FA-59381 / Subscription proration billing / Open access

Month-end anchored renewal dates: anchor day drift · case 01

After a short month, renewals stay on the 28th or 30th for the rest of the subscription.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Each renewal clamps the previous renewal's day instead of the original anchor day.

VERIFIED REPAIR

Restore the contract rule at the anchor day drift step: use `out.append([ny, nm, min(d, dim)])`.

Unsuccessful approach: The attempt snaps every late-month anchor to month end, moving a 29th or 30th anchor to the 31st.

Case contract

Input {anchor [y,m,d], n, interval months}. Renewal i (1..n) falls interval*i months after the anchor month, on the anchor day clamped to that month's length (so a 31st anchor returns to the 31st whenever possible). Return the list of [y, m, d].

Why this case matters

Month-end anchors must not drift to the 28th after February, and leap years change the clamp.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
import calendar
N = 1
observations = []
def solve(x):
    y, m, d = x['anchor']
    out = []
    for i in range(1, x['n'] + 1):
        total = (m - 1) + i * x['interval']
        ny, nm = y + total // 12, total % 12 + 1
        dim = calendar.monthrange(ny, nm)[1]
        out.append([ny, nm, min(out[-1][2] if out else d, dim)])
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('regression', {'anchor': [2025, 5, 31], 'n': 10, 'interval': 2}, [[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]), ('partial-repair probe', {'anchor': [2024, 7, 30], 'n': 7, 'interval': 3}, [[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]), ('partial-repair probe', {'anchor': [2019, 5, 29], 'n': 11, 'interval': 2}, [[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]), ('normal control', {'anchor': [2023, 3, 28], 'n': 14, 'interval': 3}, [[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]), ('normal control', {'anchor': [2025, 8, 28], 'n': 1, 'interval': 1}, [[2025, 9, 28]]), ('normal control', {'anchor': [2025, 4, 28], 'n': 8, 'interval': 12}, [[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]), ('normal control', {'anchor': [2025, 3, 31], 'n': 7, 'interval': 12}, [[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]])], [('regression', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('regression', {'anchor': [2023, 12, 31], 'n': 7, 'interval': 1}, [[2024, 1, 31], [2024, 2, 29], [2024, 3, 31], [2024, 4, 30], [2024, 5, 31], [2024, 6, 30], [2024, 7, 31]]), ('partial-repair probe', {'anchor': [2024, 2, 29], 'n': 6, 'interval': 2}, [[2024, 4, 29], [2024, 6, 29], [2024, 8, 29], [2024, 10, 29], [2024, 12, 29], [2025, 2, 28]]), ('partial-repair probe', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('normal control', {'anchor': [2025, 5, 16], 'n': 3, 'interval': 12}, [[2026, 5, 16], [2027, 5, 16], [2028, 5, 16]]), ('normal control', {'anchor': [2023, 3, 28], 'n': 12, 'interval': 1}, [[2023, 4, 28], [2023, 5, 28], [2023, 6, 28], [2023, 7, 28], [2023, 8, 28], [2023, 9, 28], [2023, 10, 28], [2023, 11, 28], [2023, 12, 28], [2024, 1, 28], [2024, 2, 28], [2024, 3, 28]]), ('normal control', {'anchor': [2023, 11, 17], 'n': 7, 'interval': 1}, [[2023, 12, 17], [2024, 1, 17], [2024, 2, 17], [2024, 3, 17], [2024, 4, 17], [2024, 5, 17], [2024, 6, 17]]), ('normal control', {'anchor': [2025, 4, 28], 'n': 7, 'interval': 1}, [[2025, 5, 28], [2025, 6, 28], [2025, 7, 28], [2025, 8, 28], [2025, 9, 28], [2025, 10, 28], [2025, 11, 28]])], [('regression', {'anchor': [2019, 1, 30], 'n': 2, 'interval': 1}, [[2019, 2, 28], [2019, 3, 30]]), ('regression', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('partial-repair probe', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('partial-repair probe', {'anchor': [2019, 12, 29], 'n': 7, 'interval': 1}, [[2020, 1, 29], [2020, 2, 29], [2020, 3, 29], [2020, 4, 29], [2020, 5, 29], [2020, 6, 29], [2020, 7, 29]]), ('normal control', {'anchor': [2019, 7, 22], 'n': 2, 'interval': 2}, [[2019, 9, 22], [2019, 11, 22]]), ('normal control', {'anchor': [2023, 8, 28], 'n': 7, 'interval': 2}, [[2023, 10, 28], [2023, 12, 28], [2024, 2, 28], [2024, 4, 28], [2024, 6, 28], [2024, 8, 28], [2024, 10, 28]]), ('normal control', {'anchor': [2025, 10, 10], 'n': 8, 'interval': 3}, [[2026, 1, 10], [2026, 4, 10], [2026, 7, 10], [2026, 10, 10], [2027, 1, 10], [2027, 4, 10], [2027, 7, 10], [2027, 10, 10]]), ('normal control', {'anchor': [2023, 8, 24], 'n': 9, 'interval': 1}, [[2023, 9, 24], [2023, 10, 24], [2023, 11, 24], [2023, 12, 24], [2024, 1, 24], [2024, 2, 24], [2024, 3, 24], [2024, 4, 24], [2024, 5, 24]])], [('regression', {'anchor': [2025, 9, 30], 'n': 9, 'interval': 1}, [[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 30], [2026, 4, 30], [2026, 5, 30], [2026, 6, 30]]), ('regression', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('partial-repair probe', {'anchor': [2019, 1, 30], 'n': 2, 'interval': 1}, [[2019, 2, 28], [2019, 3, 30]]), ('partial-repair probe', {'anchor': [2019, 10, 29], 'n': 10, 'interval': 1}, [[2019, 11, 29], [2019, 12, 29], [2020, 1, 29], [2020, 2, 29], [2020, 3, 29], [2020, 4, 29], [2020, 5, 29], [2020, 6, 29], [2020, 7, 29], [2020, 8, 29]]), ('normal control', {'anchor': [2024, 10, 28], 'n': 10, 'interval': 12}, [[2025, 10, 28], [2026, 10, 28], [2027, 10, 28], [2028, 10, 28], [2029, 10, 28], [2030, 10, 28], [2031, 10, 28], [2032, 10, 28], [2033, 10, 28], [2034, 10, 28]]), ('normal control', {'anchor': [2025, 8, 31], 'n': 3, 'interval': 2}, [[2025, 10, 31], [2025, 12, 31], [2026, 2, 28]]), ('normal control', {'anchor': [2025, 1, 31], 'n': 12, 'interval': 12}, [[2026, 1, 31], [2027, 1, 31], [2028, 1, 31], [2029, 1, 31], [2030, 1, 31], [2031, 1, 31], [2032, 1, 31], [2033, 1, 31], [2034, 1, 31], [2035, 1, 31], [2036, 1, 31], [2037, 1, 31]]), ('normal control', {'anchor': [2024, 5, 28], 'n': 10, 'interval': 1}, [[2024, 6, 28], [2024, 7, 28], [2024, 8, 28], [2024, 9, 28], [2024, 10, 28], [2024, 11, 28], [2024, 12, 28], [2025, 1, 28], [2025, 2, 28], [2025, 3, 28]])], [('regression', {'anchor': [2024, 7, 30], 'n': 9, 'interval': 1}, [[2024, 8, 30], [2024, 9, 30], [2024, 10, 30], [2024, 11, 30], [2024, 12, 30], [2025, 1, 30], [2025, 2, 28], [2025, 3, 30], [2025, 4, 30]]), ('regression', {'anchor': [2025, 1, 31], 'n': 9, 'interval': 1}, [[2025, 2, 28], [2025, 3, 31], [2025, 4, 30], [2025, 5, 31], [2025, 6, 30], [2025, 7, 31], [2025, 8, 31], [2025, 9, 30], [2025, 10, 31]]), ('partial-repair probe', {'anchor': [2019, 9, 29], 'n': 6, 'interval': 2}, [[2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29]]), ('partial-repair probe', {'anchor': [2025, 9, 30], 'n': 9, 'interval': 1}, [[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 30], [2026, 4, 30], [2026, 5, 30], [2026, 6, 30]]), ('normal control', {'anchor': [2023, 11, 28], 'n': 14, 'interval': 12}, [[2024, 11, 28], [2025, 11, 28], [2026, 11, 28], [2027, 11, 28], [2028, 11, 28], [2029, 11, 28], [2030, 11, 28], [2031, 11, 28], [2032, 11, 28], [2033, 11, 28], [2034, 11, 28], [2035, 11, 28], [2036, 11, 28], [2037, 11, 28]]), ('normal control', {'anchor': [2025, 7, 31], 'n': 8, 'interval': 6}, [[2026, 1, 31], [2026, 7, 31], [2027, 1, 31], [2027, 7, 31], [2028, 1, 31], [2028, 7, 31], [2029, 1, 31], [2029, 7, 31]]), ('normal control', {'anchor': [2023, 2, 19], 'n': 7, 'interval': 3}, [[2023, 5, 19], [2023, 8, 19], [2023, 11, 19], [2024, 2, 19], [2024, 5, 19], [2024, 8, 19], [2024, 11, 19]]), ('normal control', {'anchor': [2019, 8, 15], 'n': 8, 'interval': 6}, [[2020, 2, 15], [2020, 8, 15], [2021, 2, 15], [2021, 8, 15], [2022, 2, 15], [2022, 8, 15], [2023, 2, 15], [2023, 8, 15]])]]
for i, (label, args, expected) in enumerate(fixtures[N-1]):
    check("%s %d" % (label, i), 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 0	`[[2025, 2, 28], [2025, 3, 28], [2025, 4, 28], [2025, 5, 28], [2025, 6, 28], [2025, 7, 28], [2025, 8, 28], [2025, 9, 28]]`	`[[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]`	Failed
regression 1	`[[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 30], [2026, 3, 30], [2026, 5, 30], [2026, 7, 30], [2026, 9, 30], [2026, 11, 30], [2027, 1, 30]]`	`[[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]`	Failed
partial-repair probe 2	`[[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]`	`[[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]`	Passed
partial-repair probe 3	`[[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]`	`[[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]`	Passed
normal control 4	`[[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]`	`[[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]`	Passed
normal control 5	`[[2025, 9, 28]]`	`[[2025, 9, 28]]`	Passed
normal control 6	`[[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]`	`[[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]`	Passed
normal control 7	`[[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]]`	`[[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]]`	Passed

SHA-256 / f602ddcae9abbd1bc08756db64e81dc170fd35d93405ee073e81e852b1e87b3d

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
import calendar
N = 1
observations = []
def solve(x):
    y, m, d = x['anchor']
    out = []
    for i in range(1, x['n'] + 1):
        total = (m - 1) + i * x['interval']
        ny, nm = y + total // 12, total % 12 + 1
        dim = calendar.monthrange(ny, nm)[1]
        out.append([ny, nm, min(d, dim) if d <= 28 else dim])
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('regression', {'anchor': [2025, 5, 31], 'n': 10, 'interval': 2}, [[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]), ('partial-repair probe', {'anchor': [2024, 7, 30], 'n': 7, 'interval': 3}, [[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]), ('partial-repair probe', {'anchor': [2019, 5, 29], 'n': 11, 'interval': 2}, [[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]), ('normal control', {'anchor': [2023, 3, 28], 'n': 14, 'interval': 3}, [[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]), ('normal control', {'anchor': [2025, 8, 28], 'n': 1, 'interval': 1}, [[2025, 9, 28]]), ('normal control', {'anchor': [2025, 4, 28], 'n': 8, 'interval': 12}, [[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]), ('normal control', {'anchor': [2025, 3, 31], 'n': 7, 'interval': 12}, [[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]])], [('regression', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('regression', {'anchor': [2023, 12, 31], 'n': 7, 'interval': 1}, [[2024, 1, 31], [2024, 2, 29], [2024, 3, 31], [2024, 4, 30], [2024, 5, 31], [2024, 6, 30], [2024, 7, 31]]), ('partial-repair probe', {'anchor': [2024, 2, 29], 'n': 6, 'interval': 2}, [[2024, 4, 29], [2024, 6, 29], [2024, 8, 29], [2024, 10, 29], [2024, 12, 29], [2025, 2, 28]]), ('partial-repair probe', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('normal control', {'anchor': [2025, 5, 16], 'n': 3, 'interval': 12}, [[2026, 5, 16], [2027, 5, 16], [2028, 5, 16]]), ('normal control', {'anchor': [2023, 3, 28], 'n': 12, 'interval': 1}, [[2023, 4, 28], [2023, 5, 28], [2023, 6, 28], [2023, 7, 28], [2023, 8, 28], [2023, 9, 28], [2023, 10, 28], [2023, 11, 28], [2023, 12, 28], [2024, 1, 28], [2024, 2, 28], [2024, 3, 28]]), ('normal control', {'anchor': [2023, 11, 17], 'n': 7, 'interval': 1}, [[2023, 12, 17], [2024, 1, 17], [2024, 2, 17], [2024, 3, 17], [2024, 4, 17], [2024, 5, 17], [2024, 6, 17]]), ('normal control', {'anchor': [2025, 4, 28], 'n': 7, 'interval': 1}, [[2025, 5, 28], [2025, 6, 28], [2025, 7, 28], [2025, 8, 28], [2025, 9, 28], [2025, 10, 28], [2025, 11, 28]])], [('regression', {'anchor': [2019, 1, 30], 'n': 2, 'interval': 1}, [[2019, 2, 28], [2019, 3, 30]]), ('regression', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('partial-repair probe', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('partial-repair probe', {'anchor': [2019, 12, 29], 'n': 7, 'interval': 1}, [[2020, 1, 29], [2020, 2, 29], [2020, 3, 29], [2020, 4, 29], [2020, 5, 29], [2020, 6, 29], [2020, 7, 29]]), ('normal control', {'anchor': [2019, 7, 22], 'n': 2, 'interval': 2}, [[2019, 9, 22], [2019, 11, 22]]), ('normal control', {'anchor': [2023, 8, 28], 'n': 7, 'interval': 2}, [[2023, 10, 28], [2023, 12, 28], [2024, 2, 28], [2024, 4, 28], [2024, 6, 28], [2024, 8, 28], [2024, 10, 28]]), ('normal control', {'anchor': [2025, 10, 10], 'n': 8, 'interval': 3}, [[2026, 1, 10], [2026, 4, 10], [2026, 7, 10], [2026, 10, 10], [2027, 1, 10], [2027, 4, 10], [2027, 7, 10], [2027, 10, 10]]), ('normal control', {'anchor': [2023, 8, 24], 'n': 9, 'interval': 1}, [[2023, 9, 24], [2023, 10, 24], [2023, 11, 24], [2023, 12, 24], [2024, 1, 24], [2024, 2, 24], [2024, 3, 24], [2024, 4, 24], [2024, 5, 24]])], [('regression', {'anchor': [2025, 9, 30], 'n': 9, 'interval': 1}, [[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 30], [2026, 4, 30], [2026, 5, 30], [2026, 6, 30]]), ('regression', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('partial-repair probe', {'anchor': [2019, 1, 30], 'n': 2, 'interval': 1}, [[2019, 2, 28], [2019, 3, 30]]), ('partial-repair probe', {'anchor': [2019, 10, 29], 'n': 10, 'interval': 1}, [[2019, 11, 29], [2019, 12, 29], [2020, 1, 29], [2020, 2, 29], [2020, 3, 29], [2020, 4, 29], [2020, 5, 29], [2020, 6, 29], [2020, 7, 29], [2020, 8, 29]]), ('normal control', {'anchor': [2024, 10, 28], 'n': 10, 'interval': 12}, [[2025, 10, 28], [2026, 10, 28], [2027, 10, 28], [2028, 10, 28], [2029, 10, 28], [2030, 10, 28], [2031, 10, 28], [2032, 10, 28], [2033, 10, 28], [2034, 10, 28]]), ('normal control', {'anchor': [2025, 8, 31], 'n': 3, 'interval': 2}, [[2025, 10, 31], [2025, 12, 31], [2026, 2, 28]]), ('normal control', {'anchor': [2025, 1, 31], 'n': 12, 'interval': 12}, [[2026, 1, 31], [2027, 1, 31], [2028, 1, 31], [2029, 1, 31], [2030, 1, 31], [2031, 1, 31], [2032, 1, 31], [2033, 1, 31], [2034, 1, 31], [2035, 1, 31], [2036, 1, 31], [2037, 1, 31]]), ('normal control', {'anchor': [2024, 5, 28], 'n': 10, 'interval': 1}, [[2024, 6, 28], [2024, 7, 28], [2024, 8, 28], [2024, 9, 28], [2024, 10, 28], [2024, 11, 28], [2024, 12, 28], [2025, 1, 28], [2025, 2, 28], [2025, 3, 28]])], [('regression', {'anchor': [2024, 7, 30], 'n': 9, 'interval': 1}, [[2024, 8, 30], [2024, 9, 30], [2024, 10, 30], [2024, 11, 30], [2024, 12, 30], [2025, 1, 30], [2025, 2, 28], [2025, 3, 30], [2025, 4, 30]]), ('regression', {'anchor': [2025, 1, 31], 'n': 9, 'interval': 1}, [[2025, 2, 28], [2025, 3, 31], [2025, 4, 30], [2025, 5, 31], [2025, 6, 30], [2025, 7, 31], [2025, 8, 31], [2025, 9, 30], [2025, 10, 31]]), ('partial-repair probe', {'anchor': [2019, 9, 29], 'n': 6, 'interval': 2}, [[2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29]]), ('partial-repair probe', {'anchor': [2025, 9, 30], 'n': 9, 'interval': 1}, [[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 30], [2026, 4, 30], [2026, 5, 30], [2026, 6, 30]]), ('normal control', {'anchor': [2023, 11, 28], 'n': 14, 'interval': 12}, [[2024, 11, 28], [2025, 11, 28], [2026, 11, 28], [2027, 11, 28], [2028, 11, 28], [2029, 11, 28], [2030, 11, 28], [2031, 11, 28], [2032, 11, 28], [2033, 11, 28], [2034, 11, 28], [2035, 11, 28], [2036, 11, 28], [2037, 11, 28]]), ('normal control', {'anchor': [2025, 7, 31], 'n': 8, 'interval': 6}, [[2026, 1, 31], [2026, 7, 31], [2027, 1, 31], [2027, 7, 31], [2028, 1, 31], [2028, 7, 31], [2029, 1, 31], [2029, 7, 31]]), ('normal control', {'anchor': [2023, 2, 19], 'n': 7, 'interval': 3}, [[2023, 5, 19], [2023, 8, 19], [2023, 11, 19], [2024, 2, 19], [2024, 5, 19], [2024, 8, 19], [2024, 11, 19]]), ('normal control', {'anchor': [2019, 8, 15], 'n': 8, 'interval': 6}, [[2020, 2, 15], [2020, 8, 15], [2021, 2, 15], [2021, 8, 15], [2022, 2, 15], [2022, 8, 15], [2023, 2, 15], [2023, 8, 15]])]]
for i, (label, args, expected) in enumerate(fixtures[N-1]):
    check("%s %d" % (label, i), 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 0	`[[2025, 2, 28], [2025, 3, 31], [2025, 4, 30], [2025, 5, 31], [2025, 6, 30], [2025, 7, 31], [2025, 8, 31], [2025, 9, 30]]`	`[[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]`	Failed
regression 1	`[[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]`	`[[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]`	Passed
partial-repair probe 2	`[[2024, 10, 31], [2025, 1, 31], [2025, 4, 30], [2025, 7, 31], [2025, 10, 31], [2026, 1, 31], [2026, 4, 30]]`	`[[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]`	Failed
partial-repair probe 3	`[[2019, 7, 31], [2019, 9, 30], [2019, 11, 30], [2020, 1, 31], [2020, 3, 31], [2020, 5, 31], [2020, 7, 31], [2020, 9, 30], [2020, 11, 30], [2021, 1, 31], [2021, 3, 31]]`	`[[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]`	Failed
normal control 4	`[[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]`	`[[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]`	Passed
normal control 5	`[[2025, 9, 28]]`	`[[2025, 9, 28]]`	Passed
normal control 6	`[[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]`	`[[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]`	Passed
normal control 7	`[[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]]`	`[[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]]`	Passed

SHA-256 / 9397c9a3483fc50f83b7f8db2a0533bae49837051d010e43883a0ec7e270001e

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
import calendar
N = 1
observations = []
def solve(x):
    y, m, d = x['anchor']
    out = []
    for i in range(1, x['n'] + 1):
        total = (m - 1) + i * x['interval']
        ny, nm = y + total // 12, total % 12 + 1
        dim = calendar.monthrange(ny, nm)[1]
        out.append([ny, nm, min(d, dim)])
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('regression', {'anchor': [2025, 5, 31], 'n': 10, 'interval': 2}, [[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]), ('partial-repair probe', {'anchor': [2024, 7, 30], 'n': 7, 'interval': 3}, [[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]), ('partial-repair probe', {'anchor': [2019, 5, 29], 'n': 11, 'interval': 2}, [[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]), ('normal control', {'anchor': [2023, 3, 28], 'n': 14, 'interval': 3}, [[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]), ('normal control', {'anchor': [2025, 8, 28], 'n': 1, 'interval': 1}, [[2025, 9, 28]]), ('normal control', {'anchor': [2025, 4, 28], 'n': 8, 'interval': 12}, [[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]), ('normal control', {'anchor': [2025, 3, 31], 'n': 7, 'interval': 12}, [[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]])], [('regression', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('regression', {'anchor': [2023, 12, 31], 'n': 7, 'interval': 1}, [[2024, 1, 31], [2024, 2, 29], [2024, 3, 31], [2024, 4, 30], [2024, 5, 31], [2024, 6, 30], [2024, 7, 31]]), ('partial-repair probe', {'anchor': [2024, 2, 29], 'n': 6, 'interval': 2}, [[2024, 4, 29], [2024, 6, 29], [2024, 8, 29], [2024, 10, 29], [2024, 12, 29], [2025, 2, 28]]), ('partial-repair probe', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('normal control', {'anchor': [2025, 5, 16], 'n': 3, 'interval': 12}, [[2026, 5, 16], [2027, 5, 16], [2028, 5, 16]]), ('normal control', {'anchor': [2023, 3, 28], 'n': 12, 'interval': 1}, [[2023, 4, 28], [2023, 5, 28], [2023, 6, 28], [2023, 7, 28], [2023, 8, 28], [2023, 9, 28], [2023, 10, 28], [2023, 11, 28], [2023, 12, 28], [2024, 1, 28], [2024, 2, 28], [2024, 3, 28]]), ('normal control', {'anchor': [2023, 11, 17], 'n': 7, 'interval': 1}, [[2023, 12, 17], [2024, 1, 17], [2024, 2, 17], [2024, 3, 17], [2024, 4, 17], [2024, 5, 17], [2024, 6, 17]]), ('normal control', {'anchor': [2025, 4, 28], 'n': 7, 'interval': 1}, [[2025, 5, 28], [2025, 6, 28], [2025, 7, 28], [2025, 8, 28], [2025, 9, 28], [2025, 10, 28], [2025, 11, 28]])], [('regression', {'anchor': [2019, 1, 30], 'n': 2, 'interval': 1}, [[2019, 2, 28], [2019, 3, 30]]), ('regression', {'anchor': [2025, 1, 29], 'n': 8, 'interval': 1}, [[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]), ('partial-repair probe', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('partial-repair probe', {'anchor': [2019, 12, 29], 'n': 7, 'interval': 1}, [[2020, 1, 29], [2020, 2, 29], [2020, 3, 29], [2020, 4, 29], [2020, 5, 29], [2020, 6, 29], [2020, 7, 29]]), ('normal control', {'anchor': [2019, 7, 22], 'n': 2, 'interval': 2}, [[2019, 9, 22], [2019, 11, 22]]), ('normal control', {'anchor': [2023, 8, 28], 'n': 7, 'interval': 2}, [[2023, 10, 28], [2023, 12, 28], [2024, 2, 28], [2024, 4, 28], [2024, 6, 28], [2024, 8, 28], [2024, 10, 28]]), ('normal control', {'anchor': [2025, 10, 10], 'n': 8, 'interval': 3}, [[2026, 1, 10], [2026, 4, 10], [2026, 7, 10], [2026, 10, 10], [2027, 1, 10], [2027, 4, 10], [2027, 7, 10], [2027, 10, 10]]), ('normal control', {'anchor': [2023, 8, 24], 'n': 9, 'interval': 1}, [[2023, 9, 24], [2023, 10, 24], [2023, 11, 24], [2023, 12, 24], [2024, 1, 24], [2024, 2, 24], [2024, 3, 24], [2024, 4, 24], [2024, 5, 24]])], [('regression', {'anchor': [2025, 9, 30], 'n': 9, 'interval': 1}, [[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 30], [2026, 4, 30], [2026, 5, 30], [2026, 6, 30]]), ('regression', {'anchor': [2024, 12, 29], 'n': 10, 'interval': 1}, [[2025, 1, 29], [2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29], [2025, 10, 29]]), ('partial-repair probe', {'anchor': [2019, 1, 30], 'n': 2, 'interval': 1}, [[2019, 2, 28], [2019, 3, 30]]), ('partial-repair probe', {'anchor': [2019, 10, 29], 'n': 10, 'interval': 1}, [[2019, 11, 29], [2019, 12, 29], [2020, 1, 29], [2020, 2, 29], [2020, 3, 29], [2020, 4, 29], [2020, 5, 29], [2020, 6, 29], [2020, 7, 29], [2020, 8, 29]]), ('normal control', {'anchor': [2024, 10, 28], 'n': 10, 'interval': 12}, [[2025, 10, 28], [2026, 10, 28], [2027, 10, 28], [2028, 10, 28], [2029, 10, 28], [2030, 10, 28], [2031, 10, 28], [2032, 10, 28], [2033, 10, 28], [2034, 10, 28]]), ('normal control', {'anchor': [2025, 8, 31], 'n': 3, 'interval': 2}, [[2025, 10, 31], [2025, 12, 31], [2026, 2, 28]]), ('normal control', {'anchor': [2025, 1, 31], 'n': 12, 'interval': 12}, [[2026, 1, 31], [2027, 1, 31], [2028, 1, 31], [2029, 1, 31], [2030, 1, 31], [2031, 1, 31], [2032, 1, 31], [2033, 1, 31], [2034, 1, 31], [2035, 1, 31], [2036, 1, 31], [2037, 1, 31]]), ('normal control', {'anchor': [2024, 5, 28], 'n': 10, 'interval': 1}, [[2024, 6, 28], [2024, 7, 28], [2024, 8, 28], [2024, 9, 28], [2024, 10, 28], [2024, 11, 28], [2024, 12, 28], [2025, 1, 28], [2025, 2, 28], [2025, 3, 28]])], [('regression', {'anchor': [2024, 7, 30], 'n': 9, 'interval': 1}, [[2024, 8, 30], [2024, 9, 30], [2024, 10, 30], [2024, 11, 30], [2024, 12, 30], [2025, 1, 30], [2025, 2, 28], [2025, 3, 30], [2025, 4, 30]]), ('regression', {'anchor': [2025, 1, 31], 'n': 9, 'interval': 1}, [[2025, 2, 28], [2025, 3, 31], [2025, 4, 30], [2025, 5, 31], [2025, 6, 30], [2025, 7, 31], [2025, 8, 31], [2025, 9, 30], [2025, 10, 31]]), ('partial-repair probe', {'anchor': [2019, 9, 29], 'n': 6, 'interval': 2}, [[2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29]]), ('partial-repair probe', {'anchor': [2025, 9, 30], 'n': 9, 'interval': 1}, [[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 30], [2026, 4, 30], [2026, 5, 30], [2026, 6, 30]]), ('normal control', {'anchor': [2023, 11, 28], 'n': 14, 'interval': 12}, [[2024, 11, 28], [2025, 11, 28], [2026, 11, 28], [2027, 11, 28], [2028, 11, 28], [2029, 11, 28], [2030, 11, 28], [2031, 11, 28], [2032, 11, 28], [2033, 11, 28], [2034, 11, 28], [2035, 11, 28], [2036, 11, 28], [2037, 11, 28]]), ('normal control', {'anchor': [2025, 7, 31], 'n': 8, 'interval': 6}, [[2026, 1, 31], [2026, 7, 31], [2027, 1, 31], [2027, 7, 31], [2028, 1, 31], [2028, 7, 31], [2029, 1, 31], [2029, 7, 31]]), ('normal control', {'anchor': [2023, 2, 19], 'n': 7, 'interval': 3}, [[2023, 5, 19], [2023, 8, 19], [2023, 11, 19], [2024, 2, 19], [2024, 5, 19], [2024, 8, 19], [2024, 11, 19]]), ('normal control', {'anchor': [2019, 8, 15], 'n': 8, 'interval': 6}, [[2020, 2, 15], [2020, 8, 15], [2021, 2, 15], [2021, 8, 15], [2022, 2, 15], [2022, 8, 15], [2023, 2, 15], [2023, 8, 15]])]]
for i, (label, args, expected) in enumerate(fixtures[N-1]):
    check("%s %d" % (label, i), 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 0	`[[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]`	`[[2025, 2, 28], [2025, 3, 29], [2025, 4, 29], [2025, 5, 29], [2025, 6, 29], [2025, 7, 29], [2025, 8, 29], [2025, 9, 29]]`	Passed
regression 1	`[[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]`	`[[2025, 7, 31], [2025, 9, 30], [2025, 11, 30], [2026, 1, 31], [2026, 3, 31], [2026, 5, 31], [2026, 7, 31], [2026, 9, 30], [2026, 11, 30], [2027, 1, 31]]`	Passed
partial-repair probe 2	`[[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]`	`[[2024, 10, 30], [2025, 1, 30], [2025, 4, 30], [2025, 7, 30], [2025, 10, 30], [2026, 1, 30], [2026, 4, 30]]`	Passed
partial-repair probe 3	`[[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]`	`[[2019, 7, 29], [2019, 9, 29], [2019, 11, 29], [2020, 1, 29], [2020, 3, 29], [2020, 5, 29], [2020, 7, 29], [2020, 9, 29], [2020, 11, 29], [2021, 1, 29], [2021, 3, 29]]`	Passed
normal control 4	`[[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]`	`[[2023, 6, 28], [2023, 9, 28], [2023, 12, 28], [2024, 3, 28], [2024, 6, 28], [2024, 9, 28], [2024, 12, 28], [2025, 3, 28], [2025, 6, 28], [2025, 9, 28], [2025, 12, 28], [2026, 3, 28], [2026, 6, 28], [2026, 9, 28]]`	Passed
normal control 5	`[[2025, 9, 28]]`	`[[2025, 9, 28]]`	Passed
normal control 6	`[[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]`	`[[2026, 4, 28], [2027, 4, 28], [2028, 4, 28], [2029, 4, 28], [2030, 4, 28], [2031, 4, 28], [2032, 4, 28], [2033, 4, 28]]`	Passed
normal control 7	`[[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]]`	`[[2026, 3, 31], [2027, 3, 31], [2028, 3, 31], [2029, 3, 31], [2030, 3, 31], [2031, 3, 31], [2032, 3, 31]]`	Passed

SHA-256 / c836611b808231cb5104499a13cc53bdc4f6eff7a6efc0229deb7d3eb5b39ea8

Verification & scope

A deterministic teaching model of a stipulated billing rule. It makes no claim to reproduce any billing provider's exact behaviour and is not billing software. 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 2026-09-29T14:46:35.720880+00:00.

Case digest / 69ecaad3f2006886ef91e46dc4130c7560830c60aa2d2de8dc3020a145e1d0dc