// 文件分片與加密（Web Crypto API）
async function encryptChunk(chunk, algorithm = 'AES-GCM') {
  const key = await window.crypto.subtle.generateKey(
    { name: algorithm, length: 256 },
    true,
    ['encrypt', 'decrypt']
  );
  const iv = window.crypto.getRandomValues(new Uint8Array(12));
  const encrypted = await window.crypto.subtle.encrypt(
    { name: algorithm, iv },
    key,
    chunk
  );
  return { encrypted, iv, key }; // 實際需安全存儲密鑰
}

// 文件夾上傳（遞歸處理）
function handleFolder(entry, fileId) {
  const dirReader = entry.createReader();
  dirReader.readEntries(entries => {
    entries.forEach(subEntry => {
      if (subEntry.isFile) {
        subEntry.file(file => {
          const chunkSize = 5 * 1024 * 1024; // 5MB分片
          // 分片、加密、上傳邏輯...
        });
      } else {
        handleFolder(subEntry, `${fileId}/${subEntry.name}`); // 遞歸構建路徑
      }
    });
  });
}

// 分片上傳接口（ASP.NET Core）
[HttpPost("upload")]
public async Task UploadChunk(
    [FromForm] IFormFile fileChunk,
    [FromForm] string fileId,
    [FromForm] int chunkIndex,
    [FromForm] string algorithm)
{
    // 解密分片（示例：AES-GCM）
    var decrypted = DecryptChunk(fileChunk.OpenReadStream(), algorithm);
    
    // 存儲至OSS（帶SM4加密）
    var ossPath = $"uploads/{fileId}/{chunkIndex}.enc";
    await _ossClient.PutObjectAsync(ossPath, decrypted);
    
    // 記錄進度至SQL Server
    await _dbContext.UploadProgress.AddAsync(new UploadProgress {
        FileId = fileId,
        ChunkIndex = chunkIndex,
        Status = "completed"
    });
    await _dbContext.SaveChangesAsync();
    
    return Ok(new { status = "success" });
}

// 非打包下載（流式傳輸）
[HttpGet("download/{fileId}")]
public async Task DownloadFile(string fileId, long? rangeStart)
{
    var fileInfo = await _ossClient.GetObjectMetadataAsync($"downloads/{fileId}");
    var stream = await _ossClient.GetObjectAsync($"downloads/{fileId}");
    
    if (rangeStart.HasValue)
    {
        stream.Seek(rangeStart.Value, SeekOrigin.Begin);
        return File(stream, "application/octet-stream", fileInfo.Key, enableRangeProcessing: true);
    }
    
    return File(stream, "application/octet-stream", fileInfo.Key);
}