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FA-77326 / Calendar recurrence rules / Open access

Next due date for a recurring task after completions: regenerate base · case 01

A regenerating task completed early is rescheduled from its old due date.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The regenerated due date is computed from the previous due date instead of the completion.

THE FAILURE

The regenerated due date is computed from the previous due date instead of the completion.

Unsuccessful approach: The partial repair `max(cd, due) + step` still uses the old due date when the task is completed early.

Case contract

A recurring task first due on first_due repeats every interval_days. Completions are processed in chronological order. mode "regenerate": the next due date is completion + interval. mode "schedule": completing advances to the next scheduled due date, skipping every scheduled date on or before the completion date. If the resulting due date passes until (inclusive bound, may be None), the task has no next due date (None).

Why this case matters

Recurring calendar series are expanded into concrete instances for display, reminders and conflict checks.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import datetime as dt
import calendar
N = 1
observations = []
def solve(first_due, interval_days, mode, completions, until):
    due = dt.date.fromisoformat(first_due)
    end = dt.date.fromisoformat(until) if until else None
    step = dt.timedelta(days=interval_days)
    for c in sorted(completions):
        cd = dt.date.fromisoformat(c)
        if mode == 'regenerate':
            due = due + step
        else:
            due = due + step
            while due <= cd:
                due += step
        if end is not None and due > end:
            return None
    return due.isoformat()
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression: late completion on next due',
   ['2024-01-01', 7, 'schedule', ['2024-01-08'], None],
   '2024-01-15'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12'),
  ('late completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-20'], None], '2024-01-27'),
  ('out of order schedule', ['2024-01-01', 7, 'schedule', ['2024-01-20', '2024-01-02'], None], '2024-01-22'),
  ('multiple missed schedule', ['2024-01-01', 3, 'schedule', ['2024-01-11'], None], '2024-01-13'),
  ('until exact', ['2024-01-01', 7, 'schedule', ['2024-01-01'], '2024-01-08'], '2024-01-08'),
  ('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None)],
 [('late completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-20'], None], '2024-01-27'),
  ('out of order schedule', ['2024-01-01', 7, 'schedule', ['2024-01-20', '2024-01-02'], None], '2024-01-22'),
  ('multiple missed schedule', ['2024-01-01', 3, 'schedule', ['2024-01-11'], None], '2024-01-13'),
  ('until exact', ['2024-01-01', 7, 'schedule', ['2024-01-01'], '2024-01-08'], '2024-01-08'),
  ('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None),
  ('until regenerate', ['2024-02-01', 10, 'regenerate', ['2024-02-05'], '2024-02-15'], '2024-02-15'),
  ('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12')],
 [('multiple missed schedule', ['2024-01-01', 3, 'schedule', ['2024-01-11'], None], '2024-01-13'),
  ('until exact', ['2024-01-01', 7, 'schedule', ['2024-01-01'], '2024-01-08'], '2024-01-08'),
  ('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None),
  ('until regenerate', ['2024-02-01', 10, 'regenerate', ['2024-02-05'], '2024-02-15'], '2024-02-15'),
  ('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('completion after until but due inside',
   ['2024-03-01', 30, 'schedule', ['2024-03-05'], '2024-03-31'],
   '2024-03-31'),
  ('two schedule completions',
   ['2024-04-01', 14, 'schedule', ['2024-03-30', '2024-04-10'], None],
   '2024-04-29'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12')],
 [('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None),
  ('until regenerate', ['2024-02-01', 10, 'regenerate', ['2024-02-05'], '2024-02-15'], '2024-02-15'),
  ('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('completion after until but due inside',
   ['2024-03-01', 30, 'schedule', ['2024-03-05'], '2024-03-31'],
   '2024-03-31'),
  ('two schedule completions',
   ['2024-04-01', 14, 'schedule', ['2024-03-30', '2024-04-10'], None],
   '2024-04-29'),
  ('out of order regenerate',
   ['2024-04-01', 5, 'regenerate', ['2024-04-09', '2024-04-02'], None],
   '2024-04-14'),
  ('regression: late completion on next due',
   ['2024-01-01', 7, 'schedule', ['2024-01-08'], None],
   '2024-01-15'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12')],
 [('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('completion after until but due inside',
   ['2024-03-01', 30, 'schedule', ['2024-03-05'], '2024-03-31'],
   '2024-03-31'),
  ('two schedule completions',
   ['2024-04-01', 14, 'schedule', ['2024-03-30', '2024-04-10'], None],
   '2024-04-29'),
  ('out of order regenerate',
   ['2024-04-01', 5, 'regenerate', ['2024-04-09', '2024-04-02'], None],
   '2024-04-14'),
  ('regression: late completion on next due',
   ['2024-01-01', 7, 'schedule', ['2024-01-08'], None],
   '2024-01-15'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12'),
  ('late completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-20'], None], '2024-01-27')]]
for label, args, expected in fixtures[N-1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
regression: late completion on next due2024-01-152024-01-15Passed
early completion regenerate2024-01-172024-01-12Failed
late completion regenerate2024-01-172024-01-27Failed
out of order schedule2024-01-222024-01-22Passed
multiple missed schedule2024-01-132024-01-13Passed
until exact2024-01-082024-01-08Passed
until passedNoneNonePassed

SHA-256 / 705af698a6e8446b5882e15521ff4f927059bd4801636f853d4e42497a75b92a

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import datetime as dt
import calendar
N = 1
observations = []
def solve(first_due, interval_days, mode, completions, until):
    due = dt.date.fromisoformat(first_due)
    end = dt.date.fromisoformat(until) if until else None
    step = dt.timedelta(days=interval_days)
    for c in sorted(completions):
        cd = dt.date.fromisoformat(c)
        if mode == 'regenerate':
            due = max(cd, due) + step
        else:
            due = due + step
            while due <= cd:
                due += step
        if end is not None and due > end:
            return None
    return due.isoformat()
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression: late completion on next due',
   ['2024-01-01', 7, 'schedule', ['2024-01-08'], None],
   '2024-01-15'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12'),
  ('late completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-20'], None], '2024-01-27'),
  ('out of order schedule', ['2024-01-01', 7, 'schedule', ['2024-01-20', '2024-01-02'], None], '2024-01-22'),
  ('multiple missed schedule', ['2024-01-01', 3, 'schedule', ['2024-01-11'], None], '2024-01-13'),
  ('until exact', ['2024-01-01', 7, 'schedule', ['2024-01-01'], '2024-01-08'], '2024-01-08'),
  ('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None)],
 [('late completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-20'], None], '2024-01-27'),
  ('out of order schedule', ['2024-01-01', 7, 'schedule', ['2024-01-20', '2024-01-02'], None], '2024-01-22'),
  ('multiple missed schedule', ['2024-01-01', 3, 'schedule', ['2024-01-11'], None], '2024-01-13'),
  ('until exact', ['2024-01-01', 7, 'schedule', ['2024-01-01'], '2024-01-08'], '2024-01-08'),
  ('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None),
  ('until regenerate', ['2024-02-01', 10, 'regenerate', ['2024-02-05'], '2024-02-15'], '2024-02-15'),
  ('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12')],
 [('multiple missed schedule', ['2024-01-01', 3, 'schedule', ['2024-01-11'], None], '2024-01-13'),
  ('until exact', ['2024-01-01', 7, 'schedule', ['2024-01-01'], '2024-01-08'], '2024-01-08'),
  ('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None),
  ('until regenerate', ['2024-02-01', 10, 'regenerate', ['2024-02-05'], '2024-02-15'], '2024-02-15'),
  ('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('completion after until but due inside',
   ['2024-03-01', 30, 'schedule', ['2024-03-05'], '2024-03-31'],
   '2024-03-31'),
  ('two schedule completions',
   ['2024-04-01', 14, 'schedule', ['2024-03-30', '2024-04-10'], None],
   '2024-04-29'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12')],
 [('until passed', ['2024-01-01', 7, 'schedule', ['2024-01-01', '2024-01-08'], '2024-01-10'], None),
  ('until regenerate', ['2024-02-01', 10, 'regenerate', ['2024-02-05'], '2024-02-15'], '2024-02-15'),
  ('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('completion after until but due inside',
   ['2024-03-01', 30, 'schedule', ['2024-03-05'], '2024-03-31'],
   '2024-03-31'),
  ('two schedule completions',
   ['2024-04-01', 14, 'schedule', ['2024-03-30', '2024-04-10'], None],
   '2024-04-29'),
  ('out of order regenerate',
   ['2024-04-01', 5, 'regenerate', ['2024-04-09', '2024-04-02'], None],
   '2024-04-14'),
  ('regression: late completion on next due',
   ['2024-01-01', 7, 'schedule', ['2024-01-08'], None],
   '2024-01-15'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12')],
 [('no completions', ['2024-02-01', 10, 'schedule', [], None], '2024-02-01'),
  ('completion after until but due inside',
   ['2024-03-01', 30, 'schedule', ['2024-03-05'], '2024-03-31'],
   '2024-03-31'),
  ('two schedule completions',
   ['2024-04-01', 14, 'schedule', ['2024-03-30', '2024-04-10'], None],
   '2024-04-29'),
  ('out of order regenerate',
   ['2024-04-01', 5, 'regenerate', ['2024-04-09', '2024-04-02'], None],
   '2024-04-14'),
  ('regression: late completion on next due',
   ['2024-01-01', 7, 'schedule', ['2024-01-08'], None],
   '2024-01-15'),
  ('early completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-05'], None], '2024-01-12'),
  ('late completion regenerate', ['2024-01-10', 7, 'regenerate', ['2024-01-20'], None], '2024-01-27')]]
for label, args, expected in fixtures[N-1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
regression: late completion on next due2024-01-152024-01-15Passed
early completion regenerate2024-01-172024-01-12Failed
late completion regenerate2024-01-272024-01-27Passed
out of order schedule2024-01-222024-01-22Passed
multiple missed schedule2024-01-132024-01-13Passed
until exact2024-01-082024-01-08Passed
until passedNoneNonePassed

SHA-256 / 58d631dafbcd0310b0d82c5c2db59031eb0e941881d7ecea4eea54c7a33cfc8e

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 7 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

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Verification & scope

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.

Observations recorded using Python 3.12.14 at 2026-09-29T14:49:24.490867+00:00.

Case digest / 481f7eecb7e258cb42b0dcb3ee79a0a6efec4b9cb5956bdb4dc5a8fe15850b46