Can now decode a scan

src/ImageSharp/Formats/Jpeg/JpegDecoder.cs

@@ -25,8 +25,10 @@ namespace ImageSharp.Formats
             // {
             //    return decoder.Decode<TPixel>(stream);
             // }
-            var decoder = new Jpeg.Port.JpegDecoderCore(options, configuration);
-            return decoder.Decode<TPixel>(stream);
+            using (var decoder = new Jpeg.Port.JpegDecoderCore(options, configuration))
+            {
+                return decoder.Decode<TPixel>(stream);
+            }
         }
     }
 }

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

@@ -0,0 +1,53 @@
+// <copyright file="HuffmanTables.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 jpeg file marker
+    /// </summary>
+    internal struct FileMarker
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="FileMarker"/> struct.
+        /// </summary>
+        /// <param name="marker">The marker</param>
+        /// <param name="position">The position within the stream</param>
+        public FileMarker(ushort marker, long position)
+        {
+            this.Marker = marker;
+            this.Position = position;
+            this.Invalid = false;
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="FileMarker"/> struct.
+        /// </summary>
+        /// <param name="marker">The marker</param>
+        /// <param name="position">The position within the stream</param>
+        /// <param name="invalid">Whether the current marker is invalid</param>
+        public FileMarker(ushort marker, long position, bool invalid)
+        {
+            this.Marker = marker;
+            this.Position = position;
+            this.Invalid = invalid;
+        }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether the current marker is invalid
+        /// </summary>
+        public bool Invalid { get; set; }
+
+        /// <summary>
+        /// Gets the position of the marker within a stream
+        /// </summary>
+        public ushort Marker { get; }
+
+        /// <summary>
+        /// Gets the position of the marker within a stream
+        /// </summary>
+        public long Position { get; }
+    }
+}

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

@@ -5,11 +5,15 @@
 
 namespace ImageSharp.Formats.Jpeg.Port.Components
 {
+    using System;
+
     /// <summary>
     /// Represent a single jpeg frame
     /// </summary>
-    internal class Frame
+    internal class Frame : IDisposable
     {
+        private bool isDisposed;
+
         /// <summary>
         /// Gets or sets a value indicating whether the frame uses the extended specification
         /// </summary>
@@ -48,7 +52,7 @@ namespace ImageSharp.Formats.Jpeg.Port.Components
         /// <summary>
         /// Gets or sets the frame component collection
         /// </summary>
-        public Component[] Components { get; set; }
+        public FrameComponent[] Components { get; set; }
 
         /// <summary>
         /// Gets or sets the maximum horizontal sampling factor
@@ -69,5 +73,36 @@ namespace ImageSharp.Formats.Jpeg.Port.Components
         /// Gets or sets the number of MCU's per column
         /// </summary>
         public int McusPerColumn { get; set; }
+
+        /// <inheritdoc/>
+        public void Dispose()
+        {
+            this.Dispose(true);
+        }
+
+        /// <summary>
+        /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
+        /// </summary>
+        /// <param name="disposing">Whether to dispose of managed objects</param>
+        protected virtual void Dispose(bool disposing)
+        {
+            if (this.isDisposed)
+            {
+                return;
+            }
+
+            if (disposing)
+            {
+                foreach (FrameComponent component in this.Components)
+                {
+                    component.Dispose();
+                }
+            }
+
+            // Set large fields to null.
+            this.Components = null;
+
+            this.isDisposed = true;
+        }
     }
-}
+}

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

@@ -1,20 +1,29 @@
-// <copyright file="Component.cs" company="James Jackson-South">
+// <copyright file="FrameComponent.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 System;
+
+    using ImageSharp.Memory;
+
     /// <summary>
-    /// Represents a single color component
+    /// Represents a single frame component
     /// </summary>
-    internal struct Component
+    internal struct FrameComponent : IDisposable
     {
         /// <summary>
         /// Gets or sets the component Id
         /// </summary>
         public byte Id;
 
+        /// <summary>
+        /// TODO: What does pred stand for?
+        /// </summary>
+        public int Pred;
+
         /// <summary>
         /// Gets or sets the horizontal sampling factor.
         /// </summary>
@@ -38,7 +47,7 @@ namespace ImageSharp.Formats.Jpeg.Port.Components
         /// <summary>
         /// Gets or sets the block data
         /// </summary>
-        public short[] BlockData;
+        public Buffer<short> BlockData;
 
         /// <summary>
         /// Gets or sets the number of blocks per line
@@ -59,5 +68,11 @@ namespace ImageSharp.Formats.Jpeg.Port.Components
         /// Gets the index for the AC Huffman table
         /// </summary>
         public int ACHuffmanTableId;
+
+        /// <inheritdoc/>
+        public void Dispose()
+        {
+            this.BlockData?.Dispose();
+        }
     }
 }

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

@@ -5,7 +5,7 @@
 
 namespace ImageSharp.Formats.Jpeg.Port.Components
 {
-    using System.Collections.Generic;
+    using System.Runtime.CompilerServices;
 
     /// <summary>
     /// Represents a branch in the huffman tree
@@ -23,32 +23,31 @@ namespace ImageSharp.Formats.Jpeg.Port.Components
         public short Value;
 
         /// <summary>
-        /// The children
+        /// The children.
         /// </summary>
-        public List<HuffmanBranch> Children;
+        public HuffmanBranch[] Children;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HuffmanBranch"/> struct.
         /// </summary>
         /// <param name="value">The value</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public HuffmanBranch(short value)
-            : this(value, new List<HuffmanBranch>())
         {
+            this.Index = 0;
+            this.Value = value;
+            this.Children = new HuffmanBranch[2];
         }
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HuffmanBranch"/> struct.
         /// </summary>
         /// <param name="children">The branch children</param>
-        public HuffmanBranch(List<HuffmanBranch> children)
-            : this((short)0, children)
-        {
-        }
-
-        private HuffmanBranch(short value, List<HuffmanBranch> children)
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public HuffmanBranch(HuffmanBranch[] children)
         {
             this.Index = 0;
-            this.Value = value;
+            this.Value = -1;
             this.Children = children;
         }
     }

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

@@ -13,16 +13,16 @@ namespace ImageSharp.Formats.Jpeg.Port.Components
     /// </summary>
     internal class HuffmanTables
     {
-        private List<HuffmanBranch> first = new List<HuffmanBranch>();
+        private HuffmanBranch[] first;
 
-        private List<HuffmanBranch> second = new List<HuffmanBranch>();
+        private HuffmanBranch[] second;
 
         /// <summary>
         /// Gets or sets the table at the given index.
         /// </summary>
         /// <param name="index">The index</param>
         /// <returns>The <see cref="List{HuffmanBranch}"/></returns>
-        public List<HuffmanBranch> this[int index]
+        public HuffmanBranch[] this[int index]
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get

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

@@ -5,6 +5,8 @@
 
 namespace ImageSharp.Formats.Jpeg.Port.Components
 {
+    using System.Runtime.CompilerServices;
+
     using ImageSharp.Memory;
 
     /// <summary>
@@ -16,24 +18,30 @@ namespace ImageSharp.Formats.Jpeg.Port.Components
         /// <summary>
         /// Gets the ZigZag scan table
         /// </summary>
-        public static byte[] DctZigZag { get; } =
+        public static byte[] DctZigZag
         {
-            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
-        };
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            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.

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

@@ -0,0 +1,471 @@
+// <copyright file="ScanDecoder.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 System;
+#if DEBUG
+    using System.Diagnostics;
+#endif
+    using System.IO;
+    using System.Runtime.CompilerServices;
+
+    /// <summary>
+    /// Encapsulates a decode method. TODO: This may well be a bottleneck
+    /// </summary>
+    /// <param name="component">The component</param>
+    /// <param name="offset">The offset</param>
+    /// <param name="dcHuffmanTables">The DC Huffman tables</param>
+    /// <param name="acHuffmanTables">The AC Huffman tables</param>
+    /// <param name="stream">The input stream</param>
+    internal delegate void DecodeAction(ref FrameComponent component, int offset, HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, Stream stream);
+
+    /// <summary>
+    /// Provides the means to decode a spectral scan
+    /// </summary>
+    internal struct ScanDecoder
+    {
+        private int bitsData;
+
+        private int bitsCount;
+
+        private int specStart;
+
+        private int specEnd;
+
+        private int eobrun;
+
+        private int successiveState;
+
+        private int successiveACState;
+
+        /// <summary>
+        /// Decodes the spectral scan
+        /// </summary>
+        /// <param name="frame">The image frame</param>
+        /// <param name="stream">The input stream</param>
+        /// <param name="dcHuffmanTables">The DC Huffman tables</param>
+        /// <param name="acHuffmanTables">The AC Huffman tables</param>
+        /// <param name="components">The scan components</param>
+        /// <param name="resetInterval">The reset interval</param>
+        /// <param name="spectralStart">The spectral selection start</param>
+        /// <param name="spectralEnd">The spectral selection end</param>
+        /// <param name="successivePrev">The successive approximation bit high end</param>
+        /// <param name="successive">The successive approximation bit low end</param>
+        public void DecodeScan(Frame frame, Stream stream, HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, FrameComponent[] components, ushort resetInterval, int spectralStart, int spectralEnd, int successivePrev, int successive)
+        {
+            this.specStart = spectralStart;
+            this.specEnd = spectralEnd;
+            this.successiveState = successive;
+            bool progressive = frame.Progressive;
+            int componentsLength = components.Length;
+            int mcusPerLine = frame.McusPerLine;
+
+            // TODO: Delegate action will not be fast
+            DecodeAction decodeFn;
+
+            if (progressive)
+            {
+                if (this.specStart == 0)
+                {
+                    if (successivePrev == 0)
+                    {
+                        decodeFn = this.DecodeDCFirst;
+                    }
+                    else
+                    {
+                        decodeFn = this.DecodeDCSuccessive;
+                    }
+                }
+                else
+                {
+                    if (successivePrev == 0)
+                    {
+                        decodeFn = this.DecodeACFirst;
+                    }
+                    else
+                    {
+                        decodeFn = this.DecodeACSuccessive;
+                    }
+                }
+            }
+            else
+            {
+                decodeFn = this.DecodeBaseline;
+            }
+
+            int mcu = 0;
+            int mcuExpected;
+            if (componentsLength == 1)
+            {
+                mcuExpected = components[0].BlocksPerLine * components[0].BlocksPerColumn;
+            }
+            else
+            {
+                mcuExpected = mcusPerLine * frame.McusPerColumn;
+            }
+
+            FileMarker fileMarker;
+            while (mcu < mcuExpected)
+            {
+                // Reset interval stuff
+                int mcuToRead = resetInterval > 0 ? Math.Min(mcuExpected - mcu, resetInterval) : mcuExpected;
+                for (int i = 0; i < componentsLength; i++)
+                {
+                    ref FrameComponent c = ref components[i];
+                    c.Pred = 0;
+                }
+
+                this.eobrun = 0;
+
+                if (componentsLength == 1)
+                {
+                    ref FrameComponent component = ref components[0];
+                    for (int n = 0; n < mcuToRead; n++)
+                    {
+                        DecodeBlock(dcHuffmanTables, acHuffmanTables, ref component, decodeFn, mcu, stream);
+                        mcu++;
+                    }
+                }
+                else
+                {
+                    for (int n = 0; n < mcuToRead; n++)
+                    {
+                        for (int i = 0; i < componentsLength; i++)
+                        {
+                            ref FrameComponent component = ref components[i];
+                            int h = component.HorizontalFactor;
+                            int v = component.VerticalFactor;
+                            for (int j = 0; j < v; j++)
+                            {
+                                for (int k = 0; k < h; k++)
+                                {
+                                    DecodeMcu(dcHuffmanTables, acHuffmanTables, ref component, decodeFn, mcusPerLine, mcu, j, k, stream);
+                                }
+                            }
+                        }
+
+                        mcu++;
+                    }
+                }
+
+                // Find marker
+                this.bitsCount = 0;
+
+                // TODO: We need to make sure we are not overwriting anything here.
+                fileMarker = JpegDecoderCore.FindNextFileMarker(stream);
+
+                // Some bad images seem to pad Scan blocks with e.g. zero bytes, skip past
+                // those to attempt to find a valid marker (fixes issue4090.pdf) in original code.
+                if (fileMarker.Invalid)
+                {
+#if DEBUG
+                    Debug.WriteLine("DecodeScan - Unexpected MCU data, next marker is: " + fileMarker.Marker.ToString("X"));
+#endif
+                }
+
+                ushort marker = fileMarker.Marker;
+                if (marker <= 0xFF00)
+                {
+                    throw new ImageFormatException("Marker was not found");
+                }
+
+                if (marker >= JpegConstants.Markers.RST0 && marker <= JpegConstants.Markers.RST7)
+                {
+                    // RSTx
+                    stream.Skip(2);
+                }
+                else
+                {
+                    break;
+                }
+            }
+
+            fileMarker = JpegDecoderCore.FindNextFileMarker(stream);
+
+            // Some images include more Scan blocks than expected, skip past those and
+            // attempt to find the next valid marker (fixes issue8182.pdf) in original code.
+            if (fileMarker.Invalid)
+            {
+#if DEBUG
+                Debug.WriteLine("DecodeScan - Unexpected MCU data, next marker is: " + fileMarker.Marker.ToString("X"));
+#endif
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static int GetBlockBufferOffset(FrameComponent component, int row, int col)
+        {
+            return 64 * (((component.BlocksPerLine + 1) * row) + col);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static void DecodeMcu(HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, ref FrameComponent component, DecodeAction decode, int mcusPerLine, int mcu, int row, int col, Stream stream)
+        {
+            int mcuRow = (mcu / mcusPerLine) | 0;
+            int mcuCol = mcu % mcusPerLine;
+            int blockRow = (mcuRow * component.VerticalFactor) + row;
+            int blockCol = (mcuCol * component.HorizontalFactor) + col;
+            int offset = GetBlockBufferOffset(component, blockRow, blockCol);
+            decode(ref component, offset, dcHuffmanTables, acHuffmanTables, stream);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static void DecodeBlock(HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, ref FrameComponent component, DecodeAction decode, int mcu, Stream stream)
+        {
+            int blockRow = (mcu / component.BlocksPerLine) | 0;
+            int blockCol = mcu % component.BlocksPerLine;
+            int offset = GetBlockBufferOffset(component, blockRow, blockCol);
+            decode(ref component, offset, dcHuffmanTables, acHuffmanTables, stream);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private int ReadBit(Stream stream)
+        {
+            if (this.bitsCount > 0)
+            {
+                this.bitsCount--;
+                return (this.bitsData >> this.bitsCount) & 1;
+            }
+
+            this.bitsData = stream.ReadByte();
+            if (this.bitsData == 0xFF)
+            {
+                int nextByte = stream.ReadByte();
+                if (nextByte > 0)
+                {
+                    throw new ImageFormatException($"Unexpected marker {(this.bitsData << 8) | nextByte}");
+                }
+
+                // Unstuff 0
+            }
+
+            this.bitsCount = 7;
+            return this.bitsData >> 7;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private short DecodeHuffman(HuffmanBranch[] tree, Stream stream)
+        {
+            HuffmanBranch[] node = tree;
+            while (true)
+            {
+                int index;
+                index = this.ReadBit(stream);
+                HuffmanBranch branch = node[index];
+                node = branch.Children;
+
+                if (branch.Value > -1)
+                {
+                    return branch.Value;
+                }
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private int Receive(int length, Stream stream)
+        {
+            int n = 0;
+            while (length > 0)
+            {
+                n = (n << 1) | this.ReadBit(stream);
+                length--;
+            }
+
+            return n;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private int ReceiveAndExtend(int length, Stream stream)
+        {
+            if (length == 1)
+            {
+                return this.ReadBit(stream) == 1 ? 1 : -1;
+            }
+
+            int n = this.Receive(length, stream);
+            if (n >= 1 << (length - 1))
+            {
+                return n;
+            }
+
+            return n + (-1 << length) + 1;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void DecodeBaseline(ref FrameComponent component, int offset, HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, Stream stream)
+        {
+            int t = this.DecodeHuffman(dcHuffmanTables[component.DCHuffmanTableId], stream);
+            int diff = t == 0 ? 0 : this.ReceiveAndExtend(t, stream);
+            component.BlockData[offset] = (short)(component.Pred += diff);
+
+            int k = 1;
+            while (k < 64)
+            {
+                int rs = this.DecodeHuffman(acHuffmanTables[component.ACHuffmanTableId], stream);
+                int s = rs & 15;
+                int r = rs >> 4;
+
+                if (s == 0)
+                {
+                    if (r < 15)
+                    {
+                        break;
+                    }
+
+                    k += 16;
+                    continue;
+                }
+
+                k += r;
+                byte z = QuantizationTables.DctZigZag[k];
+                short re = (short)this.ReceiveAndExtend(s, stream);
+                component.BlockData[offset + z] = re;
+                k++;
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void DecodeDCFirst(ref FrameComponent component, int offset, HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, Stream stream)
+        {
+            int t = this.DecodeHuffman(dcHuffmanTables[component.DCHuffmanTableId], stream);
+            int diff = t == 0 ? 0 : this.ReceiveAndExtend(t, stream) << this.successiveState;
+            component.BlockData[offset] = (short)(component.Pred += diff);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void DecodeDCSuccessive(ref FrameComponent component, int offset, HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, Stream stream)
+        {
+            component.BlockData[offset] |= (short)(this.ReadBit(stream) << this.successiveState);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void DecodeACFirst(ref FrameComponent component, int offset, HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, Stream stream)
+        {
+            if (this.eobrun > 0)
+            {
+                this.eobrun--;
+                return;
+            }
+
+            int k = this.specStart;
+            int e = this.specEnd;
+            while (k <= e)
+            {
+                short rs = this.DecodeHuffman(acHuffmanTables[component.ACHuffmanTableId], stream);
+                int s = rs & 15;
+                int r = rs >> 4;
+
+                if (s == 0)
+                {
+                    if (r < 15)
+                    {
+                        this.eobrun = this.Receive(r, stream) + (1 << r) - 1;
+                        break;
+                    }
+
+                    k += 16;
+                    continue;
+                }
+
+                k += r;
+                byte z = QuantizationTables.DctZigZag[k];
+                component.BlockData[offset + z] = (short)(this.ReceiveAndExtend(s, stream) * (1 << this.successiveState));
+                k++;
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void DecodeACSuccessive(ref FrameComponent component, int offset, HuffmanTables dcHuffmanTables, HuffmanTables acHuffmanTables, Stream stream)
+        {
+            int k = this.specStart;
+            int e = this.specEnd;
+            int r = 0;
+            while (k <= e)
+            {
+                byte z = QuantizationTables.DctZigZag[k];
+                int successiveACNextValue = 0;
+                switch (this.successiveACState)
+                {
+                    case 0: // Initial state
+                        short rs = this.DecodeHuffman(acHuffmanTables[component.ACHuffmanTableId], stream);
+                        int s = rs & 15;
+                        r = rs >> 4;
+                        if (s == 0)
+                        {
+                            if (r < 15)
+                            {
+                                this.eobrun = this.Receive(r, stream) + (1 << r);
+                                this.successiveACState = 4;
+                            }
+                            else
+                            {
+                                r = 16;
+                                this.successiveACState = 1;
+                            }
+                        }
+                        else
+                        {
+                            if (s != 1)
+                            {
+                                throw new ImageFormatException("Invalid ACn encoding");
+                            }
+
+                            successiveACNextValue = this.ReceiveAndExtend(s, stream);
+                            this.successiveACState = r > 0 ? 2 : 3;
+                        }
+
+                        continue;
+                    case 1: // Skipping r zero items
+                    case 2:
+                        if (component.BlockData[offset + z] > 0)
+                        {
+                            component.BlockData[offset + z] += (short)(this.ReadBit(stream) << this.successiveState);
+                        }
+                        else
+                        {
+                            r--;
+                            if (r == 0)
+                            {
+                                this.successiveACState = this.successiveACState == 2 ? 3 : 0;
+                            }
+                        }
+
+                        break;
+                    case 3: // Set value for a zero item
+                        if (component.BlockData[offset + z] > 0)
+                        {
+                            component.BlockData[offset + z] += (short)(this.ReadBit(stream) << this.successiveState);
+                        }
+                        else
+                        {
+                            component.BlockData[offset + z] = (short)(successiveACNextValue << this.successiveState);
+                            this.successiveACState = 0;
+                        }
+
+                        break;
+                    case 4: // Eob
+                        if (component.BlockData[offset + z] > 0)
+                        {
+                            component.BlockData[offset + z] += (short)(this.ReadBit(stream) << this.successiveState);
+                        }
+
+                        break;
+                }
+
+                k++;
+            }
+
+            if (this.successiveACState == 4)
+            {
+                this.eobrun--;
+                if (this.eobrun == 0)
+                {
+                    this.successiveACState = 0;
+                }
+            }
+        }
+    }
+}

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

@@ -173,6 +173,24 @@ namespace ImageSharp.Formats.Jpeg.Port
             /// </summary>
             public const ushort SOS = 0xFFDA;
 
+            /// <summary>
+            /// Define First Restart
+            /// <remarks>
+            /// Inserted every r macroblocks, where r is the restart interval set by a DRI marker.
+            /// Not used if there was no DRI marker. The low three bits of the marker code cycle in value from 0 to 7.
+            /// </remarks>
+            /// </summary>
+            public const ushort RST0 = 0xFFD0;
+
+            /// <summary>
+            /// Define Eigth Restart
+            /// <remarks>
+            /// Inserted every r macroblocks, where r is the restart interval set by a DRI marker.
+            /// Not used if there was no DRI marker. The low three bits of the marker code cycle in value from 0 to 7.
+            /// </remarks>
+            /// </summary>
+            public const ushort RST7 = 0xFFD7;
+
             /// <summary>
             /// Contains JFIF specific markers
             /// </summary>

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

@@ -8,6 +8,7 @@ namespace ImageSharp.Formats.Jpeg.Port
     using System;
     using System.Collections.Generic;
     using System.IO;
+    using System.Runtime.CompilerServices;
 
     using ImageSharp.Common.Extensions;
     using ImageSharp.Formats.Jpeg.Port.Components;
@@ -18,7 +19,7 @@ namespace ImageSharp.Formats.Jpeg.Port
     /// Performs the jpeg decoding operation.
     /// Ported from <see href="https://github.com/mozilla/pdf.js/blob/master/src/core/jpg.js"/>
     /// </summary>
-    internal class JpegDecoderCore
+    internal class JpegDecoderCore : IDisposable
     {
         /// <summary>
         /// The decoder options.
@@ -48,10 +49,12 @@ namespace ImageSharp.Formats.Jpeg.Port
         private ushort resetInterval;
 
         /// <summary>
-        /// COntains information about the jFIF marker
+        /// Contains information about the jFIF marker
         /// </summary>
         private JFif jFif;
 
+        private bool isDisposed;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="JpegDecoderCore" /> class.
         /// </summary>
@@ -68,6 +71,109 @@ namespace ImageSharp.Formats.Jpeg.Port
         /// </summary>
         public Stream InputStream { get; private set; }
 
+        /// <summary>
+        /// Finds the next file marker within the byte stream. Not used but I'm keeping it for now for testing
+        /// </summary>
+        /// <param name="stream">The input stream</param>
+        /// <returns>The <see cref="FileMarker"/></returns>
+        public static FileMarker FindNextFileMarkerOld(Stream stream)
+        {
+            byte[] buffer = new byte[2];
+            while (true)
+            {
+                int value = stream.Read(buffer, 0, 2);
+
+                if (value == 0)
+                {
+                    return new FileMarker(JpegConstants.Markers.EOI, (int)stream.Length, true);
+                }
+
+                while (buffer[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.
+                    buffer[0] = buffer[1];
+
+                    value = stream.ReadByte();
+                    if (value == -1)
+                    {
+                        return new FileMarker(JpegConstants.Markers.EOI, (int)stream.Length, true);
+                    }
+
+                    buffer[1] = (byte)value;
+                }
+
+                return new FileMarker((ushort)((buffer[0] << 8) | buffer[1]), (int)(stream.Position - 2));
+            }
+        }
+
+        /// <summary>
+        /// Finds the next file marker within the byte stream
+        /// </summary>
+        /// <param name="stream">The input stream</param>
+        /// <returns>The <see cref="FileMarker"/></returns>
+        public static FileMarker FindNextFileMarker(Stream stream)
+        {
+            byte[] buffer = new byte[2];
+            long maxPos = stream.Length - 1;
+            long currentPos = stream.Position;
+            long newPos = currentPos;
+
+            if (currentPos >= maxPos)
+            {
+                return new FileMarker(JpegConstants.Markers.EOI, (int)stream.Length, true);
+            }
+
+            int value = stream.Read(buffer, 0, 2);
+
+            if (value == 0)
+            {
+                return new FileMarker(JpegConstants.Markers.EOI, (int)stream.Length, true);
+            }
+
+            ushort currentMarker = (ushort)((buffer[0] << 8) | buffer[1]);
+            if (currentMarker >= JpegConstants.Markers.SOF0 && currentMarker <= JpegConstants.Markers.COM)
+            {
+                return new FileMarker(currentMarker, stream.Position - 2);
+            }
+
+            value = stream.Read(buffer, 0, 2);
+
+            if (value == 0)
+            {
+                return new FileMarker(JpegConstants.Markers.EOI, (int)stream.Length, true);
+            }
+
+            ushort newMarker = (ushort)((buffer[0] << 8) | buffer[1]);
+            while (!(newMarker >= JpegConstants.Markers.SOF0 && newMarker <= JpegConstants.Markers.COM))
+            {
+                if (++newPos >= maxPos)
+                {
+                    return new FileMarker(JpegConstants.Markers.EOI, (int)stream.Length, true);
+                }
+
+                stream.Read(buffer, 0, 2);
+                newMarker = (ushort)((buffer[0] << 8) | buffer[1]);
+            }
+
+            return new FileMarker(newMarker, newPos, true);
+        }
+
         /// <summary>
         /// Decodes the image from the specified <see cref="Stream"/>  and sets
         /// the data to image.
@@ -85,27 +191,52 @@ namespace ImageSharp.Formats.Jpeg.Port
             return image;
         }
 
+        /// <inheritdoc/>
+        public void Dispose()
+        {
+            this.Dispose(true);
+        }
+
+        /// <summary>
+        /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
+        /// </summary>
+        /// <param name="disposing">Whether to dispose of managed objects</param>
+        protected virtual void Dispose(bool disposing)
+        {
+            if (!this.isDisposed)
+            {
+                if (disposing)
+                {
+                    this.frame.Dispose();
+                }
+
+                // TODO: set large fields to null.
+                this.isDisposed = true;
+            }
+        }
+
         private void ParseStream()
         {
             // Check for the Start Of Image marker.
-            ushort fileMarker = this.ReadUint16();
-            if (fileMarker != JpegConstants.Markers.SOI)
+            var fileMarker = new FileMarker(this.ReadUint16(), 0);
+            if (fileMarker.Marker != JpegConstants.Markers.SOI)
             {
                 throw new ImageFormatException("Missing SOI marker.");
             }
 
-            fileMarker = this.ReadUint16();
+            ushort marker = this.ReadUint16();
+            fileMarker = new FileMarker(marker, (int)this.InputStream.Position - 2);
 
             this.quantizationTables = new QuantizationTables();
             this.dcHuffmanTables = new HuffmanTables();
             this.acHuffmanTables = new HuffmanTables();
 
-            while (fileMarker != JpegConstants.Markers.EOI)
+            while (fileMarker.Marker != JpegConstants.Markers.EOI)
             {
                 // Get the marker length
                 int remaining = this.ReadUint16() - 2;
 
-                switch (fileMarker)
+                switch (fileMarker.Marker)
                 {
                     case JpegConstants.Markers.APP0:
                         this.ProcessApplicationHeaderMarker(remaining);
@@ -148,12 +279,12 @@ namespace ImageSharp.Formats.Jpeg.Port
                         break;
 
                     case JpegConstants.Markers.SOS:
-                        this.ProcessStartOfScan();
+                        this.ProcessStartOfScanMarker();
                         break;
                 }
 
                 // Read on
-                fileMarker = this.FindNextFileMarker();
+                fileMarker = FindNextFileMarker(this.InputStream);
             }
         }
 
@@ -281,7 +412,7 @@ namespace ImageSharp.Formats.Jpeg.Port
         /// </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)
+        private void ProcessStartOfFrameMarker(int remaining, FileMarker frameMarker)
         {
             if (this.frame != null)
             {
@@ -292,8 +423,8 @@ namespace ImageSharp.Formats.Jpeg.Port
 
             this.frame = new Frame
             {
-                Extended = frameMarker == JpegConstants.Markers.SOF1,
-                Progressive = frameMarker == JpegConstants.Markers.SOF2,
+                Extended = frameMarker.Marker == JpegConstants.Markers.SOF1,
+                Progressive = frameMarker.Marker == JpegConstants.Markers.SOF2,
                 Precision = this.temp[0],
                 Scanlines = (short)((this.temp[1] << 8) | this.temp[2]),
                 SamplesPerLine = (short)((this.temp[3] << 8) | this.temp[4]),
@@ -304,9 +435,9 @@ namespace ImageSharp.Formats.Jpeg.Port
             int maxV = 0;
             int index = 6;
 
-            // TODO: Pool this.
+            // No need to pool this. They max out at 4
             this.frame.ComponentIds = new byte[this.frame.ComponentCount];
-            this.frame.Components = new Component[this.frame.ComponentCount];
+            this.frame.Components = new FrameComponent[this.frame.ComponentCount];
 
             for (int i = 0; i < this.frame.ComponentCount; i++)
             {
@@ -389,30 +520,36 @@ namespace ImageSharp.Formats.Jpeg.Port
         /// <summary>
         /// Processes the SOS (Start of scan marker).
         /// </summary>
-        private void ProcessStartOfScan()
+        private void ProcessStartOfScanMarker()
         {
             int selectorsCount = this.InputStream.ReadByte();
-            var components = new List<Component>();
-
             for (int i = 0; i < selectorsCount; i++)
             {
                 byte componentIndex = this.frame.ComponentIds[this.InputStream.ReadByte() - 1];
-                Component component = this.frame.Components[componentIndex];
+                ref FrameComponent component = ref this.frame.Components[componentIndex];
                 int tableSpec = this.InputStream.ReadByte();
                 component.DCHuffmanTableId = tableSpec >> 4;
                 component.ACHuffmanTableId = tableSpec & 15;
-                components.Add(component);
             }
 
             this.InputStream.Read(this.temp, 0, 3);
+
             int spectralStart = this.temp[0];
             int spectralEnd = this.temp[1];
             int successiveApproximation = this.temp[2];
-        }
-
-        private int DecodeScan(List<Component> components, int spectralStart, int spectralEnd, int successivePrev, int successive)
-        {
-            return 0;
+            var scanDecoder = default(ScanDecoder);
+
+            scanDecoder.DecodeScan(
+               this.frame,
+               this.InputStream,
+               this.dcHuffmanTables,
+               this.acHuffmanTables,
+               this.frame.Components,
+               this.resetInterval,
+               spectralStart,
+               spectralEnd,
+               successiveApproximation >> 4,
+               successiveApproximation & 15);
         }
 
         /// <summary>
@@ -430,8 +567,8 @@ namespace ImageSharp.Formats.Jpeg.Port
                 length--;
             }
 
-            // TODO: Check the capacity here. Seems to max at 2
-            var code = new List<HuffmanBranch> { new HuffmanBranch(new List<HuffmanBranch>()) };
+            // TODO: Check the branch children capacity here. Seems to max at 2
+            var code = new List<HuffmanBranch> { new HuffmanBranch(-1) };
             HuffmanBranch p = code[0];
             int k = 0;
 
@@ -441,7 +578,7 @@ namespace ImageSharp.Formats.Jpeg.Port
                 for (int j = 0; j < codeLengths[i]; j++)
                 {
                     p = code.Pop();
-                    p.Children.SafeInsert(p.Index, new HuffmanBranch(values[k]));
+                    p.Children[p.Index] = new HuffmanBranch(values[k]);
                     while (p.Index > 0)
                     {
                         p = code.Pop();
@@ -451,9 +588,9 @@ namespace ImageSharp.Formats.Jpeg.Port
                     code.Add(p);
                     while (code.Count <= i)
                     {
-                        q = new HuffmanBranch(new List<HuffmanBranch>());
+                        q = new HuffmanBranch(-1);
                         code.Add(q);
-                        p.Children.SafeInsert(p.Index, new HuffmanBranch(q.Children));
+                        p.Children[p.Index] = new HuffmanBranch(q.Children);
                         p = q;
                     }
 
@@ -463,9 +600,9 @@ namespace ImageSharp.Formats.Jpeg.Port
                 if (i + 1 < length)
                 {
                     // p here points to last code
-                    q = new HuffmanBranch(new List<HuffmanBranch>());
+                    q = new HuffmanBranch(-1);
                     code.Add(q);
-                    p.Children.SafeInsert(p.Index, new HuffmanBranch(q.Children));
+                    p.Children[p.Index] = new HuffmanBranch(q.Children);
                     p = q;
                 }
             }
@@ -478,21 +615,21 @@ namespace ImageSharp.Formats.Jpeg.Port
         /// </summary>
         private void PrepareComponents()
         {
-            int mcusPerLine = this.frame.SamplesPerLine / 8 / this.frame.MaxHorizontalFactor;
-            int mcusPerColumn = this.frame.Scanlines / 8 / this.frame.MaxVerticalFactor;
+            int mcusPerLine = (int)Math.Ceiling(this.frame.SamplesPerLine / 8D / this.frame.MaxHorizontalFactor);
+            int mcusPerColumn = (int)Math.Ceiling(this.frame.Scanlines / 8D / 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 blocksPerLine = (int)Math.Ceiling(Math.Ceiling(this.frame.SamplesPerLine / 8D) * component.HorizontalFactor / this.frame.MaxHorizontalFactor);
+                int blocksPerColumn = (int)Math.Ceiling(Math.Ceiling(this.frame.Scanlines / 8D) * 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];
+                // Pooled. Disposed via frame siposal
+                component.BlockData = new Buffer<short>(blocksBufferSize);
                 component.BlocksPerLine = blocksPerLine;
                 component.BlocksPerColumn = blocksPerColumn;
             }
@@ -501,59 +638,11 @@ namespace ImageSharp.Formats.Jpeg.Port
             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 == -1)
-                    {
-                        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>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private ushort ReadUint16()
         {
             this.InputStream.Read(this.uint16Buffer, 0, 2);

tests/ImageSharp.Benchmarks/Image/DecodeJpeg.cs

@@ -23,21 +23,21 @@ namespace ImageSharp.Benchmarks.Image
         {
             if (this.jpegBytes == null)
             {
-                this.jpegBytes = File.ReadAllBytes("../ImageSharp.Tests/TestImages/Formats/Jpg/Baseline/Calliphora.jpg");
+                this.jpegBytes = File.ReadAllBytes("../../../../../../../../ImageSharp.Tests/TestImages/Formats/Jpg/Baseline/Calliphora.jpg");
             }
         }
 
-        [Benchmark(Baseline = true, Description = "System.Drawing Jpeg")]
-        public Size JpegSystemDrawing()
-        {
-            using (MemoryStream memoryStream = new MemoryStream(this.jpegBytes))
-            {
-                using (Image image = Image.FromStream(memoryStream))
-                {
-                    return image.Size;
-                }
-            }
-        }
+        //[Benchmark(Baseline = true, Description = "System.Drawing Jpeg")]
+        //public Size JpegSystemDrawing()
+        //{
+        //    using (MemoryStream memoryStream = new MemoryStream(this.jpegBytes))
+        //    {
+        //        using (Image image = Image.FromStream(memoryStream))
+        //        {
+        //            return image.Size;
+        //        }
+        //    }
+        //}
 
         [Benchmark(Description = "ImageSharp Jpeg")]
         public CoreSize JpegCore()