FA-58368 / Loan amortization schedules / Member archive

Level payment with final adjustment: monthly interest rounding · case 03

Total interest is a few cents low across the schedule.

Member previewVariant 3 · 3 implementations · 7 checks per implementation

Case contract

x = {'principal', 'rate_bp' (annual, monthly rate = bp/120000), 'months', 'round': 'nearest'|'up'}. The exact annuity payment P*r/(1-(1+r)^-n) (P/n at zero rate) is rounded half-up ('nearest') or up to the cent. Monthly interest is round_half_up(balance*bp/120000); months 1..n-1 pay the level payment and month n pays balance + interest. Return {'payment', '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: monthly interest rounding	`{"final_payment": 2500270, "payment": 2500271, "total_interest": 270}`	`{"final_payment": 2500271, "payment": 2500271, "total_interest": 271}`	Failed

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