FA-58815 / Loan amortization schedules / Member archive

Credit line average daily balance: average daily balance divisor · case 05

Average daily balance is wrong in 28 and 31 day cycles.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

x = {'rate_bp', 'basis', 'start_balance', 'cycle_days', 'events': [[day 1..cycle_days, signed amount]], 'min_bp', 'min_floor'}. Events take effect at the start of their day. Each day adds max(balance, 0) to the accumulator; interest = round_half_up(acc*bp/(10000*basis)) once per cycle. owed = end balance + interest. Minimum = 0 if owed <= 0 else min(owed, max(min_floor, interest + round_half_up(max(end balance, 0)*min_bp/10000))). Return {'interest', 'balance': owed, 'minimum', 'adb': acc // cycle_days}.

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: average daily balance divisor	`{"adb": 925333, "balance": 934563, "interest": 4563, "minimum": 13863}`	`{"adb": 991428, "balance": 934563, "interest": 4563, "minimum": 13863}`	Failed

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