credit_check.rb

num = gets.chomp

splits = num.split(//)

card_number = []

card_number << splits[-2]

card_number << splits[-4]

card_number << splits[-6]

card_number << splits[-8]

card_number << splits[-10]

card_number << splits[-12]

card_number << splits[-14]

card_number << splits[-16]

card_number.reverse!

  doubles = card_number.map do |num|
	num.to_i*2
  end

  doubled_digits = doubles.map do |num|
 	 if num > 9
 	    	num - 9
 	 else num < 9
 		num
 	 end
  end

undoubled = []

undoubled << splits[-1]

undoubled << splits[-3]

undoubled << splits[-5]

undoubled << splits[-7]

undoubled << splits[-9]

undoubled << splits[-11]

undoubled << splits[-13]

undoubled << splits[-15]

undoubled.reverse!

untouched_digits = undoubled

	remainder_digits = untouched_digits.map do |num|
 		num.to_i
 	end

sum_1 = remainder_digits.reduce(:+)

sum_2 = doubled_digits.reduce(:+)

summed_digits = sum_1 + sum_2

	if summed_digits % 10 == 0
		puts "This number is valid"
	else 
		puts "This number is not valid"
	end

# 	Valid: 5541808923795240, 4024007136512380, 6011797668867828 	Ran these numbers, and they are valid
# 	Invalid: 5541801923795240, 4024007106512380, 6011797668868728	Ran these, and they are invalid
# 	Tested a credit card and it was valid!