From 4a46ec36b374ea6145b8f8f65a6b3ce12a034003 Mon Sep 17 00:00:00 2001
From: daniel-c-harvey <the.real.daniel.harvey@gmail.com>
Date: Thu, 11 Jun 2026 06:13:20 -0400
Subject: [PATCH] fix(mp3): remove dead FrameSize field, fix CBR duration ID3
 exclusion, add MPEG2 bitrate table, pin CBR test assertions

---
 DeepDrftContent/Processors/Mp3AudioProcessor.cs | 15 ++++++++-------
 DeepDrftTests/AudioProcessorTests.cs            | 15 ++++++++++++---
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/DeepDrftContent/Processors/Mp3AudioProcessor.cs b/DeepDrftContent/Processors/Mp3AudioProcessor.cs
index d8f1296..398ea7d 100644
--- a/DeepDrftContent/Processors/Mp3AudioProcessor.cs
+++ b/DeepDrftContent/Processors/Mp3AudioProcessor.cs
@@ -14,6 +14,10 @@ public class Mp3AudioProcessor
     private static readonly int[] Mpeg1Layer3Bitrates =
         [0, 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256, 320];
 
+    // MPEG2/2.5 Layer III bitrate table (kbps), indexed by 4-bit bitrate index. 0 = free, 15 = bad.
+    private static readonly int[] Mpeg2Layer3Bitrates =
+        [0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160];
+
     private static readonly int[] Mpeg1SampleRates = [44100, 48000, 32000];
     private static readonly int[] Mpeg2SampleRates = [22050, 24000, 16000];
     private static readonly int[] Mpeg25SampleRates = [11025, 12000, 8000];
@@ -169,7 +173,8 @@ public class Mp3AudioProcessor
         };
 
         var bitrateIndex = (b2 >> 4) & 0x0F;
-        var bitrateKbps = Mpeg1Layer3Bitrates[bitrateIndex];
+        var bitrateTable = version == MpegVersion.Mpeg1 ? Mpeg1Layer3Bitrates : Mpeg2Layer3Bitrates;
+        var bitrateKbps = bitrateTable[bitrateIndex];
 
         var sampleRateIndex = (b2 >> 2) & 0x03;
         var sampleRate = version switch
@@ -179,13 +184,10 @@ public class Mp3AudioProcessor
             _ => Mpeg25SampleRates[sampleRateIndex],
         };
 
-        var paddingBit = (b2 >> 1) & 0x01;
         var channelMode = (b3 >> 6) & 0x03;
         var channels = channelMode == 3 ? 1 : 2;
         var samplesPerFrame = version == MpegVersion.Mpeg1 ? 1152 : 576;
 
-        var frameSize = (int)Math.Floor(144.0 * (bitrateKbps * 1000) / sampleRate) + paddingBit;
-
         return new FrameHeader
         {
             Version = version,
@@ -193,7 +195,6 @@ public class Mp3AudioProcessor
             SampleRate = sampleRate,
             Channels = channels,
             SamplesPerFrame = samplesPerFrame,
-            FrameSize = frameSize,
         };
     }
 
@@ -216,8 +217,9 @@ public class Mp3AudioProcessor
         }
 
         // CBR fallback: bitrate_kbps * 1000 / 8 bytes per second = bitrate_kbps * 125.
+        // Exclude the ID3v2 tag bytes (everything before frameStart) from the estimate.
         var bytesPerSecond = header.BitrateKbps * 125;
-        return bytesPerSecond > 0 ? (double)buffer.Length / bytesPerSecond : FallbackDuration;
+        return bytesPerSecond > 0 ? (double)(buffer.Length - frameStart) / bytesPerSecond : FallbackDuration;
     }
 
     /// <summary>
@@ -300,7 +302,6 @@ public class Mp3AudioProcessor
         public int SampleRate { get; init; }
         public int Channels { get; init; }
         public int SamplesPerFrame { get; init; }
-        public int FrameSize { get; init; }
     }
 
     private sealed class Mp3Metadata
diff --git a/DeepDrftTests/AudioProcessorTests.cs b/DeepDrftTests/AudioProcessorTests.cs
index 9c66bed..edac1cc 100644
--- a/DeepDrftTests/AudioProcessorTests.cs
+++ b/DeepDrftTests/AudioProcessorTests.cs
@@ -164,14 +164,23 @@ public class AudioProcessorTests
     [Test]
     public async Task Mp3_CbrMetadata_ParsedCorrectly()
     {
-        var path = await WriteAudioAsync(BuildMinimalMp3(bitrateKbps: 128, sampleRate: 44100, stereo: true), ".mp3");
+        // BuildMinimalMp3: bitrateKbps=128, sampleRate=44100, stereo=true, no Xing tag, no ID3 tag.
+        // frameSize = floor(144 * 128000 / 44100) = 417 bytes; bufferSize = max(417, 48) = 417.
+        // CBR duration = (bufferLength - frameStart) / (bitrateKbps * 125) = 417 / 16000 ≈ 0.0261 s.
+        const int bitrateKbps = 128;
+        const int sampleRate = 44100;
+        var frameSize = (int)Math.Floor(144.0 * (bitrateKbps * 1000) / sampleRate); // 417
+        var bufferSize = Math.Max(frameSize, 4 + 32 + 12); // max(417, 48) = 417
+        var expectedDuration = (double)bufferSize / (bitrateKbps * 125); // frameStart = 0 (no ID3)
+
+        var path = await WriteAudioAsync(BuildMinimalMp3(bitrateKbps: bitrateKbps, sampleRate: sampleRate, stereo: true), ".mp3");
 
         var audio = await new Mp3AudioProcessor().ProcessMp3FileAsync(path);
 
         Assert.That(audio, Is.Not.Null);
         Assert.That(audio!.Extension, Is.EqualTo(".mp3"));
-        Assert.That(audio.Duration, Is.GreaterThan(0.0));
-        Assert.That(audio.Bitrate, Is.GreaterThan(0));
+        Assert.That(audio.Bitrate, Is.EqualTo(bitrateKbps));
+        Assert.That(audio.Duration, Is.EqualTo(expectedDuration).Within(0.01));
     }
 
     [Test]