FA-58709 / Loan amortization schedules / Member archive

Accelerated biweekly schedule: biweekly periodic rate · case 04

Biweekly interest is computed as if there were 24 periods a year.

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Case contract

x = {'principal', 'rate_bp', 'months'}. The monthly payment is the exact annuity rounded half-up; the biweekly payment is round_half_up(monthly/2), paid 26 times a year. Periodic interest is round_half_up(balance*bp/260000). When balance + interest <= payment the loan pays off with that amount. Return {'payment', 'periods', '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: biweekly periodic rate	`{"final_payment": 72, "payment": 83, "periods": 72, "total_interest": 965}`	`{"final_payment": 64, "payment": 83, "periods": 71, "total_interest": 874}`	Failed

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