[Wallet] Combine change to output when possible & provide it in autocombine

-When a change address is specified through coincontrol and this adress
is already receiving an output, we now add the change to this output
-Change of autocombinedust goes to the adress being combined instead of
a change address

Author: Warrows

src/wallet.cpp

@@ -2713,11 +2713,25 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, CAmount> >& vecSend,
                     // TODO: pass in scriptChange instead of reservekey so
                     // change transaction isn't always pay-to-pivx-address
                     CScript scriptChange;
+                    bool combineChange = false;
 
                     // coin control: send change to custom address
-                    if (coinControl && !boost::get<CNoDestination>(&coinControl->destChange))
+                    if (coinControl && !boost::get<CNoDestination>(&coinControl->destChange)) {
                         scriptChange = GetScriptForDestination(coinControl->destChange);
 
+                        vector<CTxOut>::iterator it = txNew.vout.begin();
+                        for (; it != txNew.vout.end();) {
+                            if (scriptChange == it->scriptPubKey) {
+                                it->nValue += nChange;
+                                nChange = 0;
+                                reservekey.ReturnKey();
+                                combineChange = true;
+                                break;
+                            }
+                            ++it;
+                        }
+                    }
+
                     // no coin control: send change to newly generated address
                     else {
                         // Note: We use a new key here to keep it from being obvious which side is the change.
@@ -2736,18 +2750,20 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, CAmount> >& vecSend,
                         scriptChange = GetScriptForDestination(vchPubKey.GetID());
                     }
 
-                    CTxOut newTxOut(nChange, scriptChange);
-
-                    // Never create dust outputs; if we would, just
-                    // add the dust to the fee.
-                    if (newTxOut.IsDust(::minRelayTxFee)) {
-                        nFeeRet += nChange;
-                        nChange = 0;
-                        reservekey.ReturnKey();
-                    } else {
-                        // Insert change txn at random position:
-                        vector<CTxOut>::iterator position = txNew.vout.begin() + GetRandInt(txNew.vout.size() + 1);
-                        txNew.vout.insert(position, newTxOut);
+                    if (!combineChange) {
+                        CTxOut newTxOut(nChange, scriptChange);
+
+                        // Never create dust outputs; if we would, just
+                        // add the dust to the fee.
+                        if (newTxOut.IsDust(::minRelayTxFee)) {
+                            nFeeRet += nChange;
+                            nChange = 0;
+                            reservekey.ReturnKey();
+                        } else {
+                            // Insert change txn at random position:
+                            vector<CTxOut>::iterator position = txNew.vout.begin() + GetRandInt(txNew.vout.size() + 1);
+                            txNew.vout.insert(position, newTxOut);
+                        }
                     }
                 } else
                     reservekey.ReturnKey();
@@ -4036,6 +4052,14 @@ void CWallet::AutoCombineDust()
         CScript scriptPubKey = GetScriptForDestination(it->first.Get());
         vecSend.push_back(make_pair(scriptPubKey, nTotalRewardsValue));
 
+        //Send change to same address
+        CTxDestination destMyAddress;
+        if (!ExtractDestination(scriptPubKey, destMyAddress)) {
+            LogPrintf("AutoCombineDust: failed to extract destination\n");
+            continue;
+        }
+        coinControl->destChange = destMyAddress;
+
         // Create the transaction and commit it to the network
         CWalletTx wtx;
         CReserveKey keyChange(this); // this change address does not end up being used, because change is returned with coin control switch