Given a number determine whether or not it is valid per the Luhn formula.
The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.
The task is to check if a given string is valid.
Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.
4539 1488 0343 6467
The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling
4_3_ 1_8_ 0_4_ 6_6_
If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:
8569 2478 0383 3437
Then sum all of the digits:
8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80
If the sum is evenly divisible by 10, then the number is valid. This number is valid!
8273 1232 7352 0569
Double the second digits, starting from the right
7253 2262 5312 0539
Sum the digits
7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
57 is not evenly divisible by 10, so this number is not valid.
For installation and learning resources, refer to the Ruby resources page.
For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:
gem install minitest
If you would like color output, you can
require 'minitest/pride' in
the test file, or note the alternative instruction, below, for running
the test file.
Run the tests from the exercise directory using the following command:
To include color from the command line:
ruby -r minitest/pride luhn_test.rb
The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
require 'minitest/autorun' require_relative 'luhn' # Common test data version: 1.4.0 4a80663 class LuhnTest < Minitest::Test def test_single_digit_strings_can_not_be_valid # skip refute Luhn.valid?("1") end def test_a_single_zero_is_invalid skip refute Luhn.valid?("0") end def test_a_simple_valid_sin_that_remains_valid_if_reversed skip assert Luhn.valid?("059") end def test_a_simple_valid_sin_that_becomes_invalid_if_reversed skip assert Luhn.valid?("59") end def test_a_valid_canadian_sin skip assert Luhn.valid?("055 444 285") end def test_invalid_canadian_sin skip refute Luhn.valid?("055 444 286") end def test_invalid_credit_card skip refute Luhn.valid?("8273 1232 7352 0569") end def test_valid_number_with_an_even_number_of_digits skip assert Luhn.valid?("095 245 88") end def test_valid_strings_with_a_non_digit_included_become_invalid skip refute Luhn.valid?("055a 444 285") end def test_valid_strings_with_a_non_digit_added_at_the_end_become_invalid skip refute Luhn.valid?("059a") end def test_valid_strings_with_punctuation_included_become_invalid skip refute Luhn.valid?("055-444-285") end def test_valid_strings_with_symbols_included_become_invalid skip refute Luhn.valid?("055£ 444$ 285") end def test_single_zero_with_space_is_invalid skip refute Luhn.valid?(" 0") end def test_more_than_a_single_zero_is_valid skip assert Luhn.valid?("0000 0") end def test_input_digit_9_is_correctly_converted_to_output_digit_9 skip assert Luhn.valid?("091") end def test_strings_with_non_digits_is_invalid skip refute Luhn.valid?(":9") end end
class Luhn def self.valid?(number) new(number).valid? end def initialize(number) @number = sanitize(number) end def valid? return false if number.length <= 1 || number =~ /\D/ (checksum % 10).zero? end private def sanitize(number) number.strip.gsub(/\s/, "") end def checksum digits.reverse.each_slice(2).sum do |even, odd = 0| even + double(odd) end end def digits number.scan(/\d/).map(&:to_i) end def double(digit) doubled_digit = digit * 2 return doubled_digit if doubled_digit < 10 doubled_digit - 9 end attr_reader :number end
Things I learned while working on this exercise and want to remember:
1. There's no need to name variables using the type.
2. The naming of a method should hide its complexity. If we want to know more about a method, we can always read it.
3. Using indexes for side effects is a smell.
4. When doing something like sanitizing the input, we don't need to be constantly reminded that the input is sanitized. (This is related to point 2.)
5. Remember that we can pass a block to `#sum`. Also remember to check the documentation for the methods that we are thinking to use, to see what we can do and solidify past knowledge.
6. We can set default values for block arguments: `something.sum do |even, odd = 0|`
7. Don't assume that using `#map` would make something significantly less performant. If worried about performance, use `Benchmark` to check if there really is a significant difference.
Many thanks to `@ajoshguy` for his help.