From 15899c88d29c039149f701e7f0d538a49a436599 Mon Sep 17 00:00:00 2001
From: Fabian Mastenbroek <mail.fabianm@gmail.com>
Date: Wed, 29 Sep 2021 10:33:36 +0200
Subject: refactor(simulator): Lazily push changes to resource context

This change updates the SimResourceContextImpl to lazily push changes to
the resource context instead of applying them directly. The change is
picked up after the resource is updated again.
---
 .../simulator/resources/SimResourceTransformer.kt  |  4 +-
 .../resources/impl/SimResourceContextImpl.kt       | 58 ++++++++++++----------
 2 files changed, 34 insertions(+), 28 deletions(-)

diff --git a/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt b/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt
index f12ef9f1..a317f832 100644
--- a/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt
+++ b/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt
@@ -152,9 +152,7 @@ public class SimResourceTransformer(
         updateCounters(ctx, delta)
 
         return if (delegate != null) {
-            val duration = transform(ctx, delegate.onNext(this.ctx, now, delta))
-            _limit = ctx.demand
-            duration
+            transform(ctx, delegate.onNext(this.ctx, now, delta))
         } else {
             Long.MAX_VALUE
         }
diff --git a/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/impl/SimResourceContextImpl.kt b/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/impl/SimResourceContextImpl.kt
index 1ac38946..78d79434 100644
--- a/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/impl/SimResourceContextImpl.kt
+++ b/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/impl/SimResourceContextImpl.kt
@@ -69,8 +69,8 @@ internal class SimResourceContextImpl(
      * The current processing speed of the resource.
      */
     override val speed: Double
-        get() = _speed
-    private var _speed = 0.0
+        get() = _rate
+    private var _rate = 0.0
 
     /**
      * The current resource processing demand.
@@ -81,10 +81,10 @@ internal class SimResourceContextImpl(
     /**
      * The current state of the resource context.
      */
-    private var _timestamp: Long = Long.MIN_VALUE
     private var _limit: Double = 0.0
-    private var _duration: Long = Long.MAX_VALUE
-    private var _deadline: Long = Long.MAX_VALUE
+    private var _activeLimit: Double = 0.0
+    private var _deadline: Long = Long.MIN_VALUE
+    private var _lastUpdate: Long = Long.MIN_VALUE
 
     /**
      * A flag to indicate that an update is active.
@@ -152,8 +152,13 @@ internal class SimResourceContextImpl(
             return
         }
 
-        _speed = min(capacity, rate)
         _limit = rate
+
+        // Invalidate only if the active limit is change and no update is active
+        // If an update is active, it will already get picked up at the end of the update
+        if (_activeLimit != rate && !_updateActive) {
+            invalidate()
+        }
     }
 
     /**
@@ -173,45 +178,47 @@ internal class SimResourceContextImpl(
             return
         }
 
+        val lastUpdate = _lastUpdate
+        _lastUpdate = now
         _updateActive = true
 
         val reachedDeadline = _deadline <= now
-        val delta = max(0, now - _timestamp)
+        val delta = max(0, now - lastUpdate)
 
         try {
             // Update the resource counters only if there is some progress
-            if (now > _timestamp) {
-                logic.onUpdate(this, delta, _limit, reachedDeadline)
+            if (now > lastUpdate) {
+                logic.onUpdate(this, delta, _activeLimit, reachedDeadline)
             }
 
             val duration = consumer.onNext(this, now, delta)
+            val newDeadline = if (duration != Long.MAX_VALUE) now + duration else duration
 
             // Reset update flags
             _flag = 0
 
+            // Check whether the state has changed after [consumer.onNext]
             when (_state) {
                 SimResourceState.Active -> {
-                    val target = logic.onConsume(this, now, _limit, duration)
-
-                    _speed = min(capacity, _limit)
-                    _duration = duration
-                    _deadline = target
+                    logic.onConsume(this, now, _limit, duration)
 
-                    scheduleUpdate(now, target)
+                    // Schedule an update at the new deadline
+                    scheduleUpdate(now, newDeadline)
                 }
-                SimResourceState.Pending ->
-                    if (oldState != SimResourceState.Pending) {
-                        throw IllegalStateException("Illegal transition to pending state")
-                    }
-                SimResourceState.Stopped ->
-                    if (oldState != SimResourceState.Stopped) {
-                        doStop()
-                    }
+                SimResourceState.Stopped -> doStop()
+                SimResourceState.Pending -> throw IllegalStateException("Illegal transition to pending state")
             }
+
+            // Note: pending limit might be changed by [logic.onConsume], so re-fetch the value
+            val newLimit = _limit
+
+            // Flush the changes to the flow
+            _activeLimit = newLimit
+            _deadline = newDeadline
+            _rate = min(capacity, newLimit)
         } catch (cause: Throwable) {
             doFail(cause)
         } finally {
-            _timestamp = now
             _updateActive = false
         }
     }
@@ -246,7 +253,7 @@ internal class SimResourceContextImpl(
         }
     }
 
-    override fun toString(): String = "SimResourceContextImpl[capacity=$capacity]"
+    override fun toString(): String = "SimResourceContextImpl[capacity=$capacity,rate=$_rate]"
 
     /**
      * Stop the resource context.
@@ -259,6 +266,7 @@ internal class SimResourceContextImpl(
             doFail(cause)
         } finally {
             _deadline = Long.MAX_VALUE
+            _limit = 0.0
         }
     }
 
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