Function readMetadataFromHBCFooter

scripts/read.js:27–121

* 从 HBC 文件末尾的自定义 Meta 块读取元数据 * 格式: [原始HBC][MAGIC_START][LENGTH][JSON_DATA][MAGIC_END]

(buffer)

 * 格式: [原始HBC][MAGIC_START][LENGTH][JSON_DATA][MAGIC_END]
 */
function readMetadataFromHBCFooter(buffer) {
  const MAGIC = Buffer.from('RNUPDATE', 'utf8'); // 8 bytes
  const MAGIC_SIZE = 8;
  const LENGTH_SIZE = 4;

  console.log(`\n[DEBUG] Reading metadata from HBC footer...`);
  console.log(`[DEBUG] File size: ${buffer.length} bytes`);
  console.log(`[DEBUG] Last 100 bytes (hex): ${buffer.slice(-100).toString('hex')}`);
  console.log(`[DEBUG] Last 50 bytes (utf8): ${buffer.slice(-50).toString('utf8', 0, 50).replace(/[^\x20-\x7E]/g, '.')}`);

  // 检查文件是否足够大以包含 meta 块
  // 最小大小: MAGIC_START(8) + LENGTH(4) + JSON(至少2字节"{}") + MAGIC_END(8) = 22 bytes
  if (buffer.length < 22) {
    console.log(`[DEBUG] File too small: ${buffer.length} < 22`);
    return null;
  }

  // 从文件末尾向前查找最后一个 MAGIC_END
  const lastMagicIndex = buffer.lastIndexOf(MAGIC);

  console.log(`[DEBUG] MAGIC string: "${MAGIC.toString('utf8')}"`);
  console.log(`[DEBUG] Last MAGIC index: ${lastMagicIndex}`);

  if (lastMagicIndex === -1) {
    console.log(`[DEBUG] MAGIC not found in file`);
    return null;
  }

  // MAGIC_END 应该在文件末尾
  console.log(`[DEBUG] Expected MAGIC_END at: ${buffer.length - MAGIC_SIZE}`);
  console.log(`[DEBUG] Found MAGIC_END at: ${lastMagicIndex}`);

  if (lastMagicIndex + MAGIC_SIZE !== buffer.length) {
    console.warn(`⚠️  Found MAGIC but not at file end (expected ${buffer.length - MAGIC_SIZE}, found ${lastMagicIndex})`);
    return null;
  }

  // 计算 MAGIC_START 的位置
  // 从 MAGIC_END 向前：MAGIC_END(8) + JSON(?) + LENGTH(4) + MAGIC_START(8)
  const magicEndStart = lastMagicIndex;

  // 读取长度字段（在 MAGIC_END 之前的 4 字节）
  const lengthStart = magicEndStart - LENGTH_SIZE;
  if (lengthStart < MAGIC_SIZE) {
    console.log(`[DEBUG] lengthStart too small: ${lengthStart} < ${MAGIC_SIZE}`);
    return null; // 文件太小
  }

  const jsonLength = buffer.readUInt32LE(lengthStart);
  console.log(`[DEBUG] JSON length from buffer: ${jsonLength}`);

  // 验证长度是否合理（JSON 数据应该小于 10KB）
  if (jsonLength > 10240 || jsonLength < 2) {
    console.warn(`⚠️  Invalid JSON length: ${jsonLength}`);
    return null;
  }

  // 计算 JSON 数据的起始位置