Implemented log2 method

src/ImageSharp/Common/Helpers/Numerics.cs

@@ -23,6 +23,7 @@ namespace SixLabors.ImageSharp
         private const int ShuffleAlphaControl = 0b_11_11_11_11;
 #endif
 
+        // TODO: Obsolete - remove
 #if !SUPPORTS_BITOPERATIONS
         /// <summary>
         /// Gets the counts the number of bits needed to hold an integer.
@@ -45,6 +46,16 @@ namespace SixLabors.ImageSharp
         };
 #endif
 
+#if !SUPPORTS_BITOPERATIONS
+        private static ReadOnlySpan<byte> Log2DeBruijn => new byte[32]
+        {
+            00, 09, 01, 10, 13, 21, 02, 29,
+            11, 14, 16, 18, 22, 25, 03, 30,
+            08, 12, 20, 28, 15, 17, 24, 07,
+            19, 27, 23, 06, 26, 05, 04, 31
+        };
+#endif
+
         /// <summary>
         /// Determine the Greatest CommonDivisor (GCD) of two numbers.
         /// </summary>
@@ -868,5 +879,52 @@ namespace SixLabors.ImageSharp
             return bitInUnsignedInteger - BitOperations.LeadingZeroCount(number);
 #endif
         }
+
+        /// <summary>
+        /// Calculates floored log of the specified value, base 2.
+        /// Note that by convention, input value 0 returns 0 since Log(0) is undefined.
+        /// </summary>
+        /// <param name="value">The value.</param>
+        public static int Log2(uint value)
+        {
+#if SUPPORTS_BITOPERATIONS
+            return BitOperations.Log2(value);
+#else
+            return Log2SoftwareFallback(value);
+#endif
+        }
+
+#if !SUPPORTS_BITOPERATIONS
+        /// <summary>
+        /// Calculates floored log of the specified value, base 2.
+        /// Note that by convention, input value 0 returns 0 since Log(0) is undefined.
+        /// Bit hacking with deBruijn sequence, extremely fast yet does not use any intrinsics so should work on every platform.
+        /// </summary>
+        /// <remarks>
+        /// Description of this bit hacking can be found here:
+        /// https://cstheory.stackexchange.com/questions/19524/using-the-de-bruijn-sequence-to-find-the-lceil-log-2-v-rceil-of-an-integer
+        /// </remarks>
+        /// <param name="value">The value.</param>
+        private static int Log2SoftwareFallback(uint value)
+        {
+            // No AggressiveInlining due to large method size
+            // Has conventional contract 0->0 (Log(0) is undefined) by default, no need for if checking
+
+
+            // Fill trailing zeros with ones, eg 00010010 becomes 00011111
+            value |= value >> 01;
+            value |= value >> 02;
+            value |= value >> 04;
+            value |= value >> 08;
+            value |= value >> 16;
+
+            // uint.MaxValue >> 27 is always in range [0 - 31] so we use Unsafe.AddByteOffset to avoid bounds check
+            return Unsafe.AddByteOffset(
+                ref MemoryMarshal.GetReference(Log2DeBruijn),
+
+                // uint|long -> IntPtr cast on 32-bit platforms does expensive overflow checks not needed here
+                (IntPtr)(int)((value * 0x07C4ACDDu) >> 27));
+        }
+#endif
     }
 }