convert isbn13 to isbn10

convert_isbn13_10.rb

# isbn is isbn13
def to_isbn10(isbn)
  digits = isbn.slice(3,9).split("")
  sum = 0

  (0..9).each do |i|
    sum += digits[i].to_i * (10 - i)
  end

  chk_tmp = 11 - (sum % 11)
  chk_digit = ""

  if chk_tmp == 10
    chk_digit = 'x'
  elsif chk_tmp == 11
    chk_digit = 0
  else
    chk_digit = chk_tmp
  end

  digits.push(chk_digit)
  return digits.join("")
end