Add ANS hybrid uint configuration & LZ77 parameters model

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

@@ -0,0 +1,60 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Diagnostics;
+using SixLabors.ImageSharp.Formats.Jxl.Fields;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+internal sealed class JxlAnsHybridUIntConfiguration : IJxlFields
+{
+    public JxlAnsHybridUIntConfiguration(uint splitExponent = 4, uint msbInToken = 2, uint lsbInToken = 0)
+    {
+        this.SplitExponent = splitExponent;
+        this.SplitToken = 1u << (int)splitExponent;
+        this.MsbInToken = msbInToken;
+        this.LsbInToken = lsbInToken;
+
+        Debug.Assert(splitExponent >= msbInToken + lsbInToken, "Split exponent should be < msbInToken + lsbInToken");
+    }
+
+    public uint SplitExponent { get; set; }
+
+    public uint SplitToken { get; set; }
+
+    public uint MsbInToken { get; set; } // Most significant bit
+
+    public uint LsbInToken { get; set; } // Least significant bit
+
+    public uint LsbMask => (1u << (int)this.LsbInToken) - 1;
+
+    public void Encode(uint value, ref uint token, ref uint bitCount, ref uint bits)
+    {
+        if (value < this.SplitToken)
+        {
+            token = value;
+            bitCount = 0;
+            bits = 0;
+        }
+        else
+        {
+            uint n = FloorLog2Nonzero(value);
+            uint m = value - (1u << (int)n);
+
+            unchecked
+            {
+                // The following expression is quite complex.
+                // See https://github.com/libjxl/libjxl/blob/main/lib/jxl/dec_ans.h#L83C16-L86C47.
+                token = this.SplitToken +
+                (uint)(((n - this.SplitExponent) << (int)(this.MsbInToken + this.LsbInToken)) +
+                        ((m >> (int)(n - this.MsbInToken)) << (int)this.LsbInToken) +
+                        (m & ((1 << (int)this.LsbInToken) - 1)));
+
+                bitCount = n - this.MsbInToken - this.LsbInToken;
+                bits = (value >> (int)this.LsbInToken) & ((1u << (int)bitCount) - 1);
+            }
+        }
+    }
+
+    public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
+}

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

@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Formats.Jxl.Fields;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+internal sealed class JxlAnsLz77Parameters : IJxlFields
+{
+    public bool Enabled { get; set; }
+
+    public uint MinimumSymbol { get; set; }
+
+    public uint MinimumLength { get; set; }
+
+    public JxlAnsHybridUIntConfiguration LengthUintConfig { get; set; } = new(0, 0, 0);
+
+    public int NonserializedDistanceContext { get; set; }
+
+    public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
+}