storing blockchain data in a cache to reduce redundant RPCs to the ElectrumX servers

src/main/java/org/qortal/crosschain/Bitcoiny.java

@@ -55,6 +55,13 @@ public abstract class Bitcoiny implements ForeignBlockchain {
 
 	protected Coin feePerKb;
 
+	/**
+	 * Blockchain Cache
+	 *
+	 * To store blockchain data and reduce redundant RPCs to the ElectrumX servers
+	 */
+	private final BlockchainCache blockchainCache = new BlockchainCache();
+
 	// Constructors and instance
 
 	protected Bitcoiny(BitcoinyBlockchainProvider blockchainProvider, Context bitcoinjContext, String currencyCode, Coin feePerKb) {
@@ -509,8 +516,22 @@ public abstract class Bitcoiny implements ForeignBlockchain {
 					if (!historicTransactionHashes.isEmpty()) {
 						areAllKeysUnused = false;
 
-						for (TransactionHash transactionHash : historicTransactionHashes)
+						for (TransactionHash transactionHash : historicTransactionHashes) {
-							walletTransactions.add(this.getTransaction(transactionHash.txHash));
+
+							Optional<BitcoinyTransaction> walletTransaction
+									= this.blockchainCache.getTransactionByHash( transactionHash.txHash );
+
+							// if the wallet transaction is already cached
+							if(walletTransaction.isPresent() ) {
+								walletTransactions.add( walletTransaction.get() );
+							}
+							// otherwise get the transaction from the blockchain server
+							else {
+								BitcoinyTransaction transaction = getTransaction(transactionHash.txHash);
+								walletTransactions.add( transaction );
+								this.blockchainCache.addTransactionByHash(transactionHash.txHash, transaction);
+							}
+						}
 					}
 				}
 
@@ -602,17 +623,25 @@ public abstract class Bitcoiny implements ForeignBlockchain {
 				for (; ki < keys.size(); ++ki) {
 					DeterministicKey dKey = keys.get(ki);
 
-					// Check for transactions
 					Address address = Address.fromKey(this.params, dKey, ScriptType.P2PKH);
 					keySet.add(address.toString());
-					byte[] script = ScriptBuilder.createOutputScript(address).getProgram();
 
-					// Ask for transaction history - if it's empty then key has never been used
+					// if the key already has a verified transaction history
-					List<TransactionHash> historicTransactionHashes = this.getAddressTransactions(script, true);
+					if( this.blockchainCache.keyHasHistory( dKey ) ){
-
-					if (!historicTransactionHashes.isEmpty()) {
 						areAllKeysUnused = false;
 					}
+					else {
+						// Check for transactions
+						byte[] script = ScriptBuilder.createOutputScript(address).getProgram();
+
+						// Ask for transaction history - if it's empty then key has never been used
+						List<TransactionHash> historicTransactionHashes = this.getAddressTransactions(script, true);
+
+						if (!historicTransactionHashes.isEmpty()) {
+							areAllKeysUnused = false;
+							this.blockchainCache.addKeyWithHistory(dKey);
+						}
+					}
 				}
 
 				if (areAllKeysUnused) {
@@ -667,19 +696,26 @@ public abstract class Bitcoiny implements ForeignBlockchain {
 			do {
 				boolean areAllKeysUnused = true;
 
-				for (; ki < keys.size(); ++ki) {
+				for (; areAllKeysUnused && ki < keys.size(); ++ki) {
 					DeterministicKey dKey = keys.get(ki);
 
-					// Check for transactions
+					// if the key already has a verified transaction history
-					Address address = Address.fromKey(this.params, dKey, ScriptType.P2PKH);
+					if( this.blockchainCache.keyHasHistory(dKey)) {
-					byte[] script = ScriptBuilder.createOutputScript(address).getProgram();
-
-					// Ask for transaction history - if it's empty then key has never been used
-					List<TransactionHash> historicTransactionHashes = this.getAddressTransactions(script, false);
-
-					if (!historicTransactionHashes.isEmpty()) {
 						areAllKeysUnused = false;
 					}
+					else {
+						// Check for transactions
+						Address address = Address.fromKey(this.params, dKey, ScriptType.P2PKH);
+						byte[] script = ScriptBuilder.createOutputScript(address).getProgram();
+
+						// Ask for transaction history - if it's empty then key has never been used
+						List<TransactionHash> historicTransactionHashes = this.getAddressTransactions(script, true);
+
+						if (!historicTransactionHashes.isEmpty()) {
+							areAllKeysUnused = false;
+							this.blockchainCache.addKeyWithHistory(dKey);
+						}
+					}
 				}
 
 				if (areAllKeysUnused) {

src/main/java/org/qortal/crosschain/BlockchainCache.java

@@ -0,0 +1,91 @@
+package org.qortal.crosschain;
+
+import org.bitcoinj.crypto.DeterministicKey;
+
+import java.util.Optional;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentLinkedDeque;
+
+/**
+ * Class BlockchainCache
+ *
+ * Cache blockchain information to reduce redundant RPCs to the ElectrumX servers.
+ */
+public class BlockchainCache {
+
+    /**
+     * Keys With History
+     *
+     * Deterministic Keys with any transaction history.
+     */
+    private Queue<DeterministicKey> keysWithHistory = new ConcurrentLinkedDeque<>();
+
+    /**
+     * Transactions By Hash
+     *
+     * Transaction Hash -> Transaction
+     */
+    private ConcurrentHashMap<String, BitcoinyTransaction> transactionByHash = new ConcurrentHashMap<>();
+
+    /**
+     * Key History Limit
+     *
+     * If this limit is reached, the cache will be reduced.
+     */
+    private static final int KEY_HISTORY_LIMIT = 10000;
+
+    /**
+     * Transaction Limit
+     *
+     * If this limit is reached, the cache will be cleared.
+     */
+    private static final int TRANSACTION_LIMIT = 10000;
+
+    /**
+     * Add Key With History
+     *
+     * @param key a deterministic key with a verified history
+     */
+    public void addKeyWithHistory(DeterministicKey key) {
+
+        if( this.keysWithHistory.size() > KEY_HISTORY_LIMIT ) this.keysWithHistory.remove();
+
+        this.keysWithHistory.add(key);
+    }
+
+    /**
+     * Key Has History?
+     *
+     * @param key the deterministic key
+     *
+     * @return true if the key has a history, otherwise false
+     */
+    public boolean keyHasHistory( DeterministicKey key ) {
+        return this.keysWithHistory.contains(key);
+    }
+
+    /**
+     * Add Transaction By Hash
+     *
+     * @param hash the transaction hash
+     * @param transaction the transaction
+     */
+    public void addTransactionByHash( String hash, BitcoinyTransaction transaction ) {
+
+        if( this.transactionByHash.size() > TRANSACTION_LIMIT ) this.transactionByHash.clear();
+
+        this.transactionByHash.put(hash, transaction);
+    }
+
+    /**
+     * Get Transaction By Hash
+     *
+     * @param hash the transaction hash
+     *
+     * @return the transaction, empty if the hash is not in the cache
+     */
+    public Optional<BitcoinyTransaction> getTransactionByHash( String hash ) {
+        return Optional.ofNullable( this.transactionByHash.get(hash) );
+    }
+}