Sutto · December 16, 2010 03:10 · Dec 16, 2010 · Dec 16, 2010
diff --git a/luhn.rb b/luhn.rb
@@ -1,19 +1,30 @@
-def luhn_valid?(value)
-  return false unless value.is_a?(String) && !value.strip.empty?
-
-  value.gsub!(/\D/, '')
-
-  sum = 0
-
-  value.split(//).each_with_index do |number, index|
-
+def luhn_valid?(number)
+  return unless number.is_a?(String)
+  # Convert to a reversed list of digits
+  digits = number.scan(/\d/).map(&:to_i).reverse
+  # No numbers in the string
+  return false if digits.empty?
+  check_digit = digits.shift
+  # For each of the normal integers, compute the correct sum
+  final_digits = []
+  digits.each_with_index do |value, index|
+    # Second from the left is double.
     if index.even?
-      sum += (number * 2).to_s.split(//).map(&:to_i).sum
+      final_digits << (value * 2).to_s.split('').map(&:to_i)
     else
-      sum += number
+      final_digits << value
     end
-
   end
-
-  sum % 10 == 0
-end
+  # Actually compute the product, including check digit.
+  (final_digits.flatten.inject(:+) + check_digit) % 10 == 0
+end
+
+puts "#{luhn_valid?('4111 1111 1111 1111')} should be true"
+puts "#{luhn_valid?('5500 0000 0000 0004')} should be true"
+puts "#{luhn_valid?('3400 0000 0000 009')} should be true"
+puts "#{luhn_valid?('3000 0000 0000 04')} should be true"
+puts "#{luhn_valid?('3000 0000 0000 04')} should be true"
+puts "#{luhn_valid?('6011 0000 0000 0004')} should be true"
+puts "#{luhn_valid?('2014 0000 0000 009')} should be true"
+puts "#{luhn_valid?('3088 0000 0000 0009')} should be true"
+puts "#{luhn_valid?('3088 0000 0000 0008')} should be false"
diff --git a/luhn.rb b/luhn.rb
@@ -0,0 +1,19 @@
+def luhn_valid?(value)
+  return false unless value.is_a?(String) && !value.strip.empty?
+
+  value.gsub!(/\D/, '')
+
+  sum = 0
+
+  value.split(//).each_with_index do |number, index|
+
+    if index.even?
+      sum += (number * 2).to_s.split(//).map(&:to_i).sum
+    else
+      sum += number
+    end
+
+  end
+
+  sum % 10 == 0
+end
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