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Add LWW Element Set (Last Write Wins Element Set) (TheAlgorithms#4979)
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‎DIRECTORY.md

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* crdt
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* [GCounter](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/crdt/GCounter.java)
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* [GSet](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/crdt/GSet.java)
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* [LWWElementSet](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/crdt/LWWElementSet.java)
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* [PNCounter](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/crdt/PNCounter.java)
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* [TwoPSet](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/crdt/TwoPSet.java)
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* disjointsetunion
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* crdt
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* [GCounterTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/crdt/GCounterTest.java)
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* [GSetTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/crdt/GSetTest.java)
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* [LWWElementSetTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/crdt/LWWElementSetTest.java)
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* [PNCounterTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/crdt/PNCounterTest.java)
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* [TwoPSetTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/crdt/TwoPSetTest.java)
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* disjointsetunion

‎src/main/java/com/thealgorithms/datastructures/crdt/LWWElementSet.java

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package com.thealgorithms.datastructures.crdt;
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import java.util.HashMap;
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import java.util.Map;
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/**
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* Last-Write-Wins Element Set (LWWElementSet) is a state-based CRDT (Conflict-free Replicated Data Type)
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* designed for managing sets in a distributed and concurrent environment. It supports the addition and removal
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* of elements, using timestamps to determine the order of operations. The set is split into two subsets:
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* the add set for elements to be added and the remove set for elements to be removed.
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*
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* @author itakurah (Niklas Hoefflin) (https://github.com/itakurah)
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* @see <a href="https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type">Conflict-free_replicated_data_type</a>
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* @see <a href="https://github.com/itakurah">itakurah (Niklas Hoefflin)</a>
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*/
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class Element {
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String key;
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int timestamp;
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Bias bias;
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/**
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* Constructs a new Element with the specified key, timestamp and bias.
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*
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* @param key The key of the element.
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* @param timestamp The timestamp associated with the element.
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* @param bias The bias of the element (ADDS or REMOVALS).
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*/
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public Element(String key, int timestamp, Bias bias) {
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this.key = key;
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this.timestamp = timestamp;
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this.bias = bias;
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}
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}
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enum Bias {
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/**
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* ADDS bias for the add set.
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* REMOVALS bias for the remove set.
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*/
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ADDS,
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REMOVALS
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}
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class LWWElementSet {
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private final Map<String, Element> addSet;
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private final Map<String, Element> removeSet;
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/**
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* Constructs an empty LWWElementSet.
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*/
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public LWWElementSet() {
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this.addSet = new HashMap<>();
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this.removeSet = new HashMap<>();
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}
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/**
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* Adds an element to the addSet.
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*
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* @param e The element to be added.
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*/
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public void add(Element e) {
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addSet.put(e.key, e);
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}
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/**
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* Removes an element from the removeSet.
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*
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* @param e The element to be removed.
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*/
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public void remove(Element e) {
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if (lookup(e)) {
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removeSet.put(e.key, e);
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}
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}
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/**
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* Checks if an element is in the LWWElementSet by comparing timestamps in the addSet and removeSet.
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*
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* @param e The element to be checked.
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* @return True if the element is present, false otherwise.
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*/
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public boolean lookup(Element e) {
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Element inAddSet = addSet.get(e.key);
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Element inRemoveSet = removeSet.get(e.key);
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return (inAddSet != null && (inRemoveSet == null || inAddSet.timestamp > inRemoveSet.timestamp));
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}
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/**
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* Compares the LWWElementSet with another LWWElementSet to check if addSet and removeSet are a subset.
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*
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* @param other The LWWElementSet to compare.
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* @return True if the set is subset, false otherwise.
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*/
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public boolean compare(LWWElementSet other) {
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return other.addSet.keySet().containsAll(addSet.keySet()) && other.removeSet.keySet().containsAll(removeSet.keySet());
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}
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/**
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* Merges another LWWElementSet into this set by resolving conflicts based on timestamps.
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*
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* @param other The LWWElementSet to merge.
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*/
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public void merge(LWWElementSet other) {
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for (Element e : other.addSet.values()) {
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if (!addSet.containsKey(e.key) || compareTimestamps(addSet.get(e.key), e)) {
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addSet.put(e.key, e);
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}
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}
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for (Element e : other.removeSet.values()) {
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if (!removeSet.containsKey(e.key) || compareTimestamps(removeSet.get(e.key), e)) {
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removeSet.put(e.key, e);
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}
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}
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}
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/**
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* Compares timestamps of two elements based on their bias (ADDS or REMOVALS).
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*
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* @param e The first element.
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* @param other The second element.
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* @return True if the first element's timestamp is greater or the bias is ADDS and timestamps are equal.
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*/
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public boolean compareTimestamps(Element e, Element other) {
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if (!e.bias.equals(other.bias)) {
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throw new IllegalArgumentException("Invalid bias value");
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}
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Bias bias = e.bias;
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int timestampComparison = Integer.compare(e.timestamp, other.timestamp);
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if (timestampComparison == 0) {
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return !bias.equals(Bias.ADDS);
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}
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return timestampComparison < 0;
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}
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}

‎src/test/java/com/thealgorithms/datastructures/crdt/LWWElementSetTest.java

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package com.thealgorithms.datastructures.crdt;
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import static org.junit.jupiter.api.Assertions.*;
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import org.junit.jupiter.api.BeforeEach;
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import org.junit.jupiter.api.Test;
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class LWWElementSetTest {
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private LWWElementSet set;
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private final Bias bias = Bias.ADDS;
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@BeforeEach
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void setUp() {
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set = new LWWElementSet();
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}
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@Test
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void testAdd() {
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Element element = new Element("key1", 1, bias);
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set.add(element);
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assertTrue(set.lookup(element));
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}
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@Test
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void testRemove() {
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Element element = new Element("key1", 1, bias);
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set.add(element);
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set.remove(element);
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assertFalse(set.lookup(element));
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}
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@Test
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void testRemoveNonexistentElement() {
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Element element = new Element("key1", 1, bias);
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set.remove(element);
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assertFalse(set.lookup(element));
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}
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@Test
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void testLookupNonexistentElement() {
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Element element = new Element("key1", 1, bias);
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assertFalse(set.lookup(element));
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}
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@Test
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void testCompareEqualSets() {
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LWWElementSet otherSet = new LWWElementSet();
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Element element = new Element("key1", 1, bias);
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set.add(element);
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otherSet.add(element);
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assertTrue(set.compare(otherSet));
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otherSet.add(new Element("key2", 2, bias));
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assertTrue(set.compare(otherSet));
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}
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@Test
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void testCompareDifferentSets() {
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LWWElementSet otherSet = new LWWElementSet();
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Element element1 = new Element("key1", 1, bias);
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Element element2 = new Element("key2", 2, bias);
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set.add(element1);
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otherSet.add(element2);
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assertFalse(set.compare(otherSet));
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}
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@Test
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void testMerge() {
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LWWElementSet otherSet = new LWWElementSet();
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Element element1 = new Element("key1", 1, bias);
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Element element2 = new Element("key2", 2, bias);
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set.add(element1);
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otherSet.add(element2);
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set.merge(otherSet);
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assertTrue(set.lookup(element1));
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assertTrue(set.lookup(element2));
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}
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@Test
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void testCompareTimestampsEqualTimestamps() {
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LWWElementSet lwwElementSet = new LWWElementSet();
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Element e1 = new Element("key1", 10, Bias.REMOVALS);
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Element e2 = new Element("key1", 10, Bias.REMOVALS);
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assertTrue(lwwElementSet.compareTimestamps(e1, e2));
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e1 = new Element("key1", 10, Bias.ADDS);
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e2 = new Element("key1", 10, Bias.ADDS);
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assertFalse(lwwElementSet.compareTimestamps(e1, e2));
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}
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}

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