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 tests run the command `go test`

from within the exercise directory.

If the test suite contains benchmarks, you can run these with the `--bench`

and `--benchmem`

flags:

```
go test -v --bench . --benchmem
```

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.

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.

```
package luhn
// Source: exercism/problem-specifications
// Commit: 4a80663 luhn: non-digit at end is invalid
// Problem Specifications Version: 1.4.0
var testCases = []struct {
description string
input string
ok bool
}{
{
"single digit strings can not be valid",
"1",
false,
},
{
"a single zero is invalid",
"0",
false,
},
{
"a simple valid SIN that remains valid if reversed",
"059",
true,
},
{
"a simple valid SIN that becomes invalid if reversed",
"59",
true,
},
{
"a valid Canadian SIN",
"055 444 285",
true,
},
{
"invalid Canadian SIN",
"055 444 286",
false,
},
{
"invalid credit card",
"8273 1232 7352 0569",
false,
},
{
"valid number with an even number of digits",
"095 245 88",
true,
},
{
"valid strings with a non-digit included become invalid",
"055a 444 285",
false,
},
{
"valid strings with a non-digit added at the end become invalid",
"059a",
false,
},
{
"valid strings with punctuation included become invalid",
"055-444-285",
false,
},
{
"valid strings with symbols included become invalid",
"055£ 444$ 285",
false,
},
{
"single zero with space is invalid",
" 0",
false,
},
{
"more than a single zero is valid",
"0000 0",
true,
},
{
"input digit 9 is correctly converted to output digit 9",
"091",
true,
},
{
"strings with non-digits is invalid",
":9",
false,
},
}
```

```
package luhn
import "testing"
func TestValid(t *testing.T) {
for _, test := range testCases {
if ok := Valid(test.input); ok != test.ok {
t.Fatalf("Valid(%s): %s\n\t Expected: %t\n\t Got: %t", test.input, test.description, test.ok, ok)
}
}
}
func BenchmarkValid(b *testing.B) {
for i := 0; i < b.N; i++ {
Valid("2323 2005 7766 3554")
}
}
```

```
package luhn
// Valid determines whether or not given a number is valid per the Luhn formula.
func Valid(input string) bool {
index := 1
luhnSum := 0
for i := len(input) - 1; i >= 0; i-- {
c := input[i]
if c == ' ' {
continue
}
if c < '0' || c > '9' {
return false
}
num := int(c) - '0'
if index%2 == 0 {
num *= 2
if num > 9 {
num -= 9
}
}
luhnSum += num
index++
}
return index > 2 && luhnSum%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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