FA-58574 / Loan amortization schedules / Member archive

Balloon maturity payment: amortization basis · case 04

Balloon loans are quoted fully amortizing payments over the short term.

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Case contract

x = {'principal', 'rate_bp', 'amort_months', 'term_months'}. The level payment amortizes over amort_months (exact annuity, half-up). Payments run to the earlier of term and amortization end; the last one pays balance + interest (the balloon). Monthly interest is round_half_up(balance*bp/120000). Return {'payment', 'final_payment', 'balloon_excess': final - payment, 'total_interest'}.

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: amortization basis	`{"balloon_excess": 2, "final_payment": 86644, "payment": 86642, "total_interest": 39706}`	`{"balloon_excess": 990318, "final_payment": 997140, "payment": 6822, "total_interest": 72182}`	Failed

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