From fed0af3254d4c547f9002897f2ed732f0cd09e14 Mon Sep 17 00:00:00 2001
From: winscripter <142818255+winscripter@users.noreply.github.com>
Date: Thu, 16 Jul 2026 22:15:46 +0400
Subject: [PATCH] Add bit-stream reader

---
 src/ImageSharp/Formats/Jxl/IO/JxlBitReader.cs | 165 ++++++++++++++++++
 src/ImageSharp/Formats/Jxl/JxlThrowHelper.cs  |  14 ++
 2 files changed, 179 insertions(+)
 create mode 100644 src/ImageSharp/Formats/Jxl/IO/JxlBitReader.cs
 create mode 100644 src/ImageSharp/Formats/Jxl/JxlThrowHelper.cs

diff --git a/src/ImageSharp/Formats/Jxl/IO/JxlBitReader.cs b/src/ImageSharp/Formats/Jxl/IO/JxlBitReader.cs
new file mode 100644
index 0000000000..55e3ed3efd
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/IO/JxlBitReader.cs
@@ -0,0 +1,165 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Buffers.Binary;
+using System.Diagnostics;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+/// <summary>
+/// Represents a bitstream reader.
+/// </summary>
+internal sealed class JxlBitReader(ReadOnlyMemory<byte> bytes)
+{
+    private ulong buffer;
+    private uint bufferRemainingBits;
+    private int pointer;
+    private bool endOfStream;
+
+    /// <summary>
+    /// Fetches a new buffer.
+    /// </summary>
+    private void RefillCore()
+    {
+        ReadOnlySpan<byte> samplesSpan = bytes.Span;
+
+        int remaining = samplesSpan.Length - this.pointer;
+        if (remaining <= 0)
+        {
+            // we don't have any more data... mark an end of stream
+            this.buffer = 0;
+            this.bufferRemainingBits = 0;
+            this.endOfStream = true;
+            return;
+        }
+
+        if (remaining >= 8)
+        {
+            this.buffer = BinaryPrimitives.ReadUInt64LittleEndian(samplesSpan[this.pointer..]);
+            this.bufferRemainingBits = 64u;
+            this.pointer += 8;
+        }
+        else
+        {
+            ulong value = 0;
+            for (int i = 0; i < remaining; i++)
+            {
+                value |= (ulong)samplesSpan[this.pointer + i] << (8 * i);
+            }
+
+            this.buffer = value;
+            this.bufferRemainingBits = (uint)(remaining * 8);
+            this.pointer += remaining;
+        }
+    }
+
+    private void MaybeRefill()
+    {
+        if (this.bufferRemainingBits <= 0)
+        {
+            this.RefillCore();
+        }
+    }
+
+    private ulong ReadBits64Core(uint n, bool peek = false)
+    {
+        Debug.Assert(n <= 64, "Too many bits to pack into ulong");
+        this.MaybeRefill();
+
+        if (this.endOfStream)
+        {
+            JxlThrowHelper.ThrowEndOfStream();
+        }
+
+        if (n <= this.bufferRemainingBits)
+        {
+            ulong result = this.buffer & ((1UL << (int)n) - 1);
+
+            if (!peek)
+            {
+                this.buffer >>= (int)n;
+                this.bufferRemainingBits -= n;
+            }
+
+            return result;
+        }
+        else
+        {
+            uint bitsFromCurrent = this.bufferRemainingBits;
+            ulong part = this.buffer & ((1UL << (int)bitsFromCurrent) - 1);
+
+            this.buffer >>= (int)bitsFromCurrent;
+            this.bufferRemainingBits = 0;
+
+            this.RefillCore();
+
+            uint bitsFromNext = n - bitsFromCurrent;
+            ulong nextPart = this.buffer & ((1UL << (int)bitsFromNext) - 1);
+
+            if (!peek)
+            {
+                this.buffer >>= (int)bitsFromNext;
+                this.bufferRemainingBits -= bitsFromNext;
+            }
+
+            return part | (nextPart << (int)bitsFromCurrent);
+        }
+    }
+
+    private uint ReadBits32Core(uint n, bool peek = false)
+    {
+        Debug.Assert(n <= 32, "Too many bits to pack into uint");
+        this.MaybeRefill();
+
+        if (this.endOfStream)
+        {
+            JxlThrowHelper.ThrowEndOfStream();
+        }
+
+        if (n <= this.bufferRemainingBits)
+        {
+            uint result = (uint)(this.buffer & ((1UL << (int)n) - 1));
+
+            if (!peek)
+            {
+                this.buffer >>= (int)n;
+                this.bufferRemainingBits -= n;
+            }
+
+            return result;
+        }
+        else
+        {
+            uint bitsFromCurrent = this.bufferRemainingBits;
+            uint part = (uint)(this.buffer & ((1UL << (int)bitsFromCurrent) - 1));
+
+            this.buffer >>= (int)bitsFromCurrent;
+            this.bufferRemainingBits = 0;
+
+            this.RefillCore();
+
+            uint bitsFromNext = n - bitsFromCurrent;
+            uint nextPart = (uint)(this.buffer & ((1UL << (int)bitsFromNext) - 1));
+
+            if (!peek)
+            {
+                this.buffer >>= (int)bitsFromNext;
+                this.bufferRemainingBits -= bitsFromNext;
+            }
+
+            return part | (nextPart << (int)bitsFromCurrent);
+        }
+    }
+
+    public uint ReadBits32(uint bits) => this.ReadBits32Core(bits, peek: false);
+
+    public uint PeekBits32(uint bits) => this.ReadBits32Core(bits, peek: true);
+
+    public void SkipBits32(uint bits) => _ = this.ReadBits32(bits);
+
+    public ulong ReadBits64(uint bits) => this.ReadBits64Core(bits, peek: false);
+
+    public ulong PeekBits64(uint bits) => this.ReadBits64Core(bits, peek: true);
+
+    public void SkipBits64(uint bits) => _ = this.ReadBits64(bits);
+}
diff --git a/src/ImageSharp/Formats/Jxl/JxlThrowHelper.cs b/src/ImageSharp/Formats/Jxl/JxlThrowHelper.cs
new file mode 100644
index 0000000000..239d36cd01
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/JxlThrowHelper.cs
@@ -0,0 +1,14 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Diagnostics.CodeAnalysis;
+
+namespace SixLabors.ImageSharp.Formats.Jxl;
+
+internal static class JxlThrowHelper
+{
+    private static readonly EndOfStreamException EndOfStream = new();
+
+    [DoesNotReturn]
+    public static void ThrowEndOfStream() => throw EndOfStream;
+}