add consistent hashing implementations

implementations/java/consistent_hashing/ConsistentHashing.java

@@ -0,0 +1,86 @@
+package implementations.java.consistent_hashing;
+
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.*;
+
+public class ConsistentHashing {
+    private final int numReplicas; // Number of virtual nodes per server
+    private final TreeMap<Long, String> ring; // Hash ring storing virtual nodes
+    private final Set<String> servers; // Set of physical servers
+
+    public ConsistentHashing(List<String> servers, int numReplicas) {
+        this.numReplicas = numReplicas;
+        this.ring = new TreeMap<>();
+        this.servers = new HashSet<>();
+
+        // Add each server to the hash ring
+        for (String server : servers) {
+            addServer(server);
+        }
+    }
+
+    private long hash(String key) {
+        try {
+            MessageDigest md = MessageDigest.getInstance("MD5");
+            md.update(key.getBytes());
+            byte[] digest = md.digest();
+            return ((long) (digest[0] & 0xFF) << 24) |
+                   ((long) (digest[1] & 0xFF) << 16) |
+                   ((long) (digest[2] & 0xFF) << 8) |
+                   ((long) (digest[3] & 0xFF));
+        } catch (NoSuchAlgorithmException e) {
+            throw new RuntimeException("MD5 algorithm not found", e);
+        }
+    }
+
+    public void addServer(String server) {
+        servers.add(server);
+        for (int i = 0; i < numReplicas; i++) {
+            long hash = hash(server + "-" + i); // Unique hash for each virtual node
+            ring.put(hash, server);
+        }
+    }
+
+    public void removeServer(String server) {
+        if (servers.remove(server)) {
+            for (int i = 0; i < numReplicas; i++) {
+                long hash = hash(server + "-" + i);
+                ring.remove(hash);
+            }
+        }
+    }
+
+    public String getServer(String key) {
+        if (ring.isEmpty()) {
+            return null; // No servers available
+        }
+
+        long hash = hash(key);
+        // Find the closest server in a clockwise direction
+        Map.Entry<Long, String> entry = ring.ceilingEntry(hash);
+        if (entry == null) {
+            // If we exceed the highest node, wrap around to the first node
+            entry = ring.firstEntry();
+        }
+        return entry.getValue();
+    }
+
+    public static void main(String[] args) {
+        List<String> servers = Arrays.asList("S0", "S1", "S2", "S3", "S4", "S5");
+        ConsistentHashing ch = new ConsistentHashing(servers, 3);
+
+        // Step 2: Assign requests (keys) to servers
+        System.out.println("UserA is assigned to: " + ch.getServer("UserA"));
+        System.out.println("UserB is assigned to: " + ch.getServer("UserB"));
+
+        // Step 3: Add a new server dynamically
+        ch.addServer("S6");
+        System.out.println("UserA is now assigned to: " + ch.getServer("UserA"));
+
+        // Step 4: Remove a server dynamically
+        ch.removeServer("S2");
+        System.out.println("UserB is now assigned to: " + ch.getServer("UserB"));
+    }
+}
+