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diff --git a/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/network/mailbox/timeless/DefaultMailbox.java b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/network/mailbox/timeless/DefaultMailbox.java
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index 00000000..baf7270f
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+++ b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/network/mailbox/timeless/DefaultMailbox.java
@@ -0,0 +1,163 @@
+/*******************************************************************************
+ * Copyright (c) 2010-2016, Tamas Szabo, Istvan Rath and Daniel Varro
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-v20.html.
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package tools.refinery.viatra.runtime.rete.network.mailbox.timeless;
+
+import java.util.Map;
+
+import tools.refinery.viatra.runtime.matchers.tuple.Tuple;
+import tools.refinery.viatra.runtime.matchers.util.CollectionsFactory;
+import tools.refinery.viatra.runtime.matchers.util.Direction;
+import tools.refinery.viatra.runtime.rete.network.Receiver;
+import tools.refinery.viatra.runtime.rete.network.ReteContainer;
+import tools.refinery.viatra.runtime.rete.network.communication.CommunicationGroup;
+import tools.refinery.viatra.runtime.rete.network.communication.MessageSelector;
+import tools.refinery.viatra.runtime.rete.network.communication.PhasedSelector;
+import tools.refinery.viatra.runtime.rete.network.communication.Timestamp;
+import tools.refinery.viatra.runtime.rete.network.mailbox.AdaptableMailbox;
+import tools.refinery.viatra.runtime.rete.network.mailbox.Mailbox;
+
+/**
+ * Default mailbox implementation.
+ * <p>
+ * Usually, the mailbox performs counting of messages so that they can cancel each other out. However, if marked as a
+ * fall-through mailbox, than update messages are delivered directly to the receiver node to reduce overhead.
+ *
+ * @author Tamas Szabo
+ * @since 2.0
+ */
+public class DefaultMailbox implements AdaptableMailbox {
+
+    private static int SIZE_TRESHOLD = 127;
+
+    protected Map<Tuple, Integer> queue;
+    protected Map<Tuple, Integer> buffer;
+    protected final Receiver receiver;
+    protected final ReteContainer container;
+    protected boolean delivering;
+    protected Mailbox adapter;
+    protected CommunicationGroup group;
+
+    public DefaultMailbox(final Receiver receiver, final ReteContainer container) {
+        this.receiver = receiver;
+        this.container = container;
+        this.queue = CollectionsFactory.createMap();
+        this.buffer = CollectionsFactory.createMap();
+        this.adapter = this;
+    }
+
+    protected Map<Tuple, Integer> getActiveQueue() {
+        if (this.delivering) {
+            return this.buffer;
+        } else {
+            return this.queue;
+        }
+    }
+
+    @Override
+    public Mailbox getAdapter() {
+        return this.adapter;
+    }
+
+    @Override
+    public void setAdapter(final Mailbox adapter) {
+        this.adapter = adapter;
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return getActiveQueue().isEmpty();
+    }
+
+    @Override
+    public void postMessage(final Direction direction, final Tuple update, final Timestamp timestamp) {
+        final Map<Tuple, Integer> activeQueue = getActiveQueue();
+        final boolean wasEmpty = activeQueue.isEmpty();
+
+        boolean significantChange = false;
+        Integer count = activeQueue.get(update);
+        if (count == null) {
+            count = 0;
+            significantChange = true;
+        }
+
+        if (direction == Direction.DELETE) {
+            count--;
+        } else {
+            count++;
+        }
+
+        if (count == 0) {
+            activeQueue.remove(update);
+            significantChange = true;
+        } else {
+            activeQueue.put(update, count);
+        }
+
+        if (significantChange) {
+            final Mailbox targetMailbox = this.adapter;
+            final CommunicationGroup targetGroup = this.adapter.getCurrentGroup();
+
+            if (wasEmpty) {
+                targetGroup.notifyHasMessage(targetMailbox, PhasedSelector.DEFAULT);
+            } else if (activeQueue.isEmpty()) {
+                targetGroup.notifyLostAllMessages(targetMailbox, PhasedSelector.DEFAULT);
+            }
+        }
+    }
+
+    @Override
+    public void deliverAll(final MessageSelector kind) {
+        if (kind == PhasedSelector.DEFAULT) {
+            // use the buffer during delivering so that there is a clear
+            // separation between the stages
+            this.delivering = true;
+            this.receiver.batchUpdate(this.queue.entrySet(), Timestamp.ZERO);
+            this.delivering = false;
+
+            if (queue.size() > SIZE_TRESHOLD) {
+                this.queue = this.buffer;
+                this.buffer = CollectionsFactory.createMap();
+            } else {
+                this.queue.clear();
+                final Map<Tuple, Integer> tmpQueue = this.queue;
+                this.queue = this.buffer;
+                this.buffer = tmpQueue;
+            }
+        } else {
+            throw new IllegalArgumentException("Unsupported message kind " + kind);
+        }
+    }
+
+    @Override
+    public String toString() {
+        return "D_MBOX (" + this.receiver + ") " + this.getActiveQueue();
+    }
+
+    @Override
+    public Receiver getReceiver() {
+        return this.receiver;
+    }
+
+    @Override
+    public void clear() {
+        this.queue.clear();
+        this.buffer.clear();
+    }
+
+    @Override
+    public CommunicationGroup getCurrentGroup() {
+        return this.group;
+    }
+
+    @Override
+    public void setCurrentGroup(final CommunicationGroup group) {
+        this.group = group;
+    }
+
+}