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rootulp's solution

to Luhn in the Java Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code 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

in your terminal.

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
    public void testValidCanadianSINValid() {
        assertTrue(luhnValidator.isValid("055 444 285"));
    }

    @Ignore("Remove to run test")
    @Test
    public void testInvalidCanadianSINInvalid() {
        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.stream.IntStream;

public class Luhn {

    private long number;
    private long checkDigit;
    private int[] addends;
    private int checkSum;
    private boolean isValid;

    public Luhn(long number) {
        this.number = number;
        this.checkDigit = number % 10;
        this.addends = generateAddends();
        this.checkSum = IntStream.of(addends).sum();
        this.isValid = checkSum % 10 == 0;
    }

    public long getCheckDigit() {
        return checkDigit;
    }

    public int[] getAddends() {
        return addends;
    }

    public int getCheckSum() {
        return checkSum;
    }

    public boolean isValid() {
        return isValid;
    }

    private int[] generateAddends() {
        int[] reversedIntArray = splitToReversedIntArray(number);

        for (int index = 1; index < reversedIntArray.length; index++) {
            if (index % 2 != 0) {
                reversedIntArray[index] = convertDigitForAddend(reversedIntArray[index]);
            }
        }

        return reverseIntArray(reversedIntArray);
    }

    private static int[] splitToReversedIntArray(long value) {
        int[] ints = new StringBuilder(Long.toString(value))
                .reverse()
                .toString()
                .chars()
                .map(x -> Character.getNumericValue(x))
                .toArray();

        return ints;
    }

    private static int convertDigitForAddend(int value) {
        int doubled = value * 2;

        return doubled < 10 ? doubled : doubled - 9;
    }

    public static long create(long number) {
        long zeroCheckDigitNumber = number * 10;
        Luhn luhn = new Luhn(zeroCheckDigitNumber);

        if (luhn.isValid()) {
            return zeroCheckDigitNumber;
        }

        return zeroCheckDigitNumber + createCheckDigit(luhn.getCheckSum());
    }

    private static int createCheckDigit(int value) {
        int nearestTen = (int) (Math.ceil(value / 10.0) * 10);

        return nearestTen - value;
    }

    private static int[] reverseIntArray(int[] array) {
        int[] reversed = new int[array.length];

        for (int index = 0; index < array.length; index++) {
            reversed[index] = array[array.length - 1 - index];
        }

        return reversed;
    }
}

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