Meter billing — you are billed exactly what the meter read, once

meter_billing.cell

// ============================================================================
// METER BILLING — you are billed exactly what the meter read, once
// ============================================================================
//
// In December 2015 the UK regulator Ofgem fined npower £26 million for
// billing failures: estimated and duplicated invoices that didn't match
// what meters had recorded, sent to half a million customers. The root
// pattern is always the same — billing reads a copy of the meter data
// that can drift, and nothing structural ties the invoice to the register.
//
// Here conservation is by construction. Each billing cycle is a state
// machine: usage accumulates only while `accumulating`; `locked` snapshots
// the register; the ONLY write to `billed` copies the metered value at
// invoice time. You cannot invoice an unlocked register (the transition
// doesn't exist), cannot invoice twice (invoiced -> invoiced doesn't
// exist), and a settled cycle is TERMINAL — no retroactive "adjustment".
// soma verify proves the lifecycle; the audit signal pins billed==metered.
// ============================================================================

cell Billing {
    face {
        signal record_usage(meter: String, kwh: Int) -> Map
        signal lock_register(meter: String) -> Map
        signal issue_invoice(meter: String) -> Map
        signal post_payment(meter: String) -> Map
        signal audit_gap(meter: String) -> Int
        signal cycle_state(meter: String) -> String
    }

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

        billed: Map<String, Int> [persistent, consistent]
        invariant billed >= 0
    }

    state cycle {
        initial: accumulating
        accumulating -> locked     // register snapshot taken
        locked -> invoiced         // bill issued for EXACTLY the snapshot
        invoiced -> settled        // payment posted — TERMINAL: history
    }

    on record_usage(meter: String, kwh: Int) {
        if kwh <= 0 { return map("ok", false, "reason", "usage must be positive") }
        if get_status(meter) != "accumulating" {
            return map("ok", false, "reason",
                "register is locked (state: {get_status(meter)}) — no backfilled usage")
        }
        let total = (metered.get(meter) ?? 0) + kwh
        metered.set(meter, total)
        return map("ok", true, "metered", total)
    }

    on lock_register(meter: String) {
        let r = try { transition(meter, "locked") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "state", get_status(meter))
    }

    on issue_invoice(meter: String) {
        let r = try { transition(meter, "invoiced") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        // conservation by construction: the ONLY write to `billed`
        // copies the metered register, verbatim
        billed.set(meter, metered.get(meter) ?? 0)
        return map("ok", true, "billed", billed.get(meter))
    }

    on post_payment(meter: String) {
        let r = try { transition(meter, "settled") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "state", get_status(meter))
    }

    on audit_gap(meter: String) {
        return (billed.get(meter) ?? 0) - (metered.get(meter) ?? 0)
    }

    on cycle_state(meter: String) { return get_status(meter) }
}

cell test BillingTests {
    rules {
        // a normal cycle: meter ticks, register locks, invoice goes out
        assert record_usage("m1", 300).ok == true
        assert record_usage("m1", 150).ok == true

        // cannot invoice an unlocked register — the transition is absent
        assert_fails transition("m1", "invoiced")

        assert lock_register("m1").ok == true
        // once locked, no backfilled usage sneaks onto the bill
        assert record_usage("m1", 999).ok == false

        assert issue_invoice("m1").ok == true
        // CONSERVATION: billed == metered, exactly
        assert audit_gap("m1") == 0

        // cannot invoice the same cycle twice
        assert_fails transition("m1", "invoiced")

        assert post_payment("m1").ok == true
        // settled is TERMINAL: no retroactive adjustment, ever
        assert_fails transition("m1", "locked")
        assert_fails transition("m1", "accumulating")

        // negative invoices and registers are rejected by the memory
        assert_fails billed.set("m1", -50)
        assert_fails metered.set("m1", -1)
        assert audit_gap("m1") == 0
    }
}