From 7f35b1a3db1bec4b90879f56c77980106fb4ee10 Mon Sep 17 00:00:00 2001
From: winscripter <142818255+winscripter@users.noreply.github.com>
Date: Fri, 17 Jul 2026 00:39:36 +0400
Subject: [PATCH] Implement Lehmer codes

---
 .../Formats/Jxl/Processing/JxlLehmerCode.cs   | 106 ++++++++++++++++++
 1 file changed, 106 insertions(+)
 create mode 100644 src/ImageSharp/Formats/Jxl/Processing/JxlLehmerCode.cs

diff --git a/src/ImageSharp/Formats/Jxl/Processing/JxlLehmerCode.cs b/src/ImageSharp/Formats/Jxl/Processing/JxlLehmerCode.cs
new file mode 100644
index 0000000000..095e936d32
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/Processing/JxlLehmerCode.cs
@@ -0,0 +1,106 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+internal static class JxlLehmerCode
+{
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static int ValueOfLowest1Bit(int n) => n & -n;
+
+    public static bool ComputeLehmerCode(ReadOnlySpan<int> permutation, Span<uint> temp, int n, Span<uint> code)
+    {
+        temp[(n + 1)..].Clear();
+
+        for (int idx = 0; idx < n; idx++)
+        {
+            int s = permutation[idx];
+
+            uint penalty = 0u;
+            uint i = (uint)s + 1u;
+
+            while (i != 0u)
+            {
+                penalty += temp[(int)i];
+                i &= i - 1u;    // Clear lowest bit
+            }
+
+            if (s < penalty)
+            {
+                return false;
+            }
+
+            code[idx] = (uint)s - penalty;
+            i = (uint)s + 1u;
+
+            while (i < n + 1u)
+            {
+                temp[(int)i]++;
+                i += (uint)ValueOfLowest1Bit((int)i);
+            }
+        }
+
+        return true;
+    }
+
+    public static bool DecodeLehmerCode(ReadOnlySpan<uint> code, Span<uint> temp, int n, Span<int> permutation)
+    {
+        if (n == 0)
+        {
+            return false;
+        }
+
+        int log2n = CeilLog2Nonzero(n);
+        int paddedN = 1 << log2n;
+
+        for (int i = 0; i < paddedN; i++)
+        {
+            int i1 = i + 1;
+            temp[i] = (uint)ValueOfLowest1Bit(i1);
+        }
+
+        for (int i = 0; i < n; i++)
+        {
+            if (code[i] + i >= n)
+            {
+                return false;
+            }
+
+            uint rank = code[i] + 1;
+
+            int bit = paddedN;
+            int next = 0;
+
+            for (int b = 0; b <= log2n; b++)
+            {
+                int cand = next + bit;
+
+                if (cand < 1)
+                {
+                    return false;
+                }
+
+                bit >>= 1;
+
+                if (temp[cand - 1] < rank)
+                {
+                    next = cand;
+                    rank -= temp[cand - 1];
+                }
+            }
+
+            permutation[i] = next;
+
+            next++;
+            while (next <= paddedN)
+            {
+                temp[next - 1]--;
+                next += ValueOfLowest1Bit(next);
+            }
+        }
+
+        return true;
+    }
+}