Create multiple transactions with one command.

RPC send commands have a new argument now that controls whether
the amount specified can be split across multiple transactions.
It defaults to single-transaction mode for now.

SelectCoinsForOneTx: use a sliding window algorithm to pick inputs.
This will pick one small input if possible, so that if change is
produced it does not result in an ever-growing pile of tiny utxos.

src/interfaces/wallet.cpp

@@ -502,12 +502,12 @@ class WalletImpl : public Wallet
                 break;
             }
             case OUTPUT_CT:
-                if (0 != pwalletAnon->AddBlindedInputs(wtx, rtx, recipients, !fCheckFeeOnly, nFeeRet, &coin_control, fail_reason))
+                if (0 != pwalletAnon->AddBlindedInputs(wtx, rtx, recipients, !fCheckFeeOnly, 0, nFeeRet, &coin_control, fail_reason))
                     fFailed = true;
                 break;
             case OUTPUT_RINGCT:
                 if (!pwalletAnon->AddAnonInputs(wtx, rtx, recipients, !fCheckFeeOnly, nRingSize,
-                                                nInputsPerSig, nFeeRet, &coin_control, fail_reason))
+                                                nInputsPerSig, 0, nFeeRet, &coin_control, fail_reason))
                     fFailed = true;
                 break;
             default:

src/veil/ringct/anonwallet.h

@@ -227,9 +227,9 @@ class AnonWallet
             CAmount &nFeeRet, const CCoinControl *coinControl, std::string &sError, bool fZerocoinInputs, CAmount nInputValue);
 
     int AddBlindedInputs(CWalletTx &wtx, CTransactionRecord &rtx, std::vector<CTempRecipient> &vecSend, bool sign,
-            CAmount &nFeeRet, const CCoinControl *coinControl, std::string &sError);
+            size_t nMaximumInputs, CAmount &nFeeRet, const CCoinControl *coinControl, std::string &sError);
     int AddBlindedInputs_Inner(CWalletTx &wtx, CTransactionRecord &rtx, std::vector<CTempRecipient> &vecSend, bool sign,
-            CAmount &nFeeRet, const CCoinControl *coinControl, std::string &sError);
+            size_t nMaximumInputs, CAmount &nFeeRet, const CCoinControl *coinControl, std::string &sError);
 
 
     bool PlaceRealOutputs(std::vector<std::vector<int64_t> > &vMI, size_t &nSecretColumn, size_t nRingSize, std::set<int64_t> &setHave,
@@ -240,10 +240,10 @@ class AnonWallet
 
     bool IsMyAnonInput(const CTxIn& txin, COutPoint& myOutpoint);
     bool AddAnonInputs_Inner(CWalletTx &wtx, CTransactionRecord &rtx, std::vector<CTempRecipient> &vecSend,
-         bool sign, size_t nRingSize, size_t nInputsPerSig, CAmount &nFeeRet,
+         bool sign, size_t nRingSize, size_t nInputsPerSig, size_t nMaximumInputs, CAmount &nFeeRet,
          const CCoinControl *coinControl, std::string &sError, bool fZerocoinInputs, CAmount nInputValue);
     bool AddAnonInputs(CWalletTx &wtx, CTransactionRecord &rtx, std::vector<CTempRecipient> &vecSend,
-         bool sign, size_t nRingSize, size_t nInputsPerSig, CAmount &nFeeRet,
+         bool sign, size_t nRingSize, size_t nInputsPerSig, size_t nMaximumInputs, CAmount &nFeeRet,
          const CCoinControl *coinControl, std::string &sError, bool fZerocoinInputs = false,
          CAmount nInputValue = 0);
 
@@ -322,15 +322,23 @@ class AnonWallet
      * populate vCoins with vector of available COutputs.
      */
     void AvailableBlindedCoins(std::vector<COutputR>& vCoins, bool fOnlySafe=true, const CCoinControl *coinControl = nullptr, const CAmount& nMinimumAmount = 1, const CAmount& nMaximumAmount = MAX_MONEY, const CAmount& nMinimumSumAmount = MAX_MONEY, const uint64_t& nMaximumCount = 0, const int& nMinDepth = 0, const int& nMaxDepth = 0x7FFFFFFF, bool fIncludeImmature=false) const;
-    bool SelectBlindedCoins(const std::vector<COutputR>& vAvailableCoins, const CAmount& nTargetValue, std::vector<std::pair<MapRecords_t::const_iterator,unsigned int> > &setCoinsRet, CAmount &nValueRet, const CCoinControl *coinControl = nullptr) const;
+    /**
+     * Returns a list of coins for a single transaction based on the target value and allowed number of inputs.
+     */
+    bool SelectBlindedCoins(const std::vector<COutputR>& vAvailableCoins, const CAmount& nTargetValue, size_t nMaximumCount, std::vector<std::pair<MapRecords_t::const_iterator,unsigned int> > &setCoinsRet, CAmount &nValueRet, const CCoinControl *coinControl = nullptr) const;
 
     void AvailableAnonCoins(std::vector<COutputR> &vCoins, bool fOnlySafe=true, const CCoinControl *coinControl = nullptr, const CAmount& nMinimumAmount = 1, const CAmount& nMaximumAmount = MAX_MONEY, const CAmount& nMinimumSumAmount = MAX_MONEY, const uint64_t& nMaximumCount = 0, const int& nMinDepth = 0, const int& nMaxDepth = 0x7FFFFFFF, bool fIncludeImmature=false) const;
 
     /**
      * Return list of available coins and locked coins grouped by non-change output address.
      */
 
-    bool SelectCoinsMinConf(const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutputR> vCoins, std::vector<std::pair<MapRecords_t::const_iterator,unsigned int> > &setCoinsRet, CAmount &nValueRet) const;
+    bool SelectCoinsMinConf(const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutputR> &vCoins, std::vector<std::pair<MapRecords_t::const_iterator,unsigned int> > &setCoinsRet, CAmount &nValueRet) const;
+    /**
+     * Like SelectCoinsMinConf, but always selects at most nMaximumCount coins,
+     * with the intention of being put in a single tx that partially accomplishes the target to be sent.
+     */
+    bool SelectCoinsForOneTx(const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<COutputR> &vCoins, size_t nMaximumCount, std::vector<std::pair<MapRecords_t::const_iterator,unsigned int> > &setCoinsRet, CAmount &nValueRet) const;
 
     bool IsSpent(const uint256& hash, unsigned int n) const EXCLUSIVE_LOCKS_REQUIRED(cs_main);