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Markdown
79 lines
No EOL
1.6 KiB
Markdown
# Luhn
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Welcome to Luhn on Exercism's Zig Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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## Instructions
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Given a number determine whether or not it is valid per the Luhn formula.
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The [Luhn algorithm][luhn] is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.
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The task is to check if a given string is valid.
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## Validating a Number
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Strings of length 1 or less are not valid.
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Spaces are allowed in the input, but they should be stripped before checking.
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All other non-digit characters are disallowed.
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### Example 1: valid credit card number
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```text
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4539 3195 0343 6467
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```
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The first step of the Luhn algorithm is to double every second digit, starting from the right.
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We will be doubling
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```text
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4_3_ 3_9_ 0_4_ 6_6_
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```
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If doubling the number results in a number greater than 9 then subtract 9 from the product.
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The results of our doubling:
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```text
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8569 6195 0383 3437
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```
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Then sum all of the digits:
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```text
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8+5+6+9+6+1+9+5+0+3+8+3+3+4+3+7 = 80
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```
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If the sum is evenly divisible by 10, then the number is valid.
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This number is valid!
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### Example 2: invalid credit card number
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```text
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8273 1232 7352 0569
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```
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Double the second digits, starting from the right
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```text
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7253 2262 5312 0539
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```
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Sum the digits
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```text
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7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
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```
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57 is not evenly divisible by 10, so this number is not valid.
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[luhn]: https://en.wikipedia.org/wiki/Luhn_algorithm
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## Source
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### Created by
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- @ee7
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### Based on
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The Luhn Algorithm on Wikipedia - https://en.wikipedia.org/wiki/Luhn_algorithm |