require 'net/ftp'
require 'active_support/time'

#settings
delay = 30 #in minutes
ftp_delay = 1 #in minutes
ftp_server_address = 'ftp.example.com'
ftp_user = 'example_user'
ftp_password = 'example_password'
ftp_directory = 'bk1'
save_as = 'bk1_most_recent.jpg'
ftp_passive = true

def grab_latest_image(ftp_server_address, ftp_user, ftp_password,
                      ftp_passive, save_as, ftp_directory)
  ftp = Net::FTP.new(ftp_server_address, ftp_user, ftp_password)
  ftp.passive = ftp_passive
  files = ftp.chdir(ftp_directory)
  files = ftp.list('*.jpg')
  filename_most_recent = files.last.match /([0-9._-]*.jpg)/
  minutes_for_calc = filename_most_recent.to_s.match /_[0-9]{2}_([0-9]{2})/
  ftp.getbinaryfile(filename_most_recent, save_as)
  ftp.close
  return minutes_for_calc[1]
end

def calculate_delay(minutes_for_calc, ftp_delay, delay)
  current_time = Time.now.min
  wait_minutes = (minutes_for_calc.to_i + delay.to_i) - current_time.to_i
  if wait_minutes < 0
    wait_minutes *= -1
  end
  wait_minutes += ftp_delay
  return wait_minutes.minutes
end

loop do
  minutes_for_calc = grab_latest_image(ftp_server_address, ftp_user,
                                       ftp_password, ftp_passive, save_as, ftp_directory)

  calculated_delay = calculate_delay(minutes_for_calc, ftp_delay, delay)
  sleep calculated_delay
end