Handle null chunk type using enum value

Motivation
----------
Trying to narrow down perf loss in .NET 8 compared to .NET 6 specific
to stream decompression.

Modifications
-------------
Rather than using a `Nullable<ChunkType>` to track when the chunk type
is unknown, add an enum value that represents null.

Results
-------
BenchmarkDotNet v0.13.10, Windows 11 (10.0.22621.2861/22H2/2022Update/SunValley2)
12th Gen Intel Core i7-1270P, 1 CPU, 16 logical and 12 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  Job-AODULB : .NET Framework 4.8.1 (4.8.9181.0), X64 RyuJIT VectorSize=256
  Job-AZPVRO : .NET Framework 4.8.1 (4.8.9181.0), X64 RyuJIT VectorSize=256
  Job-FTJSLO : .NET 6.0.25 (6.0.2523.51912), X64 RyuJIT AVX2
  Job-XMQQAK : .NET 6.0.25 (6.0.2523.51912), X64 RyuJIT AVX2
  Job-IPTUCK : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  Job-YAGJLP : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  Job-DJZUQL : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  Job-FFPERJ : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

| Method     | Runtime            | BuildConfiguration | PGO | ReadSize | Mean     | Error   | StdDev  | Ratio | Rank |
|----------- |------------------- |------------------- |---- |--------- |---------:|--------:|--------:|------:|-----:|
| Decompress | .NET Framework 4.8 | Previous           | N   | 16384    | 340.4 us | 2.56 us | 2.39 us |  1.00 |    2 |
| Decompress | .NET Framework 4.8 | Default            | N   | 16384    | 332.2 us | 2.64 us | 2.34 us |  0.98 |    1 |
|            |                    |                    |     |          |          |         |         |       |      |
| Decompress | .NET 6.0           | Previous           | N   | 16384    | 167.5 us | 1.11 us | 1.04 us |  1.00 |    1 |
| Decompress | .NET 6.0           | Default            | N   | 16384    | 167.5 us | 2.44 us | 2.16 us |  1.00 |    1 |
|            |                    |                    |     |          |          |         |         |       |      |
| Decompress | .NET 8.0           | Previous           | N   | 16384    | 200.7 us | 1.19 us | 1.06 us |  1.00 |    2 |
| Decompress | .NET 8.0           | Default            | N   | 16384    | 191.7 us | 1.47 us | 1.30 us |  0.96 |    1 |
|            |                    |                    |     |          |          |         |         |       |      |
| Decompress | .NET 8.0           | Previous           | Y   | 16384    | 199.1 us | 0.94 us | 0.88 us |  1.00 |    2 |
| Decompress | .NET 8.0           | Default            | Y   | 16384    | 189.9 us | 0.91 us | 0.85 us |  0.95 |    1 |

Note: Benchmarks include other improvements since 1.1.3

Snappier/Internal/Constants.cs

@@ -10,7 +10,11 @@ public enum ChunkType : byte
             UncompressedData = 0x01,
             SkippableChunk = 0x80, // If this bit is set, we can safely skip the chunk if unknown
             Padding = 0xfe,
-            StreamIdentifier = 0xff
+            StreamIdentifier = 0xff,
+
+            // This is not part of the spec, but having this extra value representing null avoids
+            // the cost of wrapping in a Nullable<T>
+            Null = 0xfd,
         }
 
         public const byte Literal = 0b00;

Snappier/Internal/SnappyStreamDecompressor.cs

@@ -19,7 +19,7 @@ internal sealed class SnappyStreamDecompressor : IDisposable
 
         private readonly byte[] _scratch = new byte[ScratchBufferSize];
         private int _scratchLength;
-        private Constants.ChunkType? _chunkType;
+        private Constants.ChunkType _chunkType = Constants.ChunkType.Null;
         private int _chunkSize;
         private int _chunkBytesProcessed;
         private uint _expectedChunkCrc;
@@ -40,7 +40,7 @@ public int Decompress(Span<byte> buffer)
                 // ReSharper disable once SwitchStatementHandlesSomeKnownEnumValuesWithDefault
                 switch (_chunkType)
                 {
-                    case null:
+                    case Constants.ChunkType.Null:
                         // Not in a chunk, read the chunk type and size
 
                         uint rawChunkHeader = ReadChunkHeader(ref input);
@@ -106,7 +106,7 @@ public int Decompress(Span<byte> buffer)
                         if (_decompressor.EndOfFile)
                         {
                             // Completed reading the chunk
-                            _chunkType = null;
+                            _chunkType = Constants.ChunkType.Null;
 
                             uint crc = Crc32CAlgorithm.ApplyMask(_chunkCrc);
                             if (_expectedChunkCrc != crc)
@@ -149,7 +149,7 @@ public int Decompress(Span<byte> buffer)
                         if (_chunkBytesProcessed >= _chunkSize)
                         {
                             // Completed reading the chunk
-                            _chunkType = null;
+                            _chunkType = Constants.ChunkType.Null;
 
                             uint crc = Crc32CAlgorithm.ApplyMask(_chunkCrc);
                             if (_expectedChunkCrc != crc)
@@ -176,7 +176,7 @@ public int Decompress(Span<byte> buffer)
                         if (_chunkBytesProcessed >= _chunkSize)
                         {
                             // Completed reading the chunk
-                            _chunkType = null;
+                            _chunkType = Constants.ChunkType.Null;
                         }
 
                         break;