Convertir en paralelo

#!/usr/bin/env ruby

module ForkEach
  def fork_each(max = 4)
    pids = []
    self.each do |item|
      pids << fork { yield item }
      pids.delete(Process.wait 0) if pids.length >= max
    end
    pids.each { |pid| Process.wait pid }
  end
end

Array.send(:include, ForkEach)
Range.send(:include, ForkEach)

# código que convierte archivos en paralelo
require 'find'

prc = 4
cmd = "/Applications/calibre.app/Contents/MacOS/ebook-convert"
dir = '/Volumnes/DISCO'
ini = /\.fb2$/
fin = '.epub'

items = Find.find(dir).select do |path|
  next unless path =~ ini && File.file?(path)
  result = !File.exist?(path.sub(ini, fin))
  puts result ? "process  #{path}" : "skipping #{path}"
  result
end

print "ENTER para continuar, CTRL+C para cancelar "
gets

items.fork_each(prc) do |src|
  dst = src.sub(ini, fin)
  puts "#{cmd} #{src} → #{dst}"
  exec(cmd, src, dst)
end