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HiveMQ MQTT Client is an MQTT 5.0 and MQTT 3.1.1 compatible and feature-rich high-performance Java client library with different API flavours and backpressure support
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/*
* Copyright 2018-present HiveMQ and the HiveMQ Community
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.hivemq.client.internal.mqtt.handler.publish.outgoing;
import com.hivemq.client.internal.mqtt.datatypes.MqttTopicImpl;
import com.hivemq.client.internal.util.collections.Index;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;
import static com.hivemq.client.internal.mqtt.message.publish.MqttStatefulPublish.*;
/**
* @author Silvio Giebl
*/
public class MqttTopicAliasAutoMapping implements MqttTopicAliasMapping {
private static final byte OVERSIZE = 4;
private static final byte RETAIN = 8;
private static final byte OVERWRITE_COST_MIN = 2;
private static final byte OVERWRITE_COST_MAX = 126;
private static final byte OVERWRITE_COST_INC = 2;
private static final Index.@NotNull Spec INDEX_SPEC = new Index.Spec<>(entry -> entry.topic);
private final int topicAliasMaximum;
private final @NotNull Index map = new Index<>(INDEX_SPEC);
private @Nullable Entry lowest; // entry with lowest priority
private long accessCounter; // strictly incremented
private byte overwriteTries;
private byte overwriteCost = OVERWRITE_COST_MIN;
private byte fullOverwriteTries;
private byte fullOverwriteCost = OVERWRITE_COST_MIN;
public MqttTopicAliasAutoMapping(final int topicAliasMaximum) {
this.topicAliasMaximum = topicAliasMaximum;
}
@Override
public int getTopicAliasMaximum() {
return topicAliasMaximum;
}
@Override
public int onPublish(final @NotNull MqttTopicImpl topic) {
final long accessCounter = ++this.accessCounter;
final String topicString = topic.toString();
final Entry entry = map.get(topicString);
if (entry != null) { // entry already present
entry.access(accessCounter);
if (entry.topicAlias != DEFAULT_NO_TOPIC_ALIAS) {
if (overwriteCost > OVERWRITE_COST_MIN) {
overwriteCost--;
}
if (fullOverwriteCost > OVERWRITE_COST_MIN) {
fullOverwriteCost--;
}
if (entry.lower != null) {
if (entry.lower.topicAlias == DEFAULT_NO_TOPIC_ALIAS) {
overwriteTries = 0;
}
} else {
fullOverwriteTries = 0;
}
}
swapNewer(entry, accessCounter);
return entry.topicAlias; // topic alias is 0 if entry is part of oversize
}
final Entry newEntry = new Entry(topicString, accessCounter);
if (map.size() < topicAliasMaximum + OVERSIZE) {
if (map.size() < topicAliasMaximum) { // still unused topic aliases available
newEntry.setNewTopicAlias(map.size() + 1);
}
map.put(newEntry);
if (lowest != null) {
newEntry.higher = lowest;
lowest.lower = newEntry;
}
} else {
final Entry lowest = this.lowest;
assert lowest != null;
if (newEntry.priority(accessCounter) <= lowest.priority(accessCounter)) {
return DEFAULT_NO_TOPIC_ALIAS;
}
if (++fullOverwriteTries < fullOverwriteCost) {
return DEFAULT_NO_TOPIC_ALIAS;
}
fullOverwriteTries = 0;
if (fullOverwriteCost < OVERWRITE_COST_MAX) {
fullOverwriteCost += (byte) Math.min(OVERWRITE_COST_INC, OVERWRITE_COST_MAX - fullOverwriteCost);
}
if (lowest.topicAlias != DEFAULT_NO_TOPIC_ALIAS) {
newEntry.setNewTopicAlias(lowest.topicAlias);
}
map.remove(lowest.topic);
map.put(newEntry);
final Entry higher = lowest.higher;
newEntry.higher = higher;
if (higher != null) {
higher.lower = newEntry;
}
}
lowest = newEntry;
swapNewer(newEntry, accessCounter);
return newEntry.topicAlias;
}
private void swapNewer(final @NotNull Entry entry, final long accessCounter) {
Entry higher = entry.higher;
if (entry.higher == null) {
return;
}
Entry lower = entry.lower;
final long priority = entry.priority(accessCounter);
while (true) {
final long newerPriority = higher.priority(accessCounter);
if (newerPriority >= priority) {
break;
}
if ((entry.topicAlias == DEFAULT_NO_TOPIC_ALIAS) && (higher.topicAlias != DEFAULT_NO_TOPIC_ALIAS)) {
if (++overwriteTries < overwriteCost) {
break; // do not swap immediately if entry would overwrite the topic alias of the next entry
}
overwriteTries = 0;
if (overwriteCost < OVERWRITE_COST_MAX) {
overwriteCost += (byte) Math.min(OVERWRITE_COST_INC, OVERWRITE_COST_MAX - overwriteCost);
}
entry.setNewTopicAlias(higher.topicAlias);
higher.topicAlias = DEFAULT_NO_TOPIC_ALIAS;
}
final Entry higherHigher = higher.higher;
higher.higher = entry;
entry.lower = higher;
if (lower == null) {
higher.lower = null;
lowest = higher;
} else {
lower.higher = higher;
higher.lower = lower;
}
if (higherHigher == null) {
entry.higher = null;
break;
} else {
entry.higher = higherHigher;
higherHigher.lower = entry;
}
lower = higher;
higher = higherHigher;
}
}
@Override
public @NotNull String toString() {
final StringBuilder builder = new StringBuilder("{");
Entry entry = lowest;
while (entry != null) {
builder.append("\n ").append(entry);
entry = entry.higher;
}
return builder.append("\n}").toString();
}
static class Entry {
final @NotNull String topic;
int topicAlias;
private long used; // number of accesses, decays over time
private long access; // stamp when the entry was last accessed
@Nullable Entry higher; // entry with the next higher priority
@Nullable Entry lower; // entry with the next lower priority
Entry(final @NotNull String topic, final long accessCounter) {
this.topic = topic;
topicAlias = DEFAULT_NO_TOPIC_ALIAS;
used = 1;
access = accessCounter;
}
void setNewTopicAlias(final int topicAlias) {
this.topicAlias = topicAlias | TOPIC_ALIAS_FLAG_NEW;
}
void access(final long accessCounter) {
topicAlias &= TOPIC_ALIAS_FLAG; // clear NEW_TOPIC_ALIAS bit
used = priority(accessCounter) + 1;
access = accessCounter;
}
long priority(final long accessCounter) {
final long decay = Math.max(accessCounter - access - RETAIN, 0);
return Math.max(used - decay, 0);
}
@Override
public @NotNull String toString() {
return '{' + "topic='" + topic + '\'' +
((topicAlias == DEFAULT_NO_TOPIC_ALIAS) ? "" : (", alias=" + (topicAlias & TOPIC_ALIAS_FLAG))) +
(((topicAlias & TOPIC_ALIAS_FLAG_NEW) == 0) ? "" : ", new ") + ", used = " + used + ", access = " +
access + '}';
}
}
}