test: add MP3 fixtures for multi-value TXXX frames

Four fixtures covering null-separated multi-value TXXX fields
(2-value, 3-value, empty-slot, and single-value regression guard)
plus the Python script that generates them.

Signed-off-by: Rouzax <GitHub@mgdn.nl>

Author: Rouzax

t/generate_txxx_fixtures.py

@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+"""Generate MP3 test fixtures for Audio::Scan TXXX multi-value tests.
+
+Uses a valid MPEG frame extracted from an existing fixture as the audio
+payload, then hand-builds ID3v2.4 tags with null-separated TXXX values
+so we get exact byte-level control over the multi-value encoding.
+"""
+import struct, os
+
+OUTDIR = "/home/martijn/github/Audio-Scan/t/mp3"
+# Use an existing valid MP3 as the audio base
+BASE_MP3 = "/home/martijn/github/Audio-Scan/t/mp3/no-tags-mp1l3.mp3"
+
+
+def get_audio_payload():
+    """Read raw MPEG audio frames from an existing no-tags fixture."""
+    with open(BASE_MP3, 'rb') as f:
+        return f.read()
+
+
+def syncsafe_encode(size):
+    """Encode an integer as a 4-byte syncsafe integer (ID3v2.4 spec)."""
+    return (
+        ((size & 0x0FE00000) << 3) |
+        ((size & 0x001FC000) << 2) |
+        ((size & 0x00003F80) << 1) |
+        (size & 0x0000007F)
+    )
+
+
+def make_txxx_frame(desc, values, encoding=3):
+    """Build an ID3v2.4 TXXX frame with null-separated values.
+    encoding: 0=Latin1, 3=UTF-8
+    """
+    if encoding == 3:
+        sep = b'\x00'
+        desc_bytes = desc.encode('utf-8') + b'\x00'
+        val_bytes = sep.join(v.encode('utf-8') for v in values)
+    else:
+        sep = b'\x00'
+        desc_bytes = desc.encode('latin-1') + b'\x00'
+        val_bytes = sep.join(v.encode('latin-1') for v in values)
+
+    data = bytes([encoding]) + desc_bytes + val_bytes
+    size = len(data)
+    frame = b'TXXX' + struct.pack('>I', syncsafe_encode(size)) + b'\x00\x00' + data
+    return frame
+
+
+def make_id3v2_tag(frames):
+    """Wrap frames in an ID3v2.4 tag header."""
+    body = b''.join(frames)
+    size = len(body)
+    header = b'ID3' + b'\x04\x00' + b'\x00' + struct.pack('>I', syncsafe_encode(size))
+    return header + body
+
+
+def write_fixture(name, frames):
+    tag = make_id3v2_tag(frames)
+    audio = get_audio_payload()
+    path = os.path.join(OUTDIR, name)
+    with open(path, 'wb') as f:
+        f.write(tag + audio)
+    print(f"Wrote {path} ({len(tag) + len(audio)} bytes)")
+
+
+# Fixture 1: Two-value TXXX (ALBUMARTISTS)
+write_fixture("v2.4-txxx-multivalue.mp3", [
+    make_txxx_frame("ALBUMARTISTS", ["Artist1", "Artist2"]),
+    make_txxx_frame("ARTISTS", ["TrackArtist1", "TrackArtist2"]),
+])
+
+# Fixture 2: Three-value TXXX
+write_fixture("v2.4-txxx-multivalue-3.mp3", [
+    make_txxx_frame("ALBUMARTISTS", ["Artist1", "Artist2", "Artist3"]),
+])
+
+# Fixture 3: Multi-value with empty slot (tagger bug simulation)
+write_fixture("v2.4-txxx-multivalue-empty.mp3", [
+    make_txxx_frame("ALBUMARTISTS", ["Artist1", "", "Artist3"]),
+])
+
+# Fixture 4: Single-value TXXX (regression guard)
+write_fixture("v2.4-txxx-single.mp3", [
+    make_txxx_frame("ALBUMARTISTS", ["OnlyArtist"]),
+    make_txxx_frame("USER FRAME", ["SingleValue"]),
+])