From 51293f733e8d994c694c7b8b43f7585f7542545d Mon Sep 17 00:00:00 2001
From: saizsassin <39055732+saizsassin@users.noreply.github.com>
Date: Thu, 10 May 2018 15:01:04 +0530
Subject: [PATCH] FIFO selection wallet.cpp

This wallet.cpp is selects the UTXOs based on FIFO. Coin selection is done in selectCoins function
---
 src/wallet/wallet.cpp | 59 ++++++++++++++++++++++++-------------------
 1 file changed, 33 insertions(+), 26 deletions(-)

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 93dd40b7c..00abd0c6e 100755
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2458,12 +2458,16 @@ bool CWallet::SelectCoinsMinConf(const CAmount& nTargetValue, const int nConfMin
 
 bool CWallet::SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet, const CCoinControl* coinControl) const
 {
+	/*This selectCoins function selects the coins (UTXOs) in FIFO order i.e, the coins that is received first will be sent to spending first*/ 
+
+	//get the received time of each coin and store in set time (using set because the content will be automatically stored in ascending order of the data)
     	std::vector<COutput> vCoinsTmp(vAvailableCoins);
 	std::vector<COutput>::iterator it;
 	std::set<unsigned int> time;
 	for(it=vCoinsTmp.begin();it!=vCoinsTmp.end();it++)
-		time.insert(it->tx->nTimeReceived);
+		time.insert(it->tx->nTimeReceived); //nTimeReceived is the received time of the UTXO
 
+	//store UTXOs in vCoins in the ascending order of time
 	std::vector<COutput> vCoins;
 	vCoins.clear();
 	for(std::set<unsigned int>::iterator its = time.begin();its!=time.end();its++)
@@ -2471,7 +2475,7 @@ bool CWallet::SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAm
 			if(it->tx->nTimeReceived == *its)
 				vCoins.push_back(*it);
 	
-    // coin control -> return all selected outputs (we want all selected to go into the transaction for sure)
+    // coin control -> return all selected outputs (we want all selected to go into the transaction for sure) [manual selection] has no effect in default FIFO selection
     if (coinControl && coinControl->HasSelected() && !coinControl->fAllowOtherInputs)
     {
         for (const COutput& out : vCoins)
@@ -2484,31 +2488,31 @@ bool CWallet::SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAm
         return (nValueRet >= nTargetValue);
     }
 
-    // calculate value from preset inputs and store them
-    std::vector<CInputCoin> setPresetCoins;
-    CAmount nValueFromPresetInputs = 0;
+    // calculate value from preset inputs and store them in PresetCoins set
+    	std::vector<CInputCoin> setPresetCoins;
+    	CAmount nValueFromPresetInputs = 0;
 	setPresetCoins.clear();
 
-    std::vector<COutPoint> vPresetInputs;
-    if (coinControl)
-        coinControl->ListSelected(vPresetInputs);
-    for (const COutPoint& outpoint : vPresetInputs)
-    {
-        std::map<uint256, CWalletTx>::const_iterator it = mapWallet.find(outpoint.hash);
-        if (it != mapWallet.end())
-        {
-            const CWalletTx* pcoin = &it->second;
-            // Clearly invalid input, fail
-            if (pcoin->tx->vout.size() <= outpoint.n)
-                return false;
-            nValueFromPresetInputs += pcoin->tx->vout[outpoint.n].nValue;
-            setPresetCoins.push_back(CInputCoin(pcoin, outpoint.n));
-        } else
-            return false; // TODO: Allow non-wallet inputs
+    	std::vector<COutPoint> vPresetInputs;
+    	if (coinControl)
+        	coinControl->ListSelected(vPresetInputs);
+    	for (const COutPoint& outpoint : vPresetInputs)
+    	{
+		std::map<uint256, CWalletTx>::const_iterator it = mapWallet.find(outpoint.hash);
+		if (it != mapWallet.end())
+		{
+		    const CWalletTx* pcoin = &it->second;
+		    // Clearly invalid input, fail
+		    if (pcoin->tx->vout.size() <= outpoint.n)
+		        return false;
+		    nValueFromPresetInputs += pcoin->tx->vout[outpoint.n].nValue;
+		    setPresetCoins.push_back(CInputCoin(pcoin, outpoint.n));
+		} else
+            	return false; // TODO: Allow non-wallet inputs
     }
 	
 
-    // remove preset inputs from vCoins
+    // remove preset inputs from vCoins (utxo)
     for (std::vector<COutput>::iterator it = vCoins.begin(); it != vCoins.end() && coinControl && coinControl->HasSelected();)
     {
         if (std::find(setPresetCoins.begin(), setPresetCoins.end(), CInputCoin(it->tx, it->i)) != setPresetCoins.end())
@@ -2516,14 +2520,17 @@ bool CWallet::SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAm
         else
             ++it;
     }
-	
+    
+    //insert all preset Inputs to the setCoinsRet (returning set of selected utxo)	
     setCoinsRet.clear();
     setCoinsRet.insert(setPresetCoins.begin(), setPresetCoins.end());
     nValueRet = nValueFromPresetInputs;
 
-    if (nValueFromPresetInputs >= nTargetValue)
+    //return true if total preset input value is greater than required amount
+    if (nValueFromPresetInputs >= nTargetValue) 
         return true;
 
+    //select UTXOs from vCoins and insert into setCoinsRet (returning set of selected utxo) until required amount is obtained
     CAmount nRemainReqValue = nTargetValue - nValueFromPresetInputs;
     for (const COutput &output : vCoins)
     {
@@ -2537,13 +2544,13 @@ bool CWallet::SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAm
         nRemainReqValue -= coin.txout.nValue;
         nValueRet += coin.txout.nValue;
 
-        //return true if coins of required Value are selected
+        //return true if coins of required amount are selected
         if (nRemainReqValue <= 0)
             return true;
 
     }
 
-    //return false since coins of required Value are not selected
+    //return false since coins of required amount is not available
     return false;
 }