Add string utility function: isValidSwissSocialSecurityNumber (#78)

Add string utility function isValidSwissSocialSecurityNumber for issue
#77

---------

Co-authored-by: Sandro C. <sc@neolution.ch>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Daniele Debernardi <drebrez@gmail.com>

Author: 4 people

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `isValidSwissSocialSecurityNumber` string utility function
+
 ## [2.0.0] - 2025-07-29
 
 ### Added

src/lib/string.spec.ts

@@ -1,4 +1,4 @@
-import { isNullOrEmpty, isNullOrWhitespace, capitalize, uncapitalize, truncate } from "./string";
+import { isNullOrEmpty, isNullOrWhitespace, capitalize, uncapitalize, truncate, isValidSwissSocialSecurityNumber } from "./string";
 
 describe("string tests", () => {
   test.each([
@@ -120,4 +120,19 @@ describe("string tests", () => {
   ])("truncate without suffix parameter", (value, maxLength, expected) => {
     expect(truncate(value, maxLength)).toBe(expected);
   });
+
+  test.each([
+    [null as unknown as string, false],
+    [undefined as unknown as string, false],
+    ["7561234567891", false],
+    ["7569217076985", true],
+    ["756.92170769.85", false],
+    ["756.9217.0769.85", true],
+    ["756..9217.0769.85", false],
+    ["756.1234.5678.91", false],
+    ["test756.9217.0769.85", false],
+    ["7.56..9217...0769.85", false],
+  ])("check if the social insurance number is valid or not", (ahvNumber, expected) => {
+    expect(isValidSwissSocialSecurityNumber(ahvNumber)).toBe(expected);
+  });
 });

src/lib/string.ts

@@ -64,3 +64,66 @@ export function truncate(value: string | undefined, maxLength: number, suffix =
 
   return `${value.slice(0, maxLength)}${suffix}`;
 }
+
+/**
+ * Validation of social insurance number with checking the checksum
+ * Validation according to https://www.sozialversicherungsnummer.ch/aufbau-neu.htm
+ * @param socialInsuranceNumber The social insurance number to check
+ * Must be in one of the following formats:
+ * - "756.XXXX.XXXX.XX" with dots as separators
+ * - "756XXXXXXXXXX" with digits only
+ * @returns The result if the social insurance number is valid or not
+ */
+export function isValidSwissSocialSecurityNumber(socialInsuranceNumber: string): boolean {
+  // 1. Check if input is empty or only whitespace
+  if (isNullOrWhitespace(socialInsuranceNumber)) {
+    return false;
+  }
+
+  /**
+   * 2. Check if input matches accepted formats:
+   *    - With dots: 756.XXXX.XXXX.XX
+   *    - Without dots: 756XXXXXXXXXX
+   */
+  const socialInsuranceNumberWithDots = new RegExp(/^756\.\d{4}\.\d{4}\.\d{2}$/);
+  const socialInsuranceNumberWithoutDots = new RegExp(/^756\d{10}$/);
+
+  if (!socialInsuranceNumberWithDots.test(socialInsuranceNumber) && !socialInsuranceNumberWithoutDots.test(socialInsuranceNumber)) {
+    return false;
+  }
+
+  // 3. Remove all dots → get a string of 13 digits
+  const compactNumber = socialInsuranceNumber.replaceAll(".", "");
+
+  /**
+   * 4. Separate digits for checksum calculation
+   *    - first 12 digits: used to calculate checksum
+   *    - last digit: actual check digit
+   */
+  const digits = compactNumber.slice(0, -1);
+  const reversedDigits = [...digits].reverse().join("");
+  const reversedDigitsArray = [...reversedDigits];
+
+  /*
+   * 5. Calculate weighted sum for checksum
+   *    - Even positions (after reversing) ×3
+   *    - Odd positions ×1
+   */
+  let sum = 0;
+  for (const [i, element] of reversedDigitsArray.entries()) {
+    sum += i % 2 === 0 ? Number(element) * 3 : Number(element) * 1;
+  }
+
+  /*
+   * 6. Calculate expected check digit
+   *    - Check digit = value to reach next multiple of 10
+   */
+  const checksum = (10 - (sum % 10)) % 10;
+  const checknumber = Number.parseInt(compactNumber.slice(-1));
+
+  /*
+   * 7. Compare calculated check digit with actual last digit
+   *    - If equal → valid AHV number
+   */
+  return checksum === checknumber;
+}