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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

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.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;
    }

}

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