Add PowerOfTwo validator

It's useful when used together with the ThreadCount.nextPowerOfTwo in cases
where the number of threads must be a power of two (i.e. bit masking units of work to a given thread)

src/main/java/io/airlift/units/PowerOfTwo.java

@@ -0,0 +1,38 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.airlift.units;
+
+import javax.validation.Constraint;
+import javax.validation.Payload;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+@Target({METHOD, ANNOTATION_TYPE})
+@Retention(RUNTIME)
+@Documented
+@Constraint(validatedBy = PowerOfTwoValidator.class)
+public @interface PowerOfTwo
+{
+    String message() default "is not a power of two";
+
+    Class<?>[] groups() default {};
+
+    Class<? extends Payload>[] payload() default {};
+}

src/main/java/io/airlift/units/PowerOfTwoValidator.java

@@ -0,0 +1,30 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.airlift.units;
+
+import javax.validation.ConstraintValidator;
+import javax.validation.ConstraintValidatorContext;
+
+public class PowerOfTwoValidator
+        implements ConstraintValidator<PowerOfTwo, Integer>
+{
+    @Override
+    public void initialize(PowerOfTwo powerOfTwo) {}
+
+    @Override
+    public boolean isValid(Integer value, ConstraintValidatorContext context)
+    {
+        return value == null || Integer.bitCount(value) == 1;
+    }
+}

src/test/java/io/airlift/units/TestPowerOfTwoValidator.java

@@ -0,0 +1,87 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.airlift.units;
+
+import org.testng.annotations.Test;
+
+import static io.airlift.testing.ValidationAssertions.assertFailsValidation;
+import static io.airlift.testing.ValidationAssertions.assertValidates;
+
+public class TestPowerOfTwoValidator
+{
+    @Test
+    public void testValidator()
+    {
+        assertValid(1);
+        assertValid(2);
+        assertValid(64);
+        assertInvalid(0);
+        assertInvalid(3);
+        assertInvalid(99);
+        assertInvalid(-1);
+        assertInvalid(-2);
+        assertInvalid(-4);
+    }
+
+    @Test
+    public void testAllowsNullPowerOfTwoAnnotation()
+    {
+        assertValidates(new NullPowerOfTwoAnnotation());
+    }
+
+    private static void assertValid(int value)
+    {
+        assertValidates(new ConstrainedPowerOfTwo(value));
+    }
+
+    private static void assertInvalid(int value)
+    {
+        Object object = new ConstrainedPowerOfTwo(value);
+        assertFailsValidation(object, "unboxed", "is not a power of two", PowerOfTwo.class);
+        assertFailsValidation(object, "boxed", "is not a power of two", PowerOfTwo.class);
+    }
+
+    @SuppressWarnings("UnusedDeclaration")
+    public static class ConstrainedPowerOfTwo
+    {
+        private final int value;
+
+        public ConstrainedPowerOfTwo(int value)
+        {
+            this.value = value;
+        }
+
+        @PowerOfTwo
+        public int getUnboxed()
+        {
+            return value;
+        }
+
+        @PowerOfTwo
+        public Integer getBoxed()
+        {
+            return value;
+        }
+    }
+
+    @SuppressWarnings("UnusedDeclaration")
+    public static class NullPowerOfTwoAnnotation
+    {
+        @PowerOfTwo
+        public Integer getNull()
+        {
+            return null;
+        }
+    }
+}