feat(analysis): _expected_ticks per finestre */15 allineate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/cerbero_bite/analysis/data_audit.py

@@ -93,3 +93,29 @@ class ChainAuditReport:
     iv_null_count: int = 0
     iv_null_pct: Decimal = Decimal("0")
     depth_zero_pct: Decimal = Decimal("0")
+
+
+def _expected_ticks(since: datetime, until: datetime) -> int:
+    """Number of `*/15` ticks in ``[since, until)`` aligned to wall clock.
+
+    A tick is any UTC instant where ``minute % 15 == 0``. The first
+    tick at or after ``since`` is computed by rounding ``since`` up;
+    every subsequent tick is +15 minutes. The window is half-open on
+    the right.
+    """
+    if until <= since:
+        return 0
+    # Round `since` up to the next */15 boundary.
+    minute = since.minute
+    remainder = minute % _TICK_INTERVAL_MIN
+    if remainder == 0 and since.second == 0 and since.microsecond == 0:
+        first_tick = since
+    else:
+        bump = _TICK_INTERVAL_MIN - remainder
+        first_tick = (since + timedelta(minutes=bump)).replace(
+            second=0, microsecond=0
+        )
+    if first_tick >= until:
+        return 0
+    span = until - first_tick
+    return int(span.total_seconds() // (_TICK_INTERVAL_MIN * 60))

tests/unit/test_data_audit.py

@@ -0,0 +1,28 @@
+"""Unit tests for analysis.data_audit."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+from cerbero_bite.analysis.data_audit import _expected_ticks  # noqa: PLC2701
+
+
+def test_expected_ticks_basic() -> None:
+    since = datetime(2026, 5, 12, 12, 0, tzinfo=UTC)
+    until = datetime(2026, 5, 12, 13, 0, tzinfo=UTC)
+    # 12:00, 12:15, 12:30, 12:45 → 4 ticks before 13:00
+    assert _expected_ticks(since, until) == 4
+
+
+def test_expected_ticks_inclusive_left_exclusive_right() -> None:
+    since = datetime(2026, 5, 12, 12, 0, tzinfo=UTC)
+    until = datetime(2026, 5, 12, 12, 15, tzinfo=UTC)
+    # Only 12:00
+    assert _expected_ticks(since, until) == 1
+
+
+def test_expected_ticks_unaligned_since_rounds_up() -> None:
+    since = datetime(2026, 5, 12, 12, 7, tzinfo=UTC)
+    until = datetime(2026, 5, 12, 12, 30, tzinfo=UTC)
+    # First aligned tick after 12:07 is 12:15, then 12:30 is excluded
+    assert _expected_ticks(since, until) == 1