{"abstract":"When the remaining count is a multiple of the weekday count, the end date is one week early.","category":"Calendar recurrence rules","checks":7,"contract":"For FREQ=WEEKLY;INTERVAL;BYDAY;WKST;COUNT starting at dtstart (weeks begin on wkst, days of the first week before dtstart are skipped, active weeks every interval weeks from the dtstart week), compute the date of the final instance arithmetically, without expanding. count < 1 returns None. The result must equal the last element of the full expansion.","contract_signature":"dtstart, interval, byday, wkst, count","evaluation_group":"w2-calendar-recurrence-rules-weekly-last-occurrence-closed-form","failed_approach":"The partial repair `(rest + 1) // len(days)` rounds the other way and overshoots for most remainders.","family":"w2-calendar-recurrence-rules-weekly-last-occurrence-closed-form-full-weeks","id":"FA-77281","implementations":{"attempt":{"sha256":"9b20e30b0ef1aabbb4179f2d72e5243a777e433f22af0ff04525e0cc034bad8d","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport datetime as dt\nimport calendar\nN = 1\nobservations = []\ndef solve(dtstart, interval, byday, wkst, count):\n    start = dt.date.fromisoformat(dtstart)\n    if count < 1:\n        return None\n    days = sorted(set(byday), key=lambda w: (w - wkst) % 7)\n    week0 = start - dt.timedelta(days=(start.weekday() - wkst) % 7)\n    first_week = [w for w in days if (w - wkst) % 7 >= (start.weekday() - wkst) % 7]\n    if count <= len(first_week):\n        w = first_week[count - 1]\n        weeks = 0\n    else:\n        rest = count - len(first_week)\n        weeks = (rest + 1) // len(days)\n        w = days[(rest - 1) % len(days)]\n    last = week0 + dt.timedelta(days=weeks * interval * 7 + (w - wkst) % 7)\n    return last.isoformat()\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[('regression: WKST=SU interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 6, 4], '1997-08-31'),\n  ('WKST=MO interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 0, 4], '1997-08-24'),\n  ('multiple of weekday count', ['2024-01-01', 1, [0, 2, 4], 0, 6], '2024-01-12'),\n  ('partial first week', ['2024-01-04', 1, [0, 2, 4], 0, 5], '2024-01-15'),\n  ('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21'),\n  ('count zero', ['2024-01-01', 1, [0], 0, 0], None),\n  ('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None)],\n [('multiple of weekday count', ['2024-01-01', 1, [0, 2, 4], 0, 6], '2024-01-12'),\n  ('partial first week', ['2024-01-04', 1, [0, 2, 4], 0, 5], '2024-01-15'),\n  ('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21'),\n  ('count zero', ['2024-01-01', 1, [0], 0, 0], None),\n  ('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None),\n  ('within first week', ['2024-01-01', 2, [0, 1, 2], 0, 2], '2024-01-02'),\n  ('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23')],\n [('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21'),\n  ('count zero', ['2024-01-01', 1, [0], 0, 0], None),\n  ('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None),\n  ('within first week', ['2024-01-01', 2, [0, 1, 2], 0, 2], '2024-01-02'),\n  ('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23'),\n  ('WKST=WE interval 2', ['2024-06-04', 2, [0, 3], 2, 7], '2024-07-25'),\n  ('dtstart after all first-week days', ['2024-01-06', 2, [0, 1], 0, 5], '2024-02-12')],\n [('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None),\n  ('within first week', ['2024-01-01', 2, [0, 1, 2], 0, 2], '2024-01-02'),\n  ('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23'),\n  ('WKST=WE interval 2', ['2024-06-04', 2, [0, 3], 2, 7], '2024-07-25'),\n  ('dtstart after all first-week days', ['2024-01-06', 2, [0, 1], 0, 5], '2024-02-12'),\n  ('long series', ['2025-02-03', 2, [1, 3, 5], 0, 11], '2025-03-20'),\n  ('regression: WKST=SU interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 6, 4], '1997-08-31'),\n  ('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21')],\n [('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23'),\n  ('WKST=WE interval 2', ['2024-06-04', 2, [0, 3], 2, 7], '2024-07-25'),\n  ('dtstart after all first-week days', ['2024-01-06', 2, [0, 1], 0, 5], '2024-02-12'),\n  ('long series', ['2025-02-03', 2, [1, 3, 5], 0, 11], '2025-03-20'),\n  ('regression: WKST=SU interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 6, 4], '1997-08-31'),\n  ('WKST=MO interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 0, 4], '1997-08-24'),\n  ('multiple of weekday count', ['2024-01-01', 1, [0, 2, 4], 0, 6], '2024-01-12'),\n  ('partial first week', ['2024-01-04', 1, [0, 2, 4], 0, 5], '2024-01-15')]]\nfor label, args, expected in fixtures[N-1]:\n    check(label, solve(*args), expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"d800b0bcd3d6b26a5d7376e3ce1713ebc0c037710b9034a6852130afedde8db4","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport datetime as dt\nimport calendar\nN = 1\nobservations = []\ndef solve(dtstart, interval, byday, wkst, count):\n    start = dt.date.fromisoformat(dtstart)\n    if count < 1:\n        return None\n    days = sorted(set(byday), key=lambda w: (w - wkst) % 7)\n    week0 = start - dt.timedelta(days=(start.weekday() - wkst) % 7)\n    first_week = [w for w in days if (w - wkst) % 7 >= (start.weekday() - wkst) % 7]\n    if count <= len(first_week):\n        w = first_week[count - 1]\n        weeks = 0\n    else:\n        rest = count - len(first_week)\n        weeks = rest // len(days)\n        w = days[(rest - 1) % len(days)]\n    last = week0 + dt.timedelta(days=weeks * interval * 7 + (w - wkst) % 7)\n    return last.isoformat()\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[('regression: WKST=SU interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 6, 4], '1997-08-31'),\n  ('WKST=MO interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 0, 4], '1997-08-24'),\n  ('multiple of weekday count', ['2024-01-01', 1, [0, 2, 4], 0, 6], '2024-01-12'),\n  ('partial first week', ['2024-01-04', 1, [0, 2, 4], 0, 5], '2024-01-15'),\n  ('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21'),\n  ('count zero', ['2024-01-01', 1, [0], 0, 0], None),\n  ('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None)],\n [('multiple of weekday count', ['2024-01-01', 1, [0, 2, 4], 0, 6], '2024-01-12'),\n  ('partial first week', ['2024-01-04', 1, [0, 2, 4], 0, 5], '2024-01-15'),\n  ('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21'),\n  ('count zero', ['2024-01-01', 1, [0], 0, 0], None),\n  ('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None),\n  ('within first week', ['2024-01-01', 2, [0, 1, 2], 0, 2], '2024-01-02'),\n  ('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23')],\n [('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21'),\n  ('count zero', ['2024-01-01', 1, [0], 0, 0], None),\n  ('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None),\n  ('within first week', ['2024-01-01', 2, [0, 1, 2], 0, 2], '2024-01-02'),\n  ('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23'),\n  ('WKST=WE interval 2', ['2024-06-04', 2, [0, 3], 2, 7], '2024-07-25'),\n  ('dtstart after all first-week days', ['2024-01-06', 2, [0, 1], 0, 5], '2024-02-12')],\n [('count negative', ['2024-01-01', 2, [0, 3], 0, -1], None),\n  ('within first week', ['2024-01-01', 2, [0, 1, 2], 0, 2], '2024-01-02'),\n  ('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23'),\n  ('WKST=WE interval 2', ['2024-06-04', 2, [0, 3], 2, 7], '2024-07-25'),\n  ('dtstart after all first-week days', ['2024-01-06', 2, [0, 1], 0, 5], '2024-02-12'),\n  ('long series', ['2025-02-03', 2, [1, 3, 5], 0, 11], '2025-03-20'),\n  ('regression: WKST=SU interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 6, 4], '1997-08-31'),\n  ('interval 3 single day', ['2024-05-06', 3, [4], 0, 3], '2024-06-21')],\n [('WKST=SU weekend', ['2024-03-09', 1, [5, 6], 6, 5], '2024-03-23'),\n  ('WKST=WE interval 2', ['2024-06-04', 2, [0, 3], 2, 7], '2024-07-25'),\n  ('dtstart after all first-week days', ['2024-01-06', 2, [0, 1], 0, 5], '2024-02-12'),\n  ('long series', ['2025-02-03', 2, [1, 3, 5], 0, 11], '2025-03-20'),\n  ('regression: WKST=SU interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 6, 4], '1997-08-31'),\n  ('WKST=MO interval 2 TU,SU', ['1997-08-05', 2, [1, 6], 0, 4], '1997-08-24'),\n  ('multiple of weekday count', ['2024-01-01', 1, [0, 2, 4], 0, 6], '2024-01-12'),\n  ('partial first week', ['2024-01-04', 1, [0, 2, 4], 0, 5], '2024-01-15')]]\nfor label, args, expected in fixtures[N-1]:\n    check(label, solve(*args), expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":"Stipulated, bounded recurrence-expansion model evaluated offline on explicit fixtures; it is not a complete iCalendar implementation and makes no claim of standards conformance. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"w2-calendar-recurrence-rules-weekly-last-occurrence-closed-form-full-weeks","generated_at":"2026-09-29T14:49:23.952407+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"Recurring calendar series are expanded into concrete instances for display, reminders and conflict checks.","root_cause":"The number of full weeks after the first is computed with a plain floor division.","sha256":"018aa2f3cb7bb3666b4faa3410c50f39850d166f1c5be3091be1e5923cb2c122","title":"Closed-form final date of a COUNT-limited weekly BYDAY series: full weeks · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verified":true,"visibility":"public","verification":{"attempt":{"elapsed_ms":41.664,"exit_code":1,"observations":[{"actual":"1997-08-31","check":"regression: WKST=SU interval 2 TU,SU","expected":"1997-08-31","passed":true},{"actual":"1997-08-24","check":"WKST=MO interval 2 TU,SU","expected":"1997-08-24","passed":true},{"actual":"2024-01-12","check":"multiple of weekday count","expected":"2024-01-12","passed":true},{"actual":"2024-01-08","check":"partial first week","expected":"2024-01-15","passed":false},{"actual":"2024-07-12","check":"interval 3 single day","expected":"2024-06-21","passed":false},{"actual":null,"check":"count zero","expected":null,"passed":true},{"actual":null,"check":"count negative","expected":null,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"regression: WKST=SU interval 2 TU,SU\", \"actual\": \"1997-08-31\", \"expected\": \"1997-08-31\", \"passed\": true}, {\"check\": \"WKST=MO interval 2 TU,SU\", \"actual\": \"1997-08-24\", \"expected\": \"1997-08-24\", \"passed\": true}, {\"check\": \"multiple of weekday count\", \"actual\": \"2024-01-12\", \"expected\": \"2024-01-12\", \"passed\": true}, {\"check\": \"partial first week\", \"actual\": \"2024-01-08\", \"expected\": \"2024-01-15\", \"passed\": false}, {\"check\": \"interval 3 single day\", \"actual\": \"2024-07-12\", \"expected\": \"2024-06-21\", \"passed\": false}, {\"check\": \"count zero\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"count negative\", \"actual\": null, \"expected\": null, \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":39.625,"exit_code":1,"observations":[{"actual":"1997-08-17","check":"regression: WKST=SU interval 2 TU,SU","expected":"1997-08-31","passed":false},{"actual":"1997-08-24","check":"WKST=MO interval 2 TU,SU","expected":"1997-08-24","passed":true},{"actual":"2024-01-12","check":"multiple of weekday count","expected":"2024-01-12","passed":true},{"actual":"2024-01-08","check":"partial first week","expected":"2024-01-15","passed":false},{"actual":"2024-06-21","check":"interval 3 single day","expected":"2024-06-21","passed":true},{"actual":null,"check":"count zero","expected":null,"passed":true},{"actual":null,"check":"count negative","expected":null,"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"regression: WKST=SU interval 2 TU,SU\", \"actual\": \"1997-08-17\", \"expected\": \"1997-08-31\", \"passed\": false}, {\"check\": \"WKST=MO interval 2 TU,SU\", \"actual\": \"1997-08-24\", \"expected\": \"1997-08-24\", \"passed\": true}, {\"check\": \"multiple of weekday count\", \"actual\": \"2024-01-12\", \"expected\": \"2024-01-12\", \"passed\": true}, {\"check\": \"partial first week\", \"actual\": \"2024-01-08\", \"expected\": \"2024-01-15\", \"passed\": false}, {\"check\": \"interval 3 single day\", \"actual\": \"2024-06-21\", \"expected\": \"2024-06-21\", \"passed\": true}, {\"check\": \"count zero\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"count negative\", \"actual\": null, \"expected\": null, \"passed\": true}], \"passed\": false}\n"}},"member_only":{"stages":["fixed"],"fields":["implementations.fixed","verification.fixed","harness","repair"],"note":"The verified repair, its recorded checks, the repair description, and the scoring harness are available to members."}}