Add Move to Front transform

See inverse_mtf-inl.h

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

@@ -0,0 +1,91 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+/// <summary>
+/// Inverse Move to Front implementation
+/// </summary>
+internal static class JxlInverseMtf
+{
+    // NOTE: here we use Vector512 to store 64 bytes in a
+    // more efficient manner. However, it doesn't necessarily
+    // require 512-bit CPU vector support.
+    // If the user's CPU has 256-bit vectors, the JIT will emit
+    // such instructions for each half. Likewise, if the user's
+    // CPU only goes up to 128-bit vectors, the JIT will emit
+    // 128-bit vector code for each quarter. And if the CPU
+    // doesn't support SIMD at all, the JIT will emit scalar
+    // instructions.
+    public static void MoveToFront(Span<byte> v, byte index)
+    {
+        byte value = v[index];
+        byte i = index;
+
+        ref byte vR = ref MemoryMarshal.GetReference(v);
+
+        if (i < 4)
+        {
+            for (; i != 0; --i)
+            {
+                v[i] = v[i - 1];
+            }
+        }
+        else
+        {
+            int tail = i & 63;
+
+            if (tail != 0)
+            {
+                i -= (byte)tail;
+                Vector512<byte> vec = Vector512.LoadUnsafe(ref Unsafe.Add(ref vR, i));
+                Vector512<byte> prev = Vector512.LoadUnsafe(ref Unsafe.Add(ref vR, i + 1));
+
+                // TODO: optimize this?
+                Span<byte> maskBytes = stackalloc byte[64];
+
+                for (int j = 0; j < 64; j++)
+                {
+                    maskBytes[j] = (byte)(j < tail ? 0xFF : 0);
+                }
+
+                Vector512<byte> mask = Vector512.Create<byte>(maskBytes);
+                Vector512<byte> filter = Vector512.ConditionalSelect(mask, vec, prev);
+                filter.StoreUnsafe(ref Unsafe.Add(ref vR, i + 1));
+            }
+
+            while (i != 0)
+            {
+                i -= 64;
+                Vector512<byte> vec = Vector512.LoadUnsafe(ref Unsafe.Add(ref vR, i));
+                vec.StoreUnsafe(ref Unsafe.Add(ref vR, i + 1));
+            }
+        }
+
+        v[0] = value;
+    }
+
+    public static void InverseMoveToFrontTransform(Span<byte> v, int vLength)
+    {
+        Span<byte> mtf = stackalloc byte[256 + 64];
+        for (int i = 0; i < 256; i++)
+        {
+            mtf[i] = (byte)i;
+        }
+
+        for (int i = 0; i < vLength; i++)
+        {
+            byte index = v[i];
+            v[i] = mtf[index];
+
+            if (index != 0)
+            {
+                MoveToFront(mtf, index);
+            }
+        }
+    }
+}