DeterministicScheduler supports customizable tick precision

This change adds a constructor which takes a TimeUnit for custom
precision, but does not modify the default constructor in order to
avoid breaking existing tests which could overflow a long using
very long periods of time.

jmock/src/main/java/org/jmock/lib/concurrent/DeterministicScheduler.java

@@ -2,6 +2,7 @@
 
 import java.util.Collection;
 import java.util.List;
+import java.util.Objects;
 import java.util.concurrent.Callable;
 import java.util.concurrent.Delayed;
 import java.util.concurrent.ExecutionException;
@@ -27,7 +28,23 @@
  */
 public class DeterministicScheduler implements ScheduledExecutorService {
     private final DeltaQueue<ScheduledTask<?>> deltaQueue = new DeltaQueue<ScheduledTask<?>>();
-
+
+    private final TimeUnit tickTimeUnit;
+
+    public DeterministicScheduler() {
+        this(TimeUnit.MILLISECONDS);
+    }
+
+    /**
+     * A {@link TimeUnit} may be provided for custom tick precision. This can be helpful when operating
+     * with nanosecond or microsecond precision.
+     *
+     * @param tickTimeUnit Time unit to use for ticks.
+     */
+    public DeterministicScheduler(TimeUnit tickTimeUnit) {
+        this.tickTimeUnit = Objects.requireNonNull(tickTimeUnit, "TimeUnit is required");
+    }
+
     /**
      * Runs time forwards by a given duration, executing any commands scheduled for
      * execution during that time period, and any background tasks spawned by the 
@@ -210,7 +227,7 @@ public boolean repeats() {
         }
 
         public long getDelay(TimeUnit unit) {
-            return unit.convert(deltaQueue.delay(this), TimeUnit.MILLISECONDS);
+            return unit.convert(deltaQueue.delay(this), tickTimeUnit);
         }
 
         public int compareTo(Delayed o) {
@@ -258,7 +275,7 @@ public void run() {
     }
 
     private long toTicks(long duration, TimeUnit timeUnit) {
-        return TimeUnit.MILLISECONDS.convert(duration, timeUnit);
+        return tickTimeUnit.convert(duration, timeUnit);
     }
 
     private UnsupportedSynchronousOperationException blockingOperationsNotSupported() {

jmock/src/test/java/org/jmock/test/unit/lib/concurrent/DeterministicSchedulerTests.java

@@ -135,6 +135,19 @@ public void testGetDelayWithManyScheduledTasks() throws Exception {
         assertEquals(10, task3.getDelay(TimeUnit.SECONDS));
     }
 
+    public void testGetDelayWithManyScheduledTasksWithMicrosecondPrecision() throws Exception {
+        DeterministicScheduler microsecondScheduler = new DeterministicScheduler(TimeUnit.MICROSECONDS);
+        ScheduledFuture<?> task1 = microsecondScheduler.schedule(commandA, 10, TimeUnit.MICROSECONDS);
+        ScheduledFuture<?> task2 = microsecondScheduler.schedule(commandA, 20, TimeUnit.MICROSECONDS);
+        ScheduledFuture<?> task3 = microsecondScheduler.schedule(commandA, 15, TimeUnit.MICROSECONDS);
+
+        microsecondScheduler.tick(5, TimeUnit.MICROSECONDS);
+
+        assertEquals(5, task1.getDelay(TimeUnit.MICROSECONDS));
+        assertEquals(15, task2.getDelay(TimeUnit.MICROSECONDS));
+        assertEquals(10, task3.getDelay(TimeUnit.MICROSECONDS));
+    }
+
     public void testGetDelayOnPassedTasks() throws Exception {
         final Throwable thrown = new IllegalStateException();