From 487410e55f2e2de2de1a61fa378287d5625f7558 Mon Sep 17 00:00:00 2001
From: winscripter <142818255+winscripter@users.noreply.github.com>
Date: Wed, 15 Jul 2026 20:30:58 +0400
Subject: [PATCH] Implement field encodings, move IJxlFields

This is an implementation of field_encodings.h.

Note that I avoided implementing EnumValid() and Values() functions, as we have dedicated methods in .NET to do exactly that (Enum.IsDefined, Enum.GetValues)
---
 .../Formats/Jxl/{IO => Fields}/IJxlFields.cs  |  2 +-
 .../Formats/Jxl/Fields/JxlFieldExpressions.cs | 21 +++++++++++++++
 .../Formats/Jxl/Fields/JxlU32Distribution.cs  | 17 ++++++++++++
 .../Formats/Jxl/Fields/JxlU32Enc.cs           | 26 +++++++++++++++++++
 4 files changed, 65 insertions(+), 1 deletion(-)
 rename src/ImageSharp/Formats/Jxl/{IO => Fields}/IJxlFields.cs (90%)
 create mode 100644 src/ImageSharp/Formats/Jxl/Fields/JxlFieldExpressions.cs
 create mode 100644 src/ImageSharp/Formats/Jxl/Fields/JxlU32Distribution.cs
 create mode 100644 src/ImageSharp/Formats/Jxl/Fields/JxlU32Enc.cs

diff --git a/src/ImageSharp/Formats/Jxl/IO/IJxlFields.cs b/src/ImageSharp/Formats/Jxl/Fields/IJxlFields.cs
similarity index 90%
rename from src/ImageSharp/Formats/Jxl/IO/IJxlFields.cs
rename to src/ImageSharp/Formats/Jxl/Fields/IJxlFields.cs
index 191cc96a3e..bdc09f5f41 100644
--- a/src/ImageSharp/Formats/Jxl/IO/IJxlFields.cs
+++ b/src/ImageSharp/Formats/Jxl/Fields/IJxlFields.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.Fields;
 
 /// <summary>
 /// Abstracts enumeration of all fields into a visitor.
diff --git a/src/ImageSharp/Formats/Jxl/Fields/JxlFieldExpressions.cs b/src/ImageSharp/Formats/Jxl/Fields/JxlFieldExpressions.cs
new file mode 100644
index 0000000000..8d74c81bb8
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/Fields/JxlFieldExpressions.cs
@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Fields;
+
+internal static class JxlFieldExpressions
+{
+    public static JxlU32Distribution Value(uint value)
+    {
+        const uint directConstant = JxlU32Distribution.DirectConstant;
+
+        return new(value | directConstant);
+    }
+
+    public static JxlU32Distribution BitsOffset(uint bits, uint offset)
+        => new(((bits - 1u) & 0x1Fu) + ((offset & 0x3FFFFFFu) << 5));
+
+    public static JxlU32Distribution Bits(uint value) => BitsOffset(value, 0u);
+
+    public static int MakeBit(int index) => 1 << index;
+}
diff --git a/src/ImageSharp/Formats/Jxl/Fields/JxlU32Distribution.cs b/src/ImageSharp/Formats/Jxl/Fields/JxlU32Distribution.cs
new file mode 100644
index 0000000000..355b6f533f
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/Fields/JxlU32Distribution.cs
@@ -0,0 +1,17 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Fields;
+
+internal struct JxlU32Distribution(uint d)
+{
+    public const uint DirectConstant = 0x80000000u;
+
+    public readonly bool IsDirect => (d & DirectConstant) != 0;
+
+    public readonly uint Direct => d & (DirectConstant - 1u);
+
+    public readonly uint ExtraBits => (d & 0x1Fu) + 1u;
+
+    public readonly uint Offset => (d >> 5) & 0x3FFFFFF;
+}
diff --git a/src/ImageSharp/Formats/Jxl/Fields/JxlU32Enc.cs b/src/ImageSharp/Formats/Jxl/Fields/JxlU32Enc.cs
new file mode 100644
index 0000000000..9583f57dac
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/Fields/JxlU32Enc.cs
@@ -0,0 +1,26 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Diagnostics;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Fields;
+
+internal readonly struct JxlU32Enc
+{
+    private readonly InlineArray4<JxlU32Distribution> d = default;
+
+    public JxlU32Enc(JxlU32Distribution d0, JxlU32Distribution d1, JxlU32Distribution d2, JxlU32Distribution d3)
+    {
+        this.d[0] = d0;
+        this.d[1] = d1;
+        this.d[2] = d2;
+        this.d[3] = d3;
+    }
+
+    public JxlU32Distribution GetDistribution(int selector)
+    {
+        Debug.Assert(selector < 4, "Selector out of range");
+
+        return this.d[selector];
+    }
+}