Fixed tests, fixed compilation, added DHT marker decoding more meaningful exception messages, fixed invalid jpeg encoding

src/ImageSharp/Formats/Jpeg/Components/Encoder/HuffmanScanEncoder.cs

@@ -561,13 +561,12 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         /// this method endianness dependent.
         /// </remarks>
         [MethodImpl(InliningOptions.ShortMethod)]
-        private void FlushToStream()
+        private void FlushToStream(int endIndex)
         {
             Span<byte> emitBytes = MemoryMarshal.AsBytes(this.emitBuffer.AsSpan());
 
             int writeIdx = 0;
             int startIndex = emitBytes.Length - 1;
-            int endIndex = this.emitWriteIndex * sizeof(uint);
 
             // Some platforms may fail to eliminate this if-else branching
             // Even if it happens - buffer is flushed in big packs,
@@ -621,28 +620,25 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
             }
 
             this.target.Write(this.streamWriteBuffer, 0, writeIdx);
+        }
+
+        private void FlushToStream()
+        {
+            this.FlushToStream(this.emitWriteIndex * 4);
             this.emitWriteIndex = this.emitBuffer.Length;
         }
 
         [MethodImpl(InliningOptions.ShortMethod)]
         private void FlushRemainingBytes()
         {
-            // Flush full 4-byte blocks
-            this.FlushToStream();
-
             // Padding all 4 bytes with 1's while not corrupting initial bits stored in accumulatedBits
             // And writing only valuable count of bytes count we want to write to the output stream
             int valuableBytesCount = (int)Numerics.DivideCeil((uint)this.bitCount, 8);
             uint packedBytes = this.accumulatedBits | (uint.MaxValue >> this.bitCount);
+            this.emitBuffer[--this.emitWriteIndex] = packedBytes;
 
-            Span<byte> emitBytes = MemoryMarshal.AsBytes(this.emitBuffer.AsSpan());
-            for (int i = 0; i < valuableBytesCount; i++)
-            {
-                emitBytes[i] = (byte)((packedBytes >> ((3 - i) * 8)) & 0xff);
-            }
-
-            // Flush remaining 'tail' bytes
-            this.target.Write(emitBytes, 0, valuableBytesCount);
+            // Flush cached bytes to the output stream with padding bits
+            this.FlushToStream((this.emitWriteIndex * 4) - 4 + valuableBytesCount);
         }
     }
 }

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -1071,13 +1071,13 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                     // Types 0..1 DC..AC
                     if (tableType > 1)
                     {
-                        JpegThrowHelper.ThrowInvalidImageContentException("Bad Huffman Table type.");
+                        JpegThrowHelper.ThrowInvalidImageContentException($"Bad huffman table type: {tableType}");
                     }
 
                     // Max tables of each type
                     if (tableIndex > 3)
                     {
-                        JpegThrowHelper.ThrowInvalidImageContentException("Bad Huffman Table index.");
+                        JpegThrowHelper.ThrowInvalidImageContentException($"Bad huffman table index: {tableIndex}");
                     }
 
                     stream.Read(huffmanDataSpan, 0, 16);

tests/ImageSharp.Tests/Formats/Jpg/DCTTests.cs

@@ -2,9 +2,6 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
-using System.Numerics;
-using System.Runtime.CompilerServices;
-using System.Runtime.Intrinsics;
 #if SUPPORTS_RUNTIME_INTRINSICS
 using System.Runtime.Intrinsics.X86;
 #endif
@@ -121,24 +118,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             public void IDCT8x8_Avx(int seed)
             {
 #if SUPPORTS_RUNTIME_INTRINSICS
-                var skip = !Avx.IsSupported;
-#else
-                var skip = true;
-#endif
-
-                if (skip)
+                if (!Avx.IsSupported)
                 {
                     this.Output.WriteLine("No AVX present, skipping test!");
-                    return;
                 }
 
                 Span<float> src = Create8x8RoundedRandomFloatData(-200, 200, seed);
-                var srcBlock = default(Block8x8F);
+                Block8x8F srcBlock = default;
                 srcBlock.LoadFrom(src);
 
-                var destBlock = default(Block8x8F);
+                Block8x8F destBlock = default;
 
-                var expectedDest = new float[64];
+                float[] expectedDest = new float[64];
 
                 // reference, left part
                 ReferenceImplementations.LLM_FloatingPoint_DCT.IDCT2D8x4_32f(src, expectedDest);
@@ -149,10 +140,11 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 // testee, whole 8x8
                 FastFloatingPointDCT.IDCT8x8_Avx(ref srcBlock, ref destBlock);
 
-                var actualDest = new float[64];
+                float[] actualDest = new float[64];
                 destBlock.ScaledCopyTo(actualDest);
 
                 Assert.Equal(actualDest, expectedDest, new ApproximateFloatComparer(1f));
+#endif
             }
 
             [Theory]