Fix BitsLog2Floor

src/ImageSharp/Formats/WebP/Vp8BitReader.cs

@@ -199,22 +199,17 @@ namespace SixLabors.ImageSharp.Formats.WebP
             return x;
         }
 
+        // Returns 31 ^ clz(n) = log2(n).Returns 31 ^ clz(n) = log2(n).
         private int BitsLog2Floor(uint n)
         {
-            // Search the mask data from most significant bit (MSB) to least significant bit (LSB) for a set bit (1).
-            // https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64?view=vs-2019
-            uint mask = 1;
-            for (int y = 0; y < 32; y++)
+            int logValue = 0;
+            while (n >= 256)
             {
-                if ((mask & n) == mask)
-                {
-                    return y;
-                }
-
-                mask <<= 1;
+                logValue += 8;
+                n >>= 8;
             }
 
-            return 0;
+            return logValue + WebPConstants.LogTable8bit[n];
         }
     }
 }

src/ImageSharp/Formats/WebP/WebPConstants.cs

@@ -143,6 +143,27 @@ namespace SixLabors.ImageSharp.Formats.WebP
             0, 1, 2, 3, 6, 4, 5, 6, 6, 6, 6, 6, 6, 6, 6, 7, 0
         };
 
+        // 31 ^ clz(i)
+        public static readonly byte[] LogTable8bit =
+        {
+            0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
+            4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+            5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
+            5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
+            6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+            6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+            6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+            6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7
+        };
+
         // Paragraph 14.1
         public static readonly int[] DcTable =
         {

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -121,7 +121,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 block.Skip = (byte)this.bitReader.GetBit(dec.SkipProbability);
             }
 
-            block.IsI4x4 = this.bitReader.GetBit(145) != 0;
+            block.IsI4x4 = this.bitReader.GetBit(145) is 0;
             if (!block.IsI4x4)
             {
                 // Hardcoded 16x16 intra-mode decision tree.