Quickly get the duration of a remote MP3 without fetching the whole thing

get_remote_mp3_length.rb

require 'uri'
require 'net/http'
require 'mp3info'

# This works, but is flawed and needs work.
# Why? An ID3v2 tag is not always going to
# be within the first 8K of a file! It can
# be up to 256MB in length apparently.. so
# we need to flexibly scale. But this works
# 99% of the time..

def get_mp3_length(url)
  uri = URI(url)
  http = Net::HTTP.new(uri.host, uri.port)

  # Make a request for only the first 8K of the MP3
  request = Net::HTTP::Get.new(uri.path)
  request['Range'] = 'bytes=0-8192'
  response = http.request(request)

  # Grab out the full length of the remote file
  bytes = response.to_hash['content-range'].first[/0-\d+\/(\d+)/, 1].to_i

  # Open the MP3 chunk we have and get the bitrate info
  mp3 = Mp3Info.open(StringIO.new(response.body))
  bitrate = mp3.bitrate * 1000 / 8  
  mp3.close

  # File size divided by bitrate equals approximate length
  # Obviously, VBR can throw things off slightly
  bytes.to_f / bitrate
end

puts get_mp3_length(ARGV.first)

Hmm, seems this only works with straightforward setups, and not servers going heavy on the chunking..