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Add OR-Set (Observed-Remove Set) (TheAlgorithms#4980)
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‎DIRECTORY.md

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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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* [ORSet](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/crdt/ORSet.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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* [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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* [ORSetTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/crdt/ORSetTest.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/ORSet.java

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package com.thealgorithms.datastructures.crdt;
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import java.util.HashSet;
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import java.util.Set;
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import java.util.UUID;
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/**
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* ORSet (Observed-Removed Set) is a state-based CRDT (Conflict-free Replicated Data Type)
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* that supports both addition and removal of elements. This particular implementation follows
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* the Add-Wins strategy, meaning that in case of conflicting add and remove operations,
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* the add operation takes precedence. The merge operation of two OR-Sets ensures that
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* elements added at any replica are eventually observed at all replicas. Removed elements,
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* once observed, are never reintroduced.
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* This OR-Set implementation provides methods for adding elements, removing elements,
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* checking for element existence, retrieving the set of elements, comparing with other OR-Sets,
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* and merging with another OR-Set to create a new OR-Set containing all unique elements
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* from both sets.
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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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public class ORSet<T> {
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private final Set<Pair<T>> elements;
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private final Set<Pair<T>> tombstones;
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/**
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* Constructs an empty OR-Set.
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*/
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public ORSet() {
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this.elements = new HashSet<>();
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this.tombstones = new HashSet<>();
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}
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/**
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* Checks if the set contains the specified element.
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*
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* @param element the element to check for
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* @return true if the set contains the element, false otherwise
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*/
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public boolean contains(T element) {
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return elements.stream().anyMatch(pair -> pair.getElement().equals(element));
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}
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/**
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* Retrieves the elements in the set.
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*
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* @return a set containing the elements
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*/
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public Set<T> elements() {
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Set<T> result = new HashSet<>();
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elements.forEach(pair -> result.add(pair.getElement()));
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return result;
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}
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/**
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* Adds the specified element to the set.
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*
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* @param element the element to add
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*/
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public void add(T element) {
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String n = prepare();
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effect(element, n);
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}
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/**
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* Removes the specified element from the set.
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*
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* @param element the element to remove
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*/
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public void remove(T element) {
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Set<Pair<T>> pairsToRemove = prepare(element);
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effect(pairsToRemove);
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}
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/**
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* Collect all pairs with the specified element.
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*
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* @param element the element to collect pairs for
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* @return a set of pairs with the specified element to be removed
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*/
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private Set<Pair<T>> prepare(T element) {
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Set<Pair<T>> pairsToRemove = new HashSet<>();
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for (Pair<T> pair : elements) {
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if (pair.getElement().equals(element)) {
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pairsToRemove.add(pair);
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}
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}
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return pairsToRemove;
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}
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/**
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* Generates a unique tag for the element.
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*
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* @return the unique tag
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*/
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private String prepare() {
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return generateUniqueTag();
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}
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/**
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* Adds the element with the specified unique tag to the set.
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*
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* @param element the element to add
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* @param n the unique tag associated with the element
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*/
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private void effect(T element, String n) {
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Pair<T> pair = new Pair<>(element, n);
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elements.add(pair);
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elements.removeAll(tombstones);
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}
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/**
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* Removes the specified pairs from the set.
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*
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* @param pairsToRemove the pairs to remove
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*/
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private void effect(Set<Pair<T>> pairsToRemove) {
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elements.removeAll(pairsToRemove);
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tombstones.addAll(pairsToRemove);
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}
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/**
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* Generates a unique tag.
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*
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* @return the unique tag
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*/
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private String generateUniqueTag() {
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return UUID.randomUUID().toString();
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}
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/**
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* Compares this Add-Wins OR-Set with another OR-Set to check if elements and tombstones are a subset.
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*
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* @param other the other OR-Set to compare
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* @return true if the sets are subset, false otherwise
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*/
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public boolean compare(ORSet<T> other) {
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Set<Pair<T>> union = new HashSet<>(elements);
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union.addAll(tombstones);
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Set<Pair<T>> otherUnion = new HashSet<>(other.elements);
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otherUnion.addAll(other.tombstones);
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return otherUnion.containsAll(union) && other.tombstones.containsAll(tombstones);
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}
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/**
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* Merges this Add-Wins OR-Set with another OR-Set.
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*
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* @param other the other OR-Set to merge
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*/
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public void merge(ORSet<T> other) {
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elements.removeAll(other.tombstones);
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other.elements.removeAll(tombstones);
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elements.addAll(other.elements);
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tombstones.addAll(other.tombstones);
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}
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/**
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* Represents a pair containing an element and a unique tag.
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*
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* @param <T> the type of the element in the pair
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*/
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public static class Pair<T> {
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private final T element;
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private final String uniqueTag;
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/**
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* Constructs a pair with the specified element and unique tag.
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*
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* @param element the element in the pair
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* @param uniqueTag the unique tag associated with the element
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*/
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public Pair(T element, String uniqueTag) {
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this.element = element;
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this.uniqueTag = uniqueTag;
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}
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/**
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* Gets the element from the pair.
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*
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* @return the element
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*/
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public T getElement() {
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return element;
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}
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}
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}

‎src/test/java/com/thealgorithms/datastructures/crdt/ORSetTest.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 java.util.Set;
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import org.junit.jupiter.api.Test;
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class ORSetTest {
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@Test
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void testContains() {
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ORSet<String> orSet = new ORSet<>();
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orSet.add("A");
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assertTrue(orSet.contains("A"));
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}
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@Test
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void testAdd() {
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ORSet<String> orSet = new ORSet<>();
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orSet.add("A");
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assertTrue(orSet.contains("A"));
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}
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@Test
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void testRemove() {
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ORSet<String> orSet = new ORSet<>();
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orSet.add("A");
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orSet.add("A");
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orSet.remove("A");
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assertFalse(orSet.contains("A"));
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}
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@Test
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void testElements() {
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ORSet<String> orSet = new ORSet<>();
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orSet.add("A");
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orSet.add("B");
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assertEquals(Set.of("A", "B"), orSet.elements());
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}
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@Test
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void testCompareEqualSets() {
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ORSet<String> orSet1 = new ORSet<>();
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ORSet<String> orSet2 = new ORSet<>();
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orSet1.add("A");
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orSet2.add("A");
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orSet2.add("B");
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orSet2.add("C");
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orSet2.remove("C");
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orSet1.merge(orSet2);
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orSet2.merge(orSet1);
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orSet2.remove("B");
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assertTrue(orSet1.compare(orSet2));
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}
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@Test
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void testCompareDifferentSets() {
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ORSet<String> orSet1 = new ORSet<>();
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ORSet<String> orSet2 = new ORSet<>();
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orSet1.add("A");
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orSet2.add("B");
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assertFalse(orSet1.compare(orSet2));
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}
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@Test
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void testMerge() {
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ORSet<String> orSet1 = new ORSet<>();
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ORSet<String> orSet2 = new ORSet<>();
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orSet1.add("A");
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orSet1.add("A");
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orSet1.add("B");
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orSet1.remove("B");
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orSet2.add("B");
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orSet2.add("C");
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orSet2.remove("C");
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orSet1.merge(orSet2);
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assertTrue(orSet1.contains("A"));
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assertTrue(orSet1.contains("B"));
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}
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}

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