@@ -114,6 +114,40 @@ def to_instant(self) -> Instant:
 
         return Instant._from_untrusted_duration(self.__to_elapsed_time_since_epoch())
 
+    def with_offset(self, offset: Offset) -> OffsetDateTime:
+        """Creates a new OffsetDateTime representing the instant in time in the same calendar, but with a different
+        offset. The local date and time is adjusted accordingly.
+
+        :param offset: The new offset to use.
+        :return: The converted OffsetDateTime.
+        """
+        from ._offset_time import OffsetTime
+
+        # TODO: unchecked
+        # Slight change to the normal operation, as it's *just* about plausible that we change day
+        # twice in one direction or the other.
+        days = 0
+        nanos = self.__offset_time.nanosecond_of_day + offset.nanoseconds - self.__offset_time._offset_nanoseconds
+        if nanos >= PyodaConstants.NANOSECONDS_PER_DAY:
+            days += 1
+            nanos -= PyodaConstants.NANOSECONDS_PER_DAY
+            if nanos >= PyodaConstants.NANOSECONDS_PER_DAY:
+                days += 1
+                nanos -= PyodaConstants.NANOSECONDS_PER_DAY
+        elif nanos < 0:
+            days -= 1
+            nanos += PyodaConstants.NANOSECONDS_PER_DAY
+            if nanos < 0:
+                days -= 1
+                nanos += PyodaConstants.NANOSECONDS_PER_DAY
+        return OffsetDateTime._ctor(
+            local_date=self.__local_date if days == 0 else self.__local_date.plus_days(days),
+            offset_time=OffsetTime._ctor(
+                nanosecond_of_day=nanos,
+                offset_seconds=offset.seconds,
+            ),
+        )
+
     def __to_elapsed_time_since_epoch(self) -> Duration:
         # Equivalent to LocalDateTime.ToLocalInstant().Minus(offset)
         days: int = self.__local_date._days_since_epoch

tests/test_offset_date_time.py

@@ -0,0 +1,30 @@
+# Copyright 2024 The Pyoda Time Authors. All rights reserved.
+# Use of this source code is governed by the Apache License 2.0,
+# as found in the LICENSE.txt file.
+
+from pyoda_time import LocalDateTime, Offset, OffsetDateTime
+
+
+class TestOffsetDateTime:
+    def test_with_offset(self) -> None:
+        morning = LocalDateTime(2014, 1, 31, 9, 30)
+        original = OffsetDateTime(morning, Offset.from_hours(-8))
+        evening = LocalDateTime(2014, 1, 31, 19, 30)
+        new_offset = Offset.from_hours(2)
+        expected = OffsetDateTime(evening, new_offset)
+        assert original.with_offset(new_offset) == expected
+
+    def test_with_offset_cross_dates(self) -> None:
+        noon = OffsetDateTime(LocalDateTime(2017, 8, 22, 12, 0, 0), Offset.from_hours(0))
+        previous_night = noon.with_offset(Offset.from_hours(-14))
+        next_morning = noon.with_offset(Offset.from_hours(14))
+        assert previous_night.local_date_time == LocalDateTime(2017, 8, 21, 22, 0, 0)
+        assert next_morning.local_date_time == LocalDateTime(2017, 8, 23, 2, 0, 0)
+
+    def test_with_offset_two_days_forward_and_back(self) -> None:
+        # Go from UTC-18 to UTC+18
+        night = OffsetDateTime(LocalDateTime(2017, 8, 21, 18, 0, 0), Offset.from_hours(-18))
+        morning = night.with_offset(Offset.from_hours(18))
+        assert morning.local_date_time == LocalDateTime(2017, 8, 23, 6, 0, 0)
+        back_again = morning.with_offset(Offset.from_hours(-18))
+        assert back_again == night