FA-59831 / Subscription proration billing / Open access
Unused-time credit based on amount paid: partially paid invoices · case 01
Customers who paid part of an invoice get no credit for unused time.
ROOT CAUSE
Every non-paid status, including partially paid, is denied credit.
VERIFIED REPAIR
Restore the contract rule at the partially paid invoices step: use `if x['status'] in ('open', 'void', 'refunded'):`.
Unsuccessful approach: The attempt denies partial invoices while crediting refunded ones.
Case contract
Input {list, paid, status paid|partial|open|void|refunded, left, period}. Open, void and refunded invoices earn no unused-time credit. Paid and partially paid invoices earn paid*left/period half-up (the amount actually collected, not the list price). Return cents.
Why this case matters
Credits for unused time must not exceed what the customer actually paid for the period.
1 / The failure
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(x):
if x['status'] != 'paid':
return 0
return (x['paid'] * x['left'] * 2 + x['period']) // (2 * x['period'])
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'list': 4900, 'paid': 2450, 'status': 'partial', 'left': 24, 'period': 31}, 1897), ('regression', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('partial-repair probe', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('partial-repair probe', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 0, 'status': 'open', 'left': 4, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('regression', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('partial-repair probe', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'paid', 'left': 19, 'period': 31}, 3003), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 15, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'paid', 'left': 6, 'period': 31}, 474), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 1, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('regression', {'list': 4900, 'paid': 311, 'status': 'partial', 'left': 2, 'period': 31}, 20), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 13, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 20, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'void', 'left': 3, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 311, 'status': 'partial', 'left': 2, 'period': 31}, 20), ('regression', {'list': 1000, 'paid': 500, 'status': 'partial', 'left': 13, 'period': 31}, 210), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'open', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 29, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 2540, 'status': 'open', 'left': 27, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 6, 'period': 31}, 0)], [('regression', {'list': 1000, 'paid': 500, 'status': 'partial', 'left': 13, 'period': 31}, 210), ('regression', {'list': 9900, 'paid': 9900, 'status': 'partial', 'left': 21, 'period': 31}, 6706), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'paid', 'left': 25, 'period': 31}, 3992), ('normal control', {'list': 4900, 'paid': 786, 'status': 'open', 'left': 21, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'refunded', 'left': 28, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 3, 'period': 31}, 0)]]
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 | 0 | 1897 | Failed |
| regression 1 | 0 | 694 | Failed |
| partial-repair probe 2 | 0 | 595 | Failed |
| partial-repair probe 3 | 0 | 0 | Passed |
| normal control 4 | 0 | 0 | Passed |
| normal control 5 | 0 | 0 | Passed |
| normal control 6 | 0 | 0 | Passed |
| normal control 7 | 0 | 0 | Passed |
SHA-256 / 47f4209f337ae8c891adc373554d609e665816b0f98e9c0fc82fa18d1d537095
2 / The unsuccessful fix
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(x):
if x['status'] in ('open', 'void', 'partial'):
return 0
return (x['paid'] * x['left'] * 2 + x['period']) // (2 * x['period'])
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'list': 4900, 'paid': 2450, 'status': 'partial', 'left': 24, 'period': 31}, 1897), ('regression', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('partial-repair probe', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('partial-repair probe', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 0, 'status': 'open', 'left': 4, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('regression', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('partial-repair probe', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'paid', 'left': 19, 'period': 31}, 3003), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 15, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'paid', 'left': 6, 'period': 31}, 474), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 1, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('regression', {'list': 4900, 'paid': 311, 'status': 'partial', 'left': 2, 'period': 31}, 20), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 13, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 20, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'void', 'left': 3, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 311, 'status': 'partial', 'left': 2, 'period': 31}, 20), ('regression', {'list': 1000, 'paid': 500, 'status': 'partial', 'left': 13, 'period': 31}, 210), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'open', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 29, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 2540, 'status': 'open', 'left': 27, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 6, 'period': 31}, 0)], [('regression', {'list': 1000, 'paid': 500, 'status': 'partial', 'left': 13, 'period': 31}, 210), ('regression', {'list': 9900, 'paid': 9900, 'status': 'partial', 'left': 21, 'period': 31}, 6706), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'paid', 'left': 25, 'period': 31}, 3992), ('normal control', {'list': 4900, 'paid': 786, 'status': 'open', 'left': 21, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'refunded', 'left': 28, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 3, 'period': 31}, 0)]]
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 | 0 | 1897 | Failed |
| regression 1 | 0 | 694 | Failed |
| partial-repair probe 2 | 0 | 595 | Failed |
| partial-repair probe 3 | 149 | 0 | Failed |
| normal control 4 | 0 | 0 | Passed |
| normal control 5 | 0 | 0 | Passed |
| normal control 6 | 0 | 0 | Passed |
| normal control 7 | 0 | 0 | Passed |
SHA-256 / a281daddbbe51252d5dcd1dba6e0a800cf5b198dddb95d88e101fd56eb6b34e1
3 / The verified repair
Exit 0"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(x):
if x['status'] in ('open', 'void', 'refunded'):
return 0
return (x['paid'] * x['left'] * 2 + x['period']) // (2 * x['period'])
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression', {'list': 4900, 'paid': 2450, 'status': 'partial', 'left': 24, 'period': 31}, 1897), ('regression', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('partial-repair probe', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('partial-repair probe', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 0, 'status': 'open', 'left': 4, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 29, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 935, 'status': 'partial', 'left': 23, 'period': 31}, 694), ('regression', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('partial-repair probe', {'list': 1000, 'paid': 461, 'status': 'refunded', 'left': 10, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'paid', 'left': 19, 'period': 31}, 3003), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 15, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'paid', 'left': 6, 'period': 31}, 474), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 1, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 3688, 'status': 'partial', 'left': 5, 'period': 31}, 595), ('regression', {'list': 4900, 'paid': 311, 'status': 'partial', 'left': 2, 'period': 31}, 20), ('partial-repair probe', {'list': 4900, 'paid': 2450, 'status': 'refunded', 'left': 20, 'period': 31}, 0), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 13, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 500, 'status': 'open', 'left': 9, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 2450, 'status': 'open', 'left': 20, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'void', 'left': 3, 'period': 31}, 0)], [('regression', {'list': 4900, 'paid': 311, 'status': 'partial', 'left': 2, 'period': 31}, 20), ('regression', {'list': 1000, 'paid': 500, 'status': 'partial', 'left': 13, 'period': 31}, 210), ('partial-repair probe', {'list': 9900, 'paid': 4739, 'status': 'refunded', 'left': 24, 'period': 31}, 0), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 4900, 'status': 'open', 'left': 25, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'void', 'left': 29, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 2540, 'status': 'open', 'left': 27, 'period': 31}, 0), ('normal control', {'list': 4900, 'paid': 0, 'status': 'refunded', 'left': 6, 'period': 31}, 0)], [('regression', {'list': 1000, 'paid': 500, 'status': 'partial', 'left': 13, 'period': 31}, 210), ('regression', {'list': 9900, 'paid': 9900, 'status': 'partial', 'left': 21, 'period': 31}, 6706), ('partial-repair probe', {'list': 4900, 'paid': 1142, 'status': 'refunded', 'left': 8, 'period': 31}, 0), ('partial-repair probe', {'list': 1000, 'paid': 1000, 'status': 'refunded', 'left': 7, 'period': 31}, 0), ('normal control', {'list': 9900, 'paid': 4950, 'status': 'paid', 'left': 25, 'period': 31}, 3992), ('normal control', {'list': 4900, 'paid': 786, 'status': 'open', 'left': 21, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'refunded', 'left': 28, 'period': 31}, 0), ('normal control', {'list': 1000, 'paid': 0, 'status': 'partial', 'left': 3, 'period': 31}, 0)]]
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 | 1897 | 1897 | Passed |
| regression 1 | 694 | 694 | Passed |
| partial-repair probe 2 | 595 | 595 | Passed |
| partial-repair probe 3 | 0 | 0 | Passed |
| normal control 4 | 0 | 0 | Passed |
| normal control 5 | 0 | 0 | Passed |
| normal control 6 | 0 | 0 | Passed |
| normal control 7 | 0 | 0 | Passed |
SHA-256 / 24fc2515661a55d0f3b928e642374a5a581067e0109e833c096a6ad94b726ecd
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 2026-09-29T14:46:40.047360+00:00.
Case digest / 3be5776208b762687c4562a2e5a5e7da4983ddd376e8f39de2833db6f9a1881e