fix: Handle TD1 embedded document check digits in optional1

Some TD1 issuers embed the document number check digit inside optional1 when the check-digit slot is `<`. This update validates the embedded digit against the concatenated document number + optional prefix before falling back to the ICAO-style separator variant.

Notes:
- No official public specification found for embedded document‑number check digits in optional1.
- The anonymized test case is derived from a real example that cannot be shared.

This is related to #66.

Author: uwolfer

src/parse/__tests__/td1.test.ts

@@ -470,4 +470,39 @@ describe('parse TD1', () => {
       ],
     ]);
   });
+
+  it('TD1 with embedded document check digit', () => {
+    // Check digit is embedded in optional1
+    const MRZ = [
+      'I<PRT123456789<AAB4<<<<<<<<<<<',
+      '9001011M3001019PRT<<<<<<<<<<<2',
+      'ANON<USER<<TEST<CASE<<<<<<<<<<',
+    ];
+
+    const result = parse(MRZ);
+
+    expect(result).toMatchObject({
+      format: 'TD1',
+      valid: true,
+      documentNumber: result.fields.documentNumber,
+    });
+
+    expect(result.fields).toStrictEqual({
+      documentCode: 'I',
+      issuingState: 'PRT',
+      documentNumber: '123456789AAB',
+      documentNumberCheckDigit: '4',
+      birthDate: '900101',
+      birthDateCheckDigit: '1',
+      sex: 'male',
+      expirationDate: '300101',
+      expirationDateCheckDigit: '9',
+      nationality: 'PRT',
+      optional1: 'AAB4',
+      optional2: '',
+      compositeCheckDigit: '2',
+      lastName: 'ANON USER',
+      firstName: 'TEST CASE',
+    });
+  });
 });

src/parsers/__tests__/check.test.ts

@@ -1,6 +1,6 @@
 import { expect, test } from 'vitest';
 
-import { check } from '../check.ts';
+import { check, computeCheckDigit } from '../check.ts';
 
 test('check digits', () => {
   expect(() => check('592166117<231', 8)).not.toThrow();
@@ -11,3 +11,8 @@ test('check digits', () => {
   expect(() => check('600001795015', 2)).not.toThrow();
   expect(() => check('592166111<773', 4)).toThrow(/invalid check digit/);
 });
+
+test('compute embedded TD1 check digit', () => {
+  expect(computeCheckDigit('123456789AAB')).toBe(8);
+  expect(computeCheckDigit('123456789<AAB')).toBe(4);
+});

src/parsers/check.ts

@@ -1,4 +1,4 @@
-export function check(string: string, value: string | number) {
+export function computeCheckDigit(string: string) {
   let code = 0;
   const factors = [7, 3, 1];
   for (let i = 0; i < string.length; i++) {
@@ -10,7 +10,11 @@ export function check(string: string, value: string | number) {
     charCode *= factors[i % 3];
     code += charCode;
   }
-  code %= 10;
+  return code % 10;
+}
+
+export function check(string: string, value: string | number) {
+  const code = computeCheckDigit(string);
   if (code !== Number(value)) {
     throw new Error(`invalid check digit: ${value}. Must be ${code}`);
   }

src/parsers/parseDocumentNumberCheckDigit.ts

@@ -1,4 +1,4 @@
-import { check } from './check.ts';
+import { check, computeCheckDigit } from './check.ts';
 
 export default function parseDocumentNumberCheckDigit(
   checkDigit: string,
@@ -8,8 +8,19 @@ export default function parseDocumentNumberCheckDigit(
   if (checkDigit === '<' && optional) {
     const firstFiller = optional.indexOf('<');
     const tail = optional.slice(0, firstFiller - 1);
-    source = `${source}${tail}`;
-    checkDigit = optional.charAt(firstFiller - 1);
+    // Some TD1 issuers embed the document check digit inside optional1
+    const embeddedDigit = optional.charAt(firstFiller - 1);
+    const embeddedValid =
+      computeCheckDigit(`${source}${tail}`) === Number(embeddedDigit);
+    if (embeddedValid) {
+      return {
+        value: embeddedDigit,
+        start: firstFiller,
+        end: firstFiller + 1,
+      };
+    }
+    source = `${source}<${tail}`;
+    checkDigit = embeddedDigit;
     check(source, checkDigit);
     return {
       value: checkDigit,