fix(bg/egn): validate the embedded birth date

src/bg/egn.spec.ts

@@ -1,5 +1,9 @@
 import { validate, format } from './egn';
-import { InvalidLength, InvalidChecksum } from '../exceptions';
+import {
+  InvalidLength,
+  InvalidChecksum,
+  InvalidComponent,
+} from '../exceptions';
 
 describe('bg/egn', () => {
   it('format:752316 926 3', () => {
@@ -25,4 +29,19 @@ describe('bg/egn', () => {
 
     expect(result.error).toBeInstanceOf(InvalidChecksum);
   });
+
+  // The first six digits are a birth date whose month encodes the century
+  // (41-52 -> 2000s, 21-32 -> 1800s); an impossible date must be rejected even
+  // when the check digit is correct.
+  it('validate:2992070971 (impossible month, valid check digit)', () => {
+    const result = validate('2992070971');
+
+    expect(result.error).toBeInstanceOf(InvalidComponent);
+  });
+
+  it('validate:8019010008 (impossible month)', () => {
+    const result = validate('8019010008');
+
+    expect(result.error).toBeInstanceOf(InvalidComponent);
+  });
 });

src/bg/egn.ts

@@ -10,7 +10,7 @@
  */
 
 import * as exceptions from '../exceptions';
-import { strings } from '../util';
+import { isValidDate, strings } from '../util';
 import { Validator, ValidateReturn } from '../types';
 import { weightedSum } from '../util/checksum';
 
@@ -51,6 +51,24 @@ const impl: Validator = {
       return { isValid: false, error: new exceptions.InvalidFormat() };
     }
 
+    // The first six digits are the birth date. The month encodes the century:
+    // 41-52 -> 2000s, 21-32 -> 1800s, 01-12 -> 1900s.
+    const [yy, mm, dd] = strings.splitAt(value, 2, 4, 6);
+    let year = parseInt(yy, 10);
+    let month = parseInt(mm, 10);
+    if (month > 40) {
+      year += 2000;
+      month -= 40;
+    } else if (month > 20) {
+      year += 1800;
+      month -= 20;
+    } else {
+      year += 1900;
+    }
+    if (!isValidDate(String(year), String(month), dd)) {
+      return { isValid: false, error: new exceptions.InvalidComponent() };
+    }
+
     const [front, check] = strings.splitAt(value, -1);
 
     const sum = weightedSum(front, {