feat(spend): donate change output if below a certain value

lib/constants.dart

@@ -1,4 +1,5 @@
 import 'package:danawallet/data/enums/fiat_currency.dart';
+import 'package:danawallet/generated/rust/api/structs/amount.dart';
 import 'package:flutter/services.dart';
 
 // The default blindbit backend used
@@ -51,3 +52,9 @@ const String nameServerDevMainnet =
 // name server for other flavors that user testnet/signet
 const String nameServerDevTestnet =
     "https://test.dev.nameserver.danawallet.app/v1";
+
+// if the change is below this amount, suggest we donate
+final Amount donateBelow = Amount(field0: BigInt.from(5000));
+// signet faucet address
+const String donatePaymentCode =
+    "tsp1qqtp5ql6htn4jz4feslzspyel2n3dzmr5fxjrc69yge2w72hnh2jzzqh6g9zjzkwzzv3m79y84dwusw3sdw2akyzvxm8l2x4gagzxjlrvf5njz0j6";

lib/extensions/api_amount.dart

@@ -14,6 +14,10 @@ extension AmountExtension on Amount {
     return field0 > other.field0;
   }
 
+  bool operator <(Amount other) {
+    return field0 < other.field0;
+  }
+
   String displayBtc() {
     final btcPart = field0 ~/ BigInt.from(bitcoinUnits);
     final satsPart = (field0 % BigInt.from(bitcoinUnits));

lib/states/wallet_state.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 import 'package:danawallet/constants.dart';
 import 'package:danawallet/data/models/bip353_address.dart';
+import 'package:danawallet/extensions/api_amount.dart';
 import 'package:danawallet/extensions/date_time.dart';
 import 'package:danawallet/extensions/network.dart';
 import 'package:danawallet/extensions/outpoint.dart';
@@ -294,15 +295,30 @@ class WalletState extends ChangeNotifier {
   Future<ApiSilentPaymentUnsignedTransaction> createUnsignedTxToThisRecipient(
       Recipient recipient, int feerate) async {
     final wallet = await getWalletFromSecureStorage();
+    final changeCode = wallet.getChangeAddress();
 
     if (recipient.amount.field0 < amount.field0 - BigInt.from(546)) {
-      return wallet.createNewTransaction(
-          ownedOutputs: unspentOutputs,
-          apiRecipients: [
-            recipient,
-          ],
-          feerate: feerate.toDouble(),
-          network: network);
+      final unsignedTx = wallet.createNewTransaction(
+        ownedOutputs: unspentOutputs,
+        apiRecipients: [
+          recipient,
+        ],
+        feerate: feerate.toDouble(),
+        network: network,
+      );
+
+      final sendAmount = unsignedTx.getSendAmount(changeAddress: changeCode);
+      final changeAmount =
+          unsignedTx.getChangeAmount(changeAddress: changeCode);
+
+      if (changeAmount < sendAmount) {
+        if (changeAmount < donateBelow) {
+          // replace change payment code with donation payment code
+          return unsignedTx.replaceChangeCode(
+              changeCode: changeCode, newCode: donatePaymentCode);
+        }
+      }
+      return unsignedTx;
     } else {
       return wallet.createDrainTransaction(
           ownedOutputs: unspentOutputs,

rust/src/api/structs/unsigned_transaction.rs

@@ -11,6 +11,7 @@ use crate::api::structs::amount::Amount;
 use crate::api::structs::discovered_output::DiscoveredOutput;
 use crate::api::structs::recipient::Recipient;
 
+#[derive(Clone, Debug)]
 pub struct ApiSilentPaymentUnsignedTransaction {
     pub selected_utxos: Vec<(super::outpoint::OutPoint, DiscoveredOutput)>,
     pub recipients: Vec<Recipient>,
@@ -110,4 +111,22 @@ impl ApiSilentPaymentUnsignedTransaction {
             .cloned()
             .collect()
     }
+
+    #[frb(sync)]
+    pub fn replace_change_code(
+        &self,
+        change_code: String,
+        new_code: String,
+    ) -> anyhow::Result<Self> {
+        let mut cloned = self.clone();
+        if let Some(change_recipient) = cloned
+            .recipients
+            .iter_mut()
+            .find(|r| r.payment_code == change_code)
+        {
+            change_recipient.payment_code = new_code
+        };
+
+        Ok(cloned)
+    }
 }