# Convert ISBN as 10-digit string to 13-digit string
# Can handle dashes, returns nil if the ISBN is not 10 digits
# Also returns the ISBN if ISBN is 13 digits and begins with 978 or 979
# Por ejemplo: to13("0940016737")
def to13(isbn)
	isbn = clean(isbn)
	if isbn.length == 13 && ["978","979"].member?(isbn[0..2])
		return isbn
	elsif isbn.length != 10
		return nil
	end
	new_isbn = "978" + isbn.chop
	check = 0
	new_isbn.split(//).each_with_index do |digit, index|
		if index % 2 == 0
			check += digit.to_i * 1
		else
			check += digit.to_i * 3
		end
	end
	check = if check % 10 == 0 then 0 else 10 - (check % 10) end
	new_isbn + check.to_s
end

# Convert ISBN as 13-digit string to 10-digit string
# Can handle dashes, returns nil if the ISBN is not 13 digits
# Por ejemplo: to10("978-0-940016-73-6")
def to10(isbn)
	isbn = clean(isbn)
	return nil unless isbn.length == 13 && isbn[0..2] != "979"
	new_isbn = isbn.chop[3..(isbn.length - 1)]
	check = 0
	10.downto(2) { |n| check += new_isbn[(10 - n)..(10 - n)].to_i * n }
	check = 11 - (check % 11)
	check = "X" if check == 10
	new_isbn + check.to_s
end

# This helper method just cleans up ISBNs
def clean(isbn)
	isbn.gsub(/[^0-9X]/,'')
end