class ActiveStorage::Previewer::MuPDFPreviewer

def accept?(blob)
  pdf?(blob.content_type) && mutool_exists?
end

def draw_first_page_from(file, &block)
  draw self.class.mutool_path, "draw", "-F", "png", "-o", "-", file.path, "1", &block
end

def mutool_exists?
  return @mutool_exists unless @mutool_exists.nil?
  system mutool_path, out: File::NULL, err: File::NULL
  @mutool_exists = $?.exitstatus == 1
end

def mutool_path
  ActiveStorage.paths[:mutool] || "mutool"
end

def pdf?(content_type)
  Marcel::Magic.child? content_type, "application/pdf"
end

def preview(**options)
  download_blob_to_tempfile do |input|
    draw_first_page_from input do |output|
      yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png", **options
    end
  end
end