// Node.js 22+. Run: node idempotency-lab.mjs
// In-memory teaching fixture. No email is sent; state resets on restart.
import assert from 'node:assert/strict';
import { createHash } from 'node:crypto';
const ledger = new Map();
let queued = 0;
export async function simulatedSend(key, payload, { loseResponse = false } = {}) {
  const hash = createHash('sha256').update(JSON.stringify(payload)).digest('hex');
  const previous = ledger.get(key);
  if (previous) {
    if (previous.hash !== hash) throw new Error('409: idempotency conflict');
    return previous.result;
  }
  const result = { id: `simulated-${++queued}`, status: 'queued', mode: 'demo' };
  ledger.set(key, { hash, result }); // Commit before the response is lost.
  if (loseResponse) throw new Error('Connection closed after acceptance');
  return result;
}
const payload = { to: 'reader@example.test', subject: 'Your receipt', text: 'Order 42' };
const key = 'receipt:order-42:v1'; // Persist with the business event, before sending.
await assert.rejects(simulatedSend(key, payload, { loseResponse: true }));
const recovered = await simulatedSend(key, payload);
assert.equal(recovered.id, 'simulated-1');
assert.equal(queued, 1);
await assert.rejects(simulatedSend(key, { ...payload, text: 'Changed' }), /409/);
await Promise.all(Array.from({ length: 10 }, () => simulatedSend(key, payload)));
assert.equal(queued, 1);
console.log(JSON.stringify({ tests: 4, queued, result: recovered, sendsRealEmail: false }));