module DownloadManager
  class ParallelDownloadQueue
    DOWNLOAD_MAX_PARALLEL = ENV.fetch('DOWNLOAD_MAX_PARALLEL') { 10 }

    attr_accessor :attachments,
                  :destination,
                  :on_error

    def initialize(attachments, destination)
      @attachments = attachments
      @destination = Pathname.new(destination)
    end

    def download_all
      hydra = Typhoeus::Hydra.new(max_concurrency: DOWNLOAD_MAX_PARALLEL)

      attachments.map do |attachment, path|
        begin
          download_one(attachment: attachment,
                       path_in_download_dir: path,
                       http_client: hydra)
        rescue => e
          on_error.call(attachment, path, e)
        end
      end
      hydra.run
    end

    # can't be used with typhoeus, otherwise block is closed before the request is run by hydra
    def download_one(attachment:, path_in_download_dir:, http_client:)
      path = Pathname.new(path_in_download_dir)
      attachment_path = destination.join(path.dirname, sanitize_filename(path.basename.to_s))

      attachment_path.dirname.mkpath # defensive, do not write in undefined dir

      if attachment.is_a?(ActiveStorage::FakeAttachment)
        attachment_path.write(attachment.file.read, mode: 'wb')
      else
        request = Typhoeus::Request.new(attachment.url)
        request.on_complete do |response|
          if response.success?
            attachment_path.open(mode: "wb") do |fd|
              fd.write(response.body)
            end
          else
            attachment_path.delete if attachment_path.exist? # -> case of retries failed, must cleanup partialy downloaded file
            on_error.call(attachment, path_in_download_dir, response.code)
          end
        end
        http_client.queue(request)
      end
    end

    private

    def sanitize_filename(original_filename)
      filename = ActiveStorage::Filename.new(original_filename).sanitized

      return filename if filename.bytesize <= 255

      ext = File.extname(filename)
      basename = File.basename(filename, ext).byteslice(0, 255 - ext.bytesize)

      basename + ext
    end
  end
end