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4d47's solution

to Luhn in the Perl 6 Track

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Instructions
Test suite
Solution

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.

Validating a Number

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.

Example 1: valid credit card number

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!

Example 2: invalid credit card number

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.

Resources

Remember to check out the Perl 6 documentation and resources pages for information, tips, and examples if you get stuck.

Running the tests

There is a test suite and module included with the exercise. The test suite (a file with the extension .t) will attempt to run routines from the module (a file with the extension .pm6). Add/modify routines in the module so that the tests will pass! You can view the test data by executing the command perl6 --doc *.t (* being the name of the test suite), and run the test suite for the exercise by executing the command prove6 . in the exercise directory.

Source

The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

luhn.t

#!/usr/bin/env perl6
use v6;
use Test;
use JSON::Fast;
use lib $?FILE.IO.dirname;
use Luhn;
plan 14;

my $c-data = from-json $=pod.pop.contents;
for $c-data<cases>.values {
  given is-luhn-valid .<input><value> -> $result {
    subtest .<description>, {
      plan 2;
      isa-ok $result, Bool;
      is-deeply $result, .<expected>, 'Result matches expected';
    }
  }
}

=head2 Canonical Data
=begin code
{
  "exercise": "luhn",
  "version": "1.2.0",
  "cases": [
    {
      "description": "single digit strings can not be valid",
      "property": "valid",
      "input": {
        "value": "1"
      },
      "expected": false
    },
    {
      "description": "a single zero is invalid",
      "property": "valid",
      "input": {
        "value": "0"
      },
      "expected": false
    },
    {
      "description": "a simple valid SIN that remains valid if reversed",
      "property": "valid",
      "input": {
        "value": "059"
      },
      "expected": true
    },
    {
      "description": "a simple valid SIN that becomes invalid if reversed",
      "property": "valid",
      "input": {
        "value": "59"
      },
      "expected": true
    },
    {
      "description": "a valid Canadian SIN",
      "property": "valid",
      "input": {
        "value": "055 444 285"
      },
      "expected": true
    },
    {
      "description": "invalid Canadian SIN",
      "property": "valid",
      "input": {
        "value": "055 444 286"
      },
      "expected": false
    },
    {
      "description": "invalid credit card",
      "property": "valid",
      "input": {
        "value": "8273 1232 7352 0569"
      },
      "expected": false
    },
    {
      "description": "valid strings with a non-digit included become invalid",
      "property": "valid",
      "input": {
        "value": "055a 444 285"
      },
      "expected": false
    },
    {
      "description": "valid strings with punctuation included become invalid",
      "property": "valid",
      "input": {
        "value": "055-444-285"
      },
      "expected": false
    },
    {
      "description": "valid strings with symbols included become invalid",
      "property": "valid",
      "input": {
        "value": "055£ 444$ 285"
      },
      "expected": false
    },
    {
      "description": "single zero with space is invalid",
      "property": "valid",
      "input": {
        "value": " 0"
      },
      "expected": false
    },
    {
      "description": "more than a single zero is valid",
      "property": "valid",
      "input": {
        "value": "0000 0"
      },
      "expected": true
    },
    {
      "description": "input digit 9 is correctly converted to output digit 9",
      "property": "valid",
      "input": {
        "value": "091"
      },
      "expected": true
    },
    {
      "description": "strings with non-digits is invalid",
      "property": "valid",
      "input": {
        "value": ":9"
      },
      "expected": false
    }
  ]
}
=end code
unit module Luhn:ver<1>;

sub is-luhn-valid (Str $input --> Bool) is export {
    my @digits = $input.comb(/\S/).reverse;
    return False if @digits ~~ /\D/ || @digits.elems <= 1; 
    { $_ *= 2; $_ -= 9 if $_ > 9 } for @digits[1,3...*];
    @digits.sum %% 10
}

What can you learn from this solution?

A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that I could read more about to develop my understanding?