Published at Mar 06 2019
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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.

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.

To run the code in this exercise, you will only need to have CommandBox CLI installed. This binary runs CFML code from the command line.

To run the tests, `cd`

into the exercise folder and run the following:

```
box task run TestRunner
# Or start up a test watcher that will rerun when files change
box task run TestRunner --:watcher
```

The tests leverage a library called TestBox which supports xUnit and BDD style of testing. All test suites will be written in the BDD style which uses closures to define test specs. You won't need to worry about installing TestBox. The CLI test runner will take care of that for you. You just need to be connected to the internet the first time you run it. You can read more about it here:

https://testbox.ortusbooks.com/content/

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.

```
component extends="testbox.system.BaseSpec" {
function beforeAll(){
SUT = createObject( 'Luhn' );
}
function run(){
describe( "My Luhn class", function(){
it( 'single digit strings can not be valid', function(){
expect( SUT.valid( value='1' ) ).toBeFalse();
});
it( 'a single zero is invalid', function(){
expect( SUT.valid( value='0' ) ).toBeFalse();
});
it( 'a simple valid SIN that remains valid if reversed', function(){
expect( SUT.valid( value='059' ) ).toBeTrue();
});
it( 'a simple valid SIN that becomes invalid if reversed', function(){
expect( SUT.valid( value='59' ) ).toBeTrue();
});
it( 'a valid Canadian SIN', function(){
expect( SUT.valid( value='055 444 285' ) ).toBeTrue();
});
it( 'invalid Canadian SIN', function(){
expect( SUT.valid( value='055 444 286' ) ).toBeFalse();
});
it( 'invalid credit card', function(){
expect( SUT.valid( value='8273 1232 7352 0569' ) ).toBeFalse();
});
it( 'valid strings with a non-digit included become invalid', function(){
expect( SUT.valid( value='055a 444 285' ) ).toBeFalse();
});
it( 'valid strings with punctuation included become invalid', function(){
expect( SUT.valid( value='055-444-285' ) ).toBeFalse();
});
it( 'valid strings with symbols included become invalid', function(){
expect( SUT.valid( value='055£ 444$ 285' ) ).toBeFalse();
});
it( 'single zero with space is invalid', function(){
expect( SUT.valid( value=' 0' ) ).toBeFalse();
});
it( 'more than a single zero is valid', function(){
expect( SUT.valid( value='0000 0' ) ).toBeTrue();
});
it( 'input digit 9 is correctly converted to output digit 9', function(){
expect( SUT.valid( value='091' ) ).toBeTrue();
});
});
}
}
```

```
component extends="LuhnTest" {
function beforeAll(){
SUT = createObject( 'Solution' );
}
}
```

```
/**
* Your implmentation of the Luhn exercise
*/
component {
/**
* @returns
*/
function valid( value ) {
if (value==0) return false;
value = listToArray(value, "");
count = 1;
oddOrEven = (len(value)+1)%2;
ans = 0;
for (num in value) {
if (isNumeric(num)){
if (count%2==oddOrEven) {
if (num*2>9) num -= 4.5;
ans += num*2;
}
else ans += num;
count--;
}
else if (num!= " ") {
return false;
}
}
return ans%10==0;
}
}
```

A huge amount can be learned 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 you could read more about to improve your understanding?

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