merge: Increase minimum Java version to 17 (#115)

This pull request increases the minimum version of Java required by OpenDC to 17.
This new version of Java introduces several new features compared to our old minimum 
version (11), which we attempt to apply in this conversion.

## Implementation Notes :hammer_and_pick:

* Increase minimum Java version to Java 17 
* Use RandomGenerator as randomness source 
* Add common dispatcher interface 
* Add compatibility with Kotlin coroutines
* Use InstantSource as time source
* Re-implement SimulationScheduler as Dispatcher 
* Replace use of CoroutineContext by Dispatcher 

## External Dependencies :four_leaf_clover:

* Java 17

## Breaking API Changes :warning:

* The use of `CoroutineContext` and `Clock` as parameters of classes has been replaced 
  by the `Dispatcher` interface.
* The use of `Clock` has been replaced by `InstantSource` which does not carry time
   zone info.
* The use of `Random` and `SplittableRandom` as parameter type has been replaced 
   by `RandomGenerator`

1 files changed, 94 insertions, 0 deletions

diff --git a/opendc-common/src/main/java/org/opendc/common/util/Pacer.java b/opendc-common/src/main/java/org/opendc/common/util/Pacer.java
new file mode 100644
index 00000000..5b8d8cb0
--- /dev/null
+++ b/opendc-common/src/main/java/org/opendc/common/util/Pacer.java
@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.common.util;
+
+import java.util.function.LongConsumer;
+import org.opendc.common.Dispatcher;
+import org.opendc.common.DispatcherHandle;
+
+/**
+ * Helper class to pace the incoming scheduling requests.
+ */
+public final class Pacer {
+    private final Dispatcher dispatcher;
+    private final long quantumMs;
+    private final LongConsumer process;
+
+    /**
+     * The current {@link DispatcherHandle} representing the pending scheduling cycle.
+     */
+    private DispatcherHandle handle;
+
+    /**
+     * Construct a {@link Pacer} instance.
+     *
+     * @param dispatcher The {@link Dispatcher} to schedule future invocations.
+     * @param quantumMs The scheduling quantum in milliseconds.
+     * @param process The process to invoke for the incoming requests.
+     */
+    public Pacer(Dispatcher dispatcher, long quantumMs, LongConsumer process) {
+        this.dispatcher = dispatcher;
+        this.quantumMs = quantumMs;
+        this.process = process;
+    }
+
+    /**
+     * Determine whether a scheduling cycle is pending.
+     */
+    public boolean isPending() {
+        return handle != null;
+    }
+
+    /**
+     * Enqueue a new scheduling cycle.
+     */
+    public void enqueue() {
+        if (handle != null) {
+            return;
+        }
+
+        final Dispatcher dispatcher = this.dispatcher;
+        long quantumMs = this.quantumMs;
+        long now = dispatcher.getTimeSource().millis();
+
+        // We assume that the scheduler runs at a fixed slot every time quantum (e.g t=0, t=60, t=120).
+        // We calculate here the delay until the next scheduling slot.
+        long timeUntilNextSlot = quantumMs - (now % quantumMs);
+
+        handle = dispatcher.scheduleCancellable(timeUntilNextSlot, () -> {
+            process.accept(now + timeUntilNextSlot);
+            handle = null;
+        });
+    }
+
+    /**
+     * Cancel the currently pending scheduling cycle.
+     */
+    public void cancel() {
+        final DispatcherHandle handle = this.handle;
+        if (handle != null) {
+            this.handle = null;
+            handle.cancel();
+        }
+    }
+}