Chargeback window — disputes die at day 120, reversals happen exactly once

chargeback_window.cell

// ============================================================================
// CHARGEBACK WINDOW — disputes die at day 120, reversals happen exactly once
// ============================================================================
//
// Card networks give cardholders ~120 days to dispute a charge. Two ways
// processors lose money here: accepting a dispute after the window (the
// merchant's funds were already swept as final), and reversing the same
// charge twice when a manual agent and an automated rule both fire. Both
// are structural below: `finalized` is terminal so a late dispute has no
// edge to take, and `reversed`/`upheld` are terminal so a second reversal
// raises. The merchant ledger refuses negative balances.
// ============================================================================

cell Chargeback {
    face {
        signal capture(id: String, amount: Int) -> Map
        signal close_window(id: String, day: Int) -> Map
        signal open_dispute(id: String, day: Int) -> Map
        signal resolve(id: String, outcome: String) -> Map
        signal merchant_balance() -> Int
    }

    memory {
        merchant: Map<String, Int> [persistent, consistent]
        invariant merchant >= 0
        amounts: Map<String, Int> [persistent, consistent]
        invariant amounts >= 0
    }

    // finalized, reversed, upheld: all terminal. A finalized charge can
    // never become disputed; a reversed charge can never reverse again.
    state charge {
        initial: authorized
        authorized -> settled
        settled -> finalized
        settled -> disputed
        disputed -> reversed
        disputed -> upheld
    }

    on capture(id: String, amount: Int) {
        if amount <= 0 { return map("ok", false, "reason", "bad amount") }
        if merchant.get("acct") == () { merchant.set("acct", 0) }
        transition(id, "settled")
        amounts.set(id, amount)
        merchant.set("acct", (merchant.get("acct") ?? 0) + amount)
        return map("ok", true, "state", get_status(id))
    }

    on close_window(id: String, day: Int) {
        if day <= 120 {
            return map("ok", false, "reason", "window still open until day 120")
        }
        transition(id, "finalized")
        return map("ok", true, "state", get_status(id))
    }

    on open_dispute(id: String, day: Int) {
        if day > 120 {
            return map("ok", false, "reason", "day {day} is past the 120-day window")
        }
        transition(id, "disputed")     // raises if charge already finalized
        return map("ok", true, "state", get_status(id))
    }

    on resolve(id: String, outcome: String) {
        if outcome == "merchant_wins" {
            transition(id, "upheld")
            return map("ok", true, "state", "upheld")
        }
        transition(id, "reversed")     // money goes back exactly once
        let amt = amounts.get(id) ?? 0
        merchant.set("acct", (merchant.get("acct") ?? 0) - amt)
        return map("ok", true, "state", "reversed", "clawed_back", amt)
    }

    on merchant_balance() {
        return merchant.get("acct") ?? 0
    }
}

cell test ChargebackTests {
    rules {
        assert capture("c1", 300).ok == true
        assert capture("c2", 200).ok == true
        assert merchant_balance() == 500
        // in-window dispute on c1, cardholder wins: money moves back ONCE
        assert open_dispute("c1", 45).ok == true
        assert resolve("c1", "cardholder_wins").clawed_back == 300
        assert merchant_balance() == 200
        // THE DOUBLE REVERSAL: terminal state, second clawback has no edge
        assert_fails transition("c1", "reversed")
        assert_fails transition("c1", "disputed")
        // c2 survives the window and is finalized
        assert close_window("c2", 100).ok == false
        assert close_window("c2", 121).ok == true
        // THE LATE DISPUTE: day 130 — refused by guard AND by the machine
        assert open_dispute("c2", 130).ok == false
        assert_fails transition("c2", "disputed")
        // the merchant account cannot be driven below zero
        assert_fails merchant.set("acct", -1)
        // merchant-wins path is just as final
        assert capture("c3", 50).ok == true
        assert open_dispute("c3", 10).ok == true
        assert resolve("c3", "merchant_wins").ok == true
        assert_fails transition("c3", "reversed")
    }
}