Move, add entropy coding common functions

src/ImageSharp/Formats/Jxl/Fields/JxlU32Coder.cs

@@ -2,7 +2,7 @@
 // Licensed under the Six Labors Split License.
 
 using System.Runtime.CompilerServices;
-using SixLabors.ImageSharp.Formats.Jxl.IO;
+using SixLabors.ImageSharp.Formats.Jxl.Processing.Decoder;
 
 namespace SixLabors.ImageSharp.Formats.Jxl.Fields;
 

src/ImageSharp/Formats/Jxl/IO/JxlAnsConstants.cs → src/ImageSharp/Formats/Jxl/IO/Entropy/JxlAnsConstants.cs

@@ -1,7 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Entropy;
 
 internal static class JxlAnsConstants
 {

src/ImageSharp/Formats/Jxl/IO/JxlAnsEntry.cs → src/ImageSharp/Formats/Jxl/IO/Entropy/JxlAnsEntry.cs

@@ -3,7 +3,7 @@
 
 using System.Runtime.InteropServices;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Entropy;
 
 [StructLayout(LayoutKind.Sequential)]
 internal struct JxlAnsEntry

src/ImageSharp/Formats/Jxl/IO/JxlAnsHelper.cs → src/ImageSharp/Formats/Jxl/IO/Entropy/JxlAnsHelper.cs

@@ -5,7 +5,7 @@ using System.Buffers;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Entropy;
 
 internal static class JxlAnsHelper
 {

src/ImageSharp/Formats/Jxl/IO/JxlAnsHybridUIntConfiguration.cs → src/ImageSharp/Formats/Jxl/IO/Entropy/JxlAnsHybridUIntConfiguration.cs

@@ -4,7 +4,7 @@
 using System.Diagnostics;
 using SixLabors.ImageSharp.Formats.Jxl.Fields;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Entropy;
 
 internal sealed class JxlAnsHybridUIntConfiguration : IJxlFields
 {

src/ImageSharp/Formats/Jxl/IO/JxlAnsLz77Parameters.cs → src/ImageSharp/Formats/Jxl/IO/Entropy/JxlAnsLz77Parameters.cs

@@ -3,7 +3,7 @@
 
 using SixLabors.ImageSharp.Formats.Jxl.Fields;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Entropy;
 
 internal sealed class JxlAnsLz77Parameters : IJxlFields
 {

src/ImageSharp/Formats/Jxl/IO/JxlAnsSymbol.cs → src/ImageSharp/Formats/Jxl/IO/Entropy/JxlAnsSymbol.cs

@@ -3,7 +3,7 @@
 
 using System.Runtime.InteropServices;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Entropy;
 
 [StructLayout(LayoutKind.Sequential)]
 internal struct JxlAnsSymbol(int value, int offset, int frequency)

src/ImageSharp/Formats/Jxl/IO/FrameHeader/JxlFrameHeader.cs

@@ -7,7 +7,7 @@
 #pragma warning disable IDE0032 // Use auto property
 
 using SixLabors.ImageSharp.Formats.Jxl.Fields;
-using SixLabors.ImageSharp.Formats.Jxl.Metadata;
+using SixLabors.ImageSharp.Formats.Jxl.IO.Metadata;
 using SixLabors.ImageSharp.Formats.Jxl.Processing;
 
 namespace SixLabors.ImageSharp.Formats.Jxl.IO.FrameHeader;

src/ImageSharp/Formats/Jxl/IO/JxlHuffman.cs

@@ -0,0 +1,9 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+internal static class JxlHuffman
+{
+    
+}

src/ImageSharp/Formats/Jxl/IO/JxlHuffmanCode.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+/// <summary>
+/// A single Huffman code.
+/// </summary>
+internal struct JxlHuffmanCode
+{
+    /// <summary>
+    /// Number of bits for this symbol.
+    /// </summary>
+    public byte Bits;
+
+    /// <summary>
+    /// Symbol value/offset.
+    /// </summary>
+    public ushort Value;
+}

src/ImageSharp/Formats/Jxl/IO/JxlAnsReader.cs → src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlAnsReader.cs

@@ -3,8 +3,9 @@
 
 using System.Buffers;
 using System.Diagnostics;
+using SixLabors.ImageSharp.Formats.Jxl.IO;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Decoder;
 
 internal static class JxlAnsReader
 {

src/ImageSharp/Formats/Jxl/IO/JxlBitReader.cs → src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlBitReader.cs

@@ -4,7 +4,7 @@
 using System.Buffers.Binary;
 using System.Diagnostics;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Decoder;
 
 /// <summary>
 /// Represents a bitstream reader.

src/ImageSharp/Formats/Jxl/JxlAspectRatioHelpers.cs → src/ImageSharp/Formats/Jxl/Processing/JxlAspectRatioHelpers.cs

@@ -3,7 +3,7 @@
 
 using System.Runtime.CompilerServices;
 
-namespace SixLabors.ImageSharp.Formats.Jxl;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
 
 internal static class JxlAspectRatioHelpers
 {

src/ImageSharp/Formats/Jxl/Processing/JxlBlockContextMap.cs

@@ -23,9 +23,9 @@ internal sealed class JxlBlockContextMap
 
     public List<int>[] DcThresholds { get; } = [[], [], []];
 
-    public List<uint> QfThresholds { get; } = [];
+    public List<uint> QfThresholds { get; set; } = [];
 
-    public byte[] ContextMap { get; } = new byte[DefaultContextMap.Length];
+    public byte[] ContextMap { get; set; } = new byte[DefaultContextMap.Length];
 
     public int ContextCount { get; set; }
 

src/ImageSharp/Formats/Jxl/Processing/JxlEntropyCoder.cs

@@ -0,0 +1,120 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Jxl.Fields;
+using SixLabors.ImageSharp.Formats.Jxl.Processing.Decoder;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+/// <summary>
+/// Helper functions for use in entropy coding.
+/// </summary>
+internal static class JxlEntropyCoder
+{
+    /// <summary>
+    /// Global DC threshold distributions
+    /// </summary>
+    public static readonly JxlU32Enc DcThresholdDistributions = new(
+        JxlFieldExpressions.Bits(4),
+        JxlFieldExpressions.BitsOffset(8, 16),
+        JxlFieldExpressions.BitsOffset(16, 272),
+        JxlFieldExpressions.BitsOffset(32, 65808));
+
+    /// <summary>
+    /// Global QF threshold distributions
+    /// </summary>
+    public static readonly JxlU32Enc QfThresholdDistributions = new(
+        JxlFieldExpressions.Bits(2),
+        JxlFieldExpressions.BitsOffset(3, 4),
+        JxlFieldExpressions.BitsOffset(5, 12),
+        JxlFieldExpressions.BitsOffset(8, 44));
+
+    /// <summary>
+    /// Predicts the entropy symbol using top and left neighboring pixels.
+    /// </summary>
+    /// <param name="rowTop">Above row; set to <see cref="Span{T}.Empty"/> if missing</param>
+    /// <param name="row">Current row</param>
+    /// <param name="x">The x coordinate offset</param>
+    /// <param name="defaultValue">Default value that's used if both current and above row cannot be used</param>
+    /// <returns>The predicted coefficient using neighboring pixels.</returns>
+    public static int PredictFromTopAndLeft(
+        ReadOnlySpan<int> rowTop,
+        ReadOnlySpan<int> row,
+        int x,
+        int defaultValue)
+    {
+        if (x == 0)
+        {
+            return rowTop.Length == 0 ? defaultValue : rowTop[x];
+        }
+
+        if (rowTop.Length == 0)
+        {
+            return row[x - 1];
+        }
+
+        return (rowTop[x] + rowTop[x - 1] + 1) / 2;
+    }
+
+    public static bool DecodeBlockContextMap(Configuration configuration, JxlBitReader reader, ref JxlBlockContextMap contextMap)
+    {
+        List<int>[] dct = contextMap.DcThresholds;
+        byte[] ctxMap = contextMap.ContextMap;
+
+        bool isDefaultContextMap = reader.ReadBoolean();
+
+        if (isDefaultContextMap)
+        {
+            contextMap = new();
+            return true;
+        }
+
+        contextMap.DcContextCount = 1;
+
+        for (int j = 0; j <= 2; j++)
+        {
+            int dcThresholdCount = (int)reader.ReadBits32(4u);
+
+            dct[j] = new List<int>(dcThresholdCount);
+
+            contextMap.DcContextCount = dcThresholdCount + 1;
+
+            for (int i = 0; i < dcThresholdCount; i++)
+            {
+                dct[j][i] = JxlPackSigned.UnpackSigned(JxlU32Coder.Read(DcThresholdDistributions, reader));
+            }
+        }
+
+        int qfThresholdCount = (int)reader.ReadBits32(4u);
+
+        List<uint> qft = new(qfThresholdCount);
+
+        for (int i = 0; i < qfThresholdCount; i++)
+        {
+            qft[i] = JxlU32Coder.Read(QfThresholdDistributions, reader) + 1;
+        }
+
+        contextMap.QfThresholds = qft;
+
+        if (contextMap.DcContextCount * qft.Count > 64)
+        {
+            throw new InvalidOperationException("Invalid block context map. It is too large.");
+        }
+
+        Array.Resize(ref ctxMap, 3 * JxlForwardCoefficientOrder.OrderCount * contextMap.DcContextCount * qft.Count);
+
+        contextMap.ContextMap = ctxMap;
+
+        if (!JxlDecoderCore.DecodeContextMap(configuration, ref ctxMap, contextMap.ContextCount, reader))
+        {
+            return false;
+        }
+
+        if (contextMap.ContextCount > 16)
+        {
+            throw new InvalidOperationException("Too many distinct contexts in block context map");
+        }
+
+        return true;
+    }
+}

src/ImageSharp/Formats/Jxl/JxlFrameDimensions.cs → src/ImageSharp/Formats/Jxl/Processing/JxlFrameDimensions.cs

@@ -3,7 +3,7 @@
 
 using System.Runtime.CompilerServices;
 
-namespace SixLabors.ImageSharp.Formats.Jxl;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
 
 internal sealed class JxlFrameDimensions
 {

src/ImageSharp/Formats/Jxl/Splines/JxlControlPoint.cs → src/ImageSharp/Formats/Jxl/Processing/Splines/JxlControlPoint.cs

@@ -3,7 +3,7 @@
 
 using System.Runtime.InteropServices;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Splines;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Splines;
 
 /// <summary>
 /// A simple pair of first and second 32-bit signed integers

src/ImageSharp/Formats/Jxl/Splines/JxlQuantizedSpline.cs → src/ImageSharp/Formats/Jxl/Processing/Splines/JxlQuantizedSpline.cs

@@ -5,7 +5,7 @@ using System.Buffers;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Splines;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Splines;
 
 internal sealed class JxlQuantizedSpline : IDisposable
 {

src/ImageSharp/Formats/Jxl/Splines/JxlSpline.cs → src/ImageSharp/Formats/Jxl/Processing/Splines/JxlSpline.cs

@@ -3,7 +3,7 @@
 
 using System.Buffers;
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Splines;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Splines;
 
 internal sealed class JxlSpline : IDisposable
 {

src/ImageSharp/Formats/Jxl/Splines/JxlSplineDataView.cs → src/ImageSharp/Formats/Jxl/Processing/Splines/JxlSplineDataView.cs

@@ -1,7 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Splines;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Splines;
 
 internal sealed class JxlSplineDataView
 {

src/ImageSharp/Formats/Jxl/Splines/JxlSplineEntropyContext.cs → src/ImageSharp/Formats/Jxl/Processing/Splines/JxlSplineEntropyContext.cs

@@ -1,7 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Splines;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Splines;
 
 internal enum JxlSplineEntropyContext : byte
 {

src/ImageSharp/Formats/Jxl/Splines/JxlSplineSegment.cs → src/ImageSharp/Formats/Jxl/Processing/Splines/JxlSplineSegment.cs

@@ -1,7 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Splines;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Splines;
 
 internal struct JxlSplineSegment
 {

src/ImageSharp/Formats/Jxl/Splines/JxlSplineSegmentSpan.cs → src/ImageSharp/Formats/Jxl/Processing/Splines/JxlSplineSegmentSpan.cs

@@ -1,7 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Splines;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Splines;
 
 internal struct JxlSplineSegmentSpan(int startInclusive, int endInclusive)
 {