Add LWW Element Set (Last Write Wins Element Set) (#4979)

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

@@ -0,0 +1,138 @@
+package com.thealgorithms.datastructures.crdt;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Last-Write-Wins Element Set (LWWElementSet) is a state-based CRDT (Conflict-free Replicated Data Type)
+ * designed for managing sets in a distributed and concurrent environment. It supports the addition and removal
+ * of elements, using timestamps to determine the order of operations. The set is split into two subsets:
+ * the add set for elements to be added and the remove set for elements to be removed.
+ *
+ * @author itakurah (Niklas Hoefflin) (https://github.com/itakurah)
+ * @see <a href="https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type">Conflict-free_replicated_data_type</a>
+ * @see <a href="https://github.com/itakurah">itakurah (Niklas Hoefflin)</a>
+ */
+
+class Element {
+    String key;
+    int timestamp;
+    Bias bias;
+
+    /**
+     * Constructs a new Element with the specified key, timestamp and bias.
+     *
+     * @param key       The key of the element.
+     * @param timestamp The timestamp associated with the element.
+     * @param bias      The bias of the element (ADDS or REMOVALS).
+     */
+    public Element(String key, int timestamp, Bias bias) {
+        this.key = key;
+        this.timestamp = timestamp;
+        this.bias = bias;
+    }
+}
+
+enum Bias {
+    /**
+     * ADDS bias for the add set.
+     * REMOVALS bias for the remove set.
+     */
+    ADDS,
+    REMOVALS
+}
+
+class LWWElementSet {
+    private final Map<String, Element> addSet;
+    private final Map<String, Element> removeSet;
+
+    /**
+     * Constructs an empty LWWElementSet.
+     */
+    public LWWElementSet() {
+        this.addSet = new HashMap<>();
+        this.removeSet = new HashMap<>();
+    }
+
+    /**
+     * Adds an element to the addSet.
+     *
+     * @param e The element to be added.
+     */
+    public void add(Element e) {
+        addSet.put(e.key, e);
+    }
+
+    /**
+     * Removes an element from the removeSet.
+     *
+     * @param e The element to be removed.
+     */
+    public void remove(Element e) {
+        if (lookup(e)) {
+            removeSet.put(e.key, e);
+        }
+    }
+
+    /**
+     * Checks if an element is in the LWWElementSet by comparing timestamps in the addSet and removeSet.
+     *
+     * @param e The element to be checked.
+     * @return True if the element is present, false otherwise.
+     */
+    public boolean lookup(Element e) {
+        Element inAddSet = addSet.get(e.key);
+        Element inRemoveSet = removeSet.get(e.key);
+
+        return (inAddSet != null && (inRemoveSet == null || inAddSet.timestamp > inRemoveSet.timestamp));
+    }
+
+    /**
+     * Compares the LWWElementSet with another LWWElementSet to check if addSet and removeSet are a subset.
+     *
+     * @param other The LWWElementSet to compare.
+     * @return True if the set is subset, false otherwise.
+     */
+    public boolean compare(LWWElementSet other) {
+        return other.addSet.keySet().containsAll(addSet.keySet()) && other.removeSet.keySet().containsAll(removeSet.keySet());
+    }
+
+    /**
+     * Merges another LWWElementSet into this set by resolving conflicts based on timestamps.
+     *
+     * @param other The LWWElementSet to merge.
+     */
+    public void merge(LWWElementSet other) {
+        for (Element e : other.addSet.values()) {
+            if (!addSet.containsKey(e.key) || compareTimestamps(addSet.get(e.key), e)) {
+                addSet.put(e.key, e);
+            }
+        }
+
+        for (Element e : other.removeSet.values()) {
+            if (!removeSet.containsKey(e.key) || compareTimestamps(removeSet.get(e.key), e)) {
+                removeSet.put(e.key, e);
+            }
+        }
+    }
+
+    /**
+     * Compares timestamps of two elements based on their bias (ADDS or REMOVALS).
+     *
+     * @param e     The first element.
+     * @param other The second element.
+     * @return True if the first element's timestamp is greater or the bias is ADDS and timestamps are equal.
+     */
+    public boolean compareTimestamps(Element e, Element other) {
+        if (!e.bias.equals(other.bias)) {
+            throw new IllegalArgumentException("Invalid bias value");
+        }
+        Bias bias = e.bias;
+        int timestampComparison = Integer.compare(e.timestamp, other.timestamp);
+
+        if (timestampComparison == 0) {
+            return !bias.equals(Bias.ADDS);
+        }
+        return timestampComparison < 0;
+    }
+}