29. トランザクション

// トランザクションの commit / rollback を配列と try/catch で再現
// 送金: 口座Aから口座Bへ。途中で失敗したら全体を巻き戻す(All or Nothing)
type Account = { id: string; balance: number };

const committed: Account[] = [
  { id: "A", balance: 1000 },
  { id: "B", balance: 500 },
];

function getBalance(store: Account[], id: string): number {
  const found = store.find((a) => a.id === id);
  if (!found) throw new Error("口座が存在しません: " + id);
  return found.balance;
}

// トランザクション: 確定済みデータのコピー上で更新し、成功時だけ反映する
function runTransfer(from: string, to: string, amount: number): void {
  // スナップショット = トランザクション開始(BEGIN)
  const working: Account[] = committed.map((a) => ({ ...a }));
  try {
    const fromAcc = working.find((a) => a.id === from);
    const toAcc = working.find((a) => a.id === to);
    if (!fromAcc || !toAcc) throw new Error("口座が存在しません");

    fromAcc.balance -= amount;
    if (fromAcc.balance < 0) throw new Error("残高不足");
    toAcc.balance += amount;

    // ここまで全て成功 = COMMIT(作業コピーを確定済みへ反映)
    committed.length = 0;
    working.forEach((a) => committed.push(a));
    console.log("COMMIT: " + from + "→" + to + " " + amount + " 円を送金");
  } catch (e) {
    // 途中で失敗 = ROLLBACK(working を捨てるだけ。committed は無傷)
    if (e instanceof Error) console.log("ROLLBACK: " + e.message);
  }
}

console.log("開始: A=" + getBalance(committed, "A") + " B=" + getBalance(committed, "B"));

runTransfer("A", "B", 300); // 成功 → 確定
console.log("送金後: A=" + getBalance(committed, "A") + " B=" + getBalance(committed, "B"));

runTransfer("A", "B", 5000); // 残高不足 → 巻き戻し
console.log("失敗後: A=" + getBalance(committed, "A") + " B=" + getBalance(committed, "B"));