Improve coverage.

src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegFileMarker.cs

@@ -16,10 +16,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         /// <param name="marker">The marker</param>
         /// <param name="position">The position within the stream</param>
         public JpegFileMarker(byte marker, long position)
+            : this(marker, position, false)
         {
-            this.Marker = marker;
-            this.Position = position;
-            this.Invalid = false;
         }
 
         /// <summary>

src/ImageSharp/Formats/Jpeg/Components/Decoder/ScanDecoder.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.Runtime.CompilerServices;
+using SixLabors.ImageSharp.IO;
 
 namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 {

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -10,6 +10,7 @@ using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.Formats.Jpeg.Components;
 using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
+using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.MetaData;
 using SixLabors.ImageSharp.MetaData.Profiles.Exif;
 using SixLabors.ImageSharp.MetaData.Profiles.Icc;

src/ImageSharp/Formats/Jpeg/Components/Decoder/DoubleBufferedStreamReader.cs → src/ImageSharp/IO/DoubleBufferedStreamReader.cs

@@ -7,8 +7,7 @@ using System.Runtime.CompilerServices;
 
 using SixLabors.Memory;
 
-// TODO: This could be useful elsewhere.
-namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
+namespace SixLabors.ImageSharp.IO
 {
     /// <summary>
     /// A stream reader that add a secondary level buffer in addition to native stream buffered reading

tests/ImageSharp.Benchmarks/Codecs/Jpeg/DoubleBufferedStreams.cs

@@ -4,8 +4,7 @@
 using System;
 using System.IO;
 using BenchmarkDotNet.Attributes;
-
-using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
+using SixLabors.ImageSharp.IO;
 
 namespace SixLabors.ImageSharp.Benchmarks.Codecs.Jpeg
 {

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

@@ -0,0 +1,25 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Formats.Jpeg;
+using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Formats.Jpg
+{
+    public class JpegFileMarkerTests
+    {
+        [Fact]
+        public void MarkerConstructorAssignsProperties()
+        {
+            const byte app1 = JpegConstants.Markers.APP1;
+            const int position = 5;
+            var marker = new JpegFileMarker(app1, position);
+
+            Assert.Equal(app1, marker.Marker);
+            Assert.Equal(position, marker.Position);
+            Assert.False(marker.Invalid);
+            Assert.Equal(app1.ToString("X"), marker.ToString());
+        }
+    }
+}

tests/ImageSharp.Tests/Formats/Jpg/DoubleBufferedStreamReaderTests.cs → tests/ImageSharp.Tests/IO/DoubleBufferedStreamReaderTests.cs

@@ -3,12 +3,11 @@
 
 using System;
 using System.IO;
-
-using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
+using SixLabors.ImageSharp.IO;
 using SixLabors.Memory;
 using Xunit;
 
-namespace SixLabors.ImageSharp.Tests.Formats.Jpg
+namespace SixLabors.ImageSharp.Tests.IO
 {
     public class DoubleBufferedStreamReaderTests
     {
@@ -30,6 +29,24 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             }
         }
 
+        [Fact]
+        public void DoubleBufferedStreamReaderCanReadSingleByteFromOffset()
+        {
+            using (MemoryStream stream = this.CreateTestStream())
+            {
+                byte[] expected = stream.ToArray();
+                const int offset = 5;
+                var reader = new DoubleBufferedStreamReader(this.allocator, stream);
+                reader.Position = offset;
+
+                Assert.Equal(expected[offset], reader.ReadByte());
+
+                // We've read a whole chunk but increment by 1 in our reader.
+                Assert.Equal(stream.Position, DoubleBufferedStreamReader.ChunkLength + offset);
+                Assert.Equal(offset + 1, reader.Position);
+            }
+        }
+
         [Fact]
         public void DoubleBufferedStreamReaderCanReadSubsequentSingleByteCorrectly()
         {
@@ -139,7 +156,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 // Skip Again
                 reader.Skip(skip2);
 
-                // First Skap + First Read + Second Skip
+                // First Skip + First Read + Second Skip
                 int position = skip + plusOne + skip2;
 
                 Assert.Equal(position, reader.Position);