Begin port

src/ImageSharp/Common/Extensions/StreamExtensions.cs

@@ -27,7 +27,7 @@ namespace ImageSharp
 
             if (stream.CanSeek)
             {
-                stream.Position += count;
+                stream.Seek(count, SeekOrigin.Current); // Position += count;
             }
             else
             {

src/ImageSharp/Formats/Jpeg/JpegDecoder.cs

@@ -21,10 +21,12 @@ namespace ImageSharp.Formats
         {
             Guard.NotNull(stream, "stream");
 
-            using (JpegDecoderCore decoder = new JpegDecoderCore(options, configuration))
-            {
-                return decoder.Decode<TPixel>(stream);
-            }
+            // using (JpegDecoderCore decoder = new JpegDecoderCore(options, configuration))
+            // {
+            //    return decoder.Decode<TPixel>(stream);
+            // }
+            var decoder = new Jpeg.Port.JpegDecoderCore(options, configuration);
+            return decoder.Decode<TPixel>(stream);
         }
     }
 }

src/ImageSharp/Formats/Jpeg/Port/Components/Component.cs

@@ -0,0 +1,53 @@
+// <copyright file="Component.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Formats.Jpeg.Port.Components
+{
+    /// <summary>
+    /// Represents a single color component
+    /// </summary>
+    internal struct Component
+    {
+        /// <summary>
+        /// Gets or sets the component Id
+        /// </summary>
+        public byte Id;
+
+        /// <summary>
+        /// Gets or sets the horizontal sampling factor.
+        /// </summary>
+        public int HorizontalFactor;
+
+        /// <summary>
+        /// Gets or sets the vertical sampling factor.
+        /// </summary>
+        public int VerticalFactor;
+
+        /// <summary>
+        /// Gets or sets the identifier
+        /// </summary>
+        public byte QuantizationIdentifier;
+
+        /// <summary>
+        /// Gets or sets the quantization table
+        /// </summary>
+        public short[] QuantizationTable;
+
+        /// <summary>
+        /// Gets or sets the block data
+        /// </summary>
+        public short[] BlockData;
+
+        /// <summary>
+        /// Gets or sets the number of blocks per line
+        /// </summary>
+        public int BlocksPerLine;
+
+        /// <summary>
+        /// Gets or sets the number of blocks per column
+        /// </summary>
+        public int BlocksPerColumn;
+    }
+}

src/ImageSharp/Formats/Jpeg/Port/Components/Frame.cs

@@ -0,0 +1,73 @@
+// <copyright file="QuantizationTables.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Formats.Jpeg.Port.Components
+{
+    /// <summary>
+    /// Represent a single jpeg frame
+    /// </summary>
+    internal class Frame
+    {
+        /// <summary>
+        /// Gets or sets a value indicating whether the frame uses the extended specification
+        /// </summary>
+        public bool Extended { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether the frame uses the progressive specification
+        /// </summary>
+        public bool Progressive { get; set; }
+
+        /// <summary>
+        /// Gets or sets the precision
+        /// </summary>
+        public byte Precision { get; set; }
+
+        /// <summary>
+        /// Gets or sets the number of scanlines within the frame
+        /// </summary>
+        public short Scanlines { get; set; }
+
+        /// <summary>
+        /// Gets or sets the number of samples per scanline
+        /// </summary>
+        public short SamplesPerLine { get; set; }
+
+        /// <summary>
+        /// Gets or sets the number of components within a frame. In progressive frames this value can range from only 1 to 4
+        /// </summary>
+        public byte ComponentCount { get; set; }
+
+        /// <summary>
+        /// Gets or sets the component id collection
+        /// </summary>
+        public byte[] ComponentIds { get; set; }
+
+        /// <summary>
+        /// Gets or sets the frame component collection
+        /// </summary>
+        public Component[] Components { get; set; }
+
+        /// <summary>
+        /// Gets or sets the maximum horizontal sampling factor
+        /// </summary>
+        public int MaxHorizontalFactor { get; set; }
+
+        /// <summary>
+        /// Gets or sets the maximum vertical sampling factor
+        /// </summary>
+        public int MaxVerticalFactor { get; set; }
+
+        /// <summary>
+        /// Gets or sets the number of MCU's per line
+        /// </summary>
+        public int McusPerLine { get; set; }
+
+        /// <summary>
+        /// Gets or sets the number of MCU's per column
+        /// </summary>
+        public int McusPerColumn { get; set; }
+    }
+}

src/ImageSharp/Formats/Jpeg/Port/Components/HuffmanTables.cs

@@ -0,0 +1,15 @@
+namespace ImageSharp.Formats.Jpeg.Port.Components
+{
+    using ImageSharp.Memory;
+
+    /// <summary>
+    /// Defines a pair of huffman tables
+    /// </summary>
+    internal class HuffmanTables
+    {
+        /// <summary>
+        /// Gets or sets the quantization tables.
+        /// </summary>
+        public Fast2DArray<short> Tables { get; set; } = new Fast2DArray<short>(256, 2);
+    }
+}

src/ImageSharp/Formats/Jpeg/Port/Components/QuantizationTables.cs

@@ -0,0 +1,43 @@
+// <copyright file="QuantizationTables.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Formats.Jpeg.Port.Components
+{
+    using ImageSharp.Memory;
+
+    /// <summary>
+    /// Contains the quantization tables.
+    /// TODO: This all needs optimizing for memory. I'm just stubbing out functionality for now.
+    /// </summary>
+    internal class QuantizationTables
+    {
+        /// <summary>
+        /// Gets the ZigZag scan table
+        /// </summary>
+        public static byte[] DctZigZag { get; } =
+        {
+            0,
+            1, 8,
+            16, 9, 2,
+            3, 10, 17, 24,
+            32, 25, 18, 11, 4,
+            5, 12, 19, 26, 33, 40,
+            48, 41, 34, 27, 20, 13, 6,
+            7, 14, 21, 28, 35, 42, 49, 56,
+            57, 50, 43, 36, 29, 22, 15,
+            23, 30, 37, 44, 51, 58,
+            59, 52, 45, 38, 31,
+            39, 46, 53, 60,
+            61, 54, 47,
+            55, 62,
+            63
+        };
+
+        /// <summary>
+        /// Gets or sets the quantization tables.
+        /// </summary>
+        public Fast2DArray<short> Tables { get; set; } = new Fast2DArray<short>(64, 4);
+    }
+}

src/ImageSharp/Formats/Jpeg/Port/Huffman.cs

@@ -0,0 +1,6 @@
+namespace ImageSharp.Formats.Jpeg.Port
+{
+    class Huffman
+    {
+    }
+}

src/ImageSharp/Formats/Jpeg/Port/JpegConstants.cs

@@ -0,0 +1,185 @@
+// ReSharper disable InconsistentNaming
+namespace ImageSharp.Formats.Jpeg.Port
+{
+    /// <summary>
+    /// Contains jpeg constant values
+    /// </summary>
+    internal static class JpegConstants
+    {
+        /// <summary>
+        /// Contains marker specific constants
+        /// </summary>
+        public static class Markers
+        {
+            /// <summary>
+            /// The prefix used for all markers.
+            /// </summary>
+            public const byte Prefix = 0xFF;
+
+            /// <summary>
+            /// The Start of Image marker
+            /// </summary>
+            public const ushort SOI = 0xFFD8;
+
+            /// <summary>
+            /// The End of Image marker
+            /// </summary>
+            public const ushort EOI = 0xFFD9;
+
+            /// <summary>
+            /// Application specific marker for marking the jpeg format.
+            /// <see href="http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html"/>
+            /// </summary>
+            public const ushort APP0 = 0xFFE0;
+
+            /// <summary>
+            /// Application specific marker for marking where to store metadata.
+            /// </summary>
+            public const ushort APP1 = 0xFFE1;
+
+            /// <summary>
+            /// Application specific marker for marking where to store ICC profile information.
+            /// </summary>
+            public const ushort APP2 = 0xFFE2;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP3 = 0xFFE3;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP4 = 0xFFE4;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP5 = 0xFFE5;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP6 = 0xFFE6;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP7 = 0xFFE7;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP8 = 0xFFE8;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP9 = 0xFFE9;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP10 = 0xFFEA;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP11 = 0xFFEB;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP12 = 0xFFEC;
+
+            /// <summary>
+            /// Application specific marker.
+            /// </summary>
+            public const ushort APP13 = 0xFFED;
+
+            /// <summary>
+            /// Application specific marker used by Adobe for storing encoding information for DCT filters.
+            /// </summary>
+            public const ushort APP14 = 0xFFEE;
+
+            /// <summary>
+            /// Application specific marker used by GraphicConverter to store JPEG quality.
+            /// </summary>
+            public const ushort APP15 = 0xFFEF;
+
+            /// <summary>
+            /// The text comment marker
+            /// </summary>
+            public const ushort COM = 0xFFFE;
+
+            /// <summary>
+            /// Define Quantization Table(s) marker
+            /// <remarks>
+            /// Specifies one or more quantization tables.
+            /// </remarks>
+            /// </summary>
+            public const ushort DQT = 0xFFDB;
+
+            /// <summary>
+            /// Start of Frame (baseline DCT)
+            /// <remarks>
+            /// Indicates that this is a baseline DCT-based JPEG, and specifies the width, height, number of components,
+            /// and component subsampling (e.g., 4:2:0).
+            /// </remarks>
+            /// </summary>
+            public const ushort SOF0 = 0xFFC0;
+
+            /// <summary>
+            /// Start Of Frame (Extended Sequential DCT)
+            /// <remarks>
+            /// Indicates that this is a progressive DCT-based JPEG, and specifies the width, height, number of components,
+            /// and component subsampling (e.g., 4:2:0).
+            /// </remarks>
+            /// </summary>
+            public const ushort SOF1 = 0xFFC1;
+
+            /// <summary>
+            /// Start Of Frame (progressive DCT)
+            /// <remarks>
+            /// Indicates that this is a progressive DCT-based JPEG, and specifies the width, height, number of components,
+            /// and component subsampling (e.g., 4:2:0).
+            /// </remarks>
+            /// </summary>
+            public const ushort SOF2 = 0xFFC2;
+
+            /// <summary>
+            /// Define Huffman Table(s)
+            /// <remarks>
+            /// Specifies one or more Huffman tables.
+            /// </remarks>
+            /// </summary>
+            public const ushort DHT = 0xFFC4;
+
+            /// <summary>
+            /// Contains JFIF specific markers
+            /// </summary>
+            public static class JFif
+            {
+                /// <summary>
+                /// Represents J in ASCII
+                /// </summary>
+                public const byte J = 0x4A;
+
+                /// <summary>
+                /// Represents F in ASCII
+                /// </summary>
+                public const byte F = 0x46;
+
+                /// <summary>
+                /// Represents I in ASCII
+                /// </summary>
+                public const byte I = 0x49;
+
+                /// <summary>
+                /// Represents the null "0" marker
+                /// </summary>
+                public const byte Null = 0;
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Jpeg/Port/JpegDecoderCore.cs

@@ -0,0 +1,522 @@
+// <copyright file="JpegDecoderCore.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Formats.Jpeg.Port
+{
+    using System;
+    using System.Buffers;
+    using System.IO;
+
+    using ImageSharp.Formats.Jpeg.Port.Components;
+    using ImageSharp.PixelFormats;
+
+    /// <summary>
+    /// Performs the jpeg decoding operation.
+    /// </summary>
+    internal class JpegDecoderCore
+    {
+        /// <summary>
+        /// The decoder options.
+        /// </summary>
+        private readonly IDecoderOptions options;
+
+        /// <summary>
+        /// The global configuration
+        /// </summary>
+        private readonly Configuration configuration;
+
+        /// <summary>
+        /// Gets the temporary buffer used to store bytes read from the stream.
+        /// </summary>
+        private readonly byte[] temp = new byte[2 * 16 * 4];
+
+        private readonly byte[] uint16Buffer = new byte[2];
+
+        private QuantizationTables quantizationTables;
+
+        private HuffmanTables dcHuffmanTables;
+
+        private HuffmanTables acHuffmanTables;
+
+        private Frame frame;
+
+        /// <summary>
+        /// COntains information about the jFIF marker
+        /// </summary>
+        private JFif jFif;
+
+        /// <summary>
+        /// Whether the image has a EXIF header
+        /// </summary>
+        private bool isExif;
+
+        private int offset;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="JpegDecoderCore" /> class.
+        /// </summary>
+        /// <param name="options">The decoder options.</param>
+        /// <param name="configuration">The configuration.</param>
+        public JpegDecoderCore(IDecoderOptions options, Configuration configuration)
+        {
+            this.configuration = configuration ?? Configuration.Default;
+            this.options = options ?? new DecoderOptions();
+        }
+
+        /// <summary>
+        /// Gets the input stream.
+        /// </summary>
+        public Stream InputStream { get; private set; }
+
+        /// <summary>
+        /// Decodes the image from the specified <see cref="Stream"/>  and sets
+        /// the data to image.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="stream">The stream, where the image should be.</param>
+        /// <returns>The decoded image.</returns>
+        public Image<TPixel> Decode<TPixel>(Stream stream)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            this.InputStream = stream;
+            this.ParseStream();
+
+            var image = new Image<TPixel>(1, 1);
+            return image;
+        }
+
+        private void ParseStream()
+        {
+            // Check for the Start Of Image marker.
+            ushort fileMarker = this.ReadUint16();
+            if (fileMarker != JpegConstants.Markers.SOI)
+            {
+                throw new ImageFormatException("Missing SOI marker.");
+            }
+
+            fileMarker = this.ReadUint16();
+
+            this.quantizationTables = new QuantizationTables();
+
+            while (fileMarker != JpegConstants.Markers.EOI)
+            {
+                // Get the marker length
+                int remaining = this.ReadUint16() - 2;
+
+                switch (fileMarker)
+                {
+                    case JpegConstants.Markers.APP0:
+                        this.ProcessApplicationHeaderMarker(remaining);
+                        break;
+
+                    case JpegConstants.Markers.APP1:
+                    case JpegConstants.Markers.APP2:
+                    case JpegConstants.Markers.APP3:
+                    case JpegConstants.Markers.APP4:
+                    case JpegConstants.Markers.APP5:
+                    case JpegConstants.Markers.APP6:
+                    case JpegConstants.Markers.APP7:
+                    case JpegConstants.Markers.APP8:
+                    case JpegConstants.Markers.APP9:
+                    case JpegConstants.Markers.APP10:
+                    case JpegConstants.Markers.APP11:
+                    case JpegConstants.Markers.APP12:
+                    case JpegConstants.Markers.APP13:
+                    case JpegConstants.Markers.APP14:
+                    case JpegConstants.Markers.APP15:
+                    case JpegConstants.Markers.COM:
+                        break;
+
+                    case JpegConstants.Markers.DQT:
+                        this.ProcessDqtMarker(remaining);
+                        break;
+
+                    case JpegConstants.Markers.SOF0:
+                    case JpegConstants.Markers.SOF1:
+                    case JpegConstants.Markers.SOF2:
+                        this.ProcessStartOfFrameMarker(remaining, fileMarker);
+                        break;
+
+                    case JpegConstants.Markers.DHT:
+                        this.ProcessDefineHuffmanTablesMarker(remaining);
+                        break;
+                }
+
+                // Read on
+                fileMarker = this.FindNextFileMarker();
+            }
+        }
+
+        /// <summary>
+        /// Processes the application header containing the JFIF identifier plus extra data.
+        /// </summary>
+        /// <param name="remaining">The remaining bytes in the segment block.</param>
+        private void ProcessApplicationHeaderMarker(int remaining)
+        {
+            if (remaining < 5)
+            {
+                // Skip the application header length
+                this.InputStream.Skip(remaining);
+                return;
+            }
+
+            this.InputStream.Read(this.temp, 0, 13);
+            remaining -= 13;
+
+            bool isJfif = this.temp[0] == JpegConstants.Markers.JFif.J &&
+                          this.temp[1] == JpegConstants.Markers.JFif.F &&
+                          this.temp[2] == JpegConstants.Markers.JFif.I &&
+                          this.temp[3] == JpegConstants.Markers.JFif.F &&
+                          this.temp[4] == JpegConstants.Markers.JFif.Null;
+
+            if (isJfif)
+            {
+                this.jFif = new JFif
+                {
+                    MajorVersion = this.temp[5],
+                    MinorVersion = this.temp[6],
+                    DensityUnits = this.temp[7],
+                    XDensity = (short)((this.temp[8] << 8) | this.temp[9]),
+                    YDensity = (short)((this.temp[10] << 8) | this.temp[11])
+                };
+            }
+
+            // Skip thumbnails for now.
+            if (remaining > 0)
+            {
+                this.InputStream.Skip(remaining);
+            }
+        }
+
+        /// <summary>
+        /// Processes the Define Quantization Marker and tables. Specified in section B.2.4.1.
+        /// </summary>
+        /// <param name="remaining">The remaining bytes in the segment block.</param>
+        /// <exception cref="ImageFormatException">
+        /// Thrown if the tables do not match the header
+        /// </exception>
+        private void ProcessDqtMarker(int remaining)
+        {
+            while (remaining > 0)
+            {
+                bool done = false;
+                remaining--;
+                int quantizationTableSpec = this.InputStream.ReadByte();
+
+                if (quantizationTableSpec > 3)
+                {
+                    throw new ImageFormatException("Bad Tq index value");
+                }
+
+                switch (quantizationTableSpec >> 4)
+                {
+                    case 0:
+                        {
+                            // 8 bit values
+                            if (remaining < 64)
+                            {
+                                done = true;
+                                break;
+                            }
+
+                            this.InputStream.Read(this.temp, 0, 64);
+                            remaining -= 64;
+
+                            Span<short> tableSpan = this.quantizationTables.Tables.GetRowSpan(quantizationTableSpec);
+                            for (int j = 0; j < 64; j++)
+                            {
+                                tableSpan[QuantizationTables.DctZigZag[j]] = this.temp[j];
+                            }
+                        }
+
+                        break;
+                    case 1:
+                        {
+                            // 16 bit values
+                            if (remaining < 128)
+                            {
+                                done = true;
+                                break;
+                            }
+
+                            this.InputStream.Read(this.temp, 0, 128);
+                            remaining -= 128;
+
+                            Span<short> tableSpan = this.quantizationTables.Tables.GetRowSpan(quantizationTableSpec);
+                            for (int j = 0; j < 64; j++)
+                            {
+                                tableSpan[QuantizationTables.DctZigZag[j]] = (short)((this.temp[2 * j] << 8) | this.temp[(2 * j) + 1]);
+                            }
+                        }
+
+                        break;
+                    default:
+                        throw new ImageFormatException("Bad Tq index value");
+                }
+
+                if (done)
+                {
+                    break;
+                }
+            }
+
+            if (remaining != 0)
+            {
+                throw new ImageFormatException("DQT has wrong length");
+            }
+        }
+
+        /// <summary>
+        /// Processes the Start of Frame marker.  Specified in section B.2.2.
+        /// </summary>
+        /// <param name="remaining">The remaining bytes in the segment block.</param>
+        /// <param name="frameMarker">The current frame marker.</param>
+        private void ProcessStartOfFrameMarker(int remaining, ushort frameMarker)
+        {
+            if (this.frame != null)
+            {
+                throw new ImageFormatException("Multiple SOF markers. Only single frame jpegs supported.");
+            }
+
+            this.InputStream.Read(this.temp, 0, remaining);
+
+            this.frame = new Frame
+            {
+                Extended = frameMarker == JpegConstants.Markers.SOF1,
+                Progressive = frameMarker == JpegConstants.Markers.SOF1,
+                Precision = this.temp[0],
+                Scanlines = (short)((this.temp[1] << 8) | this.temp[2]),
+                SamplesPerLine = (short)((this.temp[3] << 8) | this.temp[4]),
+                ComponentCount = this.temp[5]
+            };
+
+            int maxH = 0;
+            int maxV = 0;
+            int index = 6;
+
+            // TODO: Pool this.
+            this.frame.ComponentIds = new byte[this.frame.ComponentCount];
+            this.frame.Components = new Component[this.frame.ComponentCount];
+
+            for (int i = 0; i < this.frame.ComponentCount; i++)
+            {
+                int h = this.temp[index + 1] >> 4;
+                int v = this.temp[index + 1] & 15;
+
+                if (maxH < h)
+                {
+                    maxH = h;
+                }
+
+                if (maxV < v)
+                {
+                    maxV = v;
+                }
+
+                ref var component = ref this.frame.Components[i];
+                component.Id = this.temp[index];
+                component.HorizontalFactor = h;
+                component.VerticalFactor = v;
+                component.QuantizationIdentifier = this.temp[index + 2];
+
+                // Don't assign the table yet.
+                index += 3;
+            }
+
+            this.frame.MaxHorizontalFactor = maxH;
+            this.frame.MaxVerticalFactor = maxV;
+            this.PrepareComponents();
+        }
+
+        /// <summary>
+        /// Processes a Define Huffman Table marker, and initializes a huffman
+        /// struct from its contents. Specified in section B.2.4.2.
+        /// </summary>
+        /// <param name="remaining">The remaining bytes in the segment block.</param>
+        private void ProcessDefineHuffmanTablesMarker(int remaining)
+        {
+            if (remaining < 17)
+            {
+                throw new ImageFormatException("DHT has wrong length");
+            }
+
+            // TODO: Pool this
+            byte[] huffmanData = new byte[remaining];
+            this.InputStream.Read(huffmanData, 0, remaining);
+
+            int o = 0;
+            for (int i = 0; i < remaining;)
+            {
+                byte huffmanTableSpec = huffmanData[i];
+                byte[] codeLengths = new byte[16];
+                int codeLengthSum = 0;
+
+                for (int j = 0; j < 16; j++)
+                {
+                    codeLengthSum += codeLengths[j] = huffmanData[o++];
+                }
+
+                short[] huffmanValues = new short[codeLengthSum];
+
+                byte[] values = null;
+                try
+                {
+                    values = ArrayPool<byte>.Shared.Rent(256);
+                    this.InputStream.Read(values, 0, codeLengthSum);
+
+                    for (int j = 0; j < codeLengthSum; j++)
+                    {
+                        huffmanValues[j] = values[o++];
+                    }
+                }
+                finally
+                {
+                    if (values != null)
+                    {
+                        ArrayPool<byte>.Shared.Return(values);
+                    }
+                }
+
+                i += 17 + codeLengthSum;
+
+                this.BuildHuffmanTable(
+                    huffmanTableSpec >> 4 == 0 ? this.dcHuffmanTables : this.acHuffmanTables,
+                    huffmanTableSpec & 15,
+                    codeLengths,
+                    huffmanValues);
+            }
+        }
+
+        private void BuildHuffmanTable(HuffmanTables tables, int index, byte[] codeLengths, short[] values)
+        {
+            Span<short> tableSpan = tables.Tables.GetRowSpan(index);
+        }
+
+        /// <summary>
+        /// Allocates the frame component blocks
+        /// </summary>
+        private void PrepareComponents()
+        {
+            int mcusPerLine = this.frame.SamplesPerLine / 8 / this.frame.MaxHorizontalFactor;
+            int mcusPerColumn = this.frame.Scanlines / 8 / this.frame.MaxVerticalFactor;
+
+            for (int i = 0; i < this.frame.ComponentCount; i++)
+            {
+                ref var component = ref this.frame.Components[i];
+                int blocksPerLine = this.frame.SamplesPerLine / 8 * component.HorizontalFactor / this.frame.MaxHorizontalFactor;
+                int blocksPerColumn = this.frame.Scanlines / 8 * component.VerticalFactor / this.frame.MaxVerticalFactor;
+                int blocksPerLineForMcu = mcusPerLine * component.HorizontalFactor;
+                int blocksPerColumnForMcu = mcusPerColumn * component.VerticalFactor;
+
+                int blocksBufferSize = 64 * blocksPerColumnForMcu * (blocksPerLineForMcu + 1);
+
+                // TODO: Pool this
+                component.BlockData = new short[blocksBufferSize];
+                component.BlocksPerLine = blocksPerLine;
+                component.BlocksPerColumn = blocksPerColumn;
+            }
+
+            this.frame.McusPerLine = mcusPerLine;
+            this.frame.McusPerColumn = mcusPerColumn;
+        }
+
+        /// <summary>
+        /// Finds the next file marker within the byte stream
+        /// </summary>
+        /// <returns>The <see cref="ushort"/></returns>
+        private ushort FindNextFileMarker()
+        {
+            while (true)
+            {
+                int value = this.InputStream.Read(this.uint16Buffer, 0, 2);
+
+                if (value <= 0)
+                {
+                    return JpegConstants.Markers.EOI;
+                }
+
+                while (this.uint16Buffer[0] != JpegConstants.Markers.Prefix)
+                {
+                    // Strictly speaking, this is a format error. However, libjpeg is
+                    // liberal in what it accepts. As of version 9, next_marker in
+                    // jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
+                    // continues to decode the stream. Even before next_marker sees
+                    // extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
+                    // bytes as it can, possibly past the end of a scan's data. It
+                    // effectively puts back any markers that it overscanned (e.g. an
+                    // "\xff\xd9" EOI marker), but it does not put back non-marker data,
+                    // and thus it can silently ignore a small number of extraneous
+                    // non-marker bytes before next_marker has a chance to see them (and
+                    // print a warning).
+                    // We are therefore also liberal in what we accept. Extraneous data
+                    // is silently ignore
+                    // This is similar to, but not exactly the same as, the restart
+                    // mechanism within a scan (the RST[0-7] markers).
+                    // Note that extraneous 0xff bytes in e.g. SOS data are escaped as
+                    // "\xff\x00", and so are detected a little further down below.
+                    this.uint16Buffer[0] = this.uint16Buffer[1];
+
+                    value = this.InputStream.ReadByte();
+                    if (value <= 0)
+                    {
+                        return JpegConstants.Markers.EOI;
+                    }
+
+                    this.uint16Buffer[1] = (byte)value;
+                }
+
+                return (ushort)((this.uint16Buffer[0] << 8) | this.uint16Buffer[1]);
+            }
+        }
+
+        /// <summary>
+        /// Reads a <see cref="ushort"/> from the stream advancing it by two bytes
+        /// </summary>
+        /// <returns>The <see cref="ushort"/></returns>
+        private ushort ReadUint16()
+        {
+            this.InputStream.Read(this.uint16Buffer, 0, 2);
+            ushort value = (ushort)((this.uint16Buffer[0] << 8) | this.uint16Buffer[1]);
+            this.offset += 2;
+            return value;
+        }
+
+        /// <summary>
+        /// Provides information about the JFIF marker segment
+        /// </summary>
+        internal struct JFif
+        {
+            /// <summary>
+            /// The major version
+            /// </summary>
+            public byte MajorVersion;
+
+            /// <summary>
+            /// The minor version
+            /// </summary>
+            public byte MinorVersion;
+
+            /// <summary>
+            /// Units for the following pixel density fields
+            ///  00 : No units; width:height pixel aspect ratio = Ydensity:Xdensity
+            ///  01 : Pixels per inch (2.54 cm)
+            ///  02 : Pixels per centimeter
+            /// </summary>
+            public byte DensityUnits;
+
+            /// <summary>
+            /// Horizontal pixel density. Must not be zero.
+            /// </summary>
+            public short XDensity;
+
+            /// <summary>
+            /// Vertical pixel density. Must not be zero.
+            /// </summary>
+            public short YDensity;
+
+            // TODO: Thumbnail?
+        }
+    }
+}

src/ImageSharp/Formats/Jpeg/Port/JpegFrame.cs

@@ -0,0 +1,33 @@
+namespace ImageSharp.Formats.Jpeg.Port
+{
+    /// <summary>
+    /// Represents a jpeg frame
+    /// </summary>
+    internal class JpegFrame
+    {
+        /// <summary>
+        /// Gets or sets a value indicating whether the fame is extended
+        /// </summary>
+        public bool Extended { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether the fame is progressive
+        /// </summary>
+        public bool Progressive { get; set; }
+
+        /// <summary>
+        /// Gets or sets the precision
+        /// </summary>
+        public byte Precision { get; set; }
+
+        /// <summary>
+        /// Gets or sets the number of scanlines within the frame
+        /// </summary>
+        public short Scanlines { get; set; }
+
+        /// <summary>
+        /// Gets or sets the number of samples per scanline
+        /// </summary>
+        public short SamplesPerLine { get; set; }
+    }
+}

src/ImageSharp/Memory/Fast2DArray{T}.cs

@@ -106,11 +106,23 @@ namespace ImageSharp.Memory
             return new Fast2DArray<T>(data);
         }
 
+        /// <summary>
+        /// Gets a <see cref="Span{T}"/> representing the row beginning from the the first item on that row.
+        /// </summary>
+        /// <param name="row">The y-coordinate of the row. Must be greater than or equal to zero and less than the height of the 2D array.</param>
+        /// <returns>The <see cref="Span{T}"/></returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Span<T> GetRowSpan(int row)
+        {
+            this.CheckCoordinates(row);
+            return new Span<T>(this.Data, row * this.Width, this.Width);
+        }
+
         /// <summary>
         /// Checks the coordinates to ensure they are within bounds.
         /// </summary>
-        /// <param name="row">The row-coordinate of the item. Must be greater than zero and smaller than the height of the array.</param>
-        /// <param name="column">The column-coordinate of the item. Must be greater than zero and smaller than the width of the array.</param>
+        /// <param name="row">The y-coordinate of the item. Must be greater than zero and smaller than the height of the array.</param>
+        /// <param name="column">The x-coordinate of the item. Must be greater than zero and smaller than the width of the array.</param>
         /// <exception cref="ArgumentOutOfRangeException">
         /// Thrown if the coordinates are not within the bounds of the array.
         /// </exception>
@@ -127,5 +139,21 @@ namespace ImageSharp.Memory
                 throw new ArgumentOutOfRangeException(nameof(column), column, $"{column} is outwith the array bounds.");
             }
         }
+
+        /// <summary>
+        /// Checks the coordinates to ensure they are within bounds.
+        /// </summary>
+        /// <param name="row">The y-coordinate of the item. Must be greater than zero and smaller than the height of the array.</param>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// Thrown if the coordinates are not within the bounds of the image.
+        /// </exception>
+        [Conditional("DEBUG")]
+        private void CheckCoordinates(int row)
+        {
+            if (row < 0 || row >= this.Height)
+            {
+                throw new ArgumentOutOfRangeException(nameof(row), row, $"{row} is outwith the array bounds.");
+            }
+        }
     }
 }