# shmibs's solution

## to Luhn in the OCaml Track

Published at Dec 14 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution was written.

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.

## Getting Started

For installation and learning resources, refer to the exercism help page.

## Installation

To work on the exercises, you will need `Opam` and `Base`. Consult opam website for instructions on how to install `opam` for your OS. Once `opam` is installed open a terminal window and run the following command to install base:

``````opam install base
``````

To run the tests you will need `OUnit`. Install it using `opam`:

``````opam install ounit
``````

## Running Tests

A Makefile is provided with a default target to compile your solution and run the tests. At the command line, type:

``````make
``````

## Interactive Shell

`utop` is a command line program which allows you to run Ocaml code interactively. The easiest way to install it is via opam:

``````opam install utop
``````

Consult utop for more detail.

## Feedback, Issues, Pull Requests

The exercism/ocaml repository on GitHub is the home for all of the Ocaml exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!

## 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.

### test.ml

``````open Base
open OUnit2
open Luhn

let assert_valid expected input _test_ctxt =
assert_equal ~printer:Bool.to_string expected (valid input)

let tests = [
"single digit strings can not be valid" >::
assert_valid false "1";
"a single zero is invalid" >::
assert_valid false "0";
"a simple valid SIN that remains valid if reversed" >::
assert_valid true "059";
"a simple valid SIN that becomes invalid if reversed" >::
assert_valid true "59";
assert_valid true "055 444 285";
assert_valid false "055 444 286";
"invalid credit card" >::
assert_valid false "8273 1232 7352 0569";
"valid strings with a non-digit included become invalid" >::
assert_valid false "055a 444 285";
"valid strings with punctuation included become invalid" >::
assert_valid false "055-444-285";
"valid strings with symbols included become invalid" >::
assert_valid false "055\194\163 444\$ 285";
"single zero with space is invalid" >::
assert_valid false " 0";
"more than a single zero is valid" >::
assert_valid true "0000 0";
"input digit 9 is correctly converted to output digit 9" >::
assert_valid true "091";
"strings with non-digits is invalid" >::
assert_valid false ":9";
]

let () =
run_test_tt_main ("luhn tests" >::: tests)``````
``````open Core

module CS = Base.Continue_or_stop

let valid (s : string) =
String.to_list_rev s
|> List.fold_until ~init:(false, 0, 0)
~f:(fun (_, len, sum) c ->
if c = ' ' then (* skip spaces *)
CS.Continue (false, len, sum)
else if (Char.is_digit c) = false then (* err on non-digits *)
CS.Stop (true, len, sum)
else (* calculate length and sum *)
let ival = if (Int.rem len 2) <> 0 then
(Char.get_digit_exn c) * 2
else
Char.get_digit_exn c
in
let ival = if ival > 9 then ival - 9 else ival in
CS.Continue (false, len + 1, sum + ival)
)
if bad || len < 2 || (Int.rem sum 10) <> 0 then
(true, len, sum)
else
(false, len, sum)
)