EXPERIMENTAL: Playing around with the idea of letting users attach a donation "per tx" (read comment in MoneroHandlerThread.java for why this is in quotes) to support MyNero development when they spend coins.

app/src/main/cpp/monerujo.cpp

@@ -233,6 +233,24 @@ std::vector<std::string> java2cpp(JNIEnv *env, jobject arrayList) {
     return result;
 }
 
+jlong getElement(JNIEnv *env, jlongArray arr_j, int element) {
+    jlong result;
+    env->GetLongArrayRegion(arr_j, element,1, &result);
+    return result;
+}
+
+std::vector<std::uint64_t> java2cpp_long(JNIEnv *env, jlongArray longArray) {
+    jint len = env->GetArrayLength(longArray);
+    std::vector<std::uint64_t> result;
+    result.reserve(len);
+    for (jint i = 0; i < len; i++) {
+        jlong amount = getElement(env, longArray, i);
+        result.emplace_back(amount);
+        env->ReleaseLongArrayElements(longArray, reinterpret_cast<jlong *>(amount), i);
+    }
+    return result;
+}
+
 std::set<std::string> java2cpp_set(JNIEnv *env, jobject arrayList) {
 
     jmethodID java_util_ArrayList_size = env->GetMethodID(class_ArrayList, "size", "()I");
@@ -1029,6 +1047,29 @@ Java_net_mynero_wallet_model_Wallet_createTransactionJ(JNIEnv *env, jobject inst
     return reinterpret_cast<jlong>(tx);
 }
 
+JNIEXPORT jlong JNICALL
+Java_net_mynero_wallet_model_Wallet_createTransactionMultDestJ(JNIEnv *env, jobject instance,
+                                                       jobject dst_addrs, jstring payment_id,
+                                                       jlongArray amounts, jint mixin_count,
+                                                       jint priority,
+                                                       jint accountIndex, jobject key_images) {
+    const std::set<std::string> _key_images = java2cpp_set(env, key_images);
+    const std::vector<std::string> _dst_addrs = java2cpp(env, dst_addrs);
+    const std::vector<std::uint64_t> _dst_amounts = java2cpp_long(env, amounts);
+    const char *_payment_id = env->GetStringUTFChars(payment_id, nullptr);
+    Monero::PendingTransaction::Priority _priority =
+            static_cast<Monero::PendingTransaction::Priority>(priority);
+    Monero::Wallet *wallet = getHandle<Monero::Wallet>(env, instance);
+
+    Monero::PendingTransaction *tx = wallet->createTransactionMultDest(_dst_addrs, _payment_id,
+                                                                       _dst_amounts, (uint32_t) mixin_count,
+                                                               _priority,
+                                                               (uint32_t) accountIndex, {}, _key_images);
+
+    env->ReleaseStringUTFChars(payment_id, _payment_id);
+    return reinterpret_cast<jlong>(tx);
+}
+
 JNIEXPORT jlong JNICALL
 Java_net_mynero_wallet_model_Wallet_estimateTransactionFee(JNIEnv *env, jobject instance,
                                                            jobject destinations, jint priority) {

app/src/main/java/net/mynero/wallet/model/TransactionOutput.java

@@ -0,0 +1,22 @@
+package net.mynero.wallet.model;
+
+import net.mynero.wallet.service.PrefService;
+import net.mynero.wallet.util.Constants;
+
+public class TransactionOutput {
+    private final String destination;
+    private final long amount;
+
+    public TransactionOutput(String destination, long amount) {
+        this.destination = destination;
+        this.amount = amount;
+    }
+
+    public String getDestination() {
+        return destination;
+    }
+
+    public long getAmount() {
+        return amount;
+    }
+}

app/src/main/java/net/mynero/wallet/model/Wallet.java

@@ -288,6 +288,14 @@ public class Wallet {
                 txData.getPreferredInputs());
     }
 
+    public PendingTransaction createSweepTransaction(String dst_addr, PendingTransaction.Priority priority, ArrayList<String> key_images) {
+        disposePendingTransaction();
+        int _priority = priority.getValue();
+        long txHandle = createSweepTransaction(dst_addr, "", 0, _priority, accountIndex, key_images);
+        pendingTransaction = new PendingTransaction(txHandle);
+        return pendingTransaction;
+    }
+
     public PendingTransaction createTransaction(String dst_addr,
                                                 long amount, PendingTransaction.Priority priority, ArrayList<String> key_images) {
         disposePendingTransaction();
@@ -302,6 +310,26 @@ public class Wallet {
         return pendingTransaction;
     }
 
+    public PendingTransaction createTransactionMultDest(ArrayList<TransactionOutput> outputs, PendingTransaction.Priority priority, ArrayList<String> key_images) {
+        disposePendingTransaction();
+        int _priority = priority.getValue();
+        ArrayList<String> destinations = new ArrayList<>();
+        long[] amounts = new long[outputs.size()];
+        for(int i = 0; i < outputs.size(); i++) {
+            TransactionOutput output = outputs.get(i);
+            destinations.add(output.getDestination());
+            amounts[i] = output.getAmount();
+        }
+        long txHandle = createTransactionMultDestJ(destinations, "", amounts, 0, _priority,
+                        accountIndex, key_images);
+        pendingTransaction = new PendingTransaction(txHandle);
+        return pendingTransaction;
+    }
+
+    private native long createTransactionMultDestJ(ArrayList<String> dst_addrs, String payment_id,
+                                           long[] amount, int mixin_count,
+                                           int priority, int accountIndex, ArrayList<String> key_images);
+
     private native long createTransactionJ(String dst_addr, String payment_id,
                                            long amount, int mixin_count,
                                            int priority, int accountIndex, ArrayList<String> key_images);

app/src/main/java/net/mynero/wallet/service/MoneroHandlerThread.java

@@ -22,11 +22,13 @@ import net.mynero.wallet.data.Node;
 import net.mynero.wallet.data.TxData;
 import net.mynero.wallet.model.CoinsInfo;
 import net.mynero.wallet.model.PendingTransaction;
+import net.mynero.wallet.model.TransactionOutput;
 import net.mynero.wallet.model.Wallet;
 import net.mynero.wallet.model.WalletListener;
 import net.mynero.wallet.model.WalletManager;
 import net.mynero.wallet.util.Constants;
 
+import java.security.SecureRandom;
 import java.util.ArrayList;
 
 
@@ -128,7 +130,7 @@ public class MoneroHandlerThread extends Thread implements WalletListener {
     }
 
     public PendingTransaction createTx(String address, String amountStr, boolean sendAll, PendingTransaction.Priority feePriority, ArrayList<String> selectedUtxos) throws Exception {
-        long amount = sendAll ? Wallet.SWEEP_ALL : Wallet.getAmountFromString(amountStr);
+        long amount = Wallet.getAmountFromString(amountStr);
         ArrayList<String> preferredInputs;
         if (selectedUtxos.isEmpty()) {
             // no inputs manually selected, we are sending from home screen most likely, or user somehow broke the app
@@ -137,7 +139,36 @@ public class MoneroHandlerThread extends Thread implements WalletListener {
             preferredInputs = selectedUtxos;
             checkSelectedAmounts(selectedUtxos, amount, sendAll);
         }
-        return wallet.createTransaction(new TxData(address, amount, 0, feePriority, preferredInputs));
+
+        if(sendAll) {
+            return wallet.createSweepTransaction(address, feePriority, preferredInputs);
+        }
+
+        boolean donatePerTx = PrefService.getInstance().getBoolean(Constants.PREF_DONATE_PER_TX, false);
+        ArrayList<TransactionOutput> outputs = new ArrayList<>();
+        outputs.add(new TransactionOutput(address, amount));
+        if(donatePerTx) {
+            float randomDonatePct = getRandomDonateAmount(0.0075f, 0.015f); // occasionally attaches a 0.75% to 1.5% fee. It is random so that not even I know how much exactly you are sending.
+            /*
+            It's also not entirely "per tx". It won't always attach it so as to not have a consistent fingerprint on-chain. When it does attach a donation,
+            it will periodically split it up into 2 outputs instead of 1.
+             */
+            int attachDonationRoll = new SecureRandom().nextInt(100);
+            if(attachDonationRoll > 75) {
+                int splitDonationRoll = new SecureRandom().nextInt(100);
+                long donateAmount = (long) (amount*randomDonatePct);
+                if(splitDonationRoll > 50) {
+                    // split
+                    long splitAmount = donateAmount / 2;
+                    outputs.add(new TransactionOutput(Constants.DONATE_ADDRESS, splitAmount));
+                    outputs.add(new TransactionOutput(Constants.DONATE_ADDRESS, splitAmount));
+                } else {
+                    outputs.add(new TransactionOutput(Constants.DONATE_ADDRESS, donateAmount));
+                }
+                checkSelectedAmounts(selectedUtxos, amount+donateAmount, false); // check that the selected UTXOs satisfy the new amount total
+            }
+        }
+        return wallet.createTransactionMultDest(outputs, feePriority, preferredInputs);
     }
 
     private void checkSelectedAmounts(ArrayList<String> selectedUtxos, long amount, boolean sendAll) throws Exception {
@@ -163,6 +194,12 @@ public class MoneroHandlerThread extends Thread implements WalletListener {
         return height % interval == 0;
     }
 
+    private float getRandomDonateAmount(float min, float max) {
+        SecureRandom rand = new SecureRandom();
+        return rand.nextFloat() * (max - min) + min;
+
+    }
+
     public interface Listener {
         void onRefresh(long height);
 

app/src/main/java/net/mynero/wallet/util/Constants.java

@@ -12,6 +12,7 @@ public class Constants {
     public static final String PREF_USES_OFFSET = "pref_uses_offset";
     public static final String PREF_STREET_MODE = "pref_street_mode";
     public static final String PREF_MONEROCHAN = "pref_monerochan";
+    public static final String PREF_DONATE_PER_TX = "pref_donate_per_tx";
 
     public static final String URI_PREFIX = "monero:";
     public static final String URI_ARG_AMOUNT = "tx_amount";