number theory: power of two

Signed-off-by: Christoph Ruegg <git@cdrnet.ch>

src/Managed.UnitTests/NumberTheoryTests/IntegerTheoryTest.cs

@@ -231,6 +231,76 @@ namespace MathNet.Numerics.UnitTests.NumberTheoryTests
                 () => IntegerTheory.CeilingToPowerOfTwo(maxPowerOfTwo - 1));
         }
 
+        [Test]
+        public void PowerOfTwoMatchesFloatingPointPower32()
+        {
+            for(int i=0; i<31; i++)
+            {
+                Assert.AreEqual(Math.Round(Math.Pow(2, i)), IntegerTheory.PowerOfTwo(i));
+            }
+        }
+
+        [Test]
+        public void PowerOfTwoMatchesFloatingPointPower64()
+        {
+            for (int i = 0; i < 63; i++)
+            {
+                Assert.AreEqual(Math.Round(Math.Pow(2, i)), IntegerTheory.PowerOfTwo((long)i));
+            }
+        }
+
+        [Test]
+        public void PowerOfTwoThrowsWhenOutOfRange32()
+        {
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo(-1));
+
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo(31));
+
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo(Int32.MinValue));
+
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo(Int32.MaxValue));
+
+            Assert.DoesNotThrow(
+                () => IntegerTheory.PowerOfTwo(30));
+
+            Assert.DoesNotThrow(
+                () => IntegerTheory.PowerOfTwo(0));
+        }
+
+        [Test]
+        public void PowerOfTwoThrowsWhenOutOfRange64()
+        {
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo((long)-1));
+
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo((long)63));
+
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo(Int64.MinValue));
+
+            Assert.Throws(
+                typeof(ArgumentOutOfRangeException),
+                () => IntegerTheory.PowerOfTwo(Int64.MaxValue));
+
+            Assert.DoesNotThrow(
+                () => IntegerTheory.PowerOfTwo((long)62));
+
+            Assert.DoesNotThrow(
+                () => IntegerTheory.PowerOfTwo((long)0));
+        }
+
         [Test]
         public void TestIsPerfectSquare32()
         {

src/Managed/NumberTheory/IntegerTheory.cs

@@ -154,6 +154,38 @@ namespace MathNet.Numerics.NumberTheory
             return number + 1;
         }
 
+        /// <summary>
+        /// Raises 2 to the provided integer exponent (0 &lt;= exponent &lt; 31).
+        /// </summary>
+        /// <param name="exponent">The exponent to raise 2 up to.</param>
+        /// <returns>2 ^ exponent.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"/>
+        public static int PowerOfTwo(this int exponent)
+        {
+            if (exponent < 0 || exponent >= 31)
+            {
+                throw new ArgumentOutOfRangeException("exponent");
+            }
+
+            return 1 << exponent;
+        }
+        
+        /// <summary>
+        /// Raises 2 to the provided integer exponent (0 &lt;= exponent &lt; 63).
+        /// </summary>
+        /// <param name="exponent">The exponent to raise 2 up to.</param>
+        /// <returns>2 ^ exponent.</returns>
+        /// <exception cref="ArgumentOutOfRangeException"/>
+        public static long PowerOfTwo(this long exponent)
+        {
+            if (exponent < 0 || exponent >= 63)
+            {
+                throw new ArgumentOutOfRangeException("exponent");
+            }
+
+            return ((long)1) << (int)exponent;
+        }
+
         /// <summary>
         /// Find out whether the provided 32 bit integer is a perfect square, i.e. a square of an integer.
         /// </summary>