From f89e4b73ca9179ace5f5f48cefcd88eef37db9ea Mon Sep 17 00:00:00 2001
From: Urs Wolfer <uwolfer@fwo.ch>
Date: Fri, 6 Mar 2026 09:32:56 +0100
Subject: [PATCH] fix: Handle TD1 embedded document check digits in optional1
 for non-compliant documents like PRT and BEL IDs

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.

This is related to #66.
---
 src/parse/__tests__/td1.test.ts              | 36 ++++++++++++++++++++
 src/parsers/__tests__/check.test.ts          |  7 +++-
 src/parsers/check.ts                         |  8 +++--
 src/parsers/parseDocumentNumberCheckDigit.ts | 21 ++++++++++--
 4 files changed, 66 insertions(+), 6 deletions(-)

diff --git a/src/parse/__tests__/td1.test.ts b/src/parse/__tests__/td1.test.ts
index 0b89373..f2e3355 100644
--- a/src/parse/__tests__/td1.test.ts
+++ b/src/parse/__tests__/td1.test.ts
@@ -470,4 +470,40 @@ describe('parse TD1', () => {
       ],
     ]);
   });
+
+  it('Belgium ID BEL-BO-03003 with embedded document check digit', () => {
+    // source: https://www.consilium.europa.eu/prado/en/BEL-BO-03003/index.html
+    // This Belgian ID has the document number check digit embedded in optional1
+    const MRZ = [
+      'IDBEL000590240<6013<<<<<<<<<<<',
+      '8512017F1311048BEL851201002007',
+      'REINARTZ<<ULRIKE<KATIA<E<<<<<<',
+    ];
+
+    const result = parse(MRZ);
+
+    expect(result).toMatchObject({
+      format: 'TD1',
+      valid: true,
+      documentNumber: result.fields.documentNumber,
+    });
+
+    expect(result.fields).toStrictEqual({
+      documentCode: 'ID',
+      issuingState: 'BEL',
+      documentNumber: '000590240601',
+      documentNumberCheckDigit: '3',
+      birthDate: '851201',
+      birthDateCheckDigit: '7',
+      sex: 'female',
+      expirationDate: '131104',
+      expirationDateCheckDigit: '8',
+      nationality: 'BEL',
+      optional1: '6013',
+      optional2: '85120100200',
+      compositeCheckDigit: '7',
+      lastName: 'REINARTZ',
+      firstName: 'ULRIKE KATIA E',
+    });
+  });
 });
diff --git a/src/parsers/__tests__/check.test.ts b/src/parsers/__tests__/check.test.ts
index 7736722..82fcf66 100644
--- a/src/parsers/__tests__/check.test.ts
+++ b/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);
+});
diff --git a/src/parsers/check.ts b/src/parsers/check.ts
index 3a249b4..af1c12f 100644
--- a/src/parsers/check.ts
+++ b/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}`);
   }
diff --git a/src/parsers/parseDocumentNumberCheckDigit.ts b/src/parsers/parseDocumentNumberCheckDigit.ts
index 6894e4f..e6d4c9b 100644
--- a/src/parsers/parseDocumentNumberCheckDigit.ts
+++ b/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 +8,23 @@ 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);
+    // Handle older non-compliant documents (e.g., PRT and BEL IDs) where the check digit
+    // is embedded in optional1 instead of following the document number directly.
+    // According to ICAO Doc 9303 Part 11 (https://www.icao.int/sites/default/files/publications/DocSeries/9303_p11_cons_en.pdf)
+    // page 88, the check digit should be calculated on the document number including
+    // any additional characters from optional1 up to (but not including) the embedded check digit.
+    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,