Emergency triage — the sickest patient is seen first, by construction

triage.cell

// ============================================================================
// EMERGENCY TRIAGE — the sickest patient is seen first, by construction
// ============================================================================
//
// ED overcrowding kills by inversion: a walk-in with a sprain gets a bed
// while a silent MI in the waiting room arrests. The Manchester / ESI
// triage standard fixes an ordering — level 1 (resuscitation) before
// level 5 (non-urgent) — but a queue implemented as plain FIFO quietly
// violates it the moment load spikes.
//
// Here "the next patient is always the most acute waiting one" is not a
// sort we hope ran; it is what `next_to_see` computes directly over the
// waiting set, and a patient's acuity is pinned to the ESI scale by a
// memory invariant (1..5). A bug that wrote acuity 0 or 9 — which would
// jump or sink someone in the order — is rejected at the slot.
//
//   soma check examples/corpus/medical/triage.cell
//   soma test  examples/corpus/medical/triage.cell
//   soma run   examples/corpus/medical/triage.cell demo
// ============================================================================

cell Triage {
    face {
        signal arrive(patient: String, esi: Int) -> Map
        signal next_to_see() -> Map
        signal see(patient: String) -> Map
        signal waiting_count() -> Int
        signal demo() -> String
    }

    memory {
        // ESI acuity per still-waiting patient. The scale is 1..5 and
        // nothing else — the ordering only means something if every value
        // is a real triage level. 0 ("more urgent than resuscitation") and
        // 6 are rejected at the write.
        acuity: Map<String, Int> [persistent, consistent]
        invariant acuity >= 1 && acuity <= 5

        seen: Map<String, Int> [persistent, consistent]
        invariant seen >= 0 && seen <= 1
    }

    on arrive(patient: String, esi: Int) {
        let w = try { acuity.set(patient, esi) }
        if w.error != () {
            return map("ok", false, "reason", "invalid ESI level: {w.error}")
        }
        return map("ok", true, "patient", patient, "esi", esi)
    }

    // The most acute waiting patient: lowest ESI number wins; ties break
    // by who has been waiting (we just take the first encountered). This
    // IS the ordering — there is no separate FIFO that could disagree.
    on next_to_see() {
        let pick = ""
        let pick_esi = 99
        for p in acuity.keys() {
            let already = seen.get(p) ?? 0
            if already == 0 {
                let e = acuity.get(p) ?? 99
                if e < pick_esi {
                    pick_esi = e
                    pick = p
                }
            }
        }
        if pick == "" { return map("ok", false, "reason", "no one waiting") }
        return map("ok", true, "patient", pick, "esi", pick_esi)
    }

    on see(patient: String) {
        let e = acuity.get(patient)
        if e == () { return map("ok", false, "reason", "unknown patient") }
        seen.set(patient, 1)
        return map("ok", true, "patient", patient)
    }

    on waiting_count() {
        let n = 0
        for p in acuity.keys() {
            if (seen.get(p) ?? 0) == 0 { n = n + 1 }
        }
        return n
    }

    on demo() {
        arrive("sprain-guy", 5)
        arrive("chest-pain", 2)
        arrive("cardiac-arrest", 1)
        arrive("headache", 4)
        let first = next_to_see()
        see(first.patient)
        let second = next_to_see()
        print("first to see:  {first}")     // cardiac-arrest, ESI 1
        print("after seeing:  {second}")    // chest-pain, ESI 2
        print("still waiting: {waiting_count()}")
        return "demo done"
    }
}

cell test TriageTests {
    rules {
        assert arrive("sprain", 5).ok == true
        assert arrive("mi", 1).ok == true
        assert arrive("fever", 3).ok == true

        // the order is acuity, never arrival: the MI is seen before the
        // sprain that arrived first
        assert next_to_see().patient == "mi"
        assert see("mi").ok == true
        assert next_to_see().patient == "fever"
        assert see("fever").ok == true
        assert next_to_see().patient == "sprain"

        assert waiting_count() == 1

        // an out-of-scale acuity that would corrupt the order is refused
        assert_fails acuity.set("ghost", 0)
        assert_fails acuity.set("ghost", 6)
        assert arrive("ghost", 9).ok == false
    }
}