Added duplicate message filter for chat transactions.

This tracks the last 3 unique (unencrypted) messages for each address and fails validation if a duplicate is sent.

src/main/java/org/qortal/chat/ChatDuplicateMessageFilter.java

@@ -0,0 +1,69 @@
+package org.qortal.chat;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class ChatDuplicateMessageFilter {
+
+    private static ChatDuplicateMessageFilter instance;
+    private volatile boolean isStopping = false;
+
+    private static final int numberOfUniqueMessagesToMonitor = 3;
+
+    // Maintain a short list of recent chat messages for each address, to save having to query the database every time
+    private Map<String, List<String>> recentMessages = new ConcurrentHashMap<>();
+
+    public ChatDuplicateMessageFilter() {
+
+    }
+
+    public static synchronized ChatDuplicateMessageFilter getInstance() {
+        if (instance == null) {
+            instance = new ChatDuplicateMessageFilter();
+        }
+
+        return instance;
+    }
+
+    public boolean isDuplicateMessage(String address, String message) {
+        boolean isDuplicateMessage;
+        boolean messagesUpdated = false;
+
+        // Add timestamp to array for address
+        List<String> messages = new ArrayList<>();
+        if (this.recentMessages.containsKey(address)) {
+            messages = this.recentMessages.get(address);
+        }
+
+        if (!messages.contains(message)) {
+            messages.add(message);
+            this.recentMessages.put(address, messages);
+            messagesUpdated = true;
+            isDuplicateMessage = false;
+        }
+        else {
+            // Can't add message because it already exists
+            isDuplicateMessage = true;
+        }
+
+        // Ensure we're not tracking more messages than intended
+        while (messages.size() > numberOfUniqueMessagesToMonitor) {
+            messages.remove(0);
+            messagesUpdated = true;
+        }
+
+        if (messagesUpdated) {
+            if (messages.size() > 0) {
+                this.recentMessages.put(address, messages);
+            }
+            else {
+                this.recentMessages.remove(address);
+            }
+        }
+
+        return isDuplicateMessage;
+    }
+
+}

src/main/java/org/qortal/transaction/ChatTransaction.java

@@ -6,6 +6,7 @@ import java.util.List;
 import org.qortal.account.Account;
 import org.qortal.account.PublicKeyAccount;
 import org.qortal.asset.Asset;
+import org.qortal.chat.ChatDuplicateMessageFilter;
 import org.qortal.chat.ChatRateLimiter;
 import org.qortal.crypto.Crypto;
 import org.qortal.crypto.MemoryPoW;
@@ -19,6 +20,7 @@ import org.qortal.repository.Repository;
 import org.qortal.transform.TransformationException;
 import org.qortal.transform.transaction.ChatTransactionTransformer;
 import org.qortal.transform.transaction.TransactionTransformer;
+import org.qortal.utils.Base58;
 
 public class ChatTransaction extends Transaction {
 
@@ -166,6 +168,14 @@ public class ChatTransaction extends Transaction {
 		if (rateLimiter.isAddressAboveRateLimit(chatTransactionData.getSender()))
 			return ValidationResult.ADDRESS_ABOVE_RATE_LIMIT;
 
+		// Check for duplicate messages (unencrypted text messages only)
+		if (!chatTransactionData.getIsEncrypted() && chatTransactionData.getIsText()) {
+			ChatDuplicateMessageFilter duplicateFilter = ChatDuplicateMessageFilter.getInstance();
+			String message58 = Base58.encode(chatTransactionData.getData());
+			if (duplicateFilter.isDuplicateMessage(chatTransactionData.getSender(), message58))
+				return ValidationResult.DUPLICATE_MESSAGE;
+		}
+
 		return ValidationResult.OK;
 	}
 

src/main/java/org/qortal/transaction/Transaction.java

@@ -249,6 +249,7 @@ public abstract class Transaction {
 		INVALID_TIMESTAMP_SIGNATURE(95),
 		ADDRESS_IN_BLACKLIST(96),
 		ADDRESS_ABOVE_RATE_LIMIT(97),
+		DUPLICATE_MESSAGE(98),
 		INVALID_BUT_OK(999),
 		NOT_YET_RELEASED(1000);