require 'net/ftp'
require 'tmpdir'
require 'thwait'

DEBUG = !ENV.fetch('DEBUG', '').empty?
CONCURRENT = ENV.fetch('CONCURRENT').to_i
FTP_HOST = ENV.fetch('FTP_HOST')
FTP_PORT = ENV.fetch('FTP_PORT').to_i
FTP_USER = ENV.fetch('FTP_USER')
FTP_PASS = ENV.fetch('FTP_PASS')
FTP_PATH = ENV.fetch('FTP_PATH')

def establish_connection
  Net::FTP.new(FTP_HOST, port: FTP_PORT, username: FTP_USER, password: FTP_PASS, passive: true, debug_mode: DEBUG)
end

content_size = establish_connection.size(FTP_PATH)
page_size = (content_size.to_f / CONCURRENT).ceil

Dir.mktmpdir do |dir|
  threads = []
  CONCURRENT.times do |i|
    offset = page_size * i
    next if offset >= content_size

    threads << Thread.start(i) do |t|
      File.open("#{dir}/#{t}", 'wb') do |page|
        ftp = establish_connection
        ftp.resume = true

        retrieved = 0
        ftp.retrbinary("RETR #{FTP_PATH}", Net::FTP::DEFAULT_BLOCKSIZE, offset) do |chunk|
          rest = page_size - retrieved
          retrievable = [rest, chunk.bytesize].min
          break if retrievable <= 0

          retrieved += page.write(chunk[0, retrievable])
        end
      end
    end
  end
  ThreadsWait.all_waits(*threads)

  File.open('downloaded', 'wb') do |output|
    CONCURRENT.times do |i|
      path = "#{dir}/#{i}"
      next unless File.exist?(path)
      IO.copy_stream(path, output)
    end
  end
end

__END__

### e.g. 495MB CentOS ISO file

ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]

1: 165.29 real         5.34 user         4.74 sys
3:  99.68 real         5.23 user         6.12 sys
5:  88.19 real         5.56 user         7.43 sys