fix(rva2): use big-endian byte order for volume adjustment

src/id3v2/parse.mjs

@@ -214,7 +214,9 @@ export function rva2Frame (buffer, version) {
 
   while (read < view.byteLength) {
     const type = view.getUint8(read)
-    const volumeadjust = view.getInt16(read + 1, true)
+    // ID3v2.4 §4.11: volume adjustment is a 16-bit signed fixed-point,
+    // big-endian like every other integer in the spec (§3).
+    const volumeadjust = view.getInt16(read + 1)
     const bitspeak = view.getUint8(read + 3)
     const length = Math.ceil(bitspeak / 8)
     const peakvolume = view.getUint8(read + 4, length)

src/id3v2/write.mjs

@@ -421,8 +421,13 @@ export function rva2Frame (values, options) {
     for (let i = 0; i < value.channels.length; i++) {
       const channel = value.channels[i]
       const type = channel.type
-      const volumeadjust = new Int16Array([channel.volumeadjust])
-      const volumeadjust8 = new Uint8Array(volumeadjust.buffer)
+      // ID3v2.4 §4.11: big-endian 16-bit signed fixed-point. Previously
+      // this used `new Int16Array([x]).buffer`, which is host byte order
+      // (little-endian on x86) — producing corrupted values on every
+      // platform and incompatible output with any standards-compliant
+      // v2.4 reader.
+      const adjust = channel.volumeadjust & 0xffff
+      const volumeadjust8 = [(adjust >> 8) & 0xff, adjust & 0xff]
       const bitspeak = channel.bitspeak
       const limit = Math.ceil(bitspeak / 8)
       const peakvolume = dataBlock(channel.peakvolume, limit)

test/id3v2/index.cjs

@@ -178,6 +178,61 @@ describe('ID3v2', function () {
       })
     })
 
+    it('Writes RVA2 volume adjustment in big-endian (§4.11)', function () {
+      // Regression test: previously the writer used `new Int16Array([v]).buffer`
+      // which is host-byte-order (little-endian on x86), producing corrupted
+      // output on every platform. Read path used `getInt16(offset, true)`,
+      // so round-trips self-matched but no other v2.4 reader could decode.
+      // This test asserts the actual bytes on disk are big-endian per spec.
+      this.mp3tag.tags.v2.RVA2 = [{
+        identification: 'test',
+        channels: [
+          { type: 1, volumeadjust: 0x1234, bitspeak: 0, peakvolume: [] },
+          { type: 2, volumeadjust: -1, bitspeak: 0, peakvolume: [] }
+        ]
+      }]
+      this.mp3tag.save({ id3v2: { version: 4 } })
+      if (this.mp3tag.error !== '') throw new Error(this.mp3tag.error)
+
+      // Inspect raw bytes after save. Find "RVA2" in the buffer and check
+      // the volume-adjust bytes for channel 1 (type=1) are 0x12 0x34 (BE).
+      const buf = new Uint8Array(this.mp3tag.buffer)
+      let offset = -1
+      for (let i = 0; i + 4 <= buf.length; i++) {
+        if (buf[i] === 0x52 && buf[i + 1] === 0x56 && buf[i + 2] === 0x41 && buf[i + 3] === 0x32) {
+          offset = i
+          break
+        }
+      }
+      if (offset < 0) throw new Error('RVA2 frame not found in saved buffer')
+      // RVA2 frame layout: 4-byte ID + 4-byte size + 2-byte flags
+      // + identification string "test" + '\0' + channel data.
+      // Channel data = type(1) + volumeadjust(2) + bitspeak(1) + peakvolume(0).
+      // Find the identification terminator.
+      const dataStart = offset + 10 + 'test'.length + 1
+      // Channel 1: positive value 0x1234 → bytes 0x12 0x34 (BE).
+      assert.strictEqual(buf[dataStart], 1, 'first channel type')
+      assert.strictEqual(buf[dataStart + 1], 0x12, 'channel 1 volume high byte (BE)')
+      assert.strictEqual(buf[dataStart + 2], 0x34, 'channel 1 volume low byte (BE)')
+      // Channel 2: negative value -1 → two's-complement 0xFFFF → 0xFF 0xFF.
+      // Crucial second assertion: a naive implementation that only swapped
+      // bytes for positive values would pass the 0x1234 check but fail
+      // here, because sign-extension and masking must also be correct.
+      assert.strictEqual(buf[dataStart + 4], 2, 'second channel type')
+      assert.strictEqual(buf[dataStart + 5], 0xff, 'channel 2 volume high byte (BE, -1)')
+      assert.strictEqual(buf[dataStart + 6], 0xff, 'channel 2 volume low byte (BE, -1)')
+
+      this.mp3tag.read()
+      if (this.mp3tag.error !== '') throw new Error(this.mp3tag.error)
+      assert.deepStrictEqual(this.mp3tag.tags.v2.RVA2, [{
+        identification: 'test',
+        channels: [
+          { type: 1, volumeadjust: 0x1234, bitspeak: 0, peakvolume: [] },
+          { type: 2, volumeadjust: -1, bitspeak: 0, peakvolume: [] }
+        ]
+      }])
+    })
+
     it('Write complex multi tag', function () {
       this.mp3tag.tags.v2.SYLT = [
         {