Type-safe CFeeRate class

Use CFeeRate instead of an int64_t for quantities that are
fee-per-size.

Helps prevent unit-conversion mismatches between the wallet,
relaying, and mining code.

16 files changed, 123 insertions, 115 deletions

diff --git a/src/core.cpp b/src/core.cpp
index aadcb44b98..6039986e6c 100644
--- a/src/core.cpp
+++ b/src/core.cpp
@@ -72,6 +72,25 @@ void CTxOut::print() const
     LogPrintf("%s\n", ToString());
 }
 
+CFeeRate::CFeeRate(int64_t nFeePaid, size_t nSize)
+{
+    if (nSize > 0)
+        nSatoshisPerK = nFeePaid*1000/nSize;
+    else
+        nSatoshisPerK = 0;
+}
+
+int64_t CFeeRate::GetFee(size_t nSize)
+{
+    return nSatoshisPerK*nSize / 1000;
+}
+
+std::string CFeeRate::ToString() const
+{
+    std::string result = FormatMoney(nSatoshisPerK) + " BTC/kB";
+    return result;
+}
+
 uint256 CTransaction::GetHash() const
 {
     return SerializeHash(*this);
diff --git a/src/core.h b/src/core.h
index ba7f691119..9fccffc4b2 100644
--- a/src/core.h
+++ b/src/core.h
@@ -112,6 +112,28 @@ public:
 
 
 
+/** Type-safe wrapper class to for fee rates
+ * (how much to pay based on transaction size)
+ */
+class CFeeRate
+{
+private:
+    int64_t nSatoshisPerK; // unit is satoshis-per-1,000-bytes
+public:
+    explicit CFeeRate(int64_t _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) { }
+    CFeeRate(int64_t nFeePaid, size_t nSize);
+    CFeeRate(const CFeeRate& other) { nSatoshisPerK = other.nSatoshisPerK; }
+
+    int64_t GetFee(size_t size); // unit returned is satoshis
+    int64_t GetFeePerK() { return GetFee(1000); } // satoshis-per-1000-bytes
+
+    friend bool operator<(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK < b.nSatoshisPerK; }
+    friend bool operator>(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK > b.nSatoshisPerK; }
+    friend bool operator==(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK == b.nSatoshisPerK; }
+
+    std::string ToString() const;
+};
+
 
 /** An output of a transaction.  It contains the public key that the next input
  * must be able to sign with to claim it.
@@ -148,17 +170,18 @@ public:
 
     uint256 GetHash() const;
 
-    bool IsDust(int64_t nMinRelayTxFee) const
+    bool IsDust(CFeeRate minRelayTxFee) const
     {
-        // "Dust" is defined in terms of CTransaction::nMinRelayTxFee,
+        // "Dust" is defined in terms of CTransaction::minRelayTxFee,
         // which has units satoshis-per-kilobyte.
         // If you'd pay more than 1/3 in fees
         // to spend something, then we consider it dust.
         // A typical txout is 34 bytes big, and will
-        // need a CTxIn of at least 148 bytes to spend,
+        // need a CTxIn of at least 148 bytes to spend:
         // so dust is a txout less than 546 satoshis 
-        // with default nMinRelayTxFee.
-        return ((nValue*1000)/(3*((int)GetSerializeSize(SER_DISK,0)+148)) < nMinRelayTxFee);
+        // with default minRelayTxFee.
+        size_t nSize = GetSerializeSize(SER_DISK,0)+148u;
+        return (nValue < 3*minRelayTxFee.GetFee(nSize));
     }
 
     friend bool operator==(const CTxOut& a, const CTxOut& b)
@@ -183,8 +206,8 @@ public:
 class CTransaction
 {
 public:
-    static int64_t nMinTxFee;
-    static int64_t nMinRelayTxFee;
+    static CFeeRate minTxFee;
+    static CFeeRate minRelayTxFee;
     static const int CURRENT_VERSION=1;
     int nVersion;
     std::vector<CTxIn> vin;
diff --git a/src/init.cpp b/src/init.cpp
index d924bd293b..1aad679b3b 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -281,8 +281,8 @@ std::string HelpMessage(HelpMessageMode hmm)
         strUsage += "  -limitfreerelay=<n>    " + _("Continuously rate-limit free transactions to <n>*1000 bytes per minute (default:15)") + "\n";
         strUsage += "  -maxsigcachesize=<n>   " + _("Limit size of signature cache to <n> entries (default: 50000)") + "\n";
     }
-    strUsage += "  -mintxfee=<amt>        " + _("Fees smaller than this are considered zero fee (for transaction creation) (default:") + " " + FormatMoney(CTransaction::nMinTxFee) + ")" + "\n";
-    strUsage += "  -minrelaytxfee=<amt>   " + _("Fees smaller than this are considered zero fee (for relaying) (default:") + " " + FormatMoney(CTransaction::nMinRelayTxFee) + ")" + "\n";
+    strUsage += "  -mintxfee=<amt>        " + _("Fees smaller than this are considered zero fee (for transaction creation) (default:") + " " + FormatMoney(CTransaction::minTxFee.GetFeePerK()) + ")" + "\n";
+    strUsage += "  -minrelaytxfee=<amt>   " + _("Fees smaller than this are considered zero fee (for relaying) (default:") + " " + FormatMoney(CTransaction::minRelayTxFee.GetFeePerK()) + ")" + "\n";
     strUsage += "  -printtoconsole        " + _("Send trace/debug info to console instead of debug.log file") + "\n";
     if (GetBoolArg("-help-debug", false))
     {
@@ -560,7 +560,7 @@ bool AppInit2(boost::thread_group& threadGroup)
     {
         int64_t n = 0;
         if (ParseMoney(mapArgs["-mintxfee"], n) && n > 0)
-            CTransaction::nMinTxFee = n;
+            CTransaction::minTxFee = CFeeRate(n);
         else
             return InitError(strprintf(_("Invalid amount for -mintxfee=<amount>: '%s'"), mapArgs["-mintxfee"]));
     }
@@ -568,7 +568,7 @@ bool AppInit2(boost::thread_group& threadGroup)
     {
         int64_t n = 0;
         if (ParseMoney(mapArgs["-minrelaytxfee"], n) && n > 0)
-            CTransaction::nMinRelayTxFee = n;
+            CTransaction::minRelayTxFee = CFeeRate(n);
         else
             return InitError(strprintf(_("Invalid amount for -minrelaytxfee=<amount>: '%s'"), mapArgs["-minrelaytxfee"]));
     }
@@ -576,10 +576,12 @@ bool AppInit2(boost::thread_group& threadGroup)
 #ifdef ENABLE_WALLET
     if (mapArgs.count("-paytxfee"))
     {
-        if (!ParseMoney(mapArgs["-paytxfee"], nTransactionFee))
+        int64_t nFeePerK = 0;
+        if (!ParseMoney(mapArgs["-paytxfee"], nFeePerK))
             return InitError(strprintf(_("Invalid amount for -paytxfee=<amount>: '%s'"), mapArgs["-paytxfee"]));
-        if (nTransactionFee > nHighTransactionFeeWarning)
+        if (nFeePerK > nHighTransactionFeeWarning)
             InitWarning(_("Warning: -paytxfee is set very high! This is the transaction fee you will pay if you send a transaction."));
+        payTxFee = CFeeRate(nFeePerK, 1000);
     }
     bSpendZeroConfChange = GetArg("-spendzeroconfchange", true);
 
diff --git a/src/main.cpp b/src/main.cpp
index 8e63e4213f..9d6c01d64e 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -50,9 +50,9 @@ bool fTxIndex = false;
 unsigned int nCoinCacheSize = 5000;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for transaction creation) */
-int64_t CTransaction::nMinTxFee = 10000;  // Override with -mintxfee
+CFeeRate CTransaction::minTxFee = CFeeRate(10000);  // Override with -mintxfee
 /** Fees smaller than this (in satoshi) are considered zero fee (for relaying and mining) */
-int64_t CTransaction::nMinRelayTxFee = 1000;
+CFeeRate CTransaction::minRelayTxFee = CFeeRate(1000);
 
 struct COrphanBlock {
     uint256 hashBlock;
@@ -543,7 +543,7 @@ bool IsStandardTx(const CTransaction& tx, string& reason)
         }
         if (whichType == TX_NULL_DATA)
             nDataOut++;
-        else if (txout.IsDust(CTransaction::nMinRelayTxFee)) {
+        else if (txout.IsDust(CTransaction::minRelayTxFee)) {
             reason = "dust";
             return false;
         }
@@ -783,10 +783,10 @@ bool CheckTransaction(const CTransaction& tx, CValidationState &state)
 
 int64_t GetMinFee(const CTransaction& tx, unsigned int nBytes, bool fAllowFree, enum GetMinFee_mode mode)
 {
-    // Base fee is either nMinTxFee or nMinRelayTxFee
-    int64_t nBaseFee = (mode == GMF_RELAY) ? tx.nMinRelayTxFee : tx.nMinTxFee;
+    // Base fee is either minTxFee or minRelayTxFee
+    CFeeRate baseFeeRate = (mode == GMF_RELAY) ? tx.minRelayTxFee : tx.minTxFee;
 
-    int64_t nMinFee = (1 + (int64_t)nBytes / 1000) * nBaseFee;
+    int64_t nMinFee = baseFeeRate.GetFee(nBytes);
 
     if (fAllowFree)
     {
@@ -800,16 +800,6 @@ int64_t GetMinFee(const CTransaction& tx, unsigned int nBytes, bool fAllowFree,
             nMinFee = 0;
     }
 
-    // This code can be removed after enough miners have upgraded to version 0.9.
-    // Until then, be safe when sending and require a fee if any output
-    // is less than CENT:
-    if (nMinFee < nBaseFee && mode == GMF_SEND)
-    {
-        BOOST_FOREACH(const CTxOut& txout, tx.vout)
-            if (txout.nValue < CENT)
-                nMinFee = nBaseFee;
-    }
-
     if (!MoneyRange(nMinFee))
         nMinFee = MAX_MONEY;
     return nMinFee;
@@ -916,10 +906,10 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
                                       hash.ToString(), nFees, txMinFee),
                              REJECT_INSUFFICIENTFEE, "insufficient fee");
 
-        // Continuously rate-limit free transactions
+        // Continuously rate-limit free (really, very-low-fee)transactions
         // This mitigates 'penny-flooding' -- sending thousands of free transactions just to
         // be annoying or make others' transactions take longer to confirm.
-        if (fLimitFree && nFees < CTransaction::nMinRelayTxFee)
+        if (fLimitFree && nFees < CTransaction::minRelayTxFee.GetFee(nSize))
         {
             static CCriticalSection csFreeLimiter;
             static double dFreeCount;
@@ -940,10 +930,10 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
             dFreeCount += nSize;
         }
 
-        if (fRejectInsaneFee && nFees > CTransaction::nMinRelayTxFee * 10000)
+        if (fRejectInsaneFee && nFees > CTransaction::minRelayTxFee.GetFee(nSize) * 10000)
             return error("AcceptToMemoryPool: : insane fees %s, %d > %d",
                          hash.ToString(),
-                         nFees, CTransaction::nMinRelayTxFee * 10000);
+                         nFees, CTransaction::minRelayTxFee.GetFee(nSize) * 10000);
 
         // Check against previous transactions
         // This is done last to help prevent CPU exhaustion denial-of-service attacks.
diff --git a/src/miner.cpp b/src/miner.cpp
index 94fc8e3888..4e131c088b 100644
--- a/src/miner.cpp
+++ b/src/miner.cpp
@@ -52,25 +52,30 @@ void SHA256Transform(void* pstate, void* pinput, const void* pinit)
         ((uint32_t*)pstate)[i] = ctx.h[i];
 }
 
-// Some explaining would be appreciated
+//
+// Unconfirmed transactions in the memory pool often depend on other
+// transactions in the memory pool. When we select transactions from the
+// pool, we select by highest priority or fee rate, so we might consider
+// transactions that depend on transactions that aren't yet in the block.
+// The COrphan class keeps track of these 'temporary orphans' while
+// CreateBlock is figuring out which transactions to include.
+//
 class COrphan
 {
 public:
     const CTransaction* ptx;
     set<uint256> setDependsOn;
     double dPriority;
-    double dFeePerKb;
+    CFeeRate feeRate;
 
-    COrphan(const CTransaction* ptxIn)
+    COrphan(const CTransaction* ptxIn) : ptx(ptxIn), feeRate(0), dPriority(0)
     {
-        ptx = ptxIn;
-        dPriority = dFeePerKb = 0;
     }
 
     void print() const
     {
-        LogPrintf("COrphan(hash=%s, dPriority=%.1f, dFeePerKb=%.1f)\n",
-               ptx->GetHash().ToString(), dPriority, dFeePerKb);
+        LogPrintf("COrphan(hash=%s, dPriority=%.1f, fee=%s)\n",
+                  ptx->GetHash().ToString(), dPriority, feeRate.ToString());
         BOOST_FOREACH(uint256 hash, setDependsOn)
             LogPrintf("   setDependsOn %s\n", hash.ToString());
     }
@@ -80,8 +85,8 @@ public:
 uint64_t nLastBlockTx = 0;
 uint64_t nLastBlockSize = 0;
 
-// We want to sort transactions by priority and fee, so:
-typedef boost::tuple<double, double, const CTransaction*> TxPriority;
+// We want to sort transactions by priority and fee rate, so:
+typedef boost::tuple<double, CFeeRate, const CTransaction*> TxPriority;
 class TxPriorityCompare
 {
     bool byFee;
@@ -210,18 +215,15 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
             unsigned int nTxSize = ::GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION);
             dPriority = tx.ComputePriority(dPriority, nTxSize);
 
-            // This is a more accurate fee-per-kilobyte than is used by the client code, because the
-            // client code rounds up the size to the nearest 1K. That's good, because it gives an
-            // incentive to create smaller transactions.
-            double dFeePerKb =  double(nTotalIn-tx.GetValueOut()) / (double(nTxSize)/1000.0);
+            CFeeRate feeRate(nTotalIn-tx.GetValueOut(), nTxSize);
 
             if (porphan)
             {
                 porphan->dPriority = dPriority;
-                porphan->dFeePerKb = dFeePerKb;
+                porphan->feeRate = feeRate;
             }
             else
-                vecPriority.push_back(TxPriority(dPriority, dFeePerKb, &mi->second.GetTx()));
+                vecPriority.push_back(TxPriority(dPriority, feeRate, &mi->second.GetTx()));
         }
 
         // Collect transactions into block
@@ -237,7 +239,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         {
             // Take highest priority transaction off the priority queue:
             double dPriority = vecPriority.front().get<0>();
-            double dFeePerKb = vecPriority.front().get<1>();
+            CFeeRate feeRate = vecPriority.front().get<1>();
             const CTransaction& tx = *(vecPriority.front().get<2>());
 
             std::pop_heap(vecPriority.begin(), vecPriority.end(), comparer);
@@ -254,7 +256,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
                 continue;
 
             // Skip free transactions if we're past the minimum block size:
-            if (fSortedByFee && (dFeePerKb < CTransaction::nMinRelayTxFee) && (nBlockSize + nTxSize >= nBlockMinSize))
+            if (fSortedByFee && (feeRate < CTransaction::minRelayTxFee) && (nBlockSize + nTxSize >= nBlockMinSize))
                 continue;
 
             // Prioritize by fee once past the priority size or we run out of high-priority
@@ -298,8 +300,8 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
 
             if (fPrintPriority)
             {
-                LogPrintf("priority %.1f feeperkb %.1f txid %s\n",
-                       dPriority, dFeePerKb, tx.GetHash().ToString());
+                LogPrintf("priority %.1f fee %s txid %s\n",
+                          dPriority, feeRate.ToString(), tx.GetHash().ToString());
             }
 
             // Add transactions that depend on this one to the priority queue
@@ -312,7 +314,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
                         porphan->setDependsOn.erase(hash);
                         if (porphan->setDependsOn.empty())
                         {
-                            vecPriority.push_back(TxPriority(porphan->dPriority, porphan->dFeePerKb, porphan->ptx));
+                            vecPriority.push_back(TxPriority(porphan->dPriority, porphan->feeRate, porphan->ptx));
                             std::push_heap(vecPriority.begin(), vecPriority.end(), comparer);
                         }
                     }
diff --git a/src/qt/coincontroldialog.cpp b/src/qt/coincontroldialog.cpp
index dc9d2afe27..e27f1bff97 100644
--- a/src/qt/coincontroldialog.cpp
+++ b/src/qt/coincontroldialog.cpp
@@ -453,7 +453,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
 
             CTxOut txout(amount, (CScript)vector<unsigned char>(24, 0));
             txDummy.vout.push_back(txout);
-            if (txout.IsDust(CTransaction::nMinRelayTxFee))
+            if (txout.IsDust(CTransaction::minRelayTxFee))
                fDust = true;
         }
     }
@@ -525,7 +525,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
         sPriorityLabel = CoinControlDialog::getPriorityLabel(dPriority);
 
         // Fee
-        int64_t nFee = nTransactionFee * (1 + (int64_t)nBytes / 1000);
+        int64_t nFee = payTxFee.GetFee(nBytes);
 
         // Min Fee
         int64_t nMinFee = GetMinFee(txDummy, nBytes, AllowFree(dPriority), GMF_SEND);
@@ -536,26 +536,11 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
         {
             nChange = nAmount - nPayFee - nPayAmount;
 
-            // if sub-cent change is required, the fee must be raised to at least CTransaction::nMinTxFee
-            if (nPayFee < CTransaction::nMinTxFee && nChange > 0 && nChange < CENT)
-            {
-                if (nChange < CTransaction::nMinTxFee) // change < 0.0001 => simply move all change to fees
-                {
-                    nPayFee += nChange;
-                    nChange = 0;
-                }
-                else
-                {
-                    nChange = nChange + nPayFee - CTransaction::nMinTxFee;
-                    nPayFee = CTransaction::nMinTxFee;
-                }
-            }
-
             // Never create dust outputs; if we would, just add the dust to the fee.
             if (nChange > 0 && nChange < CENT)
             {
                 CTxOut txout(nChange, (CScript)vector<unsigned char>(24, 0));
-                if (txout.IsDust(CTransaction::nMinRelayTxFee))
+                if (txout.IsDust(CTransaction::minRelayTxFee))
                 {
                     nPayFee += nChange;
                     nChange = 0;
@@ -610,19 +595,19 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
 
     // tool tips
     QString toolTip1 = tr("This label turns red, if the transaction size is greater than 1000 bytes.") + "<br /><br />";
-    toolTip1 += tr("This means a fee of at least %1 per kB is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::nMinTxFee)) + "<br /><br />";
+    toolTip1 += tr("This means a fee of at least %1 per kB is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::minTxFee.GetFeePerK())) + "<br /><br />";
     toolTip1 += tr("Can vary +/- 1 byte per input.");
 
     QString toolTip2 = tr("Transactions with higher priority are more likely to get included into a block.") + "<br /><br />";
     toolTip2 += tr("This label turns red, if the priority is smaller than \"medium\".") + "<br /><br />";
-    toolTip2 += tr("This means a fee of at least %1 per kB is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::nMinTxFee));
+    toolTip2 += tr("This means a fee of at least %1 per kB is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::minTxFee.GetFeePerK()));
 
     QString toolTip3 = tr("This label turns red, if any recipient receives an amount smaller than %1.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CENT)) + "<br /><br />";
-    toolTip3 += tr("This means a fee of at least %1 is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::nMinTxFee)) + "<br /><br />";
+    toolTip3 += tr("This means a fee of at least %1 is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::minTxFee.GetFeePerK())) + "<br /><br />";
     toolTip3 += tr("Amounts below 0.546 times the minimum relay fee are shown as dust.");
 
     QString toolTip4 = tr("This label turns red, if the change is smaller than %1.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CENT)) + "<br /><br />";
-    toolTip4 += tr("This means a fee of at least %1 is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::nMinTxFee));
+    toolTip4 += tr("This means a fee of at least %1 is required.").arg(BitcoinUnits::formatWithUnit(nDisplayUnit, CTransaction::minTxFee.GetFeePerK()));
 
     l5->setToolTip(toolTip1);
     l6->setToolTip(toolTip2);
diff --git a/src/qt/guiutil.cpp b/src/qt/guiutil.cpp
index 1922d228c5..4fe98251d9 100644
--- a/src/qt/guiutil.cpp
+++ b/src/qt/guiutil.cpp
@@ -210,7 +210,7 @@ bool isDust(const QString& address, qint64 amount)
     CTxDestination dest = CBitcoinAddress(address.toStdString()).Get();
     CScript script; script.SetDestination(dest);
     CTxOut txOut(amount, script);
-    return txOut.IsDust(CTransaction::nMinRelayTxFee);
+    return txOut.IsDust(CTransaction::minRelayTxFee);
 }
 
 QString HtmlEscape(const QString& str, bool fMultiLine)
diff --git a/src/qt/optionsdialog.cpp b/src/qt/optionsdialog.cpp
index 96464d2cc0..1cbf5f8810 100644
--- a/src/qt/optionsdialog.cpp
+++ b/src/qt/optionsdialog.cpp
@@ -14,7 +14,7 @@
 #include "monitoreddatamapper.h"
 #include "optionsmodel.h"
 
-#include "main.h" // for CTransaction::nMinTxFee and MAX_SCRIPTCHECK_THREADS
+#include "main.h" // for CTransaction::minTxFee and MAX_SCRIPTCHECK_THREADS
 #include "netbase.h"
 #include "txdb.h" // for -dbcache defaults
 
@@ -101,7 +101,7 @@ OptionsDialog::OptionsDialog(QWidget *parent) :
 #endif
 
     ui->unit->setModel(new BitcoinUnits(this));
-    ui->transactionFee->setSingleStep(CTransaction::nMinTxFee);
+    ui->transactionFee->setSingleStep(CTransaction::minTxFee.GetFeePerK());
 
     /* Widget-to-option mapper */
     mapper = new MonitoredDataMapper(this);
diff --git a/src/qt/optionsmodel.cpp b/src/qt/optionsmodel.cpp
index 051098315d..f3a5f37bb3 100644
--- a/src/qt/optionsmodel.cpp
+++ b/src/qt/optionsmodel.cpp
@@ -94,7 +94,7 @@ void OptionsModel::Init()
 #ifdef ENABLE_WALLET
     if (!settings.contains("nTransactionFee"))
         settings.setValue("nTransactionFee", (qint64)DEFAULT_TRANSACTION_FEE);
-    nTransactionFee = settings.value("nTransactionFee").toLongLong(); // if -paytxfee is set, this will be overridden later in init.cpp
+    payTxFee = CFeeRate(settings.value("nTransactionFee").toLongLong()); // if -paytxfee is set, this will be overridden later in init.cpp
     if (mapArgs.count("-paytxfee"))
         addOverriddenOption("-paytxfee");
 
@@ -187,15 +187,16 @@ QVariant OptionsModel::data(const QModelIndex & index, int role) const
             return settings.value("nSocksVersion", 5);
 
 #ifdef ENABLE_WALLET
-        case Fee:
-            // Attention: Init() is called before nTransactionFee is set in AppInit2()!
+        case Fee: {
+            // Attention: Init() is called before payTxFee is set in AppInit2()!
             // To ensure we can change the fee on-the-fly update our QSetting when
             // opening OptionsDialog, which queries Fee via the mapper.
-            if (nTransactionFee != settings.value("nTransactionFee").toLongLong())
-                settings.setValue("nTransactionFee", (qint64)nTransactionFee);
-            // Todo: Consider to revert back to use just nTransactionFee here, if we don't want
+            if (!(payTxFee == CFeeRate(settings.value("nTransactionFee").toLongLong(), 1000)))
+                settings.setValue("nTransactionFee", (qint64)payTxFee.GetFeePerK());
+            // Todo: Consider to revert back to use just payTxFee here, if we don't want
             // -paytxfee to update our QSettings!
             return settings.value("nTransactionFee");
+        }
         case SpendZeroConfChange:
             return settings.value("bSpendZeroConfChange");
 #endif
@@ -284,12 +285,14 @@ bool OptionsModel::setData(const QModelIndex & index, const QVariant & value, in
         }
         break;
 #ifdef ENABLE_WALLET
-        case Fee: // core option - can be changed on-the-fly
+        case Fee: { // core option - can be changed on-the-fly
             // Todo: Add is valid check  and warn via message, if not
-            nTransactionFee = value.toLongLong();
-            settings.setValue("nTransactionFee", (qint64)nTransactionFee);
+            qint64 nTransactionFee = value.toLongLong();
+            payTxFee = CFeeRate(nTransactionFee, 1000);
+            settings.setValue("nTransactionFee", nTransactionFee);
             emit transactionFeeChanged(nTransactionFee);
             break;
+        }
         case SpendZeroConfChange:
             if (settings.value("bSpendZeroConfChange") != value) {
                 settings.setValue("bSpendZeroConfChange", value);
diff --git a/src/qt/paymentserver.cpp b/src/qt/paymentserver.cpp
index 4c45585685..6165731d22 100644
--- a/src/qt/paymentserver.cpp
+++ b/src/qt/paymentserver.cpp
@@ -551,7 +551,7 @@ bool PaymentServer::processPaymentRequest(PaymentRequestPlus& request, SendCoins
 
         // Extract and check amounts
         CTxOut txOut(sendingTo.second, sendingTo.first);
-        if (txOut.IsDust(CTransaction::nMinRelayTxFee)) {
+        if (txOut.IsDust(CTransaction::minRelayTxFee)) {
             emit message(tr("Payment request error"), tr("Requested payment amount of %1 is too small (considered dust).")
                 .arg(BitcoinUnits::formatWithUnit(optionsModel->getDisplayUnit(), sendingTo.second)),
                 CClientUIInterface::MSG_ERROR);
diff --git a/src/qt/walletmodel.cpp b/src/qt/walletmodel.cpp
index 37d82ec063..87ff3db584 100644
--- a/src/qt/walletmodel.cpp
+++ b/src/qt/walletmodel.cpp
@@ -231,12 +231,6 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         return AmountExceedsBalance;
     }
 
-    if((total + nTransactionFee) > nBalance)
-    {
-        transaction.setTransactionFee(nTransactionFee);
-        return SendCoinsReturn(AmountWithFeeExceedsBalance);
-    }
-
     {
         LOCK2(cs_main, wallet->cs_wallet);
 
diff --git a/src/rpcmisc.cpp b/src/rpcmisc.cpp
index 27d6d61a36..9802445fcb 100644
--- a/src/rpcmisc.cpp
+++ b/src/rpcmisc.cpp
@@ -81,9 +81,9 @@ Value getinfo(const Array& params, bool fHelp)
     }
     if (pwalletMain && pwalletMain->IsCrypted())
         obj.push_back(Pair("unlocked_until", nWalletUnlockTime));
-    obj.push_back(Pair("paytxfee",      ValueFromAmount(nTransactionFee)));
+    obj.push_back(Pair("paytxfee",      ValueFromAmount(payTxFee.GetFeePerK())));
 #endif
-    obj.push_back(Pair("relayfee",      ValueFromAmount(CTransaction::nMinRelayTxFee)));
+    obj.push_back(Pair("relayfee",      ValueFromAmount(CTransaction::minRelayTxFee.GetFeePerK())));
     obj.push_back(Pair("errors",        GetWarnings("statusbar")));
     return obj;
 }
diff --git a/src/rpcnet.cpp b/src/rpcnet.cpp
index 63eed09b64..0eca55a472 100644
--- a/src/rpcnet.cpp
+++ b/src/rpcnet.cpp
@@ -368,7 +368,7 @@ Value getnetworkinfo(const Array& params, bool fHelp)
     obj.push_back(Pair("timeoffset",    GetTimeOffset()));
     obj.push_back(Pair("connections",   (int)vNodes.size()));
     obj.push_back(Pair("proxy",         (proxy.first.IsValid() ? proxy.first.ToStringIPPort() : string())));
-    obj.push_back(Pair("relayfee",      ValueFromAmount(CTransaction::nMinRelayTxFee)));
+    obj.push_back(Pair("relayfee",      ValueFromAmount(CTransaction::minRelayTxFee.GetFeePerK())));
     Array localAddresses;
     {
         LOCK(cs_mapLocalHost);
diff --git a/src/rpcwallet.cpp b/src/rpcwallet.cpp
index e3b35dbb04..f376ab6b63 100644
--- a/src/rpcwallet.cpp
+++ b/src/rpcwallet.cpp
@@ -1883,7 +1883,7 @@ Value settxfee(const Array& params, bool fHelp)
     if (params[0].get_real() != 0.0)
         nAmount = AmountFromValue(params[0]);        // rejects 0.0 amounts
 
-    nTransactionFee = nAmount;
+    payTxFee = CFeeRate(nAmount, 1000);
     return true;
 }
 
diff --git a/src/wallet.cpp b/src/wallet.cpp
index 89604f96ac..ef0b442e1a 100644
--- a/src/wallet.cpp
+++ b/src/wallet.cpp
@@ -16,7 +16,7 @@
 using namespace std;
 
 // Settings
-int64_t nTransactionFee = DEFAULT_TRANSACTION_FEE;
+CFeeRate payTxFee(DEFAULT_TRANSACTION_FEE);
 bool bSpendZeroConfChange = true;
 
 //////////////////////////////////////////////////////////////////////////////
@@ -1233,7 +1233,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
     {
         LOCK2(cs_main, cs_wallet);
         {
-            nFeeRet = nTransactionFee;
+            nFeeRet = payTxFee.GetFeePerK();
             while (true)
             {
                 wtxNew.vin.clear();
@@ -1246,7 +1246,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
                 BOOST_FOREACH (const PAIRTYPE(CScript, int64_t)& s, vecSend)
                 {
                     CTxOut txout(s.second, s.first);
-                    if (txout.IsDust(CTransaction::nMinRelayTxFee))
+                    if (txout.IsDust(CTransaction::minRelayTxFee))
                     {
                         strFailReason = _("Transaction amount too small");
                         return false;
@@ -1272,16 +1272,6 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
                 }
 
                 int64_t nChange = nValueIn - nValue - nFeeRet;
-                // The following if statement should be removed once enough miners
-                // have upgraded to the 0.9 GetMinFee() rules. Until then, this avoids
-                // creating free transactions that have change outputs less than
-                // CENT bitcoins.
-                if (nFeeRet < CTransaction::nMinTxFee && nChange > 0 && nChange < CENT)
-                {
-                    int64_t nMoveToFee = min(nChange, CTransaction::nMinTxFee - nFeeRet);
-                    nChange -= nMoveToFee;
-                    nFeeRet += nMoveToFee;
-                }
 
                 if (nChange > 0)
                 {
@@ -1317,7 +1307,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
 
                     // Never create dust outputs; if we would, just
                     // add the dust to the fee.
-                    if (newTxOut.IsDust(CTransaction::nMinRelayTxFee))
+                    if (newTxOut.IsDust(CTransaction::minRelayTxFee))
                     {
                         nFeeRet += nChange;
                         reservekey.ReturnKey();
@@ -1355,7 +1345,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64_t> >& vecSend,
                 dPriority = wtxNew.ComputePriority(dPriority, nBytes);
 
                 // Check that enough fee is included
-                int64_t nPayFee = nTransactionFee * (1 + (int64_t)nBytes / 1000);
+                int64_t nPayFee = payTxFee.GetFee(nBytes);
                 bool fAllowFree = AllowFree(dPriority);
                 int64_t nMinFee = GetMinFee(wtxNew, nBytes, fAllowFree, GMF_SEND);
                 if (nFeeRet < max(nPayFee, nMinFee))
@@ -1464,7 +1454,7 @@ string CWallet::SendMoneyToDestination(const CTxDestination& address, int64_t nV
     // Check amount
     if (nValue <= 0)
         return _("Invalid amount");
-    if (nValue + nTransactionFee > GetBalance())
+    if (nValue > GetBalance())
         return _("Insufficient funds");
 
     // Parse Bitcoin address
diff --git a/src/wallet.h b/src/wallet.h
index 8e2917188a..274c31157c 100644
--- a/src/wallet.h
+++ b/src/wallet.h
@@ -24,7 +24,7 @@
 #include <vector>
 
 // Settings
-extern int64_t nTransactionFee;
+extern CFeeRate payTxFee;
 extern bool bSpendZeroConfChange;
 
 // -paytxfee default