From 6ee1d85ce19a2511d684be8545cc95c3d7b28d70 Mon Sep 17 00:00:00 2001
From: fuleinist <fuleinist@users.noreply.github.com>
Date: Sat, 25 Jul 2026 11:06:09 +1000
Subject: [PATCH] fix(result_formatter): preserve full hex for checksums wider
 than 16 bits
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

ResultFormatter.checksum used ('0000' + hex).slice(-4) which truncates
any checksum > 0xFFFF to its last 4 hex digits. For example, 0xAFF5C
displayed as 0xFF5C and 0xDEADBEEF as 0xBEEF — silently wrong values
in formatted.items that downstream consumers render to users.

Replace with (value >>> 0).toString(16).padStart(4, '0') which:
- Pads values < 0x1000 to 4 hex digits (preserving prior behaviour)
- Preserves the full hex representation for wider checksums
- Coerces to unsigned via >>> 0, avoiding negative-hex display for
  32-bit CRCs whose top bit is set (companion to #458)

Add result_formatter.test.ts with 6 unit tests covering 12-bit, 16-bit,
20-bit, and 32-bit checksum round-trips.

Fixes #484
---
 lib/utils/result_formatter.test.ts | 64 ++++++++++++++++++++++++++++++
 lib/utils/result_formatter.ts      |  2 +-
 2 files changed, 65 insertions(+), 1 deletion(-)
 create mode 100644 lib/utils/result_formatter.test.ts

diff --git a/lib/utils/result_formatter.test.ts b/lib/utils/result_formatter.test.ts
new file mode 100644
index 0000000..8944e4c
--- /dev/null
+++ b/lib/utils/result_formatter.test.ts
@@ -0,0 +1,64 @@
+import { DecodeResult } from '../DecoderPluginInterface';
+import { ResultFormatter } from './result_formatter';
+
+function makeDecodeResult(): DecodeResult {
+  return {
+    decoded: true,
+    decoder: { name: 'test', type: 'pattern-match', decodeLevel: 'full' },
+    formatted: { description: 'Test', items: [] },
+    raw: {},
+    remaining: {},
+  };
+}
+
+describe('ResultFormatter.checksum', () => {
+  test('formats 16-bit checksum with 4-hex-digit padding', () => {
+    const dr = makeDecodeResult();
+    ResultFormatter.checksum(dr, 0x1234);
+    expect(dr.raw.checksum).toBe(0x1234);
+    const item = dr.formatted.items[dr.formatted.items.length - 1];
+    expect(item.type).toBe('message_checksum');
+    expect(item.value).toBe('0x1234');
+  });
+
+  test('pads small values to at least 4 hex digits', () => {
+    const dr = makeDecodeResult();
+    ResultFormatter.checksum(dr, 0x003b);
+    const item = dr.formatted.items[dr.formatted.items.length - 1];
+    expect(item.value).toBe('0x003b');
+  });
+
+  test('preserves full hex for 20-bit checksum (no truncation)', () => {
+    // 0xaff5c is 5 hex digits — old code truncated to 0xff5c
+    const dr = makeDecodeResult();
+    ResultFormatter.checksum(dr, 0xaff5c);
+    expect(dr.raw.checksum).toBe(0xaff5c);
+    const item = dr.formatted.items[dr.formatted.items.length - 1];
+    expect(item.value).toBe('0xaff5c');
+  });
+
+  test('preserves full hex for 32-bit checksum (no truncation)', () => {
+    const dr = makeDecodeResult();
+    ResultFormatter.checksum(dr, 0x123456);
+    expect(dr.raw.checksum).toBe(0x123456);
+    const item = dr.formatted.items[dr.formatted.items.length - 1];
+    expect(item.value).toBe('0x123456');
+  });
+
+  test('displays large 32-bit CRC without sign artifacts', () => {
+    // 0xDEADBEEF has the top bit set; toString(16) on a signed int would
+    // produce a negative string. >>> 0 coerces to unsigned first.
+    const dr = makeDecodeResult();
+    ResultFormatter.checksum(dr, 0xdeadbeef);
+    expect(dr.raw.checksum).toBe(0xdeadbeef);
+    const item = dr.formatted.items[dr.formatted.items.length - 1];
+    expect(item.value).toBe('0xdeadbeef');
+  });
+
+  test('pads 12-bit value to 4 digits', () => {
+    const dr = makeDecodeResult();
+    ResultFormatter.checksum(dr, 0xabc);
+    const item = dr.formatted.items[dr.formatted.items.length - 1];
+    expect(item.value).toBe('0x0abc');
+  });
+});
diff --git a/lib/utils/result_formatter.ts b/lib/utils/result_formatter.ts
index d66500d..0864cf9 100644
--- a/lib/utils/result_formatter.ts
+++ b/lib/utils/result_formatter.ts
@@ -294,7 +294,7 @@ export class ResultFormatter {
       type: 'message_checksum',
       code: 'CHECKSUM',
       label: 'Message Checksum',
-      value: '0x' + ('0000' + decodeResult.raw.checksum.toString(16)).slice(-4),
+      value: '0x' + (decodeResult.raw.checksum >>> 0).toString(16).padStart(4, '0'),
     });
   }