FA-59413 / Subscription proration billing / Member archive

Seat additions billed above the high-water mark: chronological change order · case 03

Out-of-order change logs charge seats for the wrong number of days.

Member previewVariant 3 · 3 implementations · 9 checks per implementation

Case contract

Input {price per seat per period, period_days, base_seats, changes: [[day, seat_count]]}. Changes apply in day order (same-day changes keep input order). Seats above the period high-water mark are charged price*added*(period_days - day)/period_days half-up; decreases earn no mid-cycle credit. Return [prorated charges, high-water mark, renewal seat count (the last count)].

Why this case matters

Seat-based billing charges each seat at most once per period, even when seats are removed and re-added.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression 0	`[59579, 25, 18]`	`[100201, 25, 18]`	Failed

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