From 3fec5ad02d00df08b1310248929c1a95aebc3d26 Mon Sep 17 00:00:00 2001
From: Andrew Chi <17609957+gccbb02498@users.noreply.github.com>
Date: Tue, 7 Jul 2026 17:40:24 +0800
Subject: [PATCH] fix(tw/ubn): accept 2023 relaxed check-digit rule (divisible
 by 5)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Effective 2023-04-01, Taiwan's Ministry of Finance changed the Unified
Business Number (統一編號) check-digit logic so the weighted digit sum only
needs to be divisible by 5 (previously 10), expanding the number space while
keeping the same 8-digit format. Every old, divisible-by-10 number is still
divisible by 5, so existing numbers remain valid.

- Relax the checksum from `sum % 10 === 0` to `sum % 5 === 0`.
- Keep the 7th-digit-is-7 special case (7*4=28 sums to 10), now checked
  against the mod-5 remainder.
- Add tests: an old-rule number (04595257), a new divisible-by-5 number
  (00501508), and a new 7th-digit special case (00000074).

Ref: https://www.fia.gov.tw/singlehtml/3?cntId=c4d9cff38c8642ef8872774ee9987283

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 src/tw/ubn.spec.ts | 21 +++++++++++++++++++++
 src/tw/ubn.ts      | 11 ++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/tw/ubn.spec.ts b/src/tw/ubn.spec.ts
index 83691c5c..91e03241 100644
--- a/src/tw/ubn.spec.ts
+++ b/src/tw/ubn.spec.ts
@@ -14,6 +14,27 @@ describe('tw/ubn', () => {
     expect(result.isValid && result.compact).toEqual('00501503');
   });
 
+  // Old-rule numbers (weighted digit sum divisible by 10) stay valid.
+  it('validate:04595257 (old rule, divisible by 10)', () => {
+    const result = validate('04595257');
+
+    expect(result.isValid && result.compact).toEqual('04595257');
+  });
+
+  // Since 2023-04-01 the weighted digit sum only needs to be divisible by 5.
+  it('validate:00501508 (divisible by 5, new rule)', () => {
+    const result = validate('00501508');
+
+    expect(result.isValid && result.compact).toEqual('00501508');
+  });
+
+  // 7th digit is 7: product 7*4=28 sums to 10, so a remainder of 4 also passes.
+  it('validate:00000074 (7th-digit special case, new rule)', () => {
+    const result = validate('00000074');
+
+    expect(result.isValid && result.compact).toEqual('00000074');
+  });
+
   it('validate:12345', () => {
     const result = validate('12345');
 
diff --git a/src/tw/ubn.ts b/src/tw/ubn.ts
index 5ed84474..7ba98923 100644
--- a/src/tw/ubn.ts
+++ b/src/tw/ubn.ts
@@ -5,9 +5,15 @@
  * within Taiwan for tax (VAT) purposes. The number consists of 8 digits, the
  * last being a check digit.
  *
+ * Since 2023-04-01 the Ministry of Finance relaxed the check-digit logic from
+ * "the weighted digit sum is divisible by 10" to "divisible by 5", so that the
+ * number space can be expanded while staying compatible with existing numbers
+ * (every old, divisible-by-10 number is also divisible by 5).
+ *
  * Source
  *    https://zh.wikipedia.org/wiki/統一編號
  *    https://findbiz.nat.gov.tw/fts/query/QueryBar/queryInit.do?request_locale=en
+ *    https://www.fia.gov.tw/singlehtml/3?cntId=c4d9cff38c8642ef8872774ee9987283
  *
  * ENTITY
  */
@@ -63,7 +69,10 @@ const impl: Validator = {
       .split('')
       .reduce((acc, d) => (acc + parseInt(d, 10)) % 10, 0);
 
-    if (!(sum === 0 || (sum === 9 && value[6] === '7'))) {
+    // The weighted digit sum must be divisible by 5 (was 10 before 2023-04-01).
+    // Special case: when the 7th digit is 7 its product 7*4=28 sums to 10, which
+    // may be counted as either 1 or 0, so a remainder of 4 is also accepted.
+    if (!(sum % 5 === 0 || (value[6] === '7' && (sum + 1) % 5 === 0))) {
       return { isValid: false, error: new exceptions.InvalidChecksum() };
     }