FA-58587 / Loan amortization schedules / Member archive

Balloon maturity payment: interest total scope · case 02

Total interest excludes the interest in the balloon payment.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

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: interest total scope	`{"balloon_excess": 99606, "final_payment": 100288, "payment": 682, "total_interest": 2414}`	`{"balloon_excess": 99606, "final_payment": 100288, "payment": 682, "total_interest": 3016}`	Failed

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