FA-58603 / Loan amortization schedules / Member archive

Negative amortization cap recast: deferred interest accrual · case 03

Deferred interest shrinks in months where the payment covers interest.

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Case contract

x = {'principal', 'rate_bp', 'min_payment', 'cap_pct_bp' (e.g. 11000 = 110% of principal), 'months'}. Each month interest = round_half_up(balance*bp/120000); the shortfall interest - payment (if positive) is deferred; balance += interest - payment. The first time balance*10000 exceeds principal*cap_pct_bp the loan recasts once: payment = level(balance, months - k) (exact annuity, half-up). Return {'balance', 'recast_month' or None, 'deferred', 'payment'}.

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: deferred interest accrual	`{"balance": 4, "deferred": -999996, "payment": 113575, "recast_month": 2}`	`{"balance": 4, "deferred": 20200, "payment": 113575, "recast_month": 2}`	Failed

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