FA-58447 / Loan amortization schedules / Member archive

Interest-only period with funding stub: stub month selection · case 02

Every interest-only bill is prorated like the funding month.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

Case contract

x = {'principal', 'rate_bp', 'io_months' >= 1, 'amort_months' >= 1, 'fund_day' 1..30}. IO month m pays interest round_half_up(P*bp*days/(120000*30)) with days = 31 - fund_day for the first IO month and 30 otherwise. Then the level payment on the full principal over amort_months (exact annuity, half-up) amortizes with monthly interest round_half_up(balance*bp/120000); the last month pays balance + interest. Return {'io_bills': list of IO payments, '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: stub month selection	`{"final_payment": 4703, "io_bills": [35, 35], "payment": 4718, "total_interest": 33135}`	`{"final_payment": 4703, "io_bills": [35, 1042], "payment": 4718, "total_interest": 34142}`	Failed

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