Add G-Counter (Grow-only Counter) (#4965)

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

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+package com.thealgorithms.datastructures.crdt;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * G-Counter (Grow-only Counter) is a state-based CRDT (Conflict-free Replicated Data Type)
+ * designed for tracking counts in a distributed and concurrent environment.
+ * Each process maintains its own counter, allowing only increments. The total count
+ * is obtained by summing individual process counts.
+ * This implementation supports incrementing, querying the total count,
+ * comparing with other G-Counters, and merging with another G-Counter
+ * to compute the element-wise maximum.
+ * (https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type)
+ *
+ * @author itakurah (https://github.com/itakurah)
+ */
+
+class GCounter {
+    private final Map<Integer, Integer> P;
+    private final int myId;
+    private final int n;
+
+    /**
+     * Constructs a G-Counter for a cluster of n nodes.
+     *
+     * @param n The number of nodes in the cluster.
+     */
+    public GCounter(int myId, int n) {
+        this.myId = myId;
+        this.n = n;
+        this.P = new HashMap<>();
+
+        for (int i = 0; i < n; i++) {
+            P.put(i, 0);
+        }
+    }
+
+    /**
+     * Increments the counter for the current node.
+     */
+    public void increment() {
+        P.put(myId, P.get(myId) + 1);
+    }
+
+    /**
+     * Gets the total value of the counter by summing up values from all nodes.
+     *
+     * @return The total value of the counter.
+     */
+    public int value() {
+        int sum = 0;
+        for (int v : P.values()) {
+            sum += v;
+        }
+        return sum;
+    }
+
+    /**
+     * Compares the state of this G-Counter with another G-Counter.
+     *
+     * @param other The other G-Counter to compare with.
+     * @return True if the state of this G-Counter is less than or equal to the state of the other G-Counter.
+     */
+    public boolean compare(GCounter other) {
+        for (int i = 0; i < n; i++) {
+            if (this.P.get(i) > other.P.get(i)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    /**
+     * Merges the state of this G-Counter with another G-Counter.
+     *
+     * @param other The other G-Counter to merge with.
+     */
+    public void merge(GCounter other) {
+        for (int i = 0; i < n; i++) {
+            this.P.put(i, Math.max(this.P.get(i), other.P.get(i)));
+        }
+    }
+}

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

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+package com.thealgorithms.datastructures.crdt;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+public class GCounterTest {
+    @Test
+    void increment() {
+        GCounter counter = new GCounter(0, 3);
+        counter.increment();
+        counter.increment();
+        counter.increment();
+        assertEquals(3, counter.value());
+    }
+
+    @Test
+    void merge() {
+        GCounter counter1 = new GCounter(0, 3);
+        counter1.increment();
+        GCounter counter2 = new GCounter(1, 3);
+        counter2.increment();
+        counter2.increment();
+        GCounter counter3 = new GCounter(2, 3);
+        counter3.increment();
+        counter3.increment();
+        counter3.increment();
+        counter1.merge(counter2);
+        counter1.merge(counter3);
+        counter2.merge(counter1);
+        counter3.merge(counter2);
+        assertEquals(6, counter1.value());
+        assertEquals(6, counter2.value());
+        assertEquals(6, counter3.value());
+    }
+
+    @Test
+    void compare() {
+        GCounter counter1 = new GCounter(0, 5);
+        GCounter counter2 = new GCounter(3, 5);
+        counter1.increment();
+        counter1.increment();
+        counter2.merge(counter1);
+        counter2.increment();
+        counter2.increment();
+        assertTrue(counter1.compare(counter2));
+        counter1.increment();
+        counter2.increment();
+        counter2.merge(counter1);
+        assertTrue(counter1.compare(counter2));
+        counter1.increment();
+        assertFalse(counter1.compare(counter2));
+    }
+}