refactor(#18): Provide access to latest sender

This change adds a `sender` property to the `Context` interface to
provide processes access to the sender of the latest received message.

Please note that methods like `hold()` and `interrupt()` may change the value
of this property.

2 files changed, 22 insertions, 8 deletions

diff --git a/opendc-core/src/main/kotlin/com/atlarge/opendc/simulator/Context.kt b/opendc-core/src/main/kotlin/com/atlarge/opendc/simulator/Context.kt
index c89bdb59..13170256 100644
--- a/opendc-core/src/main/kotlin/com/atlarge/opendc/simulator/Context.kt
+++ b/opendc-core/src/main/kotlin/com/atlarge/opendc/simulator/Context.kt
@@ -51,6 +51,14 @@ interface Context<S, out M> {
     val delta: Duration
 
     /**
+     * The sender of the last received message or `null` in case the process has not received any messages yet.
+     *
+     * Note that this property is only guaranteed to be correct when accessing after a single suspending call. Methods
+     * like `hold()` and `interrupt()` may change the value of this property.
+     */
+    val sender: Entity<*, *>?
+
+    /**
      * The observable state of the entity bound to this scope.
      */
     var state: S
diff --git a/opendc-kernel-omega/src/main/kotlin/com/atlarge/opendc/omega/OmegaSimulation.kt b/opendc-kernel-omega/src/main/kotlin/com/atlarge/opendc/omega/OmegaSimulation.kt
index 9c6d4ab3..bd3f4529 100644
--- a/opendc-kernel-omega/src/main/kotlin/com/atlarge/opendc/omega/OmegaSimulation.kt
+++ b/opendc-kernel-omega/src/main/kotlin/com/atlarge/opendc/omega/OmegaSimulation.kt
@@ -250,11 +250,6 @@ internal class OmegaSimulation<M>(bootstrap: Bootstrap<M>) : Simulation<M>, Boot
             get() = this@OmegaSimulation.model
 
         /**
-         * The state of the entity.
-         */
-        override var state: S = process.initialState
-
-        /**
          * The current point in simulation time.
          */
         override val time: Instant
@@ -268,9 +263,9 @@ internal class OmegaSimulation<M>(bootstrap: Bootstrap<M>) : Simulation<M>, Boot
             get() = maxOf(time - last, 0)
 
         /**
-         * The [CoroutineContext] for a [Context].
+         * The state of the entity.
          */
-        override val context: CoroutineContext = EmptyCoroutineContext
+        override var state: S = process.initialState
 
         /**
          * The observable state of an [Entity] within the simulation is provided by the context of the simulation.
@@ -279,6 +274,16 @@ internal class OmegaSimulation<M>(bootstrap: Bootstrap<M>) : Simulation<M>, Boot
             get() = context?.state ?: initialState
 
         /**
+         * The sender of the last received message or `null` in case the process has not received any messages yet.
+         */
+        override var sender: Entity<*, *>? = null
+
+        /**
+         * The [CoroutineContext] for a [Context].
+         */
+        override val context: CoroutineContext = EmptyCoroutineContext
+
+        /**
          * The continuation to resume the execution of the process.
          */
         lateinit var continuation: Continuation<Envelope<*>>
@@ -359,7 +364,8 @@ internal class OmegaSimulation<M>(bootstrap: Bootstrap<M>) : Simulation<M>, Boot
          *
          * @return The envelope containing the message.
          */
-        suspend fun receiveEnvelope(): Envelope<*> = suspendCoroutine { continuation = it }
+        suspend fun receiveEnvelope() = suspendCoroutine<Envelope<*>> { continuation = it }
+            .also { sender = it.sender }
 
         // Completion continuation implementation
         /**