# artemkorsakov's solution

## to Luhn in the Java Track

Published at Feb 06 2019 · 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.

# Running the tests

You can run all the tests for an exercise by entering

``````\$ gradle test
``````

## 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.

### LuhnValidatorTest.java

``````import org.junit.Ignore;
import org.junit.Before;
import org.junit.Test;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

public class LuhnValidatorTest {
private LuhnValidator luhnValidator;

@Before
public void setUp() {
luhnValidator = new LuhnValidator();
}

@Test
public void testSingleDigitStringInvalid() {
assertFalse(luhnValidator.isValid("1"));
}

@Ignore("Remove to run test")
@Test
public void testSingleZeroIsInvalid() {
assertFalse(luhnValidator.isValid("0"));
}

@Ignore("Remove to run test")
@Test
public void testSimpleValidSINReversedRemainsValid() {
assertTrue(luhnValidator.isValid("059"));
}

@Ignore("Remove to run test")
@Test
public void testSimpleValidSINReversedBecomesInvalid() {
assertTrue(luhnValidator.isValid("59"));
}

@Ignore("Remove to run test")
@Test
assertTrue(luhnValidator.isValid("055 444 285"));
}

@Ignore("Remove to run test")
@Test
assertFalse(luhnValidator.isValid("055 444 286"));
}

@Ignore("Remove to run test")
@Test
public void testInvalidCreditCardInvalid() {
assertFalse(luhnValidator.isValid("8273 1232 7352 0569"));
}

@Ignore("Remove to run test")
@Test
public void testStringsContainingNonDigitInvalid() {
assertFalse(luhnValidator.isValid("055a 444 285"));
}

@Ignore("Remove to run test")
@Test
public void testStringContainingPunctuationInvalid() {
assertFalse(luhnValidator.isValid("055-444-285"));
}

@Ignore("Remove to run test")
@Test
public void testStringContainingSymbolsInvalid() {
assertFalse(luhnValidator.isValid("055£ 444\$ 285"));
}

@Ignore("Remove to run test")
@Test
public void testSingleSpaceWithZeroInvalid() {
assertFalse(luhnValidator.isValid(" 0"));
}

@Ignore("Remove to run test")
@Test
public void testMoreThanSingleZeroValid() {
assertTrue(luhnValidator.isValid("0000 0"));
}

@Ignore("Remove to run test")
@Test
public void testDigitNineConvertedToOutputNine() {
assertTrue(luhnValidator.isValid("091"));
}

@Ignore("Remove to run test")
@Test
public void testStringsWithNonDigitsInvalid() {
assertFalse(luhnValidator.isValid(":9"));
}

/* The following test diverges from the canonical test data. This is because the corresponding canonical test does
* not account for Java specific functions (such as Character.getNumericValue()), which can be part of incorrect yet
* passing implementations. For more detail, check out issue #972 here:
* (https://github.com/exercism/java/issues/972).
*/
@Ignore("Remove to run test")
@Test
public void testStringContainingSymbolsInvalidJavaTrackSpecific() {
assertFalse(luhnValidator.isValid("85&"));
}
}``````
``````import java.util.regex.Matcher;
import java.util.regex.Pattern;

class LuhnValidator {

boolean isValid(String candidate) {
if (!isValidSymbols(candidate)) {
return false;
}

candidate = deleteNonDigit(candidate);
if (candidate.length() < 2) {
return false;
}

int sum = getLuhnSum(candidate);
return sum % 10 == 0;
}

private static boolean isValidSymbols(String candidate) {
Pattern p = Pattern.compile("^[0-9\\s]+\$");
Matcher m = p.matcher(candidate);
return m.matches();
}

private static String deleteNonDigit(String candidate) {
Pattern p = Pattern.compile("\\D");
Matcher m = p.matcher(candidate);
return m.replaceAll("");
}

private static int getLuhnSum(String candidate) {
int sum = 0;
for (int i = candidate.length() - 1; i >= 0; i--) {
int num = Character.getNumericValue(candidate.charAt(i));
if ((candidate.length() - i) % 2 == 0) {
num = 2 * num;
num = num > 9 ? num - 9 : num;
}
sum += num;
}
return sum;
}

}``````