Reefer setpoint — the typo that thaws forty tonnes of tuna

reefer_setpoint.cell

// ============================================================================
// REEFER SETPOINT — the typo that thaws forty tonnes of tuna
// ============================================================================
//
// Refrigerated containers are programmed by hand at the terminal: a tech
// keys the setpoint from the booking sheet. The classic, expensive typo
// is sign and unit confusion — +18 instead of -18 (cargo claims files are
// full of them), or a Fahrenheit value keyed into a Celsius controller.
// Frozen tuna, ice cream, plasma: one transposed character and the claim
// runs to hundreds of thousands of dollars, discovered three weeks later
// at the destination port.
//
// This controller's setpoint memory carries the frozen-cargo envelope as
// an invariant: nothing outside [-30, -10] C can ever BE the setpoint.
// The +18 typo isn't caught by a validator someone forgot to call on one
// code path — the memory itself refuses, and the reefer keeps its last
// safe value. Power state is a verified machine: you cannot program a
// unit that isn't on, and a tripped unit must be inspected before reuse.
// ============================================================================

cell ReeferController {
    face {
        signal power_on(unit: String) -> Map
        signal program_setpoint(unit: String, celsius: Int) -> Map
        signal trip_alarm(unit: String) -> Map
        signal inspect_unit(unit: String) -> Map
        signal setpoint_of(unit: String) -> Int
        signal unit_state(unit: String) -> String
    }

    memory {
        // frozen-cargo envelope, deg C. The +18 typo cannot exist here.
        setpoint_c: Map<String, Int> [persistent, consistent]
        invariant setpoint_c >= -30 && setpoint_c <= -10
    }

    state reefer {
        initial: unpowered
        unpowered  -> running
        running    -> unpowered
        running    -> tripped
        tripped    -> under_inspection
        under_inspection -> unpowered
    }

    on power_on(unit: String) {
        let r = try { transition(unit, "running") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "unit", unit, "state", "running")
    }

    on program_setpoint(unit: String, celsius: Int) {
        if get_status(unit) != "running" {
            let s = get_status(unit)
            return map("ok", false, "reason", "unit {unit} is {s} — cannot program")
        }
        let w = try { setpoint_c.set(unit, celsius) }
        if w.error != () {
            return map("ok", false,
                "reason", "REFUSED: {celsius} C is outside the frozen envelope [-30, -10]")
        }
        return map("ok", true, "unit", unit, "setpoint_c", celsius)
    }

    on trip_alarm(unit: String) {
        let r = try { transition(unit, "tripped") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "unit", unit, "state", "tripped")
    }

    on inspect_unit(unit: String) {
        let r = try { transition(unit, "under_inspection") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        transition(unit, "unpowered")
        return map("ok", true, "unit", unit, "state", "unpowered")
    }

    on setpoint_of(unit: String) { return setpoint_c.get(unit) ?? -18 }
    on unit_state(unit: String) { return get_status(unit) }
}

cell test FrozenEnvelope {
    rules {
        // programmed correctly for frozen tuna
        assert power_on("RF-201").ok == true
        assert program_setpoint("RF-201", -18).ok == true
        assert setpoint_of("RF-201") == -18

        // THE TYPO: +18. The memory refuses; last safe value survives.
        assert program_setpoint("RF-201", 18).ok == false
        assert setpoint_of("RF-201") == -18
        assert_fails setpoint_c.set("RF-201", 18)
        assert_fails setpoint_c.set("RF-201", -45)

        // boundary values of the envelope are legal
        assert program_setpoint("RF-201", -30).ok == true
        assert program_setpoint("RF-201", -10).ok == true
        assert program_setpoint("RF-201", -9).ok == false

        // you cannot program a unit that is not running
        assert program_setpoint("RF-999", -18).ok == false

        // a tripped unit must pass inspection before it runs again
        assert trip_alarm("RF-201").ok == true
        assert program_setpoint("RF-201", -20).ok == false
        assert_fails transition("RF-201", "running")
        assert inspect_unit("RF-201").ok == true
        assert power_on("RF-201").ok == true
    }
}