Subtract fee from amount

Fixes #2724 and #1570.

Adds the
automatically-subtract-the-fee-from-the-amount-and-send-whats-left
feature to the GUI and RPC (sendtoaddress,sendmany).

1 files changed, 18 insertions, 5 deletions

diff --git a/src/qt/coincontroldialog.cpp b/src/qt/coincontroldialog.cpp
index 3f4f082b8c..5042ff06a2 100644
--- a/src/qt/coincontroldialog.cpp
+++ b/src/qt/coincontroldialog.cpp
@@ -33,6 +33,7 @@
 using namespace std;
 QList<CAmount> CoinControlDialog::payAmounts;
 CCoinControl* CoinControlDialog::coinControl = new CCoinControl();
+bool CoinControlDialog::fSubtractFeeFromAmount = false;
 
 CoinControlDialog::CoinControlDialog(QWidget *parent) :
     QDialog(parent),
@@ -541,6 +542,11 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
         dPriority = dPriorityInputs / (nBytes - nBytesInputs + (nQuantityUncompressed * 29)); // 29 = 180 - 151 (uncompressed public keys are over the limit. max 151 bytes of the input are ignored for priority)
         sPriorityLabel = CoinControlDialog::getPriorityLabel(dPriority, mempoolEstimatePriority);
 
+        // in the subtract fee from amount case, we can tell if zero change already and subtract the bytes, so that fee calculation afterwards is accurate
+        if (CoinControlDialog::fSubtractFeeFromAmount)
+            if (nAmount - nPayAmount == 0)
+                nBytes -= 34;
+
         // Fee
         nPayFee = CWallet::GetMinimumFee(nBytes, nTxConfirmTarget, mempool);
 
@@ -556,7 +562,9 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
 
         if (nPayAmount > 0)
         {
-            nChange = nAmount - nPayFee - nPayAmount;
+            nChange = nAmount - nPayAmount;
+            if (!CoinControlDialog::fSubtractFeeFromAmount)
+                nChange -= nPayFee;
 
             // Never create dust outputs; if we would, just add the dust to the fee.
             if (nChange > 0 && nChange < CENT)
@@ -564,12 +572,17 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
                 CTxOut txout(nChange, (CScript)vector<unsigned char>(24, 0));
                 if (txout.IsDust(::minRelayTxFee))
                 {
-                    nPayFee += nChange;
-                    nChange = 0;
+                    if (CoinControlDialog::fSubtractFeeFromAmount) // dust-change will be raised until no dust
+                        nChange = txout.GetDustThreshold(::minRelayTxFee);
+                    else
+                    {
+                        nPayFee += nChange;
+                        nChange = 0;
+                    }
                 }
             }
 
-            if (nChange == 0)
+            if (nChange == 0 && !CoinControlDialog::fSubtractFeeFromAmount)
                 nBytes -= 34;
         }
 
@@ -612,7 +625,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
     {
         l3->setText(ASYMP_UTF8 + l3->text());
         l4->setText(ASYMP_UTF8 + l4->text());
-        if (nChange > 0)
+        if (nChange > 0 && !CoinControlDialog::fSubtractFeeFromAmount)
             l8->setText(ASYMP_UTF8 + l8->text());
     }