Source Code
/**
* This class impliments the
* <a href="http://en.wikipedia.org/wiki/Luhn_formula">LUHN formula</a> in order
* to check if a <a href="http://en.wikipedia.org/wiki/CUSIP">CUSIP</A>
* is valid.
* @author eggebr
*/
public class LUHN {
public static boolean isValid(String ccNum) {
char number[] = ccNum.toCharArray();
int len = number.length;
int sum = 0;
for (int i = 0; i < len - 1; i++) {
int num = mapChar(number[i]);
// Double all the odd digits
if (i % 2 != 0)
num *= 2;
// Combine digits. i.e., 16 = (1 + 6) = 7
if (num > 9)
num = (num % 10) + (num / 10);
sum += num;
}
int checkDigit = mapChar(number[number.length - 1]);
// This is the mathmatical modulus - not the remainder. i.e., 10 mod 7 = 3
int mod = (10 - (sum % 10)) % 10;
if (mod == checkDigit) {
return true;
}
return false;
}
/**
* Standard & Poor's maps A..Z to 10..35
* @param c
* @return numeric value of the letter
*/
private static int mapChar(char c) {
if (c >= '0' && c <= '9')
return c - '0';
return c - 'A' + 10;
}
}
Test case
public class LUHNTest extends TestCase {
public void testCUSIP() {
assertTrue(LUHN.isValid("31430F200"));
assertTrue(LUHN.isValid("037833100")); // Apple
assertTrue(LUHN.isValid("316549401"));
assertTrue(LUHN.isValid("022615AC2"));
assertTrue(LUHN.isValid("931142103")); // Walmart
assertTrue(LUHN.isValid("883199AQ4"));
assertTrue(LUHN.isValid("14911QGW5"));
assertTrue(LUHN.isValid("14911QHM6"));
assertTrue(LUHN.isValid("339072407"));
assertTrue(LUHN.isValid("3133XBC78"));
assertTrue(LUHN.isValid("808905AB9"));
assertFalse(LUHN.isValid("2005"));
assertFalse(LUHN.isValid("?"));
}
public void testCreditCards() {
assertTrue(Luhn.isValid("3850074292251725"));
assertTrue(Luhn.isValid("4242540460361996"));
assertTrue(Luhn.isValid("6030624550510273"));
assertTrue(Luhn.isValid(CreateNumber(16)));
assertTrue(Luhn.isValid(CreateNumber(13)));
}
private String CreateNumber(int length) {
Random random = new Random();
int[] digits = new int[length];
for (int i = 0; i < length - 1; i++) {
digits[i] = random.nextInt(10);
}
int sum = 0;
boolean alt = true;
for (int i = length - 2; i >= 0; i--) {
if (alt) {
int temp = digits[i];
temp *= 2;
if (temp > 9) {
temp -= 9;
}
sum += temp;
} else {
sum += digits[i];
}
alt = !alt;
}
int modulo = sum % 10;
if (modulo > 0) {
digits[length - 1] = 10 - modulo;
}
String out = "";
for(int n : digits) {
out += Integer.toString(n);
}
return out;
}
}