Build Field-service task lifecycle

type State = "unassigned" | "assigned" | "accepted" | "en_route" | "in_progress" | "proof_pending" | "completed" | "cancelled";
type CommandName = "create_task" | "assign" | "accept" | "start_travel" | "arrive" | "submit_proof" | "approve_proof" | "reject_proof";

type Command = {
  name: CommandName;
  aggregateId: string;
  expectedVersion: number;
  idempotencyKey: string;
};

const transitions = {
  "create_task": { from: [null], to: "unassigned", event: "task.created" },
  "assign": { from: ["unassigned", "assigned"], to: "assigned", event: "task.assigned" },
  "accept": { from: ["assigned"], to: "accepted", event: "task.accepted" },
  "start_travel": { from: ["accepted"], to: "en_route", event: "task.en_route" },
  "arrive": { from: ["en_route"], to: "in_progress", event: "task.started" },
  "submit_proof": { from: ["in_progress"], to: "proof_pending", event: "task.proof_submitted" },
  "approve_proof": { from: ["proof_pending"], to: "completed", event: "task.completed" },
  "reject_proof": { from: ["proof_pending"], to: "in_progress", event: "task.proof_rejected" },
} as const;

export function decide(current: { state: State | null; version: number }, command: Command) {
  const rule = transitions[command.name];
  if (command.expectedVersion !== current.version) throw new Error("version_conflict");
  if (!rule.from.includes(current.state as never)) throw new Error("invalid_transition");
  return {
    state: rule.to as State,
    version: current.version + 1,
    event: rule.event,
    idempotencyKey: command.idempotencyKey,
  };
}

// Persist the result, immutable event, audit row, and outbox intent atomically.
// Store the first result by idempotencyKey before executing another effect.