FLV Duration in Ruby

def duration(filename)
    f = File.open(filename, 'r')
    raise "Could not open File!" unless f

    #check for flv text
    unless f.read(3) === 'FLV'
      raise "This does not seem to be an FLV file."
    end

    #check to make sure we are a video
    f.seek(3)
    is_video = f.read(1).unpack("H*").first.hex
    unless is_video == 1
      raise "This FLV file does not contain video."
    end

    #seek to end, get size
    f.seek(0, IO::SEEK_END)
    length = f.tell

    #calculate tag length
    f.seek(-4, IO::SEEK_END)
    taglen = f.read(4).unpack("H*").first.hex

    #finally get duration
    f.seek(length - taglen, IO::SEEK_SET)
    duration = f.read(3).unpack("H*").first.hex.to_f/1000

    f.close()
    #duration returns in seconds
    duration
  end