From 91c17416f15f81587273d2253dd25bc587288ae5 Mon Sep 17 00:00:00 2001
From: Lorent Lempereur <lorent.lempereur@gmail.com>
Date: Mon, 17 Aug 2026 19:21:25 +0200
Subject: [PATCH] fix(wiegand): correct decode26 decimal input halving payload
 value

---
 CHANGELOG.md                      |  4 ++++
 src/modules/wiegand/logic.test.ts | 23 +++++++++++++++++++++++
 src/modules/wiegand/logic.ts      | 18 +++++++++++++++++-
 3 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2ffc843..344029d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+* [Bug] Wiegand Decode W26 — decimal input format returned a halved value instead of the original decimal (#1)
+
 ## [1.0.10] - 2026-04-04
 
 * [Feature] UUID Generator — UUIDv7 support (generate time-ordered UUIDs, version selector, inline timestamp display)
diff --git a/src/modules/wiegand/logic.test.ts b/src/modules/wiegand/logic.test.ts
index 49224ac..00ccbb1 100644
--- a/src/modules/wiegand/logic.test.ts
+++ b/src/modules/wiegand/logic.test.ts
@@ -162,6 +162,29 @@ describe('processWiegand', () => {
 
       expect(result?.mode).toBe('error')
     })
+
+    it('preserves the input decimal value in the decoded result', async () => {
+      const result = await processWiegand('decode26', '3268230', 'decimal')
+
+      expect(result?.mode).toBe('decode26')
+      if (result?.mode === 'decode26') {
+        expect(result.decoded?.wiegand26InDecimal).toBe(3268230)
+      }
+    })
+
+    it('round-trips with encode26 via wiegand26InDecimal', async () => {
+      const encoded = await processWiegand('encode', 'ABC 123')
+      if (encoded?.mode !== 'encode' || !encoded.encoded26) {
+        throw new Error('encode failed')
+      }
+
+      const decoded = await processWiegand('decode26', String(encoded.encoded26.wiegand26InDecimal), 'decimal')
+
+      expect(decoded).toEqual({
+        mode: 'decode26',
+        decoded: encoded.encoded26,
+      })
+    })
   })
 
   describe('decode26 mode — plate format', () => {
diff --git a/src/modules/wiegand/logic.ts b/src/modules/wiegand/logic.ts
index 6b90c57..4cf831c 100644
--- a/src/modules/wiegand/logic.ts
+++ b/src/modules/wiegand/logic.ts
@@ -28,6 +28,21 @@ export async function processWiegand(
   }
 }
 
+// anpr-wiegand only exposes decode26(hex), which strips the 2 parity bits from a raw Wiegand26
+// value but never computes them. Reconstructing correct parity bits here (same algorithm as the
+// library's internal, unexported addParityBits) lets decimal-format input produce an accurate hex.
+function popcount(value: number): number {
+  const withPairSums = value - ((value >> 1) & 0x55555555)
+  const withNibbleSums = (withPairSums & 0x33333333) + ((withPairSums >> 2) & 0x33333333)
+  return (((withNibbleSums + (withNibbleSums >> 4)) & 0x0f0f0f0f) * 0x01010101) >> 24
+}
+
+function addParityBits(payloadShiftedLeftByOne: number): number {
+  const withLeadingParity =
+    popcount((payloadShiftedLeftByOne >> 13) & 0x1fff) % 2 !== 0 ? payloadShiftedLeftByOne | (1 << 25) : payloadShiftedLeftByOne
+  return popcount((withLeadingParity >> 1) & 0xfff) % 2 === 0 ? withLeadingParity | 1 : withLeadingParity
+}
+
 async function decode26WithFormat(input: string, format: Decode26InputFormat): Promise<WiegandDecoded26 | WiegandResult> {
   switch (format) {
     case 'decimal': {
@@ -35,7 +50,8 @@ async function decode26WithFormat(input: string, format: Decode26InputFormat): P
       if (!/^\d+$/.test(input) || !Number.isInteger(asNumber) || asNumber < 0 || asNumber > 16_777_215) {
         return { mode: 'error', error: 'Invalid decimal value (must be 0-16777215)' }
       }
-      const hex = asNumber.toString(16).toUpperCase()
+      const withParityBits = addParityBits(asNumber << 1)
+      const hex = (withParityBits >>> 0).toString(16).toUpperCase().padStart(7, '0')
       return { mode: 'decode26', decoded: (await decode26(hex)) ?? null }
     }
     case 'hex':