Euclid: Log2 implementation using DeBruijn sequence lookup

src/Numerics/Euclid.cs

@@ -271,6 +271,27 @@ namespace MathNet.Numerics
             return ((long)1) << (int)exponent;
         }
 
+        /// <summary>
+        /// Evaluate the binary logarithm of an integer number.
+        /// </summary>
+        /// <remarks>Two-step method using a De Bruijn-like sequence table lookup.</remarks>
+        public static int Log2(this int number)
+        {
+            number |= number >> 1;
+            number |= number >> 2;
+            number |= number >> 4;
+            number |= number >> 8;
+            number |= number >> 16;
+
+            return MultiplyDeBruijnBitPosition[(uint)(number * 0x07C4ACDDU) >> 27];
+        }
+
+        static readonly int[] MultiplyDeBruijnBitPosition = new int[32]
+        {
+            0, 9, 1, 10, 13, 21, 2, 29, 11, 14, 16, 18, 22, 25, 3, 30,
+            8, 12, 20, 28, 15, 17, 24, 7, 19, 27, 23, 6, 26, 5, 4, 31
+        };
+
         /// <summary>
         /// Find the closest perfect power of two that is larger or equal to the provided
         /// 32 bit integer.

src/UnitTests/EuclidTests/IntegerTheoryTest.cs

@@ -442,6 +442,24 @@ namespace MathNet.Numerics.UnitTests.EuclidTests
                 () => ((long)0).PowerOfTwo());
         }
 
+        /// <summary>
+        /// Log2 matches floating point for int32.
+        /// </summary>
+        [Test]
+        public void Log2MatchesFloatingPoint32()
+        {
+            for (var i = 0; i < 31; i++)
+            {
+                int number = i.PowerOfTwo();
+                Assert.AreEqual((int)Math.Log(number, 2), number.Log2());
+                Assert.AreEqual((int)Math.Log(number + 1, 2), (number + 1).Log2());
+                if (number > 1)
+                {
+                    Assert.AreEqual((int)Math.Log(number - 1, 2), (number - 1).Log2());
+                }
+            }
+        }
+
         /// <summary>
         /// Test if int32 is perfect square.
         /// </summary>