{"abstract":"Total interest is a few cents low across the schedule.","category":"Loan amortization schedules","checks":7,"contract":"x = {'principal', 'rate_bp' (annual, monthly rate = bp/120000), 'months', 'round': 'nearest'|'up'}. The exact annuity payment P*r/(1-(1+r)^-n) (P/n at zero rate) is rounded half-up ('nearest') or up to the cent. Monthly interest is round_half_up(balance*bp/120000); months 1..n-1 pay the level payment and month n pays balance + interest. Return {'payment', 'final_payment', 'total_interest'}.","evaluation_group":"w2-loan-amortization-schedules-level-payment-and-final-adjustment","failed_approach":"Rounding any remainder up overcharges interest in most months.","family":"w2-loan-amortization-schedules-level-payment-and-final-adjustment-monthly-interest-rounding","id":"FA-58366","implementations":{"attempt":{"sha256":"e278069a6ad0fa25180e796eeb8825af08d306d67c2c64e460720a930ecb8a98","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport math\nfrom fractions import Fraction\nN = 1\nobservations = []\ndef solve(x):\n    P = x['principal']\n    n = x['months']\n    bp = x['rate_bp']\n    if bp == 0:\n        exact = Fraction(P, n)\n    else:\n        r = Fraction(bp, 120000)\n        exact = P * r / (1 - (1 + r) ** -n)\n    if x['round'] == 'up':\n        pay = math.ceil(exact)\n    else:\n        pay = math.floor(exact + Fraction(1, 2))\n    bal = P\n    total_int = 0\n    last = 0\n    for k in range(1, n + 1):\n        q, rem = divmod(bal * bp, 120000)\n        interest = q + (1 if rem else 0)\n        total_int += interest\n        if k == n:\n            last = bal + interest\n        else:\n            bal = bal + interest - pay\n    return {'payment': pay, 'final_payment': last, 'total_interest': total_int}\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 725, 'months': 360, 'round': 'up'}, {'payment': 1, 'final_payment': -1123, 'total_interest': -771}], ['control 1', {'principal': 1000000, 'rate_bp': 13, 'months': 36, 'round': 'up'}, {'payment': 27834, 'final_payment': 27808, 'total_interest': 1998}], ['control 2', {'principal': 12345, 'rate_bp': 1200, 'months': 1, 'round': 'nearest'}, {'payment': 12468, 'final_payment': 12468, 'total_interest': 123}], ['control 3', {'principal': 999999, 'rate_bp': 0, 'months': 60, 'round': 'nearest'}, {'payment': 16667, 'final_payment': 16646, 'total_interest': 0}], ['control 4', {'principal': 30000000, 'rate_bp': 0, 'months': 36, 'round': 'up'}, {'payment': 833334, 'final_payment': 833310, 'total_interest': 0}], ['control 5', {'principal': 1000000, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333334, 'total_interest': 0}], ['control 6', {'principal': 100000, 'rate_bp': 0, 'months': 360, 'round': 'up'}, {'payment': 278, 'final_payment': 198, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 2500000, 'rate_bp': 1999, 'months': 3, 'round': 'nearest'}, {'payment': 861250, 'final_payment': 861251, 'total_interest': 83751}], ['regression: monthly interest rounding, partial-repair probe', {'principal': 500, 'rate_bp': 13, 'months': 3, 'round': 'up'}, {'payment': 167, 'final_payment': 166, 'total_interest': 0}], ['control 1', {'principal': 7, 'rate_bp': 0, 'months': 60, 'round': 'nearest'}, {'payment': 0, 'final_payment': 7, 'total_interest': 0}], ['control 2', {'principal': 999999, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333333, 'total_interest': 0}], ['control 3', {'principal': 999999, 'rate_bp': 0, 'months': 36, 'round': 'nearest'}, {'payment': 27778, 'final_payment': 27769, 'total_interest': 0}], ['control 4', {'principal': 500, 'rate_bp': 13, 'months': 12, 'round': 'nearest'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}], ['control 5', {'principal': 30000000, 'rate_bp': 0, 'months': 7, 'round': 'nearest'}, {'payment': 4285714, 'final_payment': 4285716, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 2500000, 'rate_bp': 13, 'months': 1, 'round': 'nearest'}, {'payment': 2500271, 'final_payment': 2500271, 'total_interest': 271}], ['regression: monthly interest rounding, partial-repair probe', {'principal': 100000, 'rate_bp': 1999, 'months': 3, 'round': 'nearest'}, {'payment': 34450, 'final_payment': 34450, 'total_interest': 3350}], ['control 1', {'principal': 999999, 'rate_bp': 0, 'months': 12, 'round': 'nearest'}, {'payment': 83333, 'final_payment': 83336, 'total_interest': 0}], ['control 2', {'principal': 12345, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 1029, 'final_payment': 1026, 'total_interest': 0}], ['control 3', {'principal': 1000000, 'rate_bp': 0, 'months': 360, 'round': 'up'}, {'payment': 2778, 'final_payment': 2698, 'total_interest': 0}], ['control 4', {'principal': 500, 'rate_bp': 0, 'months': 7, 'round': 'nearest'}, {'payment': 71, 'final_payment': 74, 'total_interest': 0}], ['control 5', {'principal': 999999, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 83334, 'final_payment': 83325, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 13, 'months': 12, 'round': 'up'}, {'payment': 1, 'final_payment': -4, 'total_interest': 0}], ['control 1', {'principal': 7, 'rate_bp': 1999, 'months': 7, 'round': 'nearest'}, {'payment': 1, 'final_payment': 1, 'total_interest': 0}], ['control 2', {'principal': 30000000, 'rate_bp': 600, 'months': 2, 'round': 'up'}, {'payment': 15112594, 'final_payment': 15112593, 'total_interest': 225187}], ['control 3', {'principal': 30000000, 'rate_bp': 0, 'months': 360, 'round': 'nearest'}, {'payment': 83333, 'final_payment': 83453, 'total_interest': 0}], ['control 4', {'principal': 100000, 'rate_bp': 600, 'months': 3, 'round': 'up'}, {'payment': 33668, 'final_payment': 33665, 'total_interest': 1001}], ['control 5', {'principal': 500, 'rate_bp': 13, 'months': 36, 'round': 'up'}, {'payment': 14, 'final_payment': 10, 'total_interest': 0}], ['control 6', {'principal': 999999, 'rate_bp': 13, 'months': 2, 'round': 'nearest'}, {'payment': 500081, 'final_payment': 500080, 'total_interest': 162}]], [['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 600, 'months': 360, 'round': 'up'}, {'payment': 1, 'final_payment': -876, 'total_interest': -524}], ['control 1', {'principal': 30000000, 'rate_bp': 499, 'months': 2, 'round': 'nearest'}, {'payment': 15093627, 'final_payment': 15093627, 'total_interest': 187254}], ['control 2', {'principal': 999999, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333333, 'total_interest': 0}], ['control 3', {'principal': 500, 'rate_bp': 0, 'months': 2, 'round': 'up'}, {'payment': 250, 'final_payment': 250, 'total_interest': 0}], ['control 4', {'principal': 7, 'rate_bp': 600, 'months': 60, 'round': 'nearest'}, {'payment': 0, 'final_payment': 7, 'total_interest': 0}], ['control 5', {'principal': 500, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}], ['control 6', {'principal': 500, 'rate_bp': 0, 'months': 12, 'round': 'nearest'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}]]]\nfor label, args, expected in fixtures[N-1]:\n    try:\n        actual = solve(args)\n    except Exception as exc:\n        actual = 'raised ' + type(exc).__name__\n    check(label, actual, expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"13bbe5a6aa0ab60c9ab8e311763e2c834cb4be28724256236c46936dd2ad37b3","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport math\nfrom fractions import Fraction\nN = 1\nobservations = []\ndef solve(x):\n    P = x['principal']\n    n = x['months']\n    bp = x['rate_bp']\n    if bp == 0:\n        exact = Fraction(P, n)\n    else:\n        r = Fraction(bp, 120000)\n        exact = P * r / (1 - (1 + r) ** -n)\n    if x['round'] == 'up':\n        pay = math.ceil(exact)\n    else:\n        pay = math.floor(exact + Fraction(1, 2))\n    bal = P\n    total_int = 0\n    last = 0\n    for k in range(1, n + 1):\n        q, rem = divmod(bal * bp, 120000)\n        interest = q\n        total_int += interest\n        if k == n:\n            last = bal + interest\n        else:\n            bal = bal + interest - pay\n    return {'payment': pay, 'final_payment': last, 'total_interest': total_int}\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 725, 'months': 360, 'round': 'up'}, {'payment': 1, 'final_payment': -1123, 'total_interest': -771}], ['control 1', {'principal': 1000000, 'rate_bp': 13, 'months': 36, 'round': 'up'}, {'payment': 27834, 'final_payment': 27808, 'total_interest': 1998}], ['control 2', {'principal': 12345, 'rate_bp': 1200, 'months': 1, 'round': 'nearest'}, {'payment': 12468, 'final_payment': 12468, 'total_interest': 123}], ['control 3', {'principal': 999999, 'rate_bp': 0, 'months': 60, 'round': 'nearest'}, {'payment': 16667, 'final_payment': 16646, 'total_interest': 0}], ['control 4', {'principal': 30000000, 'rate_bp': 0, 'months': 36, 'round': 'up'}, {'payment': 833334, 'final_payment': 833310, 'total_interest': 0}], ['control 5', {'principal': 1000000, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333334, 'total_interest': 0}], ['control 6', {'principal': 100000, 'rate_bp': 0, 'months': 360, 'round': 'up'}, {'payment': 278, 'final_payment': 198, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 2500000, 'rate_bp': 1999, 'months': 3, 'round': 'nearest'}, {'payment': 861250, 'final_payment': 861251, 'total_interest': 83751}], ['regression: monthly interest rounding, partial-repair probe', {'principal': 500, 'rate_bp': 13, 'months': 3, 'round': 'up'}, {'payment': 167, 'final_payment': 166, 'total_interest': 0}], ['control 1', {'principal': 7, 'rate_bp': 0, 'months': 60, 'round': 'nearest'}, {'payment': 0, 'final_payment': 7, 'total_interest': 0}], ['control 2', {'principal': 999999, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333333, 'total_interest': 0}], ['control 3', {'principal': 999999, 'rate_bp': 0, 'months': 36, 'round': 'nearest'}, {'payment': 27778, 'final_payment': 27769, 'total_interest': 0}], ['control 4', {'principal': 500, 'rate_bp': 13, 'months': 12, 'round': 'nearest'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}], ['control 5', {'principal': 30000000, 'rate_bp': 0, 'months': 7, 'round': 'nearest'}, {'payment': 4285714, 'final_payment': 4285716, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 2500000, 'rate_bp': 13, 'months': 1, 'round': 'nearest'}, {'payment': 2500271, 'final_payment': 2500271, 'total_interest': 271}], ['regression: monthly interest rounding, partial-repair probe', {'principal': 100000, 'rate_bp': 1999, 'months': 3, 'round': 'nearest'}, {'payment': 34450, 'final_payment': 34450, 'total_interest': 3350}], ['control 1', {'principal': 999999, 'rate_bp': 0, 'months': 12, 'round': 'nearest'}, {'payment': 83333, 'final_payment': 83336, 'total_interest': 0}], ['control 2', {'principal': 12345, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 1029, 'final_payment': 1026, 'total_interest': 0}], ['control 3', {'principal': 1000000, 'rate_bp': 0, 'months': 360, 'round': 'up'}, {'payment': 2778, 'final_payment': 2698, 'total_interest': 0}], ['control 4', {'principal': 500, 'rate_bp': 0, 'months': 7, 'round': 'nearest'}, {'payment': 71, 'final_payment': 74, 'total_interest': 0}], ['control 5', {'principal': 999999, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 83334, 'final_payment': 83325, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 13, 'months': 12, 'round': 'up'}, {'payment': 1, 'final_payment': -4, 'total_interest': 0}], ['control 1', {'principal': 7, 'rate_bp': 1999, 'months': 7, 'round': 'nearest'}, {'payment': 1, 'final_payment': 1, 'total_interest': 0}], ['control 2', {'principal': 30000000, 'rate_bp': 600, 'months': 2, 'round': 'up'}, {'payment': 15112594, 'final_payment': 15112593, 'total_interest': 225187}], ['control 3', {'principal': 30000000, 'rate_bp': 0, 'months': 360, 'round': 'nearest'}, {'payment': 83333, 'final_payment': 83453, 'total_interest': 0}], ['control 4', {'principal': 100000, 'rate_bp': 600, 'months': 3, 'round': 'up'}, {'payment': 33668, 'final_payment': 33665, 'total_interest': 1001}], ['control 5', {'principal': 500, 'rate_bp': 13, 'months': 36, 'round': 'up'}, {'payment': 14, 'final_payment': 10, 'total_interest': 0}], ['control 6', {'principal': 999999, 'rate_bp': 13, 'months': 2, 'round': 'nearest'}, {'payment': 500081, 'final_payment': 500080, 'total_interest': 162}]], [['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 600, 'months': 360, 'round': 'up'}, {'payment': 1, 'final_payment': -876, 'total_interest': -524}], ['control 1', {'principal': 30000000, 'rate_bp': 499, 'months': 2, 'round': 'nearest'}, {'payment': 15093627, 'final_payment': 15093627, 'total_interest': 187254}], ['control 2', {'principal': 999999, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333333, 'total_interest': 0}], ['control 3', {'principal': 500, 'rate_bp': 0, 'months': 2, 'round': 'up'}, {'payment': 250, 'final_payment': 250, 'total_interest': 0}], ['control 4', {'principal': 7, 'rate_bp': 600, 'months': 60, 'round': 'nearest'}, {'payment': 0, 'final_payment': 7, 'total_interest': 0}], ['control 5', {'principal': 500, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}], ['control 6', {'principal': 500, 'rate_bp': 0, 'months': 12, 'round': 'nearest'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}]]]\nfor label, args, expected in fixtures[N-1]:\n    try:\n        actual = solve(args)\n    except Exception as exc:\n        actual = 'raised ' + type(exc).__name__\n    check(label, actual, expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"fixed":{"sha256":"5ca1df439180be1ac97b99189a4b3924301a6aceae404328be545bcb248e114c","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\nimport math\nfrom fractions import Fraction\nN = 1\nobservations = []\ndef solve(x):\n    P = x['principal']\n    n = x['months']\n    bp = x['rate_bp']\n    if bp == 0:\n        exact = Fraction(P, n)\n    else:\n        r = Fraction(bp, 120000)\n        exact = P * r / (1 - (1 + r) ** -n)\n    if x['round'] == 'up':\n        pay = math.ceil(exact)\n    else:\n        pay = math.floor(exact + Fraction(1, 2))\n    bal = P\n    total_int = 0\n    last = 0\n    for k in range(1, n + 1):\n        q, rem = divmod(bal * bp, 120000)\n        interest = q + (1 if 2 * rem >= 120000 else 0)\n        total_int += interest\n        if k == n:\n            last = bal + interest\n        else:\n            bal = bal + interest - pay\n    return {'payment': pay, 'final_payment': last, 'total_interest': total_int}\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nfixtures = [[['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 725, 'months': 360, 'round': 'up'}, {'payment': 1, 'final_payment': -1123, 'total_interest': -771}], ['control 1', {'principal': 1000000, 'rate_bp': 13, 'months': 36, 'round': 'up'}, {'payment': 27834, 'final_payment': 27808, 'total_interest': 1998}], ['control 2', {'principal': 12345, 'rate_bp': 1200, 'months': 1, 'round': 'nearest'}, {'payment': 12468, 'final_payment': 12468, 'total_interest': 123}], ['control 3', {'principal': 999999, 'rate_bp': 0, 'months': 60, 'round': 'nearest'}, {'payment': 16667, 'final_payment': 16646, 'total_interest': 0}], ['control 4', {'principal': 30000000, 'rate_bp': 0, 'months': 36, 'round': 'up'}, {'payment': 833334, 'final_payment': 833310, 'total_interest': 0}], ['control 5', {'principal': 1000000, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333334, 'total_interest': 0}], ['control 6', {'principal': 100000, 'rate_bp': 0, 'months': 360, 'round': 'up'}, {'payment': 278, 'final_payment': 198, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 2500000, 'rate_bp': 1999, 'months': 3, 'round': 'nearest'}, {'payment': 861250, 'final_payment': 861251, 'total_interest': 83751}], ['regression: monthly interest rounding, partial-repair probe', {'principal': 500, 'rate_bp': 13, 'months': 3, 'round': 'up'}, {'payment': 167, 'final_payment': 166, 'total_interest': 0}], ['control 1', {'principal': 7, 'rate_bp': 0, 'months': 60, 'round': 'nearest'}, {'payment': 0, 'final_payment': 7, 'total_interest': 0}], ['control 2', {'principal': 999999, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333333, 'total_interest': 0}], ['control 3', {'principal': 999999, 'rate_bp': 0, 'months': 36, 'round': 'nearest'}, {'payment': 27778, 'final_payment': 27769, 'total_interest': 0}], ['control 4', {'principal': 500, 'rate_bp': 13, 'months': 12, 'round': 'nearest'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}], ['control 5', {'principal': 30000000, 'rate_bp': 0, 'months': 7, 'round': 'nearest'}, {'payment': 4285714, 'final_payment': 4285716, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 2500000, 'rate_bp': 13, 'months': 1, 'round': 'nearest'}, {'payment': 2500271, 'final_payment': 2500271, 'total_interest': 271}], ['regression: monthly interest rounding, partial-repair probe', {'principal': 100000, 'rate_bp': 1999, 'months': 3, 'round': 'nearest'}, {'payment': 34450, 'final_payment': 34450, 'total_interest': 3350}], ['control 1', {'principal': 999999, 'rate_bp': 0, 'months': 12, 'round': 'nearest'}, {'payment': 83333, 'final_payment': 83336, 'total_interest': 0}], ['control 2', {'principal': 12345, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 1029, 'final_payment': 1026, 'total_interest': 0}], ['control 3', {'principal': 1000000, 'rate_bp': 0, 'months': 360, 'round': 'up'}, {'payment': 2778, 'final_payment': 2698, 'total_interest': 0}], ['control 4', {'principal': 500, 'rate_bp': 0, 'months': 7, 'round': 'nearest'}, {'payment': 71, 'final_payment': 74, 'total_interest': 0}], ['control 5', {'principal': 999999, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 83334, 'final_payment': 83325, 'total_interest': 0}]], [['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 13, 'months': 12, 'round': 'up'}, {'payment': 1, 'final_payment': -4, 'total_interest': 0}], ['control 1', {'principal': 7, 'rate_bp': 1999, 'months': 7, 'round': 'nearest'}, {'payment': 1, 'final_payment': 1, 'total_interest': 0}], ['control 2', {'principal': 30000000, 'rate_bp': 600, 'months': 2, 'round': 'up'}, {'payment': 15112594, 'final_payment': 15112593, 'total_interest': 225187}], ['control 3', {'principal': 30000000, 'rate_bp': 0, 'months': 360, 'round': 'nearest'}, {'payment': 83333, 'final_payment': 83453, 'total_interest': 0}], ['control 4', {'principal': 100000, 'rate_bp': 600, 'months': 3, 'round': 'up'}, {'payment': 33668, 'final_payment': 33665, 'total_interest': 1001}], ['control 5', {'principal': 500, 'rate_bp': 13, 'months': 36, 'round': 'up'}, {'payment': 14, 'final_payment': 10, 'total_interest': 0}], ['control 6', {'principal': 999999, 'rate_bp': 13, 'months': 2, 'round': 'nearest'}, {'payment': 500081, 'final_payment': 500080, 'total_interest': 162}]], [['regression: monthly interest rounding', {'principal': 7, 'rate_bp': 600, 'months': 360, 'round': 'up'}, {'payment': 1, 'final_payment': -876, 'total_interest': -524}], ['control 1', {'principal': 30000000, 'rate_bp': 499, 'months': 2, 'round': 'nearest'}, {'payment': 15093627, 'final_payment': 15093627, 'total_interest': 187254}], ['control 2', {'principal': 999999, 'rate_bp': 0, 'months': 3, 'round': 'nearest'}, {'payment': 333333, 'final_payment': 333333, 'total_interest': 0}], ['control 3', {'principal': 500, 'rate_bp': 0, 'months': 2, 'round': 'up'}, {'payment': 250, 'final_payment': 250, 'total_interest': 0}], ['control 4', {'principal': 7, 'rate_bp': 600, 'months': 60, 'round': 'nearest'}, {'payment': 0, 'final_payment': 7, 'total_interest': 0}], ['control 5', {'principal': 500, 'rate_bp': 0, 'months': 12, 'round': 'up'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}], ['control 6', {'principal': 500, 'rate_bp': 0, 'months': 12, 'round': 'nearest'}, {'payment': 42, 'final_payment': 38, 'total_interest': 0}]]]\nfor label, args, expected in fixtures[N-1]:\n    try:\n        actual = solve(args)\n    except Exception as exc:\n        actual = 'raised ' + type(exc).__name__\n    check(label, actual, expected)\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":"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.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"w2-loan-amortization-schedules-level-payment-and-final-adjustment-monthly-interest-rounding","generated_at":"2026-09-29T14:46:26.008175+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"Amortization engines drive borrower statements, payoff quotes and investor remittances; a misplaced rounding step, boundary or ordering rule compounds across hundreds of periods.","repair":"Round monthly interest half-up.","root_cause":"Monthly interest is truncated instead of rounded half-up.","sha256":"1137e8442f131e56b512cdb5b34362723f47bba01d46c04ec7227dd6654290eb","title":"Level payment with final adjustment: monthly interest rounding · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verification":{"attempt":{"elapsed_ms":43.175,"exit_code":1,"observations":[{"actual":{"final_payment":8,"payment":1,"total_interest":360},"check":"regression: monthly interest rounding","expected":{"final_payment":-1123,"payment":1,"total_interest":-771},"passed":false},{"actual":{"final_payment":27844,"payment":27834,"total_interest":2034},"check":"control 1","expected":{"final_payment":27808,"payment":27834,"total_interest":1998},"passed":false},{"actual":{"final_payment":12469,"payment":12468,"total_interest":124},"check":"control 2","expected":{"final_payment":12468,"payment":12468,"total_interest":123},"passed":false},{"actual":{"final_payment":16646,"payment":16667,"total_interest":0},"check":"control 3","expected":{"final_payment":16646,"payment":16667,"total_interest":0},"passed":true},{"actual":{"final_payment":833310,"payment":833334,"total_interest":0},"check":"control 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