/*

* MIT License

*

* Copyright (c) 2017-2019 Ilkka Seppälä

*

* Permission is hereby granted, free of charge, to any person obtaining a copy

* of this software and associated documentation files (the "Software"), to deal

* in the Software without restriction, including without limitation the rights

* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

* copies of the Software, and to permit persons to whom the Software is

* furnished to do so, subject to the following conditions:

*

* The above copyright notice and this permission notice shall be included in all

* copies or substantial portions of the Software.

*

* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE

* SOFTWARE.

*/

package math;

public class LuhnSnippet {

/**

* Calculates checksum for a given number with Luhn's algorithm. Works only on non-negative

* integers not greater than {@link Long#MAX_VALUE} i.e., all numbers with a maximum of 18

* digits, plus 19-digit-long numbers start with 1..8 (also some with 9, too). For

* demonstration purposes, algorithm is not optimized for efficiency.

*

* @param num number whose checksum is to be calculated

* @return checksum value for num

* @see <a href="https://patents.google.com/patent/US2950048A">Hans P. LUHN's patent US2950048A</a>

* @see <a href="https://en.wikipedia.org/wiki/Luhn_algorithm">Luhn algorithm on Wikipedia</a>

*/

public static int calculateLuhnChecksum(long num) {

if (num < 0) {

throw new IllegalArgumentException("Non-negative numbers only.");

}

final var numStr = String.valueOf(num);

var sum = 0;

var isOddPosition = true;

// Loop on digits of numStr from right to left.

for (var i = numStr.length() - 1; i >= 0; i--) {

final var digit = Integer.parseInt(Character.toString(numStr.charAt(i)));

final var substituteDigit = (isOddPosition ? 2 : 1) * digit;

final var tensPlaceDigit = substituteDigit / 10;

final var onesPlaceDigit = substituteDigit % 10;

sum += tensPlaceDigit + onesPlaceDigit;

isOddPosition = !isOddPosition;

}

final var checksumDigit = (10 - (sum % 10)) % 10;

// Outermost modulus handles edge case `num = 0`.

return checksumDigit;

}

}