Merge pull request #52078 from Expensify/cmartins-fixAlgo

[CP Staging] Fix credit card validation

(cherry picked from commit 83f2258)

(CP triggered by luacmartins)

src/components/AddPaymentCard/PaymentCardForm.tsx

@@ -177,18 +177,26 @@ function PaymentCardForm({
     };
 
     const onChangeCardNumber = useCallback((newValue: string) => {
-        // replace all characters that are not spaces or digits
+        // Replace all characters that are not spaces or digits
         let validCardNumber = newValue.replace(/[^\d ]/g, '');
 
-        // gets only the first 16 digits if the inputted number have more digits than that
+        // Gets only the first 16 digits if the inputted number have more digits than that
         validCardNumber = validCardNumber.match(/(?:\d *){1,16}/)?.[0] ?? '';
 
-        // add the spacing between every 4 digits
-        validCardNumber =
-            validCardNumber
-                .replace(/ /g, '')
-                .match(/.{1,4}/g)
-                ?.join(' ') ?? '';
+        // Remove all spaces to simplify formatting
+        const cleanedNumber = validCardNumber.replace(/ /g, '');
+
+        // Check if the number is a potential Amex card (starts with 34 or 37 and has up to 15 digits)
+        const isAmex = /^3[47]\d{0,13}$/.test(cleanedNumber);
+
+        // Format based on Amex or standard 4-4-4-4 pattern
+        if (isAmex) {
+            // Format as 4-6-5 for Amex
+            validCardNumber = cleanedNumber.replace(/(\d{1,4})(\d{1,6})?(\d{1,5})?/, (match, p1, p2, p3) => [p1, p2, p3].filter(Boolean).join(' '));
+        } else {
+            // Format as 4-4-4-4 for non-Amex
+            validCardNumber = cleanedNumber.match(/.{1,4}/g)?.join(' ') ?? '';
+        }
 
         setCardNumber(validCardNumber);
     }, []);

src/libs/ValidationUtils.ts

@@ -20,16 +20,24 @@ import StringUtils from './StringUtils';
  */
 function validateCardNumber(value: string): boolean {
     let sum = 0;
-    for (let i = 0; i < value.length; i++) {
-        let intVal = parseInt(value.substr(i, 1), 10);
-        if (i % 2 === 0) {
+    let shouldDouble = false;
+
+    // Loop through the card number from right to left
+    for (let i = value.length - 1; i >= 0; i--) {
+        let intVal = parseInt(value[i], 10);
+
+        // Double every second digit from the right
+        if (shouldDouble) {
             intVal *= 2;
             if (intVal > 9) {
-                intVal = 1 + (intVal % 10);
+                intVal -= 9;
             }
         }
+
         sum += intVal;
+        shouldDouble = !shouldDouble;
     }
+
     return sum % 10 === 0;
 }