From e5f10f2667634c83ea2bd95e0298cc5ae8cf1b9b Mon Sep 17 00:00:00 2001
From: ZGaetano <zgaetano@wilddragon.net>
Date: Sat, 13 Jun 2026 00:21:55 -0400
Subject: [PATCH] test(engine): pin audio FourCC constants to ASCII packing
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Adds regression tests that would have caught the PCMs16 FourCC typo
(0x73334d50 'PM3s' vs correct 0x734D4350 'PCMs'). The prior tests
referenced the symbol on both sides of the assertion, so a wrong literal
stayed self-consistent and green. These new tests:

  1. Assert each public FourCC constant equals the little-endian ASCII
     packing of its documented characters (the same convention the NDI
     SDK and the video FourCCs use).
  2. Feed a raw PCM16 buffer tagged with the *literal* 0x734D4350 and
     assert the peak is decoded — proving a real Teams legacy sender's
     FourCC routes to the int16 decoder rather than the unknown branch.
---
 .../Pipeline/AudioPeakComputerTests.cs        | 55 ++++++++++++++++++-
 1 file changed, 52 insertions(+), 3 deletions(-)

diff --git a/src/tests/Dragon-ISO.Engine.Tests/Pipeline/AudioPeakComputerTests.cs b/src/tests/Dragon-ISO.Engine.Tests/Pipeline/AudioPeakComputerTests.cs
index 382af71..52a43f0 100644
--- a/src/tests/Dragon-ISO.Engine.Tests/Pipeline/AudioPeakComputerTests.cs
+++ b/src/tests/Dragon-ISO.Engine.Tests/Pipeline/AudioPeakComputerTests.cs
@@ -22,7 +22,7 @@ public class AudioPeakComputerTests
     [Fact]
     public void UnknownFourCC_ReturnsZero_RatherThanThrow()
     {
-        // Receiver loop must never crash on an unrecognized format — better to
+        // Receiver loop must never crash on an unrecognized format — better to
         // show silence on the meter than to take down the pipeline.
         var floats = new[] { 0.5f, -0.5f };
         var bytes = AsBytes(floats);
@@ -77,7 +77,7 @@ public class AudioPeakComputerTests
     public void Fltp_TotalSamplesSmallerThanBuffer_OnlyConsumesReportedRange()
     {
         // The reported range covers only the first 3 floats. The 4th
-        // (largest) is past `totalSamples` and must be ignored — otherwise we'd
+        // (largest) is past `totalSamples` and must be ignored — otherwise we'd
         // be reading beyond what the source said it wrote.
         var floats = new[] { 0.1f, -0.2f, 0.3f, 0.99f };
         var bytes = AsBytes(floats);
@@ -128,10 +128,59 @@ public class AudioPeakComputerTests
         var samples = new[] { (short)0, (short)16384, (short)-16383 };
         var bytes = AsBytes(samples);
         var peak = AudioPeakComputer.ComputePeak(bytes, AudioPeakComputer.FourCC_PCMs16, samples.Length);
-        // 16384 / 32767 ≈ 0.500015; tolerate small precision drift.
+        // 16384 / 32767 ≈ 0.500015; tolerate small precision drift.
         Assert.InRange(peak, 0.49, 0.51);
     }
 
+    // ============================================================
+    // FourCC constant integrity — regression guard
+    // ============================================================
+    //
+    // The NDI SDK packs FourCCs little-endian: the first ASCII character
+    // lands in the least-significant byte. These tests pin each public
+    // constant to the little-endian packing of its documented characters,
+    // so a transposed/typo'd literal (the PCMs16 0x73334d50 → 0x734D4350
+    // fix) fails loudly instead of silently routing real audio to the
+    // "unknown format → 0.0" branch.
+
+    [Fact]
+    public void FourCC_FLTP_MatchesAsciiPacking()
+    {
+        AudioPeakComputer.FourCC_FLTP.Should().Be(Pack('F', 'L', 'T', 'p'));
+    }
+
+    [Fact]
+    public void FourCC_FLT_MatchesAsciiPacking()
+    {
+        AudioPeakComputer.FourCC_FLT.Should().Be(Pack('F', 'L', 'T', ' '));
+    }
+
+    [Fact]
+    public void FourCC_PCMs16_MatchesAsciiPacking()
+    {
+        // Regression: was 0x73334d50 ('P','M','3','s'), should be 'P','C','M','s'.
+        AudioPeakComputer.FourCC_PCMs16.Should().Be(Pack('P', 'C', 'M', 's'));
+        AudioPeakComputer.FourCC_PCMs16.Should().Be(0x734D4350u);
+    }
+
+    [Fact]
+    public void Pcms16_DecodedViaLiteralFourCC_NotJustSymbol()
+    {
+        // Belt-and-braces: feed a PCM16 buffer tagged with the *literal* FourCC
+        // a real legacy Teams/NDI sender emits, not the symbol. If the constant
+        // ever drifts from the wire value again, this peak collapses to 0.0
+        // because ComputePeak falls through to the unknown branch.
+        const uint wirePcms16 = 0x734D4350; // little-endian 'P','C','M','s'
+        var samples = new[] { (short)0, (short)16384, (short)-200 };
+        var bytes = AsBytes(samples);
+        var peak = AudioPeakComputer.ComputePeak(bytes, wirePcms16, samples.Length);
+        peak.Should().BeInRange(0.49, 0.51);
+    }
+
+    /// <summary>Little-endian ASCII FourCC packing: first char in the low byte.</summary>
+    private static uint Pack(char a, char b, char c, char d) =>
+        (uint)a | ((uint)b << 8) | ((uint)c << 16) | ((uint)d << 24);
+
     private static byte[] AsBytes<T>(T[] arr) where T : struct
     {
         var span = MemoryMarshal.Cast<T, byte>(arr.AsSpan());