Implement JxlAnsHelper.GetPopulationCountPrecision

See ans_common.h

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

@@ -0,0 +1,25 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Buffers;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using static SixLabors.ImageSharp.Formats.Jxl.IO.JxlAnsConstants;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+internal static class JxlAnsHelper
+{
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static int GetPopulationCountPrecision(int logCount, int shift)
+    {
+        int r = Math.Min(logCount, shift - ((AnsLogTableSize - logCount) >> 1));
+
+        if (r < 0)
+        {
+            return 0;
+        }
+
+        return r;
+    }
+}