rakefile.rb

require 'fileutils'

FLAC_CMD = '/usr/bin/flac'
LAME_CMD = '/usr/bin/lame'

NUMBER_OF_THREADS = 6

desc "Convert all FLAC's to mp3s and delete source" 
task :convert  do

  base_path = File.expand_path('..', __FILE__)

  convert_cmds = Dir["#{base_path}/**/*.flac"].inject([]) do |cmds, flac|
    output_directory = File.dirname flac

    file_name = File.basename(flac, '.flac')

    wav_file = File.join(output_directory, "#{file_name}.wav")
    mp3_file = File.join(output_directory, "#{file_name}.mp3")

    decode_cmd = %Q{#{FLAC_CMD} -f -d -o "#{wav_file}" "#{flac}"}
    encode_cmd = %Q{#{LAME_CMD} --quiet -h -b 320 --cbr "#{wav_file}" "#{mp3_file}"}
    clean_up_cmd = %Q{rm "#{wav_file}" && rm "#{flac}"}

    convert_cmd = "#{decode_cmd} && #{encode_cmd} && #{clean_up_cmd}"
    cmds << convert_cmd
    cmds
  end

  queues = []
  convert_cmds.each_index do |i|
    num = i % NUMBER_OF_THREADS
    queues[num] ||= []
    queues[num] << convert_cmds[i]
  end

  threads = []

  queues.each do |queue|
    threads << Thread.new(queue) do |q|
      q.each {|cmd| `#{cmd}`}
    end
  end

  threads.each {|thread| thread.join}

end