I was working with hardware that was sequential in coming off a production line. Looking at the hardwares’ serial number, I realized the numbers were mostly sequential, with the exception of the suffix on the serial number. I realized that I was looking at numbers suffixed with a checksum. I tried out a few checksum algorithms to figure out which one fit. This was only being done from curiosity.
One algorithm of special interest was Luhn’s algorithm. This is the algorithm that is used with IMEI, credit card numbers, and Canadian social security numbers. It’s a useful public domain algorithm. I’ve collected a few implementations of it here.
C++
bool pasesLuhnCheck(const string& cardNo)
{
int nDigits = cardNo.length();
int nSum = 0, isSecond = false;
for (int i = nDigits - 1; i >= 0; i--) {
int d = cardNo[i] - '0';
if (isSecond == true)
d = d * 2;
// We add two digits to handle
// cases that make two digits after
// doubling
nSum += d / 10;
nSum += d % 10;
isSecond = !isSecond;
}
return (nSum % 10 == 0);
}
C#
public static bool CheckLuhnParity(string digits)
{
bool isValid = false;
if (!string.IsNullOrEmpty(digits))
{
long sum = 0;
int parity = digits.Length % 2;
for (int i = 0; i < digits.Length; i++)
{
int digit = digits[^(i + 1)] - '0';
sum += (i % 2 == parity) ? Luhn(digit) : digit;
}
isValid = (sum % 10) == 0;
}
return isValid;
}
Kotlin
fun IsValidLuhnString(entry:String): Boolean {
var sum = 0
var isSecond = false
for( i in entry.length-1..0) {
val multiplier = if(isSecond) { 2} else {1}
val digit = entry[i].digitToInt() * multiplier
sum += digit / 10;
sum += digit % 10
isSecond = !isSecond
}
return (sum % 10) == 0
}
JavaScript
const luhnCheck =( num, radix) => {
if(radix == null) {
radix = 10
}
const arr = `${num}`
.split('')
.reverse()
.map(x => Number.parseInt(x, radix));
const lastDigit = arr.shift();
let sum = arr.reduce(
(acc, val, i) =>
i % 2 !== 0 ? acc + val : acc + ((val *= 2) > 9 ? val - 9 : val),
0
);
sum += lastDigit;
return sum % 10 === 0;
};
You can find a JavaScript/HTML implementation of it at https://j2inet.github.io/apps/luhns. Though this example doesn’t engage in any network communication, while I encourage you to try it out, please don’t use your bank card. Better yet, if you want to examine the code locally, there is also a download link on that page that contains all of the source code.
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