Match Game MP3 Decoding Timing

Util/Audio/Clip/MP3MonoLayoutReader.cs

@@ -0,0 +1,73 @@
+using System;
+
+namespace ADOFAIRenderer.Util.Audio.Clip;
+
+/// <summary>
+/// <see cref="IPCMReader"/> that reproduces the game's output for mono MP3 files.
+///
+/// MP3Sharp (Assembly-CSharp-firstpass, MP3Sharp.Buffer16BitStereo) writes every frame in stereo layout and fills only the
+/// channel 0 slot for mono streams, while MP3Stream.ChannelCount reports 1. The game creates a 1-channel clip from that data,
+/// so a mono MP3 plays as [s0, 0, s1, 0, ...]: twice as long, at half speed and one octave lower
+/// (confirmed in game with TestMaps/mp3_mono, 2026-10-13).
+///
+/// Output: 1 channel at the inner reader's sample rate. Frame 2k is inner frame k, frame 2k + 1 is 0.
+/// <see cref="TotalFrames"/> = 2 * inner TotalFrames.
+/// </summary>
+internal sealed class MP3MonoLayoutReader : IPCMReader
+{
+    private readonly IPCMReader _inner;
+    private float[] _innerBuffer = Array.Empty<float>();
+
+    /// <param name="inner">Mono reader of the MP3 file with the game's MP3 timing (<see cref="FFmpegPCMReader.IsMP3"/>). Owned by this reader.</param>
+    public MP3MonoLayoutReader(IPCMReader inner)
+    {
+        if (inner.Channels != 1) throw new ArgumentException("Inner reader must be mono.", nameof(inner));
+        _inner = inner;
+        Position = inner.Position * 2;
+    }
+
+    public int Channels => 1;
+    public int SampleRate => _inner.SampleRate;
+    public long TotalFrames => _inner.TotalFrames * 2;
+    public bool IsForwardOnly => _inner.IsForwardOnly;
+    public long Position { get; private set; }
+
+    /// <summary>Moves to <paramref name="frame"/>; the inner reader moves to the inner frame of the next even position.</summary>
+    public void Seek(long frame)
+    {
+        _inner.Seek((frame + 1) / 2);
+        Position = frame;
+    }
+
+    public int Read(float[] buffer, int frameCount)
+    {
+        var limit = (int)Math.Min(frameCount, TotalFrames - Position);
+        var written = 0;
+        while (written < limit)
+        {
+            if ((Position & 1) == 1)
+            {
+                buffer[written++] = 0f;
+                Position++;
+                continue;
+            }
+
+            var want = (limit - written + 1) / 2;
+            if (_innerBuffer.Length < want) _innerBuffer = new float[want];
+            var got = _inner.Read(_innerBuffer, want);
+            if (got == 0) break;
+
+            for (var i = 0; i < got && written < limit; i++)
+            {
+                buffer[written++] = _innerBuffer[i];
+                Position++;
+                if (written >= limit) break;
+                buffer[written++] = 0f;
+                Position++;
+            }
+        }
+        return written;
+    }
+
+    public void Dispose() => _inner.Dispose();
+}