Anchor-date cycle position arithmetic (Feature 05, task 2)

presence.cycle_week_index: ((target_date - anchor_date).days // 7) %
cycle_length_weeks — anchor-date arithmetic ONLY (D-004).  Python
floor division makes pre-anchor dates walk the cycle backwards
(day -1 is the last week), never inverting; the modulo of a negative
stays non-negative, so the index is always a valid week index.  The
target date passes through the model's strict plain-date parsing.
Tests: formula offsets, three-week pattern wrap, backwards walking,
day-by-day stability across three Dec-31 -> Jan-1 boundaries
including 2020 (verified 53 ISO weeks — the parity-inversion
regression), and a source guard that the module never reads ISO week
numbers.  Suite: 980 passed.

custom_components/nestquest/presence.py

@@ -298,3 +298,22 @@ class PresenceOverride:
     def covers(self, date: datetime.date) -> bool:
         """Return True when the override applies to ``date``."""
         return self.start_date <= date <= self.end_date
+
+
+def cycle_week_index(
+    schedule: PresenceSchedule, target_date: object
+) -> int:
+    """Return the cycle week index ``target_date`` falls in.
+
+    Anchor-date arithmetic ONLY (D-004): ``((target_date -
+    anchor_date).days // 7) % cycle_length_weeks``.  Python's floor
+    division makes dates BEFORE the anchor behave correctly — they
+    walk the cycle backwards (day -1 is the last week of the cycle,
+    day -8 wraps one further) instead of inverting, which is exactly
+    what ISO week parity does wrong in 53-week years.  The modulo of
+    a negative in Python is non-negative, so the index is always a
+    valid week index.
+    """
+    target = _parse_date(target_date, "target_date")
+    days = (target - schedule.anchor_date).days
+    return (days // 7) % schedule.cycle_length_weeks

tests/test_presence.py

@@ -9,6 +9,7 @@ from custom_components.nestquest.presence import (
     MAX_CYCLE_LENGTH_WEEKS,
     PresenceOverride,
     PresenceSchedule,
+    cycle_week_index,
 )
 
 
@@ -260,3 +261,76 @@ def test_schedule_pattern_is_immutable_after_construction() -> None:
     # The validated contents are still readable and encode correctly.
     assert schedule.pattern[0] == frozenset({0, 2})
     assert schedule.encode() == "0,2|1"
+
+
+# ---------------------------------------------------------------------------
+# Cycle arithmetic: anchor-date only, never ISO week parity
+# ---------------------------------------------------------------------------
+
+
+def test_cycle_week_index_follows_the_anchor_formula() -> None:
+    schedule = PresenceSchedule(1, 2, ANCHOR, {0: {0}, 1: {1}})
+    # The anchor day starts week 0; each whole 7 days steps the cycle.
+    offsets = {0: 0, 1: 0, 6: 0, 7: 1, 8: 1, 13: 1, 14: 0, 15: 0, 22: 1}
+    for offset, expected in offsets.items():
+        assert (
+            cycle_week_index(
+                schedule, ANCHOR + datetime.timedelta(days=offset)
+            )
+            == expected
+        ), f"offset {offset}"
+
+
+def test_cycle_week_index_before_the_anchor_walks_backwards() -> None:
+    """Python floor division: day -1 is the LAST week of the cycle,
+    wrapping backwards — never inverting."""
+    schedule = PresenceSchedule(1, 2, ANCHOR, {0: {0}, 1: {1}})
+    assert cycle_week_index(schedule, ANCHOR - datetime.timedelta(days=1)) == 1
+    assert cycle_week_index(schedule, ANCHOR - datetime.timedelta(days=7)) == 1
+    assert cycle_week_index(schedule, ANCHOR - datetime.timedelta(days=8)) == 0
+
+
+def test_cycle_week_index_three_week_pattern() -> None:
+    schedule = PresenceSchedule(
+        1, 3, ANCHOR, {0: {0}, 1: {1}, 2: {2}}
+    )
+    offsets = [0, 7, 14, 21, 28]
+    assert [cycle_week_index(schedule, ANCHOR + datetime.timedelta(days=o)) for o in offsets] == [
+        0, 1, 2, 0, 1
+    ]
+
+
+def test_cycle_week_index_stable_across_three_year_boundaries() -> None:
+    """The regression this exists for: walking day by day through a
+    December 31st -> January 1st boundary, the cycle index steps by 0
+    or +1 (mod cycle length) and NEVER resets or inverts — including
+    2020, a 53-ISO-week year, where ISO week parity flips and would
+    invert a two-week custody schedule on January 1st."""
+    schedule = PresenceSchedule(
+        1, 2, datetime.date(2019, 1, 6), {0: {0, 2, 4}, 1: {1, 3}}
+    )
+    for year in (2019, 2020, 2021):
+        day = datetime.date(year, 12, 24)
+        while day < datetime.date(year + 1, 1, 8):
+            current = cycle_week_index(schedule, day)
+            following = cycle_week_index(
+                schedule, day + datetime.timedelta(days=1)
+            )
+            assert following in (current, (current + 1) % 2), (
+                f"{day} -> {day + datetime.timedelta(days=1)}: {current} "
+                f"-> {following} breaks the cycle across the year boundary"
+            )
+            day += datetime.timedelta(days=1)
+    # And 2020 really is the 53-ISO-week year this guards against.
+    assert datetime.date(2020, 12, 31).isocalendar()[1] == 53
+
+
+def test_module_uses_no_iso_week_functions() -> None:
+    """D-004 guardrail: cycle math never reads ISO week numbers."""
+    from pathlib import Path
+
+    source = Path(
+        __import__("custom_components.nestquest.presence", fromlist=["__file__"]).__file__
+    ).read_text()
+    assert "isocalendar" not in source
+    assert "isoweek" not in source