feat: Add testkit for testing behavior

This change adds a testkit for synchronously testking Behavior
implementations.

7 files changed, 621 insertions, 0 deletions

diff --git a/odcsim-testkit/build.gradle.kts b/odcsim-testkit/build.gradle.kts
new file mode 100644
index 00000000..8b19019a
--- /dev/null
+++ b/odcsim-testkit/build.gradle.kts
@@ -0,0 +1,43 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019 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.
+ */
+
+/* Build configuration */
+apply(from = "../gradle/kotlin.gradle")
+plugins {
+    `java-library`
+}
+
+/* Project configuration */
+repositories {
+    jcenter()
+}
+
+val junitJupiterVersion: String by extra
+
+dependencies {
+    api(project(":odcsim-core"))
+
+    implementation(kotlin("stdlib"))
+    implementation("org.junit.jupiter:junit-jupiter-api:$junitJupiterVersion")
+}
diff --git a/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/BehaviorTestKit.kt b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/BehaviorTestKit.kt
new file mode 100644
index 00000000..65782d52
--- /dev/null
+++ b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/BehaviorTestKit.kt
@@ -0,0 +1,138 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019 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 com.atlarge.odcsim.testkit
+
+import com.atlarge.odcsim.ActorContext
+import com.atlarge.odcsim.ActorPath
+import com.atlarge.odcsim.ActorRef
+import com.atlarge.odcsim.Behavior
+import com.atlarge.odcsim.Instant
+import com.atlarge.odcsim.isAlive
+import com.atlarge.odcsim.testkit.internal.BehaviorTestKitImpl
+import java.util.UUID
+
+/**
+ * Utensils for synchronous testing of [Behavior] instances.
+ *
+ * @param T The shape of the messages the behavior accepts.
+ */
+interface BehaviorTestKit<T : Any> {
+    /**
+     * The current point in simulation time.
+     */
+    val time: Instant
+
+    /**
+     * The current behavior of the simulated actor.
+     */
+    val behavior: Behavior<T>
+
+    /**
+     * The self reference to the simulated actor inside the test kit.
+     */
+    val ref: ActorRef<T>
+
+    /**
+     * The context of the simulated actor.
+     */
+    val context: ActorContext<T>
+
+    /**
+     * The inbox of the simulated actor.
+     */
+    val inbox: TestInbox<T>
+
+    /**
+     * A flag indicating whether the [Behavior] instance is still alive.
+     */
+    val isAlive: Boolean get() = behavior.isAlive
+
+    /**
+     * Interpret the specified message at the current point in simulation time using the current behavior.
+     *
+     * @return `true` if the message was handled by the behavior, `false` if it was unhandled.
+     */
+    fun run(msg: T): Boolean
+
+    /**
+     * Interpret the oldest message in the inbox using the current behavior.
+     *
+     * @return `true` if the message was handled by the behavior, `false` if it was unhandled.
+     * @throws NoSuchElementException if the actor's inbox is empty.
+     */
+    fun runOne(): Boolean
+
+    /**
+     * Interpret the messages in the inbox until the specified point in simulation time is reached.
+     *
+     * @param time The time until which the messages in the inbox should be processed.
+     */
+    fun runTo(time: Instant)
+
+    /**
+     * Create an anonymous [TestInbox] for receiving messages.
+     */
+    fun <U : Any> createInbox(): TestInbox<U>
+
+    /**
+     * Get the child inbox for the child with the given name, or fail if there is no child with the given name
+     * spawned
+     */
+    fun <U : Any> childInbox(name: String): TestInbox<U>
+
+    /**
+     * Get the child inbox for the child referenced by [ref], or fail if it is not a child of this behavior.
+     */
+    fun <U : Any> childInbox(ref: ActorRef<U>): TestInbox<U>
+
+    /**
+     * Obtain the [BehaviorTestKit] for the child with the given name, or fail if there is no child with the given
+     * name spawned.
+     */
+    fun <U : Any> childTestKit(name: String): BehaviorTestKit<U>
+
+    /**
+     * Obtain the [BehaviorTestKit] for the given child [ActorRef].
+     */
+    fun <U : Any> childTestKit(ref: ActorRef<U>): BehaviorTestKit<U>
+
+    companion object {
+        /**
+         * Create a [BehaviorTestKit] instance for the specified [Behavior].
+         *
+         * @param behavior The behavior for which a test kit should be created.
+         */
+        operator fun <T : Any> invoke(behavior: Behavior<T>): BehaviorTestKit<T> =
+            BehaviorTestKitImpl(behavior, ActorPath.Root(name = "/" + UUID.randomUUID().toString()))
+
+        /**
+         * Create a [BehaviorTestKit] instance for the specified [Behavior].
+         *
+         * @param behavior The behavior for which a test kit should be created.
+         */
+        @JvmStatic
+        fun <T : Any> create(behavior: Behavior<T>): BehaviorTestKit<T> = invoke(behavior)
+    }
+}
diff --git a/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/TestInbox.kt b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/TestInbox.kt
new file mode 100644
index 00000000..de240143
--- /dev/null
+++ b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/TestInbox.kt
@@ -0,0 +1,90 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019 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 com.atlarge.odcsim.testkit
+
+import com.atlarge.odcsim.ActorRef
+import com.atlarge.odcsim.Envelope
+
+/**
+ * A helper class for testing messages sent to an [ActorRef].
+ *
+ * @param T The shape of the messages the inbox accepts.
+ */
+interface TestInbox<T : Any> {
+    /**
+     * The actor reference of this inbox.
+     */
+    val ref: ActorRef<T>
+
+    /**
+     * A flag to indicate whether the inbox contains any messages.
+     */
+    val hasMessages: Boolean
+
+    /**
+     * Receive the oldest message from the inbox and remove it.
+     *
+     * @return The message that has been received.
+     * @throws NoSuchElementException if the inbox is empty.
+     */
+    fun receiveMessage(): T = receiveEnvelope().message
+
+    /**
+     * Receive the oldest message from the inbox and remove it.
+     *
+     * @return The envelope containing the message that has been received.
+     * @throws NoSuchElementException if the inbox is empty.
+     */
+    fun receiveEnvelope(): Envelope<T>
+
+    /**
+     * Receive all messages from the inbox and empty it.
+     *
+     * @return The list of messages in the inbox.
+     */
+    fun receiveAll(): List<Envelope<T>>
+
+    /**
+     * Clear all messages from the inbox.
+     */
+    fun clear()
+
+    /**
+     * Assert that the oldest message is equal to the [expected] message and remove
+     * it from the inbox.
+     *
+     * @param expected The expected message to be the oldest in the inbox.
+     */
+    fun expectMessage(expected: T)
+
+    /**
+     * Assert that the oldest message is equal to the [expected] message and remove
+     * it from the inbox.
+     *
+     * @param expected The expected message to be the oldest in the inbox.
+     * @param message The failure message to fail with.
+     */
+    fun expectMessage(expected: T, message: String)
+}
diff --git a/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/ActorContextStub.kt b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/ActorContextStub.kt
new file mode 100644
index 00000000..d11d47a3
--- /dev/null
+++ b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/ActorContextStub.kt
@@ -0,0 +1,89 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019 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 com.atlarge.odcsim.testkit.internal
+
+import com.atlarge.odcsim.ActorContext
+import com.atlarge.odcsim.ActorRef
+import com.atlarge.odcsim.ActorSystem
+import com.atlarge.odcsim.Behavior
+import com.atlarge.odcsim.Duration
+import com.atlarge.odcsim.Instant
+import com.atlarge.odcsim.internal.logging.LoggerImpl
+import org.slf4j.Logger
+
+/**
+ * A stubbed [ActorContext] implementation for synchronous behavior testing.
+ *
+ * @property owner The owner of this context.
+ */
+internal class ActorContextStub<T : Any>(private val owner: BehaviorTestKitImpl<T>) : ActorContext<T> {
+    /**
+     * The children of this context.
+     */
+    val children = HashMap<String, BehaviorTestKitImpl<*>>()
+
+    override val self: ActorRef<T>
+        get() = owner.ref
+
+    override val time: Instant
+        get() = owner.time
+
+    override val system: ActorSystem<*> by lazy {
+        ActorSystemStub(owner)
+    }
+
+    override val log: Logger by lazy {
+        LoggerImpl(this)
+    }
+
+    override fun <U : Any> send(ref: ActorRef<U>, msg: U, after: Duration) {
+        if (ref !is TestInboxImpl.ActorRefImpl) {
+            throw IllegalArgumentException("The referenced ActorRef is not part of the test kit")
+        }
+
+        ref.send(msg, after)
+    }
+
+    override fun <U : Any> spawn(behavior: Behavior<U>, name: String): ActorRef<U> {
+        val btk = BehaviorTestKitImpl(behavior, self.path.child(name))
+        children[name] = btk
+        return btk.ref
+    }
+
+    override fun stop(child: ActorRef<*>): Boolean {
+        if (child.path.parent != self.path) {
+            // This is not a child of this actor
+            return false
+        }
+        children -= child.path.name
+        return true
+    }
+
+    override fun sync(target: ActorRef<*>) {}
+
+    override fun unsync(target: ActorRef<*>) {}
+
+    override fun isSync(target: ActorRef<*>): Boolean = true
+}
diff --git a/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/ActorSystemStub.kt b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/ActorSystemStub.kt
new file mode 100644
index 00000000..f61c1d76
--- /dev/null
+++ b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/ActorSystemStub.kt
@@ -0,0 +1,52 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019 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 com.atlarge.odcsim.testkit.internal
+
+import com.atlarge.odcsim.ActorPath
+import com.atlarge.odcsim.ActorSystem
+import com.atlarge.odcsim.Duration
+import com.atlarge.odcsim.Instant
+
+/**
+ * A stubbed [ActorSystem] for synchronous testing of behavior.
+ *
+ * @property owner The owner of this actor system.
+ */
+internal class ActorSystemStub<T : Any>(private val owner: BehaviorTestKitImpl<T>) : ActorSystem<T> {
+    override val time: Instant
+        get() = owner.time
+
+    override val name: String
+        get() = owner.ref.path.name
+
+    override fun run(until: Duration) = throw IllegalStateException("Cannot run ActorSystem within actor")
+
+    override fun send(msg: T, after: Duration) = owner.context.send(owner.context.self, msg, after)
+
+    override fun terminate() {}
+
+    override val path: ActorPath
+        get() = owner.ref.path
+}
diff --git a/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/BehaviorTestKitImpl.kt b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/BehaviorTestKitImpl.kt
new file mode 100644
index 00000000..5b6669bb
--- /dev/null
+++ b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/BehaviorTestKitImpl.kt
@@ -0,0 +1,110 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019 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 com.atlarge.odcsim.testkit.internal
+
+import com.atlarge.odcsim.ActorPath
+import com.atlarge.odcsim.ActorRef
+import com.atlarge.odcsim.Behavior
+import com.atlarge.odcsim.Instant
+import com.atlarge.odcsim.internal.BehaviorInterpreter
+import com.atlarge.odcsim.testkit.BehaviorTestKit
+import com.atlarge.odcsim.testkit.TestInbox
+import java.util.UUID
+import kotlin.math.max
+
+/**
+ * Default implementation of the [BehaviorTestKit] interface.
+ *
+ * @param initialBehavior The initial behavior to initialize the actor of the test kit with.
+ * @param path The path to the actor.
+ */
+internal class BehaviorTestKitImpl<T : Any>(initialBehavior: Behavior<T>,
+                                            path: ActorPath) : BehaviorTestKit<T> {
+    /**
+     * The [BehaviorInterpreter] used to interpret incoming messages.
+     */
+    private val interpreter = BehaviorInterpreter(initialBehavior)
+
+    /**
+     * A flag to indicate whether the behavior was initially started.
+     */
+    private var isStarted: Boolean = false
+
+    override var time: Instant = .0
+        private set
+
+    override val inbox: TestInbox<T> = TestInboxImpl(this, path)
+
+    override val behavior: Behavior<T> get() = interpreter.behavior
+
+    override val ref: ActorRef<T> = inbox.ref
+
+    override val context: ActorContextStub<T> = ActorContextStub(this)
+
+    override fun run(msg: T): Boolean {
+        if (!isStarted) {
+            isStarted = true
+            interpreter.start(context)
+        }
+
+        return interpreter.interpretMessage(context, msg)
+    }
+
+    override fun runOne(): Boolean {
+        val (delivery, msg) = inbox.receiveEnvelope()
+        time = max(time, delivery)
+        return run(msg)
+    }
+
+    override fun runTo(time: Instant) {
+        while (inbox.hasMessages && this.time <= time) {
+            runOne()
+        }
+        this.time = time
+    }
+
+    override fun <U : Any> createInbox(): TestInbox<U> {
+        return TestInboxImpl(this, ref.path.child(UUID.randomUUID().toString()))
+    }
+
+    override fun <U : Any> childInbox(name: String): TestInbox<U> = childTestKit<U>(name).inbox
+
+    override fun <U : Any> childInbox(ref: ActorRef<U>): TestInbox<U> = childTestKit(ref).inbox
+
+    override fun <U : Any> childTestKit(name: String): BehaviorTestKit<U> {
+        @Suppress("UNCHECKED_CAST")
+        return context.children[ref.path.name] as BehaviorTestKitImpl<U>? ?: throw IllegalArgumentException("$ref is not a child of $this")
+    }
+
+    override fun <U : Any> childTestKit(ref: ActorRef<U>): BehaviorTestKit<U> {
+        val btk = childTestKit<U>(ref.path.name)
+
+        if (btk.ref != ref) {
+            throw IllegalArgumentException("$ref is not a child of $this")
+        }
+
+        return btk
+    }
+}
diff --git a/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/TestInboxImpl.kt b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/TestInboxImpl.kt
new file mode 100644
index 00000000..2f939eaa
--- /dev/null
+++ b/odcsim-testkit/src/main/kotlin/com/atlarge/odcsim/testkit/internal/TestInboxImpl.kt
@@ -0,0 +1,99 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019 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 com.atlarge.odcsim.testkit.internal
+
+import com.atlarge.odcsim.ActorPath
+import com.atlarge.odcsim.ActorRef
+import com.atlarge.odcsim.Envelope
+import com.atlarge.odcsim.Instant
+import com.atlarge.odcsim.testkit.TestInbox
+import org.junit.jupiter.api.Assertions.assertEquals
+import java.util.PriorityQueue
+
+/**
+ * A helper class for testing messages sent to an [ActorRef].
+ *
+ * @param owner The owner of the inbox.
+ * @param path The path to the test inbox.
+ * @param T The shape of the messages the inbox accepts.
+ */
+internal class TestInboxImpl<T : Any>(private val owner: BehaviorTestKitImpl<*>, path: ActorPath) : TestInbox<T> {
+    /**
+     * The queue of received messages.
+     */
+    private val inbox = PriorityQueue<Envelope<T>>()
+
+    /**
+     * The identifier for the next message to be scheduled.
+     */
+    private var nextId: Long = 0
+
+    override val ref: ActorRef<T> = ActorRefImpl(path)
+
+    override val hasMessages: Boolean
+        get() = inbox.isNotEmpty()
+
+    override fun receiveEnvelope(): Envelope<T> = inbox.remove()
+
+    override fun receiveAll(): List<Envelope<T>> = inbox.toList().also { inbox.clear() }
+
+    override fun clear() = inbox.clear()
+
+    override fun expectMessage(expected: T) = assertEquals(expected, receiveMessage())
+
+    override fun expectMessage(expected: T, message: String) = assertEquals(expected, receiveMessage(), message)
+
+    internal inner class ActorRefImpl(override val path: ActorPath) : ActorRef<T> {
+        /**
+         * Send the specified message to the actor this reference is pointing to after the specified delay.
+         *
+         * @param msg The message to send.
+         * @param after The delay before the message is received.
+         */
+        fun send(msg: T, after: Instant) {
+            inbox.add(EnvelopeImpl(nextId++, owner.time + after, msg))
+        }
+    }
+
+    /**
+     * A wrapper around a message that has been scheduled for processing.
+     *
+     * @property id The identifier of the message to keep the priority queue stable.
+     * @property time The point in time to deliver the message.
+     * @property message The message to wrap.
+     */
+    private inner class EnvelopeImpl(val id: Long,
+                                     override val time: Instant,
+                                     override val message: T) : Envelope<T> {
+        override fun compareTo(other: Envelope<*>): Int {
+            val cmp = super.compareTo(other)
+            return if (cmp == 0 && other is EnvelopeImpl)
+                id.compareTo(other.id)
+            else
+                cmp
+        }
+    }
+
+}