FA-77246 / Calendar recurrence rules / Open access

Rendering a recurrence rule as a human-readable sentence: negative ordinal · case 01

BYDAY=-1FR is rendered as "the 1st to last Friday".

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

ROOT CAUSE

Every negative ordinal uses the "to last" form, including -1.

VERIFIED REPAIR

At the negative ordinal decision use `'last' if ordinal == -1 else nth(-ordinal) + ' to last'`, leaving the other decision sites of the model unchanged.

Unsuccessful approach: The partial repair `'last'` renders every negative ordinal as "last", so -2 loses its position.

Case contract

House style: "every <unit>" for interval 1, else "every <n> <unit>s"; weekday lists are in Monday-first order without duplicates and joined as "A, B and C"; the exact set MO..FR renders as "on weekdays"; an ordinal renders "on the 2nd Tuesday", -1 as "on the last Friday" and -n (n>1) as "on the 2nd to last Friday"; count renders ", once" for 1 and ", <n> times" otherwise.

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

N = 1
observations = []
def solve(freq, interval, byday, ordinal, count):
    unit = {'DAILY': 'day', 'WEEKLY': 'week', 'MONTHLY': 'month', 'YEARLY': 'year'}[freq]
    text = 'every ' + unit if interval == 1 else 'every %d %ss' % (interval, unit)
    names = {'MO': 'Monday', 'TU': 'Tuesday', 'WE': 'Wednesday', 'TH': 'Thursday', 'FR': 'Friday', 'SA': 'Saturday', 'SU': 'Sunday'}
    order = ['MO', 'TU', 'WE', 'TH', 'FR', 'SA', 'SU']
    def nth(n):
        return str(n) + {1: 'st', 2: 'nd', 3: 'rd'}.get(n, 'th')
    if ordinal:
        word = (nth(-ordinal) + ' to last') if ordinal < 0 else nth(ordinal)
        text += ' on the ' + word + ' ' + names[byday[0]]
    elif byday:
        if set(byday) == {'MO', 'TU', 'WE', 'TH', 'FR'}:
            text += ' on weekdays'
        else:
            words = [names[d] for d in sorted(set(byday), key=order.index)]
            text += ' on ' + (words[0] if len(words) == 1 else ', '.join(words[:-1]) + ' and ' + words[-1])
    if count:
        text += ', ' + ('once' if count == 1 else '%d times' % count)
    return text
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('biweekly single day once', ['WEEKLY', 2, ['TU'], 0, 1], 'every 2 weeks on Tuesday, once'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday'),
  ('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times')],
 [('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday'),
  ('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times')],
 [('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')],
 [('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('two days unordered', ['WEEKLY', 1, ['TH', 'TU'], 0, 1], 'every week on Tuesday and Thursday, once'),
  ('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')],
 [('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('two days unordered', ['WEEKLY', 1, ['TH', 'TU'], 0, 1], 'every week on Tuesday and Thursday, once'),
  ('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('biweekly single day once', ['WEEKLY', 2, ['TU'], 0, 1], 'every 2 weeks on Tuesday, once'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')]]
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 fixture	Actual	Expected	Outcome
regression: weekly three days	`every week on Monday, Wednesday and Friday`	`every week on Monday, Wednesday and Friday`	Passed
biweekly single day once	`every 2 weeks on Tuesday, once`	`every 2 weeks on Tuesday, once`	Passed
last friday monthly	`every month on the 1st to last Friday, 6 times`	`every month on the last Friday, 6 times`	Failed
second to last sunday	`every 3 months on the 2nd to last Sunday`	`every 3 months on the 2nd to last Sunday`	Passed
second tuesday	`every month on the 2nd Tuesday, 10 times`	`every month on the 2nd Tuesday, 10 times`	Passed
weekdays exact	`every week on weekdays`	`every week on weekdays`	Passed
weekdays reordered	`every week on weekdays, 3 times`	`every week on weekdays, 3 times`	Passed

SHA-256 / 1427b5e6afae94c8742609456305da21885de916c374207fb4f51538bf00f46c

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(freq, interval, byday, ordinal, count):
    unit = {'DAILY': 'day', 'WEEKLY': 'week', 'MONTHLY': 'month', 'YEARLY': 'year'}[freq]
    text = 'every ' + unit if interval == 1 else 'every %d %ss' % (interval, unit)
    names = {'MO': 'Monday', 'TU': 'Tuesday', 'WE': 'Wednesday', 'TH': 'Thursday', 'FR': 'Friday', 'SA': 'Saturday', 'SU': 'Sunday'}
    order = ['MO', 'TU', 'WE', 'TH', 'FR', 'SA', 'SU']
    def nth(n):
        return str(n) + {1: 'st', 2: 'nd', 3: 'rd'}.get(n, 'th')
    if ordinal:
        word = ('last') if ordinal < 0 else nth(ordinal)
        text += ' on the ' + word + ' ' + names[byday[0]]
    elif byday:
        if set(byday) == {'MO', 'TU', 'WE', 'TH', 'FR'}:
            text += ' on weekdays'
        else:
            words = [names[d] for d in sorted(set(byday), key=order.index)]
            text += ' on ' + (words[0] if len(words) == 1 else ', '.join(words[:-1]) + ' and ' + words[-1])
    if count:
        text += ', ' + ('once' if count == 1 else '%d times' % count)
    return text
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('biweekly single day once', ['WEEKLY', 2, ['TU'], 0, 1], 'every 2 weeks on Tuesday, once'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday'),
  ('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times')],
 [('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday'),
  ('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times')],
 [('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')],
 [('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('two days unordered', ['WEEKLY', 1, ['TH', 'TU'], 0, 1], 'every week on Tuesday and Thursday, once'),
  ('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')],
 [('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('two days unordered', ['WEEKLY', 1, ['TH', 'TU'], 0, 1], 'every week on Tuesday and Thursday, once'),
  ('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('biweekly single day once', ['WEEKLY', 2, ['TU'], 0, 1], 'every 2 weeks on Tuesday, once'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')]]
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 fixture	Actual	Expected	Outcome
regression: weekly three days	`every week on Monday, Wednesday and Friday`	`every week on Monday, Wednesday and Friday`	Passed
biweekly single day once	`every 2 weeks on Tuesday, once`	`every 2 weeks on Tuesday, once`	Passed
last friday monthly	`every month on the last Friday, 6 times`	`every month on the last Friday, 6 times`	Passed
second to last sunday	`every 3 months on the last Sunday`	`every 3 months on the 2nd to last Sunday`	Failed
second tuesday	`every month on the 2nd Tuesday, 10 times`	`every month on the 2nd Tuesday, 10 times`	Passed
weekdays exact	`every week on weekdays`	`every week on weekdays`	Passed
weekdays reordered	`every week on weekdays, 3 times`	`every week on weekdays, 3 times`	Passed

SHA-256 / fc732bbb83b73eb5129fb81608e61d839153b6039c313bc1affe95f950fdb85d

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(freq, interval, byday, ordinal, count):
    unit = {'DAILY': 'day', 'WEEKLY': 'week', 'MONTHLY': 'month', 'YEARLY': 'year'}[freq]
    text = 'every ' + unit if interval == 1 else 'every %d %ss' % (interval, unit)
    names = {'MO': 'Monday', 'TU': 'Tuesday', 'WE': 'Wednesday', 'TH': 'Thursday', 'FR': 'Friday', 'SA': 'Saturday', 'SU': 'Sunday'}
    order = ['MO', 'TU', 'WE', 'TH', 'FR', 'SA', 'SU']
    def nth(n):
        return str(n) + {1: 'st', 2: 'nd', 3: 'rd'}.get(n, 'th')
    if ordinal:
        word = ('last' if ordinal == -1 else nth(-ordinal) + ' to last') if ordinal < 0 else nth(ordinal)
        text += ' on the ' + word + ' ' + names[byday[0]]
    elif byday:
        if set(byday) == {'MO', 'TU', 'WE', 'TH', 'FR'}:
            text += ' on weekdays'
        else:
            words = [names[d] for d in sorted(set(byday), key=order.index)]
            text += ' on ' + (words[0] if len(words) == 1 else ', '.join(words[:-1]) + ' and ' + words[-1])
    if count:
        text += ', ' + ('once' if count == 1 else '%d times' % count)
    return text
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('biweekly single day once', ['WEEKLY', 2, ['TU'], 0, 1], 'every 2 weeks on Tuesday, once'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday'),
  ('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times')],
 [('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday'),
  ('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times')],
 [('second tuesday', ['MONTHLY', 1, ['TU'], 2, 10], 'every month on the 2nd Tuesday, 10 times'),
  ('weekdays exact', ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'FR'], 0, 0], 'every week on weekdays'),
  ('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')],
 [('weekdays reordered',
   ['WEEKLY', 1, ['FR', 'MO', 'TU', 'WE', 'TH'], 0, 3],
   'every week on weekdays, 3 times'),
  ('five days with saturday',
   ['WEEKLY', 1, ['MO', 'TU', 'WE', 'TH', 'SA'], 0, 0],
   'every week on Monday, Tuesday, Wednesday, Thursday and Saturday'),
  ('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('two days unordered', ['WEEKLY', 1, ['TH', 'TU'], 0, 1], 'every week on Tuesday and Thursday, once'),
  ('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')],
 [('duplicate days', ['WEEKLY', 3, ['SU', 'MO', 'SU'], 0, 2], 'every 3 weeks on Monday and Sunday, 2 times'),
  ('daily with count one', ['DAILY', 1, [], 0, 1], 'every day, once'),
  ('yearly interval', ['YEARLY', 4, [], 0, 0], 'every 4 years'),
  ('two days unordered', ['WEEKLY', 1, ['TH', 'TU'], 0, 1], 'every week on Tuesday and Thursday, once'),
  ('regression: weekly three days',
   ['WEEKLY', 1, ['MO', 'WE', 'FR'], 0, 0],
   'every week on Monday, Wednesday and Friday'),
  ('biweekly single day once', ['WEEKLY', 2, ['TU'], 0, 1], 'every 2 weeks on Tuesday, once'),
  ('last friday monthly', ['MONTHLY', 1, ['FR'], -1, 6], 'every month on the last Friday, 6 times'),
  ('second to last sunday', ['MONTHLY', 3, ['SU'], -2, 0], 'every 3 months on the 2nd to last Sunday')]]
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 fixture	Actual	Expected	Outcome
regression: weekly three days	`every week on Monday, Wednesday and Friday`	`every week on Monday, Wednesday and Friday`	Passed
biweekly single day once	`every 2 weeks on Tuesday, once`	`every 2 weeks on Tuesday, once`	Passed
last friday monthly	`every month on the last Friday, 6 times`	`every month on the last Friday, 6 times`	Passed
second to last sunday	`every 3 months on the 2nd to last Sunday`	`every 3 months on the 2nd to last Sunday`	Passed
second tuesday	`every month on the 2nd Tuesday, 10 times`	`every month on the 2nd Tuesday, 10 times`	Passed
weekdays exact	`every week on weekdays`	`every week on weekdays`	Passed
weekdays reordered	`every week on weekdays, 3 times`	`every week on weekdays, 3 times`	Passed

SHA-256 / 7d3a5ca99c02404df8596d41dfc18685fb6e485bd708f90723cb6f31d0b24e59

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 2026-09-29T14:49:23.839486+00:00.

Case digest / afeb1ea45da2f3a1e50e328d194f72ff71cbe07a780b7fb36833d7224fa76393