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60 lines (50 loc) · 1.35 KB
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package org.luhn;
public class Luhn {
/**
* Validate a number string using Luhn algorithm
*
* @param numberString
* @return
*/
public static boolean validate(String numberString) {
return checkSum(numberString) == 0;
}
/**
* Generate check digit for a number string. Assumes check digit or a place
* holder is already appended at end of the string.
*
* @param numberString
* @return
*/
public static int checkSum(String numberString) {
return checkSum(numberString, false);
}
/**
* Generate check digit for a number string.
*
* @param numberString
* @param noCheckDigit
* Whether check digit is present or not. True if no check Digit
* is appended.
* @return
*/
public static int checkSum(String numberString, boolean noCheckDigit) {
int sum = 0, checkDigit = 0;
if(!noCheckDigit)
numberString = numberString.substring(0, numberString.length()-1);
boolean isDouble = true;
for (int i = numberString.length() - 1; i >= 0; i--) {
int k = Integer.parseInt(String.valueOf(numberString.charAt(i)));
sum += sumToSingleDigit((k * (isDouble ? 2 : 1)));
isDouble = !isDouble;
}
if ((sum % 10) > 0)
checkDigit = (10 - (sum % 10));
return checkDigit;
}
private static int sumToSingleDigit(int k) {
if (k < 10)
return k;
return sumToSingleDigit(k / 10) + (k % 10);
}
}