Update MIDI FS SysEx parsing logic and add unit tests for page incrementing

src/lib/midi/fsSysex.ts

@@ -136,9 +136,11 @@ export function buildSysExGetFileDataRequest(page: number) {
 }
 
 export function parseSysExGetFileDataResponse(bytes: Uint8Array) {
+  const page = (bytes[0] << 8) | bytes[1];
+
   return {
-    page: (bytes[0] << 8) | bytes[1],
-    nextPage: ((bytes[0] << 8) | (bytes[1] + 1)) & 0xffff,
+    page,
+    nextPage: (page + 1) & 0xffff,
     data: bytes.subarray(2),
   };
 }

tests/midi-fs-sysex.test.ts

@@ -0,0 +1,18 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { parseSysExGetFileDataResponse } from '../src/lib/midi/fsSysex';
+
+test('increments the complete uint16 file page across byte boundaries', () => {
+  const response = parseSysExGetFileDataResponse(new Uint8Array([0x01, 0xff, 0xaa, 0xbb]));
+
+  assert.equal(response.page, 0x01ff);
+  assert.equal(response.nextPage, 0x0200);
+  assert.deepEqual(response.data, new Uint8Array([0xaa, 0xbb]));
+});
+
+test('wraps the file page only after uint16 max', () => {
+  const response = parseSysExGetFileDataResponse(new Uint8Array([0xff, 0xff]));
+
+  assert.equal(response.page, 0xffff);
+  assert.equal(response.nextPage, 0);
+});