From ffcd8792080790f2041a6f0ab275adb0a9beaf92 Mon Sep 17 00:00:00 2001
From: Sean Gilligan <sean@msgilligan.com>
Date: Mon, 22 Apr 2024 11:19:19 -0700
Subject: [PATCH] ForwardingService: inline `CoinSelector` creation into
 `forward()`

This also moves and adds comments.
---
 .../bitcoinj/examples/ForwardingService.java  | 23 ++++++-------------
 1 file changed, 7 insertions(+), 16 deletions(-)

diff --git a/examples/src/main/java/org/bitcoinj/examples/ForwardingService.java b/examples/src/main/java/org/bitcoinj/examples/ForwardingService.java
index b90c225a0..336bc29cf 100644
--- a/examples/src/main/java/org/bitcoinj/examples/ForwardingService.java
+++ b/examples/src/main/java/org/bitcoinj/examples/ForwardingService.java
@@ -17,7 +17,6 @@
 package org.bitcoinj.examples;
 
 import org.bitcoinj.base.BitcoinNetwork;
-import org.bitcoinj.base.Sha256Hash;
 import org.bitcoinj.base.Address;
 import org.bitcoinj.base.Coin;
 import org.bitcoinj.base.AddressParser;
@@ -160,16 +159,20 @@ public class ForwardingService implements Closeable {
     }
 
     /**
-     * Forward an incoming transaction by creating a new transaction, signing, and sending to the specified address.
+     * Forward an incoming transaction by creating a new transaction, signing, and sending to the specified address. The
+     * inputs for the new transaction should only come from the incoming transaction, so we use a custom {@link CoinSelector}
+     * that only selects wallet UTXOs with the correct parent transaction ID.
      * @param wallet The active wallet
      * @param incomingTx the received transaction
      * @param forwardingAddress the address to send to
      * @return A future for a TransactionBroadcast object that completes when relay is acknowledged by peers
      */
     private CompletableFuture<TransactionBroadcast> forward(Wallet wallet, Transaction incomingTx, Address forwardingAddress) {
-        // Send coins received in incomingTx onwards by sending exactly the outputs that have been sent to us
+        // Send coins received in incomingTx onwards by sending exactly the UTXOs we have just received.
+        // We're not truly emptying the wallet because we're limiting the available outputs with a CoinSelector.
         SendRequest sendRequest = SendRequest.emptyWallet(forwardingAddress);
-        sendRequest.coinSelector = forwardingCoinSelector(incomingTx.getTxId());
+        // Use a CoinSelector that only returns wallet UTXOs from the incoming transaction.
+        sendRequest.coinSelector = CoinSelector.fromPredicate(output -> Objects.equals(output.getParentTransactionHash(), incomingTx.getTxId()));
         System.out.printf("Creating outgoing transaction for %s...\n", forwardingAddress);
         return wallet.sendTransaction(sendRequest)
                 .thenCompose(broadcast -> {
@@ -181,16 +184,4 @@ public class ForwardingService implements Closeable {
     static String getPrefix(BitcoinNetwork network) {
         return String.format("forwarding-service-%s", network.toString());
     }
-
-    /**
-     * Create a CoinSelector that only returns outputs from a given parent transaction.
-     * <p>
-     * This is using the idea of partial function application to create a 2-argument function for coin selection
-     * with a third, fixed argument of the transaction id.
-     * @param parentTxId The parent transaction hash
-     * @return a coin selector
-     */
-    static CoinSelector forwardingCoinSelector(Sha256Hash parentTxId) {
-        return CoinSelector.fromPredicate(output -> Objects.equals(output.getParentTransactionHash(), parentTxId));
-    }
 }