RingBuffer: Fix off-by-one error in gap calculation

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/sdk/timeseries/_ringbuffer/buffer.py

@@ -82,7 +82,7 @@ def __init__(
         self._gaps: list[Gap] = []
         self._datetime_newest: datetime = self._DATETIME_MIN
         self._datetime_oldest: datetime = self._DATETIME_MAX
-        self._time_range: timedelta = (len(self._buffer) - 1) * sampling_period
+        self._full_time_range: timedelta = len(self._buffer) * self._sampling_period
 
     @property
     def sampling_period(self) -> timedelta:
@@ -145,7 +145,9 @@ def update(self, sample: Sample[QuantityT]) -> None:
         # Update timestamps
         prev_newest = self._datetime_newest
         self._datetime_newest = max(self._datetime_newest, timestamp)
-        self._datetime_oldest = self._datetime_newest - self._time_range
+        self._datetime_oldest = self._datetime_newest - (
+            self._full_time_range - self._sampling_period
+        )
 
         # Update data
         value: float = np.nan if sample.value is None else sample.value.base_value
@@ -293,7 +295,7 @@ def _update_gaps(
 
         if not new_missing:
             # Replace all gaps with one if we went far into then future
-            if self._datetime_newest - newest >= self._time_range:
+            if self._datetime_newest - newest >= self._full_time_range:
                 self._gaps = [
                     Gap(start=self._datetime_oldest, end=self._datetime_newest)
                 ]

tests/timeseries/test_ringbuffer.py

@@ -335,3 +335,22 @@ def test_ringbuffer_empty_buffer() -> None:
             sampling_period=timedelta(seconds=1),
             align_to=datetime(1, 1, 1),
         )
+
+
+def test_off_by_one_gap_logic_bug() -> None:
+    """Test off by one bug in the gap calculation."""
+    buffer = OrderedRingBuffer(
+        np.empty(shape=2, dtype=float),
+        sampling_period=timedelta(seconds=1),
+        align_to=datetime(1, 1, 1, tzinfo=timezone.utc),
+    )
+
+    base_time = datetime(2023, 1, 1, tzinfo=timezone.utc)
+
+    times = [base_time, base_time + timedelta(seconds=1)]
+
+    buffer.update(Sample(times[0], Quantity(1.0)))
+    buffer.update(Sample(times[1], Quantity(2.0)))
+
+    assert buffer.is_missing(times[0]) is False
+    assert buffer.is_missing(times[1]) is False

tests/timeseries/test_ringbuffer_serialization.py

@@ -61,8 +61,6 @@ def load_dump_test(dumped: rb.OrderedRingBuffer[Any], path: str) -> None:
     # pylint: disable=protected-access
     assert dumped._gaps == loaded._gaps
     # pylint: disable=protected-access
-    assert dumped._time_range == loaded._time_range
-    # pylint: disable=protected-access
     assert dumped._sampling_period == loaded._sampling_period
     # pylint: disable=protected-access
     assert dumped._time_index_alignment == loaded._time_index_alignment