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FA-58761 / Loan amortization schedules / Open access

Installment due date calendar: anchor day drift · case 01

After February every due date sticks to the 28th.

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

ROOT CAUSE

The clamped day replaces the anchor day for later months.

VERIFIED REPAIR

Clamp each month from the original anchor day.

Unsuccessful approach: Clamping every month to 28 moves due dates that fit in the month.

Case contract

x = {'first_due': [y, m, d] (d may exceed the month length), 'count', 'roll': 'none'|'following'|'modified_following', 'holidays': ISO dates}. Installment i falls in month m+i on the anchor day d clamped to the month length (later months return to d). Unless roll is 'none', a Saturday, Sunday or holiday rolls forward to the next business day; under modified_following a roll that leaves the month instead rolls back to the previous business day. Return ISO date strings.

Why this case matters

Amortization engines drive borrower statements, payoff quotes and investor remittances; a misplaced rounding step, boundary or ordering rule compounds across hundreds of periods.

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['first_due']
    hol = set(x['holidays'])
    def bad(t):
        return t.weekday() >= 5 or t.isoformat() in hol
    out = []
    for i in range(x['count']):
        mm = m - 1 + i
        yy = y + mm // 12
        mm = mm % 12 + 1
        dd = min(d, calendar.monthrange(yy, mm)[1])
        d = dd
        date = datetime.date(yy, mm, dd)
        adj = date
        if x['roll'] != 'none':
            while bad(adj):
                adj += datetime.timedelta(days=1)
            if x['roll'] == 'modified_following' and adj.month != date.month:
                adj = date
                while bad(adj):
                    adj -= datetime.timedelta(days=1)
        out.append(adj.isoformat())
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression: anchor day drift', {'first_due': [2023, 3, 31], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-03-29']}, ['2023-03-31', '2023-04-28', '2023-05-31', '2023-06-30', '2023-07-31', '2023-08-31']], ['control 1', {'first_due': [2023, 9, 1], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-03-29', '2024-05-31']}, ['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']], ['control 2', {'first_due': [2024, 7, 28], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-12-31', '2024-01-01']}, ['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']], ['control 3', {'first_due': [2024, 11, 15], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']], ['control 4', {'first_due': [2024, 2, 5], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-03-29', '2024-05-31', '2023-12-25']}, ['2024-02-05', '2024-03-05', '2024-04-05']], ['control 5', {'first_due': [2023, 8, 15], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-02-29', '2023-06-30']}, ['2023-08-15', '2023-09-15', '2023-10-16']], ['control 6', {'first_due': [2024, 3, 15], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-05-31']}, ['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 13, 'roll': 'following', 'holidays': ['2024-01-01', '2024-12-31']}, ['2023-10-30', '2023-11-30', '2024-01-02', '2024-01-30', '2024-02-29', '2024-04-01', '2024-04-30', '2024-05-30', '2024-07-01', '2024-07-30', '2024-08-30', '2024-09-30', '2024-10-30']], ['control 1', {'first_due': [2024, 9, 5], 'count': 13, 'roll': 'following', 'holidays': []}, ['2024-09-05', '2024-10-07', '2024-11-05', '2024-12-05', '2025-01-06', '2025-02-05', '2025-03-05', '2025-04-07', '2025-05-05', '2025-06-05', '2025-07-07', '2025-08-05', '2025-09-05']], ['control 2', {'first_due': [2023, 4, 5], 'count': 6, 'roll': 'following', 'holidays': ['2024-01-01', '2024-05-31', '2023-06-30']}, ['2023-04-05', '2023-05-05', '2023-06-05', '2023-07-05', '2023-08-07', '2023-09-05']], ['control 3', {'first_due': [2023, 6, 5], 'count': 6, 'roll': 'none', 'holidays': ['2024-01-01']}, ['2023-06-05', '2023-07-05', '2023-08-05', '2023-09-05', '2023-10-05', '2023-11-05']], ['control 4', {'first_due': [2023, 11, 1], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-12-31', '2023-06-30', '2024-05-31']}, ['2023-11-01', '2023-12-01', '2024-01-01']], ['control 5', {'first_due': [2023, 7, 30], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-12-31']}, ['2023-07-31', '2023-08-30', '2023-09-29', '2023-10-30', '2023-11-30', '2023-12-29']], ['control 6', {'first_due': [2023, 5, 5], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-03-29', '2024-02-29']}, ['2023-05-05', '2023-06-05', '2023-07-05', '2023-08-05', '2023-09-05', '2023-10-05', '2023-11-05', '2023-12-05', '2024-01-05', '2024-02-05', '2024-03-05', '2024-04-05', '2024-05-05']]], [['regression: anchor day drift', {'first_due': [2023, 8, 30], 'count': 13, 'roll': 'following', 'holidays': []}, ['2023-08-30', '2023-10-02', '2023-10-30', '2023-11-30', '2024-01-01', '2024-01-30', '2024-02-29', '2024-04-01', '2024-04-30', '2024-05-30', '2024-07-01', '2024-07-30', '2024-08-30']], ['control 1', {'first_due': [2023, 9, 1], 'count': 3, 'roll': 'modified_following', 'holidays': []}, ['2023-09-01', '2023-10-02', '2023-11-01']], ['control 2', {'first_due': [2024, 5, 1], 'count': 13, 'roll': 'none', 'holidays': []}, ['2024-05-01', '2024-06-01', '2024-07-01', '2024-08-01', '2024-09-01', '2024-10-01', '2024-11-01', '2024-12-01', '2025-01-01', '2025-02-01', '2025-03-01', '2025-04-01', '2025-05-01']], ['control 3', {'first_due': [2024, 10, 30], 'count': 3, 'roll': 'following', 'holidays': ['2024-05-31', '2023-06-30', '2024-03-29']}, ['2024-10-30', '2024-12-02', '2024-12-30']], ['control 4', {'first_due': [2023, 3, 15], 'count': 13, 'roll': 'none', 'holidays': []}, ['2023-03-15', '2023-04-15', '2023-05-15', '2023-06-15', '2023-07-15', '2023-08-15', '2023-09-15', '2023-10-15', '2023-11-15', '2023-12-15', '2024-01-15', '2024-02-15', '2024-03-15']], ['control 5', {'first_due': [2023, 2, 5], 'count': 3, 'roll': 'none', 'holidays': ['2024-03-29']}, ['2023-02-05', '2023-03-05', '2023-04-05']], ['control 6', {'first_due': [2024, 11, 15], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-05-31']}, ['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15', '2025-05-15', '2025-06-16', '2025-07-15', '2025-08-15', '2025-09-15', '2025-10-15', '2025-11-17']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 6, 'roll': 'none', 'holidays': ['2024-01-01']}, ['2023-10-30', '2023-11-30', '2023-12-30', '2024-01-30', '2024-02-29', '2024-03-30']], ['control 1', {'first_due': [2024, 8, 15], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2024-08-15', '2024-09-16', '2024-10-15', '2024-11-15', '2024-12-16', '2025-01-15']], ['control 2', {'first_due': [2023, 8, 1], 'count': 3, 'roll': 'following', 'holidays': []}, ['2023-08-01', '2023-09-01', '2023-10-02']], ['control 3', {'first_due': [2023, 2, 28], 'count': 3, 'roll': 'following', 'holidays': ['2023-12-25', '2024-05-31']}, ['2023-02-28', '2023-03-28', '2023-04-28']], ['control 4', {'first_due': [2024, 9, 28], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-01-01', '2024-05-31']}, ['2024-09-28', '2024-10-28', '2024-11-28', '2024-12-28', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-28', '2025-07-28', '2025-08-28', '2025-09-28']], ['control 5', {'first_due': [2024, 1, 5], 'count': 3, 'roll': 'none', 'holidays': ['2024-03-29', '2024-05-31']}, ['2024-01-05', '2024-02-05', '2024-03-05']], ['control 6', {'first_due': [2023, 2, 28], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-05-31']}, ['2023-02-28', '2023-03-28', '2023-04-28', '2023-05-28', '2023-06-28', '2023-07-28', '2023-08-28', '2023-09-28', '2023-10-28', '2023-11-28', '2023-12-28', '2024-01-28', '2024-02-28']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 13, 'roll': 'none', 'holidays': ['2024-01-01', '2024-03-29']}, ['2023-10-30', '2023-11-30', '2023-12-30', '2024-01-30', '2024-02-29', '2024-03-30', '2024-04-30', '2024-05-30', '2024-06-30', '2024-07-30', '2024-08-30', '2024-09-30', '2024-10-30']], ['control 1', {'first_due': [2024, 1, 29], 'count': 6, 'roll': 'none', 'holidays': []}, ['2024-01-29', '2024-02-29', '2024-03-29', '2024-04-29', '2024-05-29', '2024-06-29']], ['control 2', {'first_due': [2024, 8, 5], 'count': 3, 'roll': 'modified_following', 'holidays': []}, ['2024-08-05', '2024-09-05', '2024-10-07']], ['control 3', {'first_due': [2024, 9, 30], 'count': 6, 'roll': 'none', 'holidays': ['2024-02-29']}, ['2024-09-30', '2024-10-30', '2024-11-30', '2024-12-30', '2025-01-30', '2025-02-28']], ['control 4', {'first_due': [2023, 10, 28], 'count': 3, 'roll': 'following', 'holidays': ['2024-12-31']}, ['2023-10-30', '2023-11-28', '2023-12-28']], ['control 5', {'first_due': [2023, 6, 30], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2023-06-30', '2023-07-31', '2023-08-30', '2023-09-29', '2023-10-30', '2023-11-30']], ['control 6', {'first_due': [2023, 2, 15], 'count': 13, 'roll': 'none', 'holidays': ['2024-02-29', '2024-12-31']}, ['2023-02-15', '2023-03-15', '2023-04-15', '2023-05-15', '2023-06-15', '2023-07-15', '2023-08-15', '2023-09-15', '2023-10-15', '2023-11-15', '2023-12-15', '2024-01-15', '2024-02-15']]]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'raised ' + type(exc).__name__
    check(label, actual, 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: anchor day drift['2023-03-31', '2023-04-28', '2023-05-30', '2023-06-30', '2023-07-31', '2023-08-30']['2023-03-31', '2023-04-28', '2023-05-31', '2023-06-30', '2023-07-31', '2023-08-31']Failed
control 1['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']Passed
control 2['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']Passed
control 3['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']Passed
control 4['2024-02-05', '2024-03-05', '2024-04-05']['2024-02-05', '2024-03-05', '2024-04-05']Passed
control 5['2023-08-15', '2023-09-15', '2023-10-16']['2023-08-15', '2023-09-15', '2023-10-16']Passed
control 6['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']Passed

SHA-256 / fd69cc2618075af65186b80ae700b76dd37ae199cc2acc6873cc39bb00d67f4e

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['first_due']
    hol = set(x['holidays'])
    def bad(t):
        return t.weekday() >= 5 or t.isoformat() in hol
    out = []
    for i in range(x['count']):
        mm = m - 1 + i
        yy = y + mm // 12
        mm = mm % 12 + 1
        dd = min(d, 28)
        date = datetime.date(yy, mm, dd)
        adj = date
        if x['roll'] != 'none':
            while bad(adj):
                adj += datetime.timedelta(days=1)
            if x['roll'] == 'modified_following' and adj.month != date.month:
                adj = date
                while bad(adj):
                    adj -= datetime.timedelta(days=1)
        out.append(adj.isoformat())
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression: anchor day drift', {'first_due': [2023, 3, 31], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-03-29']}, ['2023-03-31', '2023-04-28', '2023-05-31', '2023-06-30', '2023-07-31', '2023-08-31']], ['control 1', {'first_due': [2023, 9, 1], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-03-29', '2024-05-31']}, ['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']], ['control 2', {'first_due': [2024, 7, 28], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-12-31', '2024-01-01']}, ['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']], ['control 3', {'first_due': [2024, 11, 15], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']], ['control 4', {'first_due': [2024, 2, 5], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-03-29', '2024-05-31', '2023-12-25']}, ['2024-02-05', '2024-03-05', '2024-04-05']], ['control 5', {'first_due': [2023, 8, 15], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-02-29', '2023-06-30']}, ['2023-08-15', '2023-09-15', '2023-10-16']], ['control 6', {'first_due': [2024, 3, 15], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-05-31']}, ['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 13, 'roll': 'following', 'holidays': ['2024-01-01', '2024-12-31']}, ['2023-10-30', '2023-11-30', '2024-01-02', '2024-01-30', '2024-02-29', '2024-04-01', '2024-04-30', '2024-05-30', '2024-07-01', '2024-07-30', '2024-08-30', '2024-09-30', '2024-10-30']], ['control 1', {'first_due': [2024, 9, 5], 'count': 13, 'roll': 'following', 'holidays': []}, ['2024-09-05', '2024-10-07', '2024-11-05', '2024-12-05', '2025-01-06', '2025-02-05', '2025-03-05', '2025-04-07', '2025-05-05', '2025-06-05', '2025-07-07', '2025-08-05', '2025-09-05']], ['control 2', {'first_due': [2023, 4, 5], 'count': 6, 'roll': 'following', 'holidays': ['2024-01-01', '2024-05-31', '2023-06-30']}, ['2023-04-05', '2023-05-05', '2023-06-05', '2023-07-05', '2023-08-07', '2023-09-05']], ['control 3', {'first_due': [2023, 6, 5], 'count': 6, 'roll': 'none', 'holidays': ['2024-01-01']}, ['2023-06-05', '2023-07-05', '2023-08-05', '2023-09-05', '2023-10-05', '2023-11-05']], ['control 4', {'first_due': [2023, 11, 1], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-12-31', '2023-06-30', '2024-05-31']}, ['2023-11-01', '2023-12-01', '2024-01-01']], ['control 5', {'first_due': [2023, 7, 30], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-12-31']}, ['2023-07-31', '2023-08-30', '2023-09-29', '2023-10-30', '2023-11-30', '2023-12-29']], ['control 6', {'first_due': [2023, 5, 5], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-03-29', '2024-02-29']}, ['2023-05-05', '2023-06-05', '2023-07-05', '2023-08-05', '2023-09-05', '2023-10-05', '2023-11-05', '2023-12-05', '2024-01-05', '2024-02-05', '2024-03-05', '2024-04-05', '2024-05-05']]], [['regression: anchor day drift', {'first_due': [2023, 8, 30], 'count': 13, 'roll': 'following', 'holidays': []}, ['2023-08-30', '2023-10-02', '2023-10-30', '2023-11-30', '2024-01-01', '2024-01-30', '2024-02-29', '2024-04-01', '2024-04-30', '2024-05-30', '2024-07-01', '2024-07-30', '2024-08-30']], ['control 1', {'first_due': [2023, 9, 1], 'count': 3, 'roll': 'modified_following', 'holidays': []}, ['2023-09-01', '2023-10-02', '2023-11-01']], ['control 2', {'first_due': [2024, 5, 1], 'count': 13, 'roll': 'none', 'holidays': []}, ['2024-05-01', '2024-06-01', '2024-07-01', '2024-08-01', '2024-09-01', '2024-10-01', '2024-11-01', '2024-12-01', '2025-01-01', '2025-02-01', '2025-03-01', '2025-04-01', '2025-05-01']], ['control 3', {'first_due': [2024, 10, 30], 'count': 3, 'roll': 'following', 'holidays': ['2024-05-31', '2023-06-30', '2024-03-29']}, ['2024-10-30', '2024-12-02', '2024-12-30']], ['control 4', {'first_due': [2023, 3, 15], 'count': 13, 'roll': 'none', 'holidays': []}, ['2023-03-15', '2023-04-15', '2023-05-15', '2023-06-15', '2023-07-15', '2023-08-15', '2023-09-15', '2023-10-15', '2023-11-15', '2023-12-15', '2024-01-15', '2024-02-15', '2024-03-15']], ['control 5', {'first_due': [2023, 2, 5], 'count': 3, 'roll': 'none', 'holidays': ['2024-03-29']}, ['2023-02-05', '2023-03-05', '2023-04-05']], ['control 6', {'first_due': [2024, 11, 15], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-05-31']}, ['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15', '2025-05-15', '2025-06-16', '2025-07-15', '2025-08-15', '2025-09-15', '2025-10-15', '2025-11-17']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 6, 'roll': 'none', 'holidays': ['2024-01-01']}, ['2023-10-30', '2023-11-30', '2023-12-30', '2024-01-30', '2024-02-29', '2024-03-30']], ['control 1', {'first_due': [2024, 8, 15], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2024-08-15', '2024-09-16', '2024-10-15', '2024-11-15', '2024-12-16', '2025-01-15']], ['control 2', {'first_due': [2023, 8, 1], 'count': 3, 'roll': 'following', 'holidays': []}, ['2023-08-01', '2023-09-01', '2023-10-02']], ['control 3', {'first_due': [2023, 2, 28], 'count': 3, 'roll': 'following', 'holidays': ['2023-12-25', '2024-05-31']}, ['2023-02-28', '2023-03-28', '2023-04-28']], ['control 4', {'first_due': [2024, 9, 28], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-01-01', '2024-05-31']}, ['2024-09-28', '2024-10-28', '2024-11-28', '2024-12-28', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-28', '2025-07-28', '2025-08-28', '2025-09-28']], ['control 5', {'first_due': [2024, 1, 5], 'count': 3, 'roll': 'none', 'holidays': ['2024-03-29', '2024-05-31']}, ['2024-01-05', '2024-02-05', '2024-03-05']], ['control 6', {'first_due': [2023, 2, 28], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-05-31']}, ['2023-02-28', '2023-03-28', '2023-04-28', '2023-05-28', '2023-06-28', '2023-07-28', '2023-08-28', '2023-09-28', '2023-10-28', '2023-11-28', '2023-12-28', '2024-01-28', '2024-02-28']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 13, 'roll': 'none', 'holidays': ['2024-01-01', '2024-03-29']}, ['2023-10-30', '2023-11-30', '2023-12-30', '2024-01-30', '2024-02-29', '2024-03-30', '2024-04-30', '2024-05-30', '2024-06-30', '2024-07-30', '2024-08-30', '2024-09-30', '2024-10-30']], ['control 1', {'first_due': [2024, 1, 29], 'count': 6, 'roll': 'none', 'holidays': []}, ['2024-01-29', '2024-02-29', '2024-03-29', '2024-04-29', '2024-05-29', '2024-06-29']], ['control 2', {'first_due': [2024, 8, 5], 'count': 3, 'roll': 'modified_following', 'holidays': []}, ['2024-08-05', '2024-09-05', '2024-10-07']], ['control 3', {'first_due': [2024, 9, 30], 'count': 6, 'roll': 'none', 'holidays': ['2024-02-29']}, ['2024-09-30', '2024-10-30', '2024-11-30', '2024-12-30', '2025-01-30', '2025-02-28']], ['control 4', {'first_due': [2023, 10, 28], 'count': 3, 'roll': 'following', 'holidays': ['2024-12-31']}, ['2023-10-30', '2023-11-28', '2023-12-28']], ['control 5', {'first_due': [2023, 6, 30], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2023-06-30', '2023-07-31', '2023-08-30', '2023-09-29', '2023-10-30', '2023-11-30']], ['control 6', {'first_due': [2023, 2, 15], 'count': 13, 'roll': 'none', 'holidays': ['2024-02-29', '2024-12-31']}, ['2023-02-15', '2023-03-15', '2023-04-15', '2023-05-15', '2023-06-15', '2023-07-15', '2023-08-15', '2023-09-15', '2023-10-15', '2023-11-15', '2023-12-15', '2024-01-15', '2024-02-15']]]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'raised ' + type(exc).__name__
    check(label, actual, 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: anchor day drift['2023-03-28', '2023-04-28', '2023-05-29', '2023-06-28', '2023-07-28', '2023-08-28']['2023-03-31', '2023-04-28', '2023-05-31', '2023-06-30', '2023-07-31', '2023-08-31']Failed
control 1['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']Passed
control 2['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']Passed
control 3['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']Passed
control 4['2024-02-05', '2024-03-05', '2024-04-05']['2024-02-05', '2024-03-05', '2024-04-05']Passed
control 5['2023-08-15', '2023-09-15', '2023-10-16']['2023-08-15', '2023-09-15', '2023-10-16']Passed
control 6['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']Passed

SHA-256 / 997312aaf95d875c936e0b006d96584826157bcad2294863443b31463ba87628

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['first_due']
    hol = set(x['holidays'])
    def bad(t):
        return t.weekday() >= 5 or t.isoformat() in hol
    out = []
    for i in range(x['count']):
        mm = m - 1 + i
        yy = y + mm // 12
        mm = mm % 12 + 1
        dd = min(d, calendar.monthrange(yy, mm)[1])
        date = datetime.date(yy, mm, dd)
        adj = date
        if x['roll'] != 'none':
            while bad(adj):
                adj += datetime.timedelta(days=1)
            if x['roll'] == 'modified_following' and adj.month != date.month:
                adj = date
                while bad(adj):
                    adj -= datetime.timedelta(days=1)
        out.append(adj.isoformat())
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression: anchor day drift', {'first_due': [2023, 3, 31], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-03-29']}, ['2023-03-31', '2023-04-28', '2023-05-31', '2023-06-30', '2023-07-31', '2023-08-31']], ['control 1', {'first_due': [2023, 9, 1], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-03-29', '2024-05-31']}, ['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']], ['control 2', {'first_due': [2024, 7, 28], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-12-31', '2024-01-01']}, ['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']], ['control 3', {'first_due': [2024, 11, 15], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']], ['control 4', {'first_due': [2024, 2, 5], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-03-29', '2024-05-31', '2023-12-25']}, ['2024-02-05', '2024-03-05', '2024-04-05']], ['control 5', {'first_due': [2023, 8, 15], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-02-29', '2023-06-30']}, ['2023-08-15', '2023-09-15', '2023-10-16']], ['control 6', {'first_due': [2024, 3, 15], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-05-31']}, ['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 13, 'roll': 'following', 'holidays': ['2024-01-01', '2024-12-31']}, ['2023-10-30', '2023-11-30', '2024-01-02', '2024-01-30', '2024-02-29', '2024-04-01', '2024-04-30', '2024-05-30', '2024-07-01', '2024-07-30', '2024-08-30', '2024-09-30', '2024-10-30']], ['control 1', {'first_due': [2024, 9, 5], 'count': 13, 'roll': 'following', 'holidays': []}, ['2024-09-05', '2024-10-07', '2024-11-05', '2024-12-05', '2025-01-06', '2025-02-05', '2025-03-05', '2025-04-07', '2025-05-05', '2025-06-05', '2025-07-07', '2025-08-05', '2025-09-05']], ['control 2', {'first_due': [2023, 4, 5], 'count': 6, 'roll': 'following', 'holidays': ['2024-01-01', '2024-05-31', '2023-06-30']}, ['2023-04-05', '2023-05-05', '2023-06-05', '2023-07-05', '2023-08-07', '2023-09-05']], ['control 3', {'first_due': [2023, 6, 5], 'count': 6, 'roll': 'none', 'holidays': ['2024-01-01']}, ['2023-06-05', '2023-07-05', '2023-08-05', '2023-09-05', '2023-10-05', '2023-11-05']], ['control 4', {'first_due': [2023, 11, 1], 'count': 3, 'roll': 'modified_following', 'holidays': ['2024-12-31', '2023-06-30', '2024-05-31']}, ['2023-11-01', '2023-12-01', '2024-01-01']], ['control 5', {'first_due': [2023, 7, 30], 'count': 6, 'roll': 'modified_following', 'holidays': ['2024-12-31']}, ['2023-07-31', '2023-08-30', '2023-09-29', '2023-10-30', '2023-11-30', '2023-12-29']], ['control 6', {'first_due': [2023, 5, 5], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-03-29', '2024-02-29']}, ['2023-05-05', '2023-06-05', '2023-07-05', '2023-08-05', '2023-09-05', '2023-10-05', '2023-11-05', '2023-12-05', '2024-01-05', '2024-02-05', '2024-03-05', '2024-04-05', '2024-05-05']]], [['regression: anchor day drift', {'first_due': [2023, 8, 30], 'count': 13, 'roll': 'following', 'holidays': []}, ['2023-08-30', '2023-10-02', '2023-10-30', '2023-11-30', '2024-01-01', '2024-01-30', '2024-02-29', '2024-04-01', '2024-04-30', '2024-05-30', '2024-07-01', '2024-07-30', '2024-08-30']], ['control 1', {'first_due': [2023, 9, 1], 'count': 3, 'roll': 'modified_following', 'holidays': []}, ['2023-09-01', '2023-10-02', '2023-11-01']], ['control 2', {'first_due': [2024, 5, 1], 'count': 13, 'roll': 'none', 'holidays': []}, ['2024-05-01', '2024-06-01', '2024-07-01', '2024-08-01', '2024-09-01', '2024-10-01', '2024-11-01', '2024-12-01', '2025-01-01', '2025-02-01', '2025-03-01', '2025-04-01', '2025-05-01']], ['control 3', {'first_due': [2024, 10, 30], 'count': 3, 'roll': 'following', 'holidays': ['2024-05-31', '2023-06-30', '2024-03-29']}, ['2024-10-30', '2024-12-02', '2024-12-30']], ['control 4', {'first_due': [2023, 3, 15], 'count': 13, 'roll': 'none', 'holidays': []}, ['2023-03-15', '2023-04-15', '2023-05-15', '2023-06-15', '2023-07-15', '2023-08-15', '2023-09-15', '2023-10-15', '2023-11-15', '2023-12-15', '2024-01-15', '2024-02-15', '2024-03-15']], ['control 5', {'first_due': [2023, 2, 5], 'count': 3, 'roll': 'none', 'holidays': ['2024-03-29']}, ['2023-02-05', '2023-03-05', '2023-04-05']], ['control 6', {'first_due': [2024, 11, 15], 'count': 13, 'roll': 'modified_following', 'holidays': ['2024-05-31']}, ['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15', '2025-05-15', '2025-06-16', '2025-07-15', '2025-08-15', '2025-09-15', '2025-10-15', '2025-11-17']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 6, 'roll': 'none', 'holidays': ['2024-01-01']}, ['2023-10-30', '2023-11-30', '2023-12-30', '2024-01-30', '2024-02-29', '2024-03-30']], ['control 1', {'first_due': [2024, 8, 15], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2024-08-15', '2024-09-16', '2024-10-15', '2024-11-15', '2024-12-16', '2025-01-15']], ['control 2', {'first_due': [2023, 8, 1], 'count': 3, 'roll': 'following', 'holidays': []}, ['2023-08-01', '2023-09-01', '2023-10-02']], ['control 3', {'first_due': [2023, 2, 28], 'count': 3, 'roll': 'following', 'holidays': ['2023-12-25', '2024-05-31']}, ['2023-02-28', '2023-03-28', '2023-04-28']], ['control 4', {'first_due': [2024, 9, 28], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-01-01', '2024-05-31']}, ['2024-09-28', '2024-10-28', '2024-11-28', '2024-12-28', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-28', '2025-07-28', '2025-08-28', '2025-09-28']], ['control 5', {'first_due': [2024, 1, 5], 'count': 3, 'roll': 'none', 'holidays': ['2024-03-29', '2024-05-31']}, ['2024-01-05', '2024-02-05', '2024-03-05']], ['control 6', {'first_due': [2023, 2, 28], 'count': 13, 'roll': 'none', 'holidays': ['2023-06-30', '2024-05-31']}, ['2023-02-28', '2023-03-28', '2023-04-28', '2023-05-28', '2023-06-28', '2023-07-28', '2023-08-28', '2023-09-28', '2023-10-28', '2023-11-28', '2023-12-28', '2024-01-28', '2024-02-28']]], [['regression: anchor day drift', {'first_due': [2023, 10, 30], 'count': 13, 'roll': 'none', 'holidays': ['2024-01-01', '2024-03-29']}, ['2023-10-30', '2023-11-30', '2023-12-30', '2024-01-30', '2024-02-29', '2024-03-30', '2024-04-30', '2024-05-30', '2024-06-30', '2024-07-30', '2024-08-30', '2024-09-30', '2024-10-30']], ['control 1', {'first_due': [2024, 1, 29], 'count': 6, 'roll': 'none', 'holidays': []}, ['2024-01-29', '2024-02-29', '2024-03-29', '2024-04-29', '2024-05-29', '2024-06-29']], ['control 2', {'first_due': [2024, 8, 5], 'count': 3, 'roll': 'modified_following', 'holidays': []}, ['2024-08-05', '2024-09-05', '2024-10-07']], ['control 3', {'first_due': [2024, 9, 30], 'count': 6, 'roll': 'none', 'holidays': ['2024-02-29']}, ['2024-09-30', '2024-10-30', '2024-11-30', '2024-12-30', '2025-01-30', '2025-02-28']], ['control 4', {'first_due': [2023, 10, 28], 'count': 3, 'roll': 'following', 'holidays': ['2024-12-31']}, ['2023-10-30', '2023-11-28', '2023-12-28']], ['control 5', {'first_due': [2023, 6, 30], 'count': 6, 'roll': 'modified_following', 'holidays': []}, ['2023-06-30', '2023-07-31', '2023-08-30', '2023-09-29', '2023-10-30', '2023-11-30']], ['control 6', {'first_due': [2023, 2, 15], 'count': 13, 'roll': 'none', 'holidays': ['2024-02-29', '2024-12-31']}, ['2023-02-15', '2023-03-15', '2023-04-15', '2023-05-15', '2023-06-15', '2023-07-15', '2023-08-15', '2023-09-15', '2023-10-15', '2023-11-15', '2023-12-15', '2024-01-15', '2024-02-15']]]]
for label, args, expected in fixtures[N-1]:
    try:
        actual = solve(args)
    except Exception as exc:
        actual = 'raised ' + type(exc).__name__
    check(label, actual, 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: anchor day drift['2023-03-31', '2023-04-28', '2023-05-31', '2023-06-30', '2023-07-31', '2023-08-31']['2023-03-31', '2023-04-28', '2023-05-31', '2023-06-30', '2023-07-31', '2023-08-31']Passed
control 1['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']['2023-09-01', '2023-10-02', '2023-11-01', '2023-12-01', '2024-01-01', '2024-02-01', '2024-03-01', '2024-04-01', '2024-05-01', '2024-06-03', '2024-07-01', '2024-08-01', '2024-09-02']Passed
control 2['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']['2024-07-29', '2024-08-28', '2024-09-30', '2024-10-28', '2024-11-28', '2024-12-30', '2025-01-28', '2025-02-28', '2025-03-28', '2025-04-28', '2025-05-28', '2025-06-30', '2025-07-28']Passed
control 3['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']['2024-11-15', '2024-12-16', '2025-01-15', '2025-02-17', '2025-03-17', '2025-04-15']Passed
control 4['2024-02-05', '2024-03-05', '2024-04-05']['2024-02-05', '2024-03-05', '2024-04-05']Passed
control 5['2023-08-15', '2023-09-15', '2023-10-16']['2023-08-15', '2023-09-15', '2023-10-16']Passed
control 6['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']['2024-03-15', '2024-04-15', '2024-05-15', '2024-06-17', '2024-07-15', '2024-08-15']Passed

SHA-256 / df0e7dd0e4570e28e48406953307543398804b8fbebf8d5fd2758ac3c006d1fd

Verification & scope

A deterministic bounded teaching model with stipulated toy lending rules stated in the contract; money is integer cents and rates are basis points; it makes no claim of conformance to any regulation, servicing standard or product. 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 2026-09-29T14:46:29.771741+00:00.

Case digest / 16584c2084afcbeb841139df5557c4bb2a791ec260a58f9f6f43301c624b652a