// 檔案系統與時鐘由外部注入，核心不直接碰真實環境
export interface Fs {
  read(path: string): Promise<string | null>
  /** 原子寫入：先寫暫存檔再改名 */
  write(path: string, data: string): Promise<void>
}

export type Clock = () => number

export function memoryFs(): Fs & { files: Map<string, string> } {
  const files = new Map<string, string>()
  return {
    files,
    async read(p) {
      return files.get(p) ?? null
    },
    async write(p, d) {
      files.set(p, d)
    },
  }
}

export async function readJson<T>(fs: Fs, path: string, fallback: T): Promise<T> {
  const raw = await fs.read(path)
  if (raw === null) return fallback
  try {
    return JSON.parse(raw) as T
  } catch {
    return fallback
  }
}

export const writeJson = (fs: Fs, path: string, value: unknown) => fs.write(path, JSON.stringify(value, null, 2))