[wallet] Refactor to use new MIN_CHANGE

* Introduce new constant MIN_CHANGE and use it instead of the
hardcoded "CENT"
* Add test case for MIN_CHANGE
* Introduce new constant for -mintxfee default:
  DEFAULT_TRANSACTION_MINFEE = 1000

4 files changed, 85 insertions, 56 deletions

diff --git a/src/qt/coincontroldialog.cpp b/src/qt/coincontroldialog.cpp
index 51008ad2de..74bc8dbec9 100644
--- a/src/qt/coincontroldialog.cpp
+++ b/src/qt/coincontroldialog.cpp
@@ -567,7 +567,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
                 nChange -= nPayFee;
 
             // Never create dust outputs; if we would, just add the dust to the fee.
-            if (nChange > 0 && nChange < CENT)
+            if (nChange > 0 && nChange < MIN_CHANGE)
             {
                 CTxOut txout(nChange, (CScript)std::vector<unsigned char>(24, 0));
                 if (txout.IsDust(::minRelayTxFee))
diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index a5bc52b8dc..8b9292bd14 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -117,7 +117,7 @@ BOOST_AUTO_TEST_CASE(coin_selection_tests)
 
         // try making 34 cents from 1,2,5,10,20 - we can't do it exactly
         BOOST_CHECK( wallet.SelectCoinsMinConf(34 * CENT, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_GT(nValueRet, 34 * CENT);         // but should get more than 34 cents
+        BOOST_CHECK_EQUAL(nValueRet, 35 * CENT);       // but 35 cents is closest
         BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);     // the best should be 20+10+5.  it's incredibly unlikely the 1 or 2 got included (but possible)
 
         // when we try making 7 cents, the smaller coins (1,2,5) are enough.  We should see just 2+5
@@ -185,33 +185,34 @@ BOOST_AUTO_TEST_CASE(coin_selection_tests)
         BOOST_CHECK_EQUAL(nValueRet, 2 * COIN);  // we should get 2 BTC in 1 coin
         BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
 
-        // empty the wallet and start again, now with fractions of a cent, to test sub-cent change avoidance
+        // empty the wallet and start again, now with fractions of a cent, to test small change avoidance
+
         empty_wallet();
-        add_coin(0.1*CENT);
-        add_coin(0.2*CENT);
-        add_coin(0.3*CENT);
-        add_coin(0.4*CENT);
-        add_coin(0.5*CENT);
-
-        // try making 1 cent from 0.1 + 0.2 + 0.3 + 0.4 + 0.5 = 1.5 cents
-        // we'll get sub-cent change whatever happens, so can expect 1.0 exactly
-        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * CENT, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * CENT);
+        add_coin(0.1*MIN_CHANGE);
+        add_coin(0.2*MIN_CHANGE);
+        add_coin(0.3*MIN_CHANGE);
+        add_coin(0.4*MIN_CHANGE);
+        add_coin(0.5*MIN_CHANGE);
+
+        // try making 1 * MIN_CHANGE from the 1.5 * MIN_CHANGE
+        // we'll get change smaller than MIN_CHANGE whatever happens, so can expect MIN_CHANGE exactly
+        BOOST_CHECK( wallet.SelectCoinsMinConf(MIN_CHANGE, 1, 1, vCoins, setCoinsRet, nValueRet));
+        BOOST_CHECK_EQUAL(nValueRet, MIN_CHANGE);
 
-        // but if we add a bigger coin, making it possible to avoid sub-cent change, things change:
-        add_coin(1111*CENT);
+        // but if we add a bigger coin, small change is avoided
+        add_coin(1111*MIN_CHANGE);
 
-        // try making 1 cent from 0.1 + 0.2 + 0.3 + 0.4 + 0.5 + 1111 = 1112.5 cents
-        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * CENT, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * CENT); // we should get the exact amount
+        // try making 1 from 0.1 + 0.2 + 0.3 + 0.4 + 0.5 + 1111 = 1112.5
+        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * MIN_CHANGE, 1, 1, vCoins, setCoinsRet, nValueRet));
+        BOOST_CHECK_EQUAL(nValueRet, 1 * MIN_CHANGE); // we should get the exact amount
 
-        // if we add more sub-cent coins:
-        add_coin(0.6*CENT);
-        add_coin(0.7*CENT);
+        // if we add more small coins:
+        add_coin(0.6*MIN_CHANGE);
+        add_coin(0.7*MIN_CHANGE);
 
-        // and try again to make 1.0 cents, we can still make 1.0 cents
-        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * CENT, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * CENT); // we should get the exact amount
+        // and try again to make 1.0 * MIN_CHANGE
+        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * MIN_CHANGE, 1, 1, vCoins, setCoinsRet, nValueRet));
+        BOOST_CHECK_EQUAL(nValueRet, 1 * MIN_CHANGE); // we should get the exact amount
 
         // run the 'mtgox' test (see http://blockexplorer.com/tx/29a3efd3ef04f9153d47a990bd7b048a4b2d213daaa5fb8ed670fb85f13bdbcf)
         // they tried to consolidate 10 50k coins into one 500k coin, and ended up with 50k in change
@@ -223,45 +224,65 @@ BOOST_AUTO_TEST_CASE(coin_selection_tests)
         BOOST_CHECK_EQUAL(nValueRet, 500000 * COIN); // we should get the exact amount
         BOOST_CHECK_EQUAL(setCoinsRet.size(), 10U); // in ten coins
 
-        // if there's not enough in the smaller coins to make at least 1 cent change (0.5+0.6+0.7 < 1.0+1.0),
+        // if there's not enough in the smaller coins to make at least 1 * MIN_CHANGE change (0.5+0.6+0.7 < 1.0+1.0),
         // we need to try finding an exact subset anyway
 
         // sometimes it will fail, and so we use the next biggest coin:
         empty_wallet();
-        add_coin(0.5 * CENT);
-        add_coin(0.6 * CENT);
-        add_coin(0.7 * CENT);
-        add_coin(1111 * CENT);
-        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * CENT, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1111 * CENT); // we get the bigger coin
+        add_coin(0.5 * MIN_CHANGE);
+        add_coin(0.6 * MIN_CHANGE);
+        add_coin(0.7 * MIN_CHANGE);
+        add_coin(1111 * MIN_CHANGE);
+        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * MIN_CHANGE, 1, 1, vCoins, setCoinsRet, nValueRet));
+        BOOST_CHECK_EQUAL(nValueRet, 1111 * MIN_CHANGE); // we get the bigger coin
         BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
 
         // but sometimes it's possible, and we use an exact subset (0.4 + 0.6 = 1.0)
         empty_wallet();
-        add_coin(0.4 * CENT);
-        add_coin(0.6 * CENT);
-        add_coin(0.8 * CENT);
-        add_coin(1111 * CENT);
-        BOOST_CHECK( wallet.SelectCoinsMinConf(1 * CENT, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1 * CENT);   // we should get the exact amount
+        add_coin(0.4 * MIN_CHANGE);
+        add_coin(0.6 * MIN_CHANGE);
+        add_coin(0.8 * MIN_CHANGE);
+        add_coin(1111 * MIN_CHANGE);
+        BOOST_CHECK( wallet.SelectCoinsMinConf(MIN_CHANGE, 1, 1, vCoins, setCoinsRet, nValueRet));
+        BOOST_CHECK_EQUAL(nValueRet, MIN_CHANGE);   // we should get the exact amount
         BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U); // in two coins 0.4+0.6
 
-        // test avoiding sub-cent change
+        // test avoiding small change
         empty_wallet();
-        add_coin(0.0005 * COIN);
-        add_coin(0.01 * COIN);
-        add_coin(1 * COIN);
+        add_coin(0.05 * MIN_CHANGE);
+        add_coin(1    * MIN_CHANGE);
+        add_coin(100  * MIN_CHANGE);
 
-        // trying to make 1.0001 from these three coins
-        BOOST_CHECK( wallet.SelectCoinsMinConf(1.0001 * COIN, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1.0105 * COIN);   // we should get all coins
+        // trying to make 100.01 from these three coins
+        BOOST_CHECK( wallet.SelectCoinsMinConf(100.01 * MIN_CHANGE, 1, 1, vCoins, setCoinsRet, nValueRet));
+        BOOST_CHECK_EQUAL(nValueRet, 101.05 * MIN_CHANGE);   // we should get all coins
         BOOST_CHECK_EQUAL(setCoinsRet.size(), 3U);
 
-        // but if we try to make 0.999, we should take the bigger of the two small coins to avoid sub-cent change
-        BOOST_CHECK( wallet.SelectCoinsMinConf(0.999 * COIN, 1, 1, vCoins, setCoinsRet, nValueRet));
-        BOOST_CHECK_EQUAL(nValueRet, 1.01 * COIN);   // we should get 1 + 0.01
+        // but if we try to make 99.9, we should take the bigger of the two small coins to avoid small change
+        BOOST_CHECK( wallet.SelectCoinsMinConf(99.9 * MIN_CHANGE, 1, 1, vCoins, setCoinsRet, nValueRet));
+        BOOST_CHECK_EQUAL(nValueRet, 101 * MIN_CHANGE);
         BOOST_CHECK_EQUAL(setCoinsRet.size(), 2U);
 
+        // test with many inputs
+        for (CAmount amt=1500; amt < COIN; amt*=10) {
+             empty_wallet();
+             // Create 676 inputs (= MAX_STANDARD_TX_SIZE / 148 bytes per input)
+             for (uint16_t j = 0; j < 676; j++)
+                 add_coin(amt);
+             BOOST_CHECK(wallet.SelectCoinsMinConf(2000, 1, 1, vCoins, setCoinsRet, nValueRet));
+             if (amt - 2000 < MIN_CHANGE) {
+                 // needs more than one input:
+                 uint16_t returnSize = std::ceil((2000.0 + MIN_CHANGE)/amt);
+                 CAmount returnValue = amt * returnSize;
+                 BOOST_CHECK_EQUAL(nValueRet, returnValue);
+                 BOOST_CHECK_EQUAL(setCoinsRet.size(), returnSize);
+             } else {
+                 // one input is sufficient:
+                 BOOST_CHECK_EQUAL(nValueRet, amt);
+                 BOOST_CHECK_EQUAL(setCoinsRet.size(), 1U);
+             }
+        }
+
         // test randomness
         {
             empty_wallet();
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index c3b1172201..2414532f75 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -47,7 +47,7 @@ bool fPayAtLeastCustomFee = true;
  * Fees smaller than this (in satoshi) are considered zero fee (for transaction creation)
  * Override with -mintxfee
  */
-CFeeRate CWallet::minTxFee = CFeeRate(1000);
+CFeeRate CWallet::minTxFee = CFeeRate(DEFAULT_TRANSACTION_MINFEE);
 
 /** @defgroup mapWallet
  *
@@ -1498,9 +1498,6 @@ CAmount CWallet::GetImmatureWatchOnlyBalance() const
     return nTotal;
 }
 
-/**
- * populate vCoins with vector of available COutputs.
- */
 void CWallet::AvailableCoins(vector<COutput>& vCoins, bool fOnlyConfirmed, const CCoinControl *coinControl, bool fIncludeZeroValue) const
 {
     vCoins.clear();
@@ -1620,7 +1617,7 @@ bool CWallet::SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int
             nValueRet += coin.first;
             return true;
         }
-        else if (n < nTargetValue + CENT)
+        else if (n < nTargetValue + MIN_CHANGE)
         {
             vValue.push_back(coin);
             nTotalLower += n;
@@ -1655,14 +1652,14 @@ bool CWallet::SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int
     vector<char> vfBest;
     CAmount nBest;
 
-    ApproximateBestSubset(vValue, nTotalLower, nTargetValue, vfBest, nBest, 1000);
-    if (nBest != nTargetValue && nTotalLower >= nTargetValue + CENT)
-        ApproximateBestSubset(vValue, nTotalLower, nTargetValue + CENT, vfBest, nBest, 1000);
+    ApproximateBestSubset(vValue, nTotalLower, nTargetValue, vfBest, nBest);
+    if (nBest != nTargetValue && nTotalLower >= nTargetValue + MIN_CHANGE)
+        ApproximateBestSubset(vValue, nTotalLower, nTargetValue + MIN_CHANGE, vfBest, nBest);
 
     // If we have a bigger coin and (either the stochastic approximation didn't find a good solution,
     //                                   or the next bigger coin is closer), return the bigger coin
     if (coinLowestLarger.second.first &&
-        ((nBest != nTargetValue && nBest < nTargetValue + CENT) || coinLowestLarger.first <= nBest))
+        ((nBest != nTargetValue && nBest < nTargetValue + MIN_CHANGE) || coinLowestLarger.first <= nBest))
     {
         setCoinsRet.insert(coinLowestLarger.second);
         nValueRet += coinLowestLarger.first;
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index bd30b67b09..143ca22bdc 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -41,8 +41,12 @@ extern bool fPayAtLeastCustomFee;
 static const CAmount DEFAULT_TRANSACTION_FEE = 0;
 //! -paytxfee will warn if called with a higher fee than this amount (in satoshis) per KB
 static const CAmount nHighTransactionFeeWarning = 0.01 * COIN;
+//! -mintxfee default
+static const CAmount DEFAULT_TRANSACTION_MINFEE = 1000;
 //! -maxtxfee default
 static const CAmount DEFAULT_TRANSACTION_MAXFEE = 0.1 * COIN;
+//! minimum change amount
+static const CAmount MIN_CHANGE = CENT;
 //! -txconfirmtarget default
 static const unsigned int DEFAULT_TX_CONFIRM_TARGET = 2;
 //! -maxtxfee will warn if called with a higher fee than this amount (in satoshis)
@@ -544,7 +548,14 @@ public:
     //! check whether we are allowed to upgrade (or already support) to the named feature
     bool CanSupportFeature(enum WalletFeature wf) { AssertLockHeld(cs_wallet); return nWalletMaxVersion >= wf; }
 
+    /**
+     * populate vCoins with vector of available COutputs.
+     */
     void AvailableCoins(std::vector<COutput>& vCoins, bool fOnlyConfirmed=true, const CCoinControl *coinControl = NULL, bool fIncludeZeroValue=false) const;
+    /**
+     * Select coins until nTargetValue is reached. Return the actual value
+     * and the corresponding coin set.
+     */
     bool SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int nConfTheirs, std::vector<COutput> vCoins, std::set<std::pair<const CWalletTx*,unsigned int> >& setCoinsRet, CAmount& nValueRet) const;
 
     bool IsSpent(const uint256& hash, unsigned int n) const;