refactor: Replace use of CoroutineContext by Dispatcher

This change replaces the use of `CoroutineContext` for passing the
`SimulationDispatcher` across the different modules of OpenDC by the
lightweight `Dispatcher` interface of the OpenDC common module.

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();
+        }
+    }
+}