3 files changed, 220 insertions, 0 deletions
diff --git a/opendc-utils/build.gradle.kts b/opendc-utils/build.gradle.kts
index 800b374d..858aa64c 100644
--- a/opendc-utils/build.gradle.kts
+++ b/opendc-utils/build.gradle.kts
@@ -26,6 +26,7 @@ description = "Utilities used across OpenDC modules"
 plugins {
     `kotlin-library-conventions`
     `testing-conventions`
+    `jacoco-conventions`
 }
 
 dependencies {
diff --git a/opendc-utils/src/main/kotlin/org/opendc/utils/Pacer.kt b/opendc-utils/src/main/kotlin/org/opendc/utils/Pacer.kt
new file mode 100644
index 00000000..07d3cb87
--- /dev/null
+++ b/opendc-utils/src/main/kotlin/org/opendc/utils/Pacer.kt
@@ -0,0 +1,92 @@
+/*
+ * 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.utils
+
+import kotlinx.coroutines.*
+import java.lang.Runnable
+import java.time.Clock
+import kotlin.coroutines.ContinuationInterceptor
+import kotlin.coroutines.CoroutineContext
+
+/**
+ * Helper class to pace the incoming scheduling requests.
+ *
+ * @param context The [CoroutineContext] in which the pacer runs.
+ * @param clock The virtual simulation clock.
+ * @param quantum The scheduling quantum.
+ * @param process The process to invoke for the incoming requests.
+ */
+public class Pacer(
+    private val context: CoroutineContext,
+    private val clock: Clock,
+    private val quantum: Long,
+    private val process: (Long) -> Unit
+) {
+    /**
+     * The [Delay] instance that provides scheduled execution of [Runnable]s.
+     */
+    @OptIn(InternalCoroutinesApi::class)
+    private val delay =
+        requireNotNull(context[ContinuationInterceptor] as? Delay) { "Invalid CoroutineDispatcher: no delay implementation" }
+
+    /**
+     * The current [DisposableHandle] representing the pending scheduling cycle.
+     */
+    private var handle: DisposableHandle? = null
+
+    /**
+     * Determine whether a scheduling cycle is pending.
+     */
+    public val isPending: Boolean get() = handle != null
+
+    /**
+     * Enqueue a new scheduling cycle.
+     */
+    public fun enqueue() {
+        if (handle != null) {
+            return
+        }
+
+        val quantum = quantum
+        val now = clock.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.
+        val timeUntilNextSlot = quantum - (now % quantum)
+
+        @OptIn(InternalCoroutinesApi::class)
+        handle = delay.invokeOnTimeout(timeUntilNextSlot, {
+            process(now + timeUntilNextSlot)
+            handle = null
+        }, context)
+    }
+
+    /**
+     * Cancel the currently pending scheduling cycle.
+     */
+    public fun cancel() {
+        val handle = handle ?: return
+        this.handle = null
+        handle.dispose()
+    }
+}
diff --git a/opendc-utils/src/test/kotlin/org/opendc/utils/PacerTest.kt b/opendc-utils/src/test/kotlin/org/opendc/utils/PacerTest.kt
new file mode 100644
index 00000000..b8419e80
--- /dev/null
+++ b/opendc-utils/src/test/kotlin/org/opendc/utils/PacerTest.kt
@@ -0,0 +1,127 @@
+/*
+ * 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.utils
+
+import kotlinx.coroutines.delay
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+import org.opendc.simulator.core.runBlockingSimulation
+import java.time.Clock
+import kotlin.coroutines.EmptyCoroutineContext
+
+/**
+ * Test suite for the [Pacer] class.
+ */
+class PacerTest {
+    @Test
+    fun testEmptyContext() {
+        assertThrows<IllegalArgumentException> { Pacer(EmptyCoroutineContext, Clock.systemUTC(), 100) {} }
+    }
+
+    @Test
+    fun testSingleEnqueue() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+        }
+
+        assertEquals(1, count) { "Process should execute once" }
+    }
+
+    @Test
+    fun testCascade() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+            pacer.enqueue()
+
+            assertTrue(pacer.isPending)
+        }
+
+        assertEquals(1, count) { "Process should execute once" }
+    }
+
+    @Test
+    fun testCancel() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+            pacer.cancel()
+
+            assertFalse(pacer.isPending)
+        }
+
+        assertEquals(0, count) { "Process should never execute " }
+    }
+
+    @Test
+    fun testCancelWithoutPending() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            assertFalse(pacer.isPending)
+            assertDoesNotThrow { pacer.cancel() }
+
+            pacer.enqueue()
+        }
+
+        assertEquals(1, count) { "Process should execute once" }
+    }
+
+    @Test
+    fun testSubsequent() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+            delay(100)
+            pacer.enqueue()
+        }
+
+        assertEquals(2, count) { "Process should execute twice" }
+    }
+}