FAILURE MAP
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FA-76921 / Calendar recurrence rules / Open access

Monthly BYSETPOS selection from a weekday set: position index · case 01

BYSETPOS=1 selects the second weekday of the month.

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

ROOT CAUSE

Positive positions are used as zero-based indexes.

THE FAILURE

Positive positions are used as zero-based indexes.

Unsuccessful approach: The partial repair `p - 1 if p > 0 else len(cand) + p - 1` fixes positive positions but shifts negative positions one day earlier.

Case contract

FREQ=MONTHLY;BYDAY=<weekdays>;BYSETPOS=<positions>. For each month from the dtstart month, the candidate set is every day of the whole month whose weekday is listed, in date order. Positions are 1-based, negative from the end; out-of-range positions are ignored and positions naming the same day yield it once. Selected days are emitted chronologically; days before dtstart are dropped after selection. Return the first count ISO dates.

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(dtstart, weekdays, setpos, count):
    start = dt.date.fromisoformat(dtstart)
    out = []
    for k in range(240):
        mi = start.month - 1 + k
        y, m = start.year + mi // 12, mi % 12 + 1
        dim = calendar.monthrange(y, m)[1]
        cand = [x for x in [dt.date(y, m, d) for d in range(1, dim + 1)] if x.weekday() in weekdays and x >= dt.date(y, m, 1)]
        picked = []
        for p in setpos:
            idx = p if p > 0 else len(cand) + p
            if 0 <= idx < len(cand) and cand[idx] not in picked:
                picked.append(cand[idx])
        for d in sorted(picked):
            if d < start:
                continue
            out.append(d.isoformat())
            if len(out) >= count:
                return out
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression: last weekday of month',
   ['2024-01-01', [0, 1, 2, 3, 4], [-1], 4],
   ['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30']),
  ('first weekday, dtstart mid-month',
   ['2024-01-10', [0, 1, 2, 3, 4], [1], 3],
   ['2024-02-01', '2024-03-01', '2024-04-01']),
  ('first and last in reverse order',
   ['2024-03-01', [0, 1, 2, 3, 4], [-1, 1], 4],
   ['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30']),
  ('coinciding positions',
   ['2024-01-01', [0], [1, -5], 4],
   ['2024-01-01', '2024-02-05', '2024-03-04', '2024-04-01']),
  ('three positions with repeat',
   ['2024-04-01', [2], [1, -1, -5], 5],
   ['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']),
  ('out of range negative', ['2024-01-01', [5, 6], [-7, 1], 3], ['2024-01-06', '2024-01-07', '2024-02-03']),
  ('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30'])],
 [('first and last in reverse order',
   ['2024-03-01', [0, 1, 2, 3, 4], [-1, 1], 4],
   ['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30']),
  ('coinciding positions',
   ['2024-01-01', [0], [1, -5], 4],
   ['2024-01-01', '2024-02-05', '2024-03-04', '2024-04-01']),
  ('three positions with repeat',
   ['2024-04-01', [2], [1, -1, -5], 5],
   ['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']),
  ('out of range negative', ['2024-01-01', [5, 6], [-7, 1], 3], ['2024-01-06', '2024-01-07', '2024-02-03']),
  ('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30']),
  ('second weekend day', ['2024-06-15', [5, 6], [2], 3], ['2024-07-07', '2024-08-04', '2024-09-07']),
  ('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27'])],
 [('three positions with repeat',
   ['2024-04-01', [2], [1, -1, -5], 5],
   ['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']),
  ('out of range negative', ['2024-01-01', [5, 6], [-7, 1], 3], ['2024-01-06', '2024-01-07', '2024-02-03']),
  ('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30']),
  ('second weekend day', ['2024-06-15', [5, 6], [2], 3], ['2024-07-07', '2024-08-04', '2024-09-07']),
  ('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27']),
  ('position 3 of TU,TH mid month',
   ['2024-09-12', [1, 3], [3, -1], 4],
   ['2024-09-26', '2024-10-08', '2024-10-31', '2024-11-12']),
  ('december rollover',
   ['2024-12-01', [4], [1, -1], 4],
   ['2024-12-06', '2024-12-27', '2025-01-03', '2025-01-31'])],
 [('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30']),
  ('second weekend day', ['2024-06-15', [5, 6], [2], 3], ['2024-07-07', '2024-08-04', '2024-09-07']),
  ('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27']),
  ('position 3 of TU,TH mid month',
   ['2024-09-12', [1, 3], [3, -1], 4],
   ['2024-09-26', '2024-10-08', '2024-10-31', '2024-11-12']),
  ('december rollover',
   ['2024-12-01', [4], [1, -1], 4],
   ['2024-12-06', '2024-12-27', '2025-01-03', '2025-01-31']),
  ('dtstart equals picked day', ['2024-05-31', [0, 1, 2, 3, 4], [-1], 2], ['2024-05-31', '2024-06-28']),
  ('regression: last weekday of month',
   ['2024-01-01', [0, 1, 2, 3, 4], [-1], 4],
   ['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30'])],
 [('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27']),
  ('position 3 of TU,TH mid month',
   ['2024-09-12', [1, 3], [3, -1], 4],
   ['2024-09-26', '2024-10-08', '2024-10-31', '2024-11-12']),
  ('december rollover',
   ['2024-12-01', [4], [1, -1], 4],
   ['2024-12-06', '2024-12-27', '2025-01-03', '2025-01-31']),
  ('dtstart equals picked day', ['2024-05-31', [0, 1, 2, 3, 4], [-1], 2], ['2024-05-31', '2024-06-28']),
  ('regression: last weekday of month',
   ['2024-01-01', [0, 1, 2, 3, 4], [-1], 4],
   ['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30']),
  ('first weekday, dtstart mid-month',
   ['2024-01-10', [0, 1, 2, 3, 4], [1], 3],
   ['2024-02-01', '2024-03-01', '2024-04-01']),
  ('first and last in reverse order',
   ['2024-03-01', [0, 1, 2, 3, 4], [-1, 1], 4],
   ['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30'])]]
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: last weekday of month['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30']['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30']Passed
first weekday, dtstart mid-month['2024-02-02', '2024-03-04', '2024-04-02']['2024-02-01', '2024-03-01', '2024-04-01']Failed
first and last in reverse order['2024-03-04', '2024-03-29', '2024-04-02', '2024-04-30']['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30']Failed
coinciding positions['2024-01-01', '2024-01-08', '2024-02-12', '2024-03-11']['2024-01-01', '2024-02-05', '2024-03-04', '2024-04-01']Failed
three positions with repeat['2024-04-10', '2024-04-24', '2024-05-01', '2024-05-08', '2024-05-29']['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']Failed
out of range negative['2024-01-07', '2024-02-04', '2024-03-03']['2024-01-06', '2024-01-07', '2024-02-03']Failed
out of range positive[]['2024-04-29', '2024-07-29', '2024-09-30']Failed

SHA-256 / 104534bce370a7183ce24e8b24bcb970950091b9d4a6e477af89120bca266aaa

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(dtstart, weekdays, setpos, count):
    start = dt.date.fromisoformat(dtstart)
    out = []
    for k in range(240):
        mi = start.month - 1 + k
        y, m = start.year + mi // 12, mi % 12 + 1
        dim = calendar.monthrange(y, m)[1]
        cand = [x for x in [dt.date(y, m, d) for d in range(1, dim + 1)] if x.weekday() in weekdays and x >= dt.date(y, m, 1)]
        picked = []
        for p in setpos:
            idx = p - 1 if p > 0 else len(cand) + p - 1
            if 0 <= idx < len(cand) and cand[idx] not in picked:
                picked.append(cand[idx])
        for d in sorted(picked):
            if d < start:
                continue
            out.append(d.isoformat())
            if len(out) >= count:
                return out
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression: last weekday of month',
   ['2024-01-01', [0, 1, 2, 3, 4], [-1], 4],
   ['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30']),
  ('first weekday, dtstart mid-month',
   ['2024-01-10', [0, 1, 2, 3, 4], [1], 3],
   ['2024-02-01', '2024-03-01', '2024-04-01']),
  ('first and last in reverse order',
   ['2024-03-01', [0, 1, 2, 3, 4], [-1, 1], 4],
   ['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30']),
  ('coinciding positions',
   ['2024-01-01', [0], [1, -5], 4],
   ['2024-01-01', '2024-02-05', '2024-03-04', '2024-04-01']),
  ('three positions with repeat',
   ['2024-04-01', [2], [1, -1, -5], 5],
   ['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']),
  ('out of range negative', ['2024-01-01', [5, 6], [-7, 1], 3], ['2024-01-06', '2024-01-07', '2024-02-03']),
  ('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30'])],
 [('first and last in reverse order',
   ['2024-03-01', [0, 1, 2, 3, 4], [-1, 1], 4],
   ['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30']),
  ('coinciding positions',
   ['2024-01-01', [0], [1, -5], 4],
   ['2024-01-01', '2024-02-05', '2024-03-04', '2024-04-01']),
  ('three positions with repeat',
   ['2024-04-01', [2], [1, -1, -5], 5],
   ['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']),
  ('out of range negative', ['2024-01-01', [5, 6], [-7, 1], 3], ['2024-01-06', '2024-01-07', '2024-02-03']),
  ('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30']),
  ('second weekend day', ['2024-06-15', [5, 6], [2], 3], ['2024-07-07', '2024-08-04', '2024-09-07']),
  ('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27'])],
 [('three positions with repeat',
   ['2024-04-01', [2], [1, -1, -5], 5],
   ['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']),
  ('out of range negative', ['2024-01-01', [5, 6], [-7, 1], 3], ['2024-01-06', '2024-01-07', '2024-02-03']),
  ('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30']),
  ('second weekend day', ['2024-06-15', [5, 6], [2], 3], ['2024-07-07', '2024-08-04', '2024-09-07']),
  ('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27']),
  ('position 3 of TU,TH mid month',
   ['2024-09-12', [1, 3], [3, -1], 4],
   ['2024-09-26', '2024-10-08', '2024-10-31', '2024-11-12']),
  ('december rollover',
   ['2024-12-01', [4], [1, -1], 4],
   ['2024-12-06', '2024-12-27', '2025-01-03', '2025-01-31'])],
 [('out of range positive', ['2024-02-01', [0], [5], 3], ['2024-04-29', '2024-07-29', '2024-09-30']),
  ('second weekend day', ['2024-06-15', [5, 6], [2], 3], ['2024-07-07', '2024-08-04', '2024-09-07']),
  ('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27']),
  ('position 3 of TU,TH mid month',
   ['2024-09-12', [1, 3], [3, -1], 4],
   ['2024-09-26', '2024-10-08', '2024-10-31', '2024-11-12']),
  ('december rollover',
   ['2024-12-01', [4], [1, -1], 4],
   ['2024-12-06', '2024-12-27', '2025-01-03', '2025-01-31']),
  ('dtstart equals picked day', ['2024-05-31', [0, 1, 2, 3, 4], [-1], 2], ['2024-05-31', '2024-06-28']),
  ('regression: last weekday of month',
   ['2024-01-01', [0, 1, 2, 3, 4], [-1], 4],
   ['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30'])],
 [('second to last weekday',
   ['2024-07-01', [0, 1, 2, 3, 4], [-2], 3],
   ['2024-07-30', '2024-08-29', '2024-09-27']),
  ('position 3 of TU,TH mid month',
   ['2024-09-12', [1, 3], [3, -1], 4],
   ['2024-09-26', '2024-10-08', '2024-10-31', '2024-11-12']),
  ('december rollover',
   ['2024-12-01', [4], [1, -1], 4],
   ['2024-12-06', '2024-12-27', '2025-01-03', '2025-01-31']),
  ('dtstart equals picked day', ['2024-05-31', [0, 1, 2, 3, 4], [-1], 2], ['2024-05-31', '2024-06-28']),
  ('regression: last weekday of month',
   ['2024-01-01', [0, 1, 2, 3, 4], [-1], 4],
   ['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30']),
  ('first weekday, dtstart mid-month',
   ['2024-01-10', [0, 1, 2, 3, 4], [1], 3],
   ['2024-02-01', '2024-03-01', '2024-04-01']),
  ('first and last in reverse order',
   ['2024-03-01', [0, 1, 2, 3, 4], [-1, 1], 4],
   ['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30'])]]
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: last weekday of month['2024-01-30', '2024-02-28', '2024-03-28', '2024-04-29']['2024-01-31', '2024-02-29', '2024-03-29', '2024-04-30']Failed
first weekday, dtstart mid-month['2024-02-01', '2024-03-01', '2024-04-01']['2024-02-01', '2024-03-01', '2024-04-01']Passed
first and last in reverse order['2024-03-01', '2024-03-28', '2024-04-01', '2024-04-29']['2024-03-01', '2024-03-29', '2024-04-01', '2024-04-30']Failed
coinciding positions['2024-01-01', '2024-02-05', '2024-03-04', '2024-04-01']['2024-01-01', '2024-02-05', '2024-03-04', '2024-04-01']Passed
three positions with repeat['2024-04-03', '2024-04-17', '2024-05-01', '2024-05-22', '2024-06-05']['2024-04-03', '2024-04-24', '2024-05-01', '2024-05-29', '2024-06-05']Failed
out of range negative['2024-01-06', '2024-02-03', '2024-03-02']['2024-01-06', '2024-01-07', '2024-02-03']Failed
out of range positive['2024-04-29', '2024-07-29', '2024-09-30']['2024-04-29', '2024-07-29', '2024-09-30']Passed

SHA-256 / c1fb9e3dec33d69fa4745bc5a40534354cc4c1d5466d0a84467bad9b7e4a956e

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.

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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 2026-09-29T14:49:20.635182+00:00.

Case digest / cd73eadb06ca682854f5f6f3cbc98457fdc8da106a715ed64dc90749c2f23c30