def calculate_checksum(file)
      @lookup_checksums[file] = Digest::MD5.hexdigest(File.read(file, mode: "rb")) rescue ""
    end
def calculate_checksum(file)
      @lookup_checksums[file] = Digest::MD5.hexdigest(File.read(file, mode: "rb")) rescue ""
    end
Calculate and return a file's checksum.

 @param file [String] The file name.

 @return [String] The file's checksum.