merge: Address technical debt in codebase

This pull request removes some of the existing technical debt in the OpenDC codebase.

Close #51

## Implementation Notes :hammer_and_pick:

* Add Pacer to pace scheduling cycles.
* Rename utils module to common module
* Optimize TimerScheduler
* Remove opendc-platform module
* Disallow duplicate UIDs for SimHost

## Breaking API Changes :warning:

* TimerScheduler does not implement `AutoCloseable` anymore.

2 files changed, 260 insertions, 0 deletions

diff --git a/opendc-common/src/test/kotlin/org/opendc/common/util/PacerTest.kt b/opendc-common/src/test/kotlin/org/opendc/common/util/PacerTest.kt
new file mode 100644
index 00000000..1cd435f6
--- /dev/null
+++ b/opendc-common/src/test/kotlin/org/opendc/common/util/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.common.util
+
+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" }
+    }
+}
diff --git a/opendc-common/src/test/kotlin/org/opendc/common/util/TimerSchedulerTest.kt b/opendc-common/src/test/kotlin/org/opendc/common/util/TimerSchedulerTest.kt
new file mode 100644
index 00000000..01f61f92
--- /dev/null
+++ b/opendc-common/src/test/kotlin/org/opendc/common/util/TimerSchedulerTest.kt
@@ -0,0 +1,133 @@
+/*
+ * 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 kotlinx.coroutines.ExperimentalCoroutinesApi
+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
+
+/**
+ * A test suite for the [TimerScheduler] class.
+ */
+@OptIn(ExperimentalCoroutinesApi::class)
+internal class TimerSchedulerTest {
+    @Test
+    fun testEmptyContext() {
+        assertThrows<IllegalArgumentException> { TimerScheduler<Unit>(EmptyCoroutineContext, Clock.systemUTC()) }
+    }
+
+    @Test
+    fun testBasicTimer() {
+        runBlockingSimulation {
+            val scheduler = TimerScheduler<Int>(coroutineContext, clock)
+
+            scheduler.startSingleTimer(0, 1000) {
+                assertEquals(1000, clock.millis())
+            }
+
+            assertTrue(scheduler.isTimerActive(0))
+            assertFalse(scheduler.isTimerActive(1))
+        }
+    }
+
+    @Test
+    fun testCancelNonExisting() {
+        runBlockingSimulation {
+            val scheduler = TimerScheduler<Int>(coroutineContext, clock)
+
+            scheduler.cancel(1)
+        }
+    }
+
+    @Test
+    fun testCancelExisting() {
+        runBlockingSimulation {
+            val scheduler = TimerScheduler<Int>(coroutineContext, clock)
+
+            scheduler.startSingleTimer(0, 1000) {
+                fail()
+            }
+
+            scheduler.startSingleTimer(1, 100) {
+                scheduler.cancel(0)
+
+                assertEquals(100, clock.millis())
+            }
+        }
+    }
+
+    @Test
+    fun testCancelAll() {
+        runBlockingSimulation {
+            val scheduler = TimerScheduler<Int>(coroutineContext, clock)
+
+            scheduler.startSingleTimer(0, 1000) { fail() }
+            scheduler.startSingleTimer(1, 100) { fail() }
+            scheduler.cancelAll()
+        }
+    }
+
+    @Test
+    fun testOverride() {
+        runBlockingSimulation {
+            val scheduler = TimerScheduler<Int>(coroutineContext, clock)
+
+            scheduler.startSingleTimer(0, 1000) { fail() }
+
+            scheduler.startSingleTimer(0, 200) {
+
+                assertEquals(200, clock.millis())
+            }
+        }
+    }
+
+    @Test
+    fun testOverrideBlock() {
+        runBlockingSimulation {
+            val scheduler = TimerScheduler<Int>(coroutineContext, clock)
+
+            scheduler.startSingleTimer(0, 1000) { fail() }
+
+            scheduler.startSingleTimer(0, 1000) {
+                assertEquals(1000, clock.millis())
+            }
+        }
+    }
+
+    @Test
+    fun testNegativeDelay() {
+        runBlockingSimulation {
+            val scheduler = TimerScheduler<Int>(coroutineContext, clock)
+
+            assertThrows<IllegalArgumentException> {
+                scheduler.startSingleTimer(1, -1) {
+                    fail()
+                }
+            }
+        }
+    }
+}