Add delta to ClockUtil.

Prevent delta calculation when start time isn't available.

Signed-off-by: Achim Kraus <achim.kraus@cloudcoap.net>

element-connector/src/main/java/org/eclipse/californium/elements/util/ClockUtil.java

@@ -15,6 +15,8 @@
  ******************************************************************************/
 package org.eclipse.californium.elements.util;
 
+import java.util.concurrent.TimeUnit;
+
 /**
  * Clock utility. Provides a {@linkplain ClockUtil#handler handler}.
  * 
@@ -79,4 +81,21 @@ public static void setRealtimeHandler(Realtime systemHandler) {
 	public static long nanoRealtime() {
 		return handler.nanoRealtime();
 	}
+
+	/**
+	 * Calculate the delta from the provided past nano-realtime.
+	 * 
+	 * @param pastNanoRealtime past value of {@link #nanoRealtime()}. Maybe
+	 *            {@code 0}, if the past nano-realtime isn't available.
+	 * @param unit unit of result
+	 * @return the difference of the current nano-realtime to the provided one,
+	 *         or {@code 0}, if {@code 0} is provided.
+	 * @since 3.11
+	 */
+	public static long delta(long pastNanoRealtime, TimeUnit unit) {
+		if (pastNanoRealtime > 0) {
+			return unit.convert(handler.nanoRealtime() - pastNanoRealtime, TimeUnit.NANOSECONDS);
+		}
+		return 0;
+	}
 }

element-connector/src/test/java/org/eclipse/californium/elements/util/ClockUtilTest.java

@@ -0,0 +1,55 @@
+/********************************************************************************
+ * Copyright (c) 2024 Contributors to the Eclipse Foundation
+ * 
+ * See the NOTICE file(s) distributed with this work for additional
+ * information regarding copyright ownership.
+ * 
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * https://www.eclipse.org/legal/epl-2.0, or the Eclipse Distribution License
+ * v1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ * 
+ * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+ ********************************************************************************/
+package org.eclipse.californium.elements.util;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.Matchers.greaterThanOrEqualTo;
+import static org.hamcrest.Matchers.is;
+
+import java.util.concurrent.TimeUnit;
+
+import org.eclipse.californium.elements.category.Small;
+import org.junit.Test;
+import org.junit.experimental.categories.Category;
+
+@Category(Small.class)
+public class ClockUtilTest {
+
+	@Test
+	public void testNanoRealtime() {
+		long now1 = ClockUtil.nanoRealtime();
+		try {
+			Thread.sleep(200);
+			long now2 = ClockUtil.nanoRealtime();
+			assertThat(now2, is(greaterThanOrEqualTo(now1)));
+		} catch (InterruptedException e) {
+		}
+	}
+
+	@Test
+	public void testDelta() {
+		long now = ClockUtil.nanoRealtime();
+		try {
+			Thread.sleep(200);
+			long delta = ClockUtil.delta(now, TimeUnit.MILLISECONDS);
+			assertThat(delta, is(greaterThan(100L)));
+		} catch (InterruptedException e) {
+		}
+		long delta2 = ClockUtil.delta(0, TimeUnit.MILLISECONDS);
+		assertThat(delta2, is(0L));
+	}
+
+}