fix: Luhn generation algorithms and tests (#980)

Co-authored-by: ST-DDT <[email protected]>

2 files changed, 44 insertions, 21 deletions

diff --git a/src/modules/helpers/index.ts b/src/modules/helpers/index.ts
index ac302c6d..a68b66ae 100644
--- a/src/modules/helpers/index.ts
+++ b/src/modules/helpers/index.ts
@@ -1,4 +1,5 @@
 import type { Faker } from '../..';
+import { luhnCheckValue } from './luhn-check';
 
 /**
  * Module with various helper methods that transform the method input rather than returning values from locales.
@@ -141,30 +142,10 @@ export class Helpers {
   ): string {
     // default values required for calling method without arguments
 
-    // Function calculating the Luhn checksum of a number string
-    const getCheckBit = (number: number[]) => {
-      number.reverse();
-      number = number.map((num, index) => {
-        if (index % 2 === 0) {
-          num *= 2;
-          if (num > 9) {
-            num -= 9;
-          }
-        }
-        return num;
-      });
-      const sum = number.reduce((prev, curr) => prev + curr);
-      return sum % 10;
-    };
-
     string = this.regexpStyleStringParse(string); // replace [4-9] with a random number in range etc...
     string = this.replaceSymbolWithNumber(string, symbol); // replace ### with random numbers
 
-    const numberList = string
-      .replace(/\D/g, '')
-      .split('')
-      .map((num) => parseInt(num));
-    const checkNum = getCheckBit(numberList);
+    const checkNum = luhnCheckValue(string);
     return string.replace('L', String(checkNum));
   }
 
diff --git a/src/modules/helpers/luhn-check.ts b/src/modules/helpers/luhn-check.ts
new file mode 100644
index 00000000..9cfcc577
--- /dev/null
+++ b/src/modules/helpers/luhn-check.ts
@@ -0,0 +1,42 @@
+/**
+ * Checks that the given string passes the luhn algorithm.
+ *
+ * @param str The string to validate.
+ */
+export function luhnCheck(str: string): boolean {
+  return luhnChecksum(str) === 0;
+}
+
+/**
+ * Calculates the luhn check value for the given string.
+ *
+ * @param str The string to calculate the check value for.
+ * May contain the `L` placeholder at the end.
+ */
+export function luhnCheckValue(str: string): number {
+  const checksum = luhnChecksum(str.replace(/L?$/, '0'));
+  return checksum === 0 ? 0 : 10 - checksum;
+}
+
+/**
+ * Calculates the luhn checksum value for the given value.
+ *
+ * @param str The string to generate the checksum for.
+ */
+function luhnChecksum(str: string): number {
+  str = str.replace(/[\s-]/g, '');
+  let sum = 0;
+  let alternate = false;
+  for (let i = str.length - 1; i >= 0; i--) {
+    let n = parseInt(str.substring(i, i + 1));
+    if (alternate) {
+      n *= 2;
+      if (n > 9) {
+        n = (n % 10) + 1;
+      }
+    }
+    sum += n;
+    alternate = !alternate;
+  }
+  return sum % 10;
+}