FA-58837 / Loan amortization schedules / Member archive

Graduated payment schedule: peak tracking · case 02

Peak balance disclosures report only the original principal.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

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: peak tracking	`{"end_balance": 105921, "peak_balance": 100000, "year_payments": [500, 550]}`	`{"end_balance": 105921, "peak_balance": 105921, "year_payments": [500, 550]}`	Failed

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