FA-58823 / Loan amortization schedules / Member archive

Graduated payment schedule: year boundary · case 03

Payments step up in the twelfth month of each year.

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Case contract

x = {'principal', 'rate_bp', 'months', 'initial_payment', 'step_bp', 'steps'}. Month m (1-based) is in year t = (m-1)//12. For t < steps+1 the year payment starts at initial_payment and each new year multiplies the previous year payment by (10000+step_bp)/10000, rounded half-up. At the start of year steps+1 (if the loan runs that long) the payment is re-leveled on the balance over the remaining months (exact annuity, half-up) and stays fixed. Interest round_half_up(balance*bp/120000); balance += interest - payment (it may grow). Return {'year_payments', 'peak_balance', 'end_balance'}.

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.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: year boundary	`{"end_balance": 0, "peak_balance": 1170424, "year_payments": [3000, 3225, 3467, 1179202]}`	`{"end_balance": 6, "peak_balance": 1115035, "year_payments": [3000, 3225, 97511]}`	Failed

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