FA-58489 / Loan amortization schedules / Member archive

Payment application waterfall: threshold boundary · case 04

A payment exactly equal to the threshold is parked in suspense.

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Case contract

x = {'due': {'interest', 'principal', 'late_fee', 'fees'}, 'payment', 'suspense', 'threshold', 'balance'}. Available = payment + suspense. If available < threshold, everything goes to suspense and nothing is applied. Otherwise apply in order interest, principal, late_fee, fees, then curtail principal with the remainder up to balance - applied principal; the rest is refunded. Return {'applied', 'curtailment', 'refund', 'suspense', 'new_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: threshold boundary	`{"applied": {"fees": 0, "interest": 0, "late_fee": 0, "principal": 0}, "curtailment": 0, "new_balance": 2000, "refund": 0, "suspense": 1000}`	`{"applied": {"fees": 0, "interest": 1000, "late_fee": 0, "principal": 0}, "curtailment": 0, "new_balance": 2000, "refund": 0, "suspense": 0}`	Failed

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