Add validation to Util::log2

Changed logic to take advantage of Integer::numberOfLeadingZeros which cuts down the code.

Fixes #443

src/main/java/com/lmax/disruptor/util/Util.java

@@ -91,18 +91,16 @@ public static Sequence[] getSequencesFor(final EventProcessor... processors)
      * Calculate the log base 2 of the supplied integer, essentially reports the location
      * of the highest bit.
      *
-     * @param i Value to calculate log2 for.
+     * @param value Positive value to calculate log2 for.
      * @return The log2 value
      */
-    public static int log2(final int i)
+    public static int log2(final int value)
     {
-        long value = i;
-        int r = 0;
-        while ((value >>= 1) != 0)
+        if (value < 1)
         {
-            ++r;
+            throw new IllegalArgumentException("value must be a positive number");
         }
-        return r;
+        return Integer.SIZE - Integer.numberOfLeadingZeros(value) - 1;
     }
 
     /**

src/test/java/com/lmax/disruptor/util/UtilTest.java

@@ -19,6 +19,7 @@
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 public final class UtilTest
 {
@@ -52,4 +53,22 @@ public void shouldReturnLongMaxWhenNoEventProcessors()
 
         assertEquals(Long.MAX_VALUE, Util.getMinimumSequence(sequences));
     }
+
+    @Test
+    void shouldThrowErrorIfValuePassedToLog2FunctionIsNotPositive()
+    {
+        assertThrows(IllegalArgumentException.class, () -> Util.log2(0));
+        assertThrows(IllegalArgumentException.class, () -> Util.log2(-1));
+        assertThrows(IllegalArgumentException.class, () -> Util.log2(Integer.MIN_VALUE));
+    }
+
+    @Test
+    void shouldCalculateCorrectlyIntegerFlooredLog2()
+    {
+        assertEquals(0, Util.log2(1));
+        assertEquals(1, Util.log2(2));
+        assertEquals(1, Util.log2(3));
+        assertEquals(10, Util.log2(1024));
+        assertEquals(30, Util.log2(Integer.MAX_VALUE));
+    }
 }