From 63bba49ef4c9203cb996ca5cb45dc16dde934251 Mon Sep 17 00:00:00 2001
From: ikyokushin <vla.genya@mail.ru>
Date: Fri, 19 Feb 2021 12:36:35 +1000
Subject: [PATCH] Stake modifier work Change MAKX_MONEY value from 21M to 10M.
 Sync improvements for older clients. TimeHash wrapper for stake modifier hash
 timestamps (fixes old uint256 sorting issue). Some further work to stake
 modifier genetion (WIP) Added some debug code for debugging stake modier
 generation :q q

---
 src/amount.h       |   2 +-
 src/chain.h        |   1 +
 src/init.cpp       |   2 +-
 src/pos.cpp        | 572 ++++++++++++++++++++++++++++++++++++++++++++-
 src/validation.cpp |  19 ++
 src/validation.h   |   2 +-
 6 files changed, 587 insertions(+), 11 deletions(-)

diff --git a/src/amount.h b/src/amount.h
index 47968e80b1..b42ebe43cd 100644
--- a/src/amount.h
+++ b/src/amount.h
@@ -22,7 +22,7 @@ static const CAmount COIN = 100000000;
  * critical; in unusual circumstances like a(nother) overflow bug that allowed
  * for the creation of coins out of thin air modification could lead to a fork.
  * */
-static const CAmount MAX_MONEY = 21000000 * COIN;
+static const CAmount MAX_MONEY = 10000000 * COIN;
 inline bool MoneyRange(const CAmount& nValue) { return (nValue >= 0 && nValue <= MAX_MONEY); }
 
 #endif //  BITCOIN_AMOUNT_H
diff --git a/src/chain.h b/src/chain.h
index 6b376aeac3..2dd66a9fe0 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -69,6 +69,7 @@ class CBlockFileInfo
          nUndoSize = 0;
          nFlags = 0;
          nStakeModifier = 0;
+         nStakeModifierChecksum = 0;
          hashProofOfStake = arith_uint256();
          prevoutStake.SetNull();
          nStakeTime = 0;
diff --git a/src/init.cpp b/src/init.cpp
index 1149a827ec..7fe3f5b1ff 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -59,7 +59,7 @@
 #include <util/threadnames.h>
 #include <util/translation.h>
 #include <validation.h>
-
+#include <pos.h>
 #include <validationinterface.h>
 #include <walletinitinterface.h>
 
diff --git a/src/pos.cpp b/src/pos.cpp
index 5a3b20b755..f0aa576f88 100644
--- a/src/pos.cpp
+++ b/src/pos.cpp
@@ -3,17 +3,573 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include "pos.h"
+#include <pos.h>
 
-#include "arith_uint256.h"
-#include "chain.h"
-#include "primitives/block.h"
-#include "uint256.h"
-#include "util.h"
+#include <arith_uint256.h>
+#include <chain.h>
+#include <primitives/block.h>
+#include <uint256.h>
+#include <utilmoneystr.h>
+#include <policy/policy.h>
+#include <addrman.h>
+#include <util.h>
+#include <streams.h>
+#include <hash.h>
+#include <miner.h>
+#include <utiltime.h>
+#include <chainparams.h>
+#include <consensus/validation.h>
+#include <script/interpreter.h>
+#include <index/txindex.h>
+#include <validation.h>
 
+// wrapper class for block timestamp/hash pair to enable custom sorting that matches the legacy Cloak codebase
+class TimeHash : public std::pair<int64_t, arith_uint256>
+{
+public:
+    TimeHash(int64_t time, arith_uint256 hash)
+    {
+        this->first = time;
+        this->second = hash;
+    }
+
+    friend inline bool operator<(const TimeHash& a, const TimeHash& b)
+    {
+        return a.first < b.first || (a.first == b.first && a.second < b.second);
+    }       
+};
+
+// Hashed
+typedef std::vector<TimeHash> TimestampedHashes;
+
+std::set<std::pair<COutPoint, unsigned int> > setStakeSeen;
+std::map<uint256, uint256> mapProofOfStake;
+// Hard checkpoints of stake modifiers to ensure they are deterministic
+static std::map<int, unsigned int> mapStakeModifierCheckpoints = {{0, 0xfd11f4e7u}};
+
+// miner's coin stake reward based on nBits and coin age spent (coin-days)
+// simple algorithm, not depend on the diff
+int64_t GetProofOfStakeReward(int64_t nCoinAge, unsigned int nBits, unsigned int nTime, int nHeight)
+{
+    int64_t nRewardCoinYear;
+    nRewardCoinYear = MAX_MINT_PROOF_OF_STAKE;
+    int64_t nSubsidy = nCoinAge * nRewardCoinYear / 365;
+
+    //if (gArgs.GetBoolArg("-printcreation", false))
+     //   LogPrint("GetProofOfStakeReward(): create=%s nCoinAge=%u nBits=%d\n", FormatMoney(nSubsidy).c_str(), nCoinAge, nBits);
+
+    return nSubsidy;
+}
+
+// Get selection interval section (in seconds)
+static int64_t GetStakeModifierSelectionIntervalSection(int nSection)
+{
+    assert(nSection >= 0 && nSection < 64);
+    unsigned int nModifierInterval = Params().GetConsensus().nStakeModifierInterval;
+    int64_t a = nModifierInterval * 63 / (63 + ((63 - nSection) * (MODIFIER_INTERVAL_RATIO - 1)));
+    return a;
+}
+
+// Get stake modifier selection interval (in seconds)
+static int64_t GetStakeModifierSelectionInterval()
+{
+    int64_t nSelectionInterval = 0;
+    for (int nSection = 0; nSection < 64; nSection++)
+    {
+        nSelectionInterval += GetStakeModifierSelectionIntervalSection(nSection);
+    }
+    return nSelectionInterval;
+}
+
+// Get the last stake modifier and its generation time from a given block
+static bool GetLastStakeModifier(const CBlockIndex* pindex, uint64_t& nStakeModifier, int64_t& nModifierTime)
+{
+    if (!pindex)
+        return error("GetLastStakeModifier: null pindex");
+    while (pindex && pindex->pprev && !pindex->GeneratedStakeModifier())
+        pindex = pindex->pprev;
+    if (!pindex->GeneratedStakeModifier())
+        return error("GetLastStakeModifier: no generation at genesis block");
+    nStakeModifier = pindex->nStakeModifier;
+    nModifierTime = pindex->GetBlockTime();
+    return true;
+}
+
+// select a block from the candidate blocks in vSortedByTimestamp, excluding
+// already selected blocks in vSelectedBlocks, and with timestamp up to
+// nSelectionIntervalStop.
+static bool SelectBlockFromCandidates(
+    TimestampedHashes& vSortedByTimestamp,
+    std::map<uint256, const CBlockIndex*>& mapSelectedBlocks,
+    int64_t nSelectionIntervalStop, uint64_t nStakeModifierPrev,
+    const CBlockIndex** pindexSelected)
+{
+    bool fSelected = false;
+    arith_uint256 hashBest = arith_uint256();
+
+    *pindexSelected = (const CBlockIndex*)0;
+
+    std::vector<const CBlockIndex*> indexes;
+    for (const std::pair<int64_t, arith_uint256>& item : vSortedByTimestamp) 
+    {
+        if (!mapBlockIndex.count(ArithToUint256(item.second)))
+            return error("SelectBlockFromCandidates: failed to find block index for candidate block %s", item.second.ToString().c_str());
+        const CBlockIndex* pindex = mapBlockIndex[ArithToUint256(item.second)];
+        indexes.push_back(pindex);
+        if (fSelected && pindex->GetBlockTime() > nSelectionIntervalStop)
+            break;
+        if (mapSelectedBlocks.count(pindex->GetBlockHash()) > 0)
+            continue;
+        // compute the selection hash by hashing its proof-hash and the
+        // previous proof-of-stake modifier
+        uint256 hashProof = pindex->IsProofOfStake() ? ArithToUint256(pindex->hashProofOfStake) : pindex->GetBlockHash();
+        std::string strHashProof = hashProof.GetHex();
+        CDataStream ss(SER_GETHASH, 0);
+        ss << hashProof << nStakeModifierPrev;        
+        arith_uint256 hashSelection = UintToArith256(Hash(ss.begin(), ss.end()));
+        std::string strHashSelection = hashSelection.GetHex();
+        // the selection hash is divided by 2**32 so that proof-of-stake block
+        // is always favored over proof-of-work block. this is to preserve
+        // the energy efficiency property
+        if (pindex->IsProofOfStake())
+            hashSelection >>= 32;
+                
+        if (fSelected && hashSelection < hashBest)
+        {
+            hashBest = hashSelection;
+            *pindexSelected = (const CBlockIndex*)pindex;
+        }
+        else if (!fSelected)
+        {
+            fSelected = true;
+            hashBest = hashSelection;
+            *pindexSelected = (const CBlockIndex*)pindex;
+        }
+    }
+    if (gArgs.GetBoolArg("-printstakemodifier", false))
+        LogPrintf("SelectBlockFromCandidates: selection hash=%s\n", hashBest.ToString().c_str());
+    return fSelected;
+}
 
+// Stake Modifier (hash modifier of proof-of-stake):
+// The purpose of stake modifier is to prevent a txout (coin) owner from
+// computing future proof-of-stake generated by this txout at the time
+// of transaction confirmation. To meet kernel protocol, the txout
+// must hash with a future stake modifier to generate the proof.
+// Stake modifier consists of bits each of which is contributed from a
+// selected block of a given block group in the past.
+// The selection of a block is based on a hash of the block's proof-hash and
+// the previous stake modifier.
+// Stake modifier is recomputed at a fixed time interval instead of every 
+// block. This is to make it difficult for an attacker to gain control of
+// additional bits in the stake modifier, even after generating a chain of
+// blocks.
+bool ComputeNextStakeModifier(const CBlockIndex* pindexPrev, uint64_t& nStakeModifier, bool& fGeneratedStakeModifier)
+{
+    nStakeModifier = 0;
+    fGeneratedStakeModifier = false;
+    if (!pindexPrev)
+    {
+        fGeneratedStakeModifier = true;
+        return true;  // genesis block's modifier is 0
+    }
+    // First find current stake modifier and its generation block time
+    // if it's not old enough, return the same stake modifier
+    int64_t nModifierTime = 0;
+    if (!GetLastStakeModifier(pindexPrev, nStakeModifier, nModifierTime))
+        return error("ComputeNextStakeModifier: unable to get last modifier");
+    
+    LogPrint(BCLog::SELECTCOINS, "ComputeNextStakeModifier: prev modifier=0x%016x time=%s\n", nStakeModifier, DateTimeStrFormat("%Y-%m-%d %H:%M:%S", nModifierTime).c_str());
+
+    unsigned int nModifierInterval = Params().GetConsensus().nStakeModifierInterval;
+    
+    if (nModifierTime / nModifierInterval >= pindexPrev->GetBlockTime() / nModifierInterval)
+        return true;
+
+    // Sort candidate blocks by timestamp
+    TimestampedHashes vSortedByTimestamp;
+    vSortedByTimestamp.reserve(64 * nModifierInterval / Params().GetConsensus().nStakeTargetSpacing);
+    int64_t nSelectionInterval = GetStakeModifierSelectionInterval();
+    int64_t nSelectionIntervalStart = (pindexPrev->GetBlockTime() / nModifierInterval) * nModifierInterval - nSelectionInterval;
+    const CBlockIndex* pindex = pindexPrev;
+    int index = 0;
+    while (pindex && pindex->GetBlockTime() >= nSelectionIntervalStart)
+    {
+        //LogPrintf("Adding %d height = %d hash = %s\n", index, pindex->nHeight, pindex->GetBlockHash().GetHex().c_str());
+        uint256 hash = pindex->GetBlockHash();
+        vSortedByTimestamp.push_back(TimeHash(pindex->GetBlockTime(), UintToArith256(hash)));
+        pindex = pindex->pprev;
+        index++;
+    }
+    int nHeightFirstCandidate = pindex ? (pindex->nHeight + 1) : 0;
+
+    reverse(vSortedByTimestamp.begin(), vSortedByTimestamp.end());   
+    // custom sort the hashed timestamps
+    sort(vSortedByTimestamp.begin(), vSortedByTimestamp.end());
+
+    // Select 64 blocks from candidate blocks to generate stake modifier
+    uint64_t nStakeModifierNew = 0;
+    int64_t nSelectionIntervalStop = nSelectionIntervalStart;
+    std::map<uint256, const CBlockIndex*> mapSelectedBlocks;
+    for (int nRound = 0; nRound < std::min(64, (int)vSortedByTimestamp.size()); nRound++)
+    { 
+        // add an interval section to the current selection round
+        nSelectionIntervalStop += GetStakeModifierSelectionIntervalSection(nRound);
+        // select a block from the candidates of current round
+        if (!SelectBlockFromCandidates(vSortedByTimestamp, mapSelectedBlocks, nSelectionIntervalStop, nStakeModifier, &pindex))
+            return error("ComputeNextStakeModifier: unable to select block at round %d", nRound);
+
+        // write the entropy bit of the selected block
+        nStakeModifierNew |= (((uint64_t)pindex->GetStakeEntropyBit()) << nRound);
+        // add the selected block from candidates to selected list
+        mapSelectedBlocks.insert(std::make_pair(pindex->GetBlockHash(), pindex));
+        if (gArgs.GetBoolArg("-printstakemodifier", false))
+            LogPrintf("ComputeNextStakeModifier: selected round %d stop=%s height=%d bit=%d sm=%lu\n",
+                nRound, DateTimeStrFormat("%Y-%m-%d %H:%M:%S", nSelectionIntervalStop).c_str(), pindex->nHeight, pindex->GetStakeEntropyBit(), nStakeModifierNew);
+    }
+    // Print selection map for visualization of the selected blocks
+    if (gArgs.GetBoolArg("-printstakemodifier", false))
+    {
+        std::string strSelectionMap = "";
+        // '-' indicates proof-of-work blocks not selected
+        strSelectionMap.insert(0, pindexPrev->nHeight - nHeightFirstCandidate + 1, '-');
+        pindex = pindexPrev;
+        while (pindex && pindex->nHeight >= nHeightFirstCandidate)
+        {
+            // '=' indicates proof-of-stake blocks not selected
+            if (pindex->IsProofOfStake())
+                strSelectionMap.replace(pindex->nHeight - nHeightFirstCandidate, 1, "=");
+            pindex = pindex->pprev;
+        }
+        
+        for(const std::pair<uint256, const CBlockIndex*>& item : mapSelectedBlocks)
+        {
+            // 'S' indicates selected proof-of-stake blocks
+            // 'W' indicates selected proof-of-work blocks
+            strSelectionMap.replace(item.second->nHeight - nHeightFirstCandidate, 1, item.second->IsProofOfStake() ? "S" : "W");
+        }
+        LogPrint(BCLog::SELECTCOINS, "ComputeNextStakeModifier: selection height [%d, %d] map %s\n", nHeightFirstCandidate, pindexPrev->nHeight, strSelectionMap.c_str());
+    }
+    if (gArgs.GetBoolArg("-printstakemodifier", false))
+    {
+        LogPrint(BCLog::SELECTCOINS, "ComputeNextStakeModifier: new modifier=0x%016x time=%s\n", nStakeModifierNew, DateTimeStrFormat("%Y-%m-%d %H:%M:%S", pindexPrev->GetBlockTime()).c_str());
+    }
+
+    nStakeModifier = nStakeModifierNew;
+    fGeneratedStakeModifier = true;
+    return true;
+}
+
+// Get stake modifier checksum
+unsigned int GetStakeModifierChecksum(const CBlockIndex* pindex)
+{
+    assert(pindex->pprev || pindex->GetBlockHash() == Params().GenesisBlock().GetHash());
+    // Hash previous checksum with flags, hashProofOfStake and nStakeModifier
+    CDataStream ss(SER_GETHASH, 0);
+    if (pindex->pprev)
+        ss << pindex->pprev->nStakeModifierChecksum;
+    ss << pindex->nFlags << pindex->hashProofOfStake << pindex->nStakeModifier;
+    arith_uint256 hashChecksum = UintToArith256(Hash(ss.begin(), ss.end()));
+    hashChecksum >>= (256 - 32);
+    if (gArgs.GetBoolArg("-printstakemodifier", false))
+        LogPrint(BCLog::SELECTCOINS, "stake checksum : 0x % 016x", ArithToUint256(hashChecksum).GetUint64(0));
+    return hashChecksum.GetLow64();
+}
 
-bool CheckProofOfStake(const CTransaction& tx, unsigned int nBits, uint256& hashProofOfStake)
+// The stake modifier used to hash for a stake kernel is chosen as the stake
+// modifier about a selection interval later than the coin generating the kernel
+static bool GetKernelStakeModifier(uint256 hashBlockFrom, uint64_t& nStakeModifier, int& nStakeModifierHeight, int64_t& nStakeModifierTime, bool fPrintProofOfStake)
 {
+    nStakeModifier = 0;
+    if (!mapBlockIndex.count(hashBlockFrom))
+        return error("GetKernelStakeModifier() : block not indexed");
+    const CBlockIndex* pindexFrom = mapBlockIndex[hashBlockFrom];
+    nStakeModifierHeight = pindexFrom->nHeight;
+    nStakeModifierTime = pindexFrom->GetBlockTime();
+    int64_t nStakeModifierSelectionInterval = GetStakeModifierSelectionInterval();
+    const CBlockIndex* pindex = pindexFrom;
+
+    // loop to find the stake modifier later by a selection interval
+    while (nStakeModifierTime < pindexFrom->GetBlockTime() + nStakeModifierSelectionInterval)
+    {
+        if (!chainActive.Next(pindex))
+        {   // reached best block; may happen if node is behind on block chain
+            if (fPrintProofOfStake || (pindex->GetBlockTime() + Params().GetConsensus().nStakeMinAge - nStakeModifierSelectionInterval > GetAdjustedTime()))
+                return error("GetKernelStakeModifier() : reached best block %s at height %d from block %s",
+                    pindex->GetBlockHash().ToString().c_str(), pindex->nHeight, hashBlockFrom.ToString().c_str());
+            else
+            {
+                    LogPrint(BCLog::ALL, ">> nStakeModifierTime = %d, pindexFrom->GetBlockTime() = %d, nStakeModifierSelectionInterval = %d\n",
+                        nStakeModifierTime, pindexFrom->GetBlockTime(), nStakeModifierSelectionInterval);
+
+                return false;
+            }
+        }
+        pindex = chainActive.Next(pindex);
+        if (pindex->GeneratedStakeModifier())
+        {            
+            nStakeModifierHeight = pindex->nHeight;
+            nStakeModifierTime = pindex->GetBlockTime();
+        }
+    }
+    nStakeModifier = pindex->nStakeModifier;
+    return true;
+}
+
+// Check stake modifier hard checkpoints
+bool CheckStakeModifierCheckpoints(int nHeight, unsigned int nStakeModifierChecksum)
+{
+    if (Params().NetworkIDString() == "test") return true; // Testnet has no checkpoints
+    if (mapStakeModifierCheckpoints.count(nHeight))
+    {
+        return nStakeModifierChecksum == mapStakeModifierCheckpoints[nHeight];
+    }
+    return true;
+}
+
+// ppcoin kernel protocol
+// coinstake must meet hash target according to the protocol:
+// kernel (input 0) must meet the formula
+//     hash(nStakeModifier + txPrev.block.nTime + txPrev.offset + txPrev.nTime + txPrev.vout.n + nTime) < bnTarget * nCoinDayWeight
+// this ensures that the chance of getting a coinstake is proportional to the
+// amount of coin age one owns.
+// The reason this hash is chosen is the following:
+//   nStakeModifier: 
+//       (v0.3) scrambles computation to make it very difficult to precompute
+//              future proof-of-stake at the time of the coin's confirmation
+//       (v0.2) nBits (deprecated): encodes all past block timestamps
+//   txPrev.block.nTime: prevent nodes from guessing a good timestamp to
+//                       generate transaction for future advantage
+//   txPrev.offset: offset of txPrev inside block, to reduce the chance of 
+//                  nodes generating coinstake at the same time
+//   txPrev.nTime: reduce the chance of nodes generating coinstake at the same
+//                 time
+//   txPrev.vout.n: output number of txPrev, to reduce the chance of nodes
+//                  generating coinstake at the same time
+//   block/tx hash should not be used here as they can be generated in vast
+//   quantities so as to generate blocks faster, degrading the system back into
+//   a proof-of-work situation.
+//
+bool CheckStakeKernelHash(unsigned int nBits, CBlockIndex* pindexPrev, unsigned int nTxPrevOffset, const CTransactionRef txPrev, const COutPoint& prevout, unsigned int nTimeTx, uint256& hashProofOfStake,  bool fPrintProofOfStake)
+{
+    if (nTimeTx < txPrev->nTime)  // Transaction timestamp violation
+        return error("CheckStakeKernelHash() : nTime violation");
+
+    unsigned int nTimeBlockFrom = pindexPrev->GetBlockTime();
+    if (nTimeBlockFrom + Params().GetConsensus().nStakeMinAge > nTimeTx) // Min age requirement
+        return error("CheckStakeKernelHash() : min age violation");
+
+    arith_uint256 bnTargetPerCoinDay;
+    bnTargetPerCoinDay.SetCompact(nBits);
+    int64_t nValueIn = txPrev->vout[prevout.n].nValue;
+
+    // v0.3 protocol kernel hash weight starts from 0 at the min age
+    // this change increases active coins participating the hash and helps
+    // to secure the network when proof-of-stake difficulty is low
+    int64_t nTimeWeight = std::min((int64_t)nTimeTx - txPrev->nTime, (int64_t)Params().GetConsensus().nStakeMaxAge) - Params().GetConsensus().nStakeMinAge;
+    arith_uint512 bnCoinDayWeight = arith_uint512(nValueIn) * nTimeWeight / (COIN / 100) / (24 * 60 * 60);
+
+    // We need to convert to uint512 to prevent overflow when multiplying by 1st block coins
+    std::string bnTargetPerCoinDayHex = bnTargetPerCoinDay.GetHex();
+    arith_uint512 targetProofOfStake512(bnTargetPerCoinDayHex);
+    targetProofOfStake512 *= bnCoinDayWeight;
+    
+    // Calculate hash
+    CDataStream ss(SER_GETHASH, 0);
+    uint64_t nStakeModifier = 0;
+    int nStakeModifierHeight = 0;
+    int64_t nStakeModifierTime = 0;
+
+    if (!GetKernelStakeModifier(pindexPrev->GetBlockHash(), nStakeModifier, nStakeModifierHeight, nStakeModifierTime, fPrintProofOfStake))
+    {
+        LogPrint(BCLog::ALL, ">>> CheckStakeKernelHash: GetKernelStakeModifier return false\n");
+        return false; 
+    }
+
+    if (fPrintProofOfStake)
+        LogPrint(BCLog::ALL, ">>> CheckStakeKernelHash: passed GetKernelStakeModifier\n");
+
+    // create PoS hash
+    ss << nStakeModifier;
+    ss << nTimeBlockFrom << nTxPrevOffset << txPrev->nTime << prevout.n << nTimeTx;
+    hashProofOfStake = Hash(ss.begin(), ss.end());
+    if (fPrintProofOfStake)
+    {
+        LogPrint(BCLog::ALL, "CheckStakeKernelHash() : using modifier 0x%d at height=%d timestamp=%s for block from height=%d timestamp=%s\n",
+            nStakeModifier, nStakeModifierHeight,
+            DateTimeStrFormat("%Y-%m-%d %H:%M:%S", nStakeModifierTime).c_str(),
+            pindexPrev->nHeight,
+            DateTimeStrFormat("%Y-%m-%d %H:%M:%S", pindexPrev->GetBlockTime()).c_str());
+        LogPrint(BCLog::ALL, "CheckStakeKernelHash() : check protocol=%s modifier=0x%d nTimeBlockFrom=%u nTxPrevOffset=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
+            "0.3",
+            nStakeModifier,
+            nTimeBlockFrom, nTxPrevOffset, txPrev->nTime, prevout.n, nTimeTx,
+            hashProofOfStake.ToString().c_str());
+    }
+    
+    // We need to convert type so it can be compared to target
+    base_uint<512> hashProofOfStake512(hashProofOfStake.GetHex());
+    
+    // Now check if proof-of-stake hash meets target protocol
+    if (hashProofOfStake512 > targetProofOfStake512)
+    {
+        LogPrint(BCLog::ALL, ">>> bnCoinDayWeight = %s, bnTargetPerCoinDay=%s\n", bnCoinDayWeight.ToString().c_str(), bnTargetPerCoinDay.ToString().c_str());
+        LogPrint(BCLog::ALL, ">>> CheckStakeKernelHash - hashProofOfStake too much\n");
+        return false;
+    }
+
+    if (fPrintProofOfStake)
+    {
+        LogPrint(BCLog::ALL, "CheckStakeKernelHash() : using modifier 0x%d at height=%d timestamp=%s for block from height=%d timestamp=%s\n",
+            nStakeModifier, nStakeModifierHeight,
+            DateTimeStrFormat("%Y-%m-%d %H:%M:%S", nStakeModifierTime).c_str(),
+            mapBlockIndex[pindexPrev->GetBlockHash()]->nHeight,
+            DateTimeStrFormat("%Y-%m-%d %H:%M:%S", pindexPrev->GetBlockTime()).c_str());
+
+        LogPrint(BCLog::ALL, "CheckStakeKernelHash() : pass protocol=%s modifier=0x%d nTimeBlockFrom=%u nTxPrevOffset=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
+            "0.3",
+            nStakeModifier,
+            nTimeBlockFrom, nTxPrevOffset, txPrev->nTime, prevout.n, nTimeTx,
+            hashProofOfStake.ToString().c_str());
+    }
+    return true;
+}
+
+// ppcoin: total coin age spent in block, in the unit of coin-days.
+bool GetCoinAgeBlock(uint64_t& nCoinAge, CBlock block)
+{
+    nCoinAge = 0;
+    for (const CTransactionRef& tx : block.vtx)
+    {
+        uint64_t nTxCoinAge;
+        if (GetCoinAge(nTxCoinAge, tx))
+            nCoinAge += nTxCoinAge;
+        else
+            return false;
+    }
+
+    if (nCoinAge == 0) // block coin age minimum 1 coin-day
+        nCoinAge = 1;
+
+    //if (fDebug && GetBoolArg("-printcoinage"))
+    //    printf("block coin age total nCoinDays=%" PRI64d "\n", nCoinAge);
+    return true;
+}
+
+// ppcoin: total coin age spent in transaction, in the unit of coin-days.
+// Only those coins meeting minimum age requirement counts. As those
+// transactions not in main chain are not currently indexed so we
+// might not find out about their coin age. Older transactions are 
+// guaranteed to be in main chain by sync-checkpoint. This rule is
+// introduced to help nodes establish a consistent view of the coin
+// age (trust score) of competing branches.
+bool GetCoinAge(uint64_t& nCoinAge, CTransactionRef tx) 
+{
+    arith_uint512 bnCentSecond = 0;  // coin age in the unit of cent-seconds
+    nCoinAge = 0;
+
+    if (tx->IsCoinBase())
+        return true;
+
+    //BOOST_FOREACH(const CTxIn& txin, vin)
+    for (const CTxIn& txin : tx->vin) {
+        // First try finding the previous transaction in database
+        CTransactionRef txPrev;
+        uint256 hashPrevBlock = uint256();
+
+        /*
+            if (!GetTransaction(txin.prevout.hash, txPrev, Params().GetConsensus(), hashPrevBlock, true))
+            return error("CheckProofOfStake() : INFO: read txPrev failed");  // previous transaction not in main chain, may occur during initial download
+        */
+
+        // get input tx and block ref containing for block including input tx
+        if (!GetTransaction(txin.prevout.hash, txPrev, Params().GetConsensus(), hashPrevBlock, true))
+            continue;  // previous transaction not in main chain
+
+        if (tx->nTime < txPrev->nTime)
+            return false;  // Transaction timestamp violation
+
+        // Read block header
+        CBlockIndex* pblockindex = mapBlockIndex[hashPrevBlock];
+        CBlock blockPrev;
+        CDiskBlockPos blockPos = pblockindex->GetBlockPos();
+
+        if (!ReadBlockFromDisk(blockPrev, blockPos, Params().GetConsensus()))
+            return false; // unable to read block of previous transaction
+
+        if (blockPrev.GetBlockTime() + Params().GetConsensus().nStakeMinAge > tx->nTime)
+            continue; // only count coins meeting min age requirement
+
+        int64_t nValueIn = txPrev->vout[txin.prevout.n].nValue;
+        bnCentSecond += arith_uint512(nValueIn) * (tx->nTime - txPrev->nTime) / CENT;
+
+        //if (gArgs.GetArg("-printcoinage", false))
+        //    printf("coin age nValueIn=%ld nTimeDiff=%d bnCentSecond=%s\n", nValueIn, nTime - txPrev.nTime, bnCentSecond.ToString().c_str());
+    }
+
+    arith_uint512 bnCoinDay = bnCentSecond * CENT / COIN / (24 * 60 * 60);
+    if (gArgs.GetArg("-printcoinage", false))
+        printf("coin age bnCoinDay=%s\n", bnCoinDay.ToString().c_str());
+    nCoinAge = bnCoinDay.GetLow64();
+    return true;
+}
+
+// Check kernel hash target and coinstake signature
+bool CheckProofOfStake(const CTransactionRef tx, unsigned int nBits, uint256& hashProofOfStake, std::vector<CScriptCheck> *pvChecks, bool fCHeckSignature)
+{
+    if (!tx->IsCoinStake())
+        return error("CheckProofOfStake() : called on non-coinstake %s", tx->GetHash().ToString().c_str());
+
+    // Kernel (input 0) must match the stake hash target per coin age (nBits)
+    const CTxIn& txin = tx->vin[0];
+
+    // First try finding the previous transaction in database
+    CTransactionRef txPrev;
+    uint256 hashPrevBlock = uint256();
+
+    // get input tx and block ref containing for block including input tx
+    if (!GetTransaction(txin.prevout.hash, txPrev, Params().GetConsensus(), hashPrevBlock, true))
+        return error("CheckProofOfStake() : INFO: read txPrev failed");  // previous transaction not in main chain, may occur during initial download
+        
+    CCoinsViewCache inputs(pcoinsTip.get());
+
+    std::string hashBlockPrev = hashPrevBlock.GetHex();
+    std::string hashPrevTxIn = txin.prevout.hash.GetHex();
+    
+    if (fCHeckSignature)
+    {
+        PrecomputedTransactionData txdata(*tx);
+        const COutPoint &prevout = tx->vin[0].prevout;
+        const CTxOut& coinsOutput = inputs.AccessCoin(prevout).out;
+        //assert(coins);
+
+        // Verify signature
+        CScriptCheck check(coinsOutput, *tx, 0, SCRIPT_VERIFY_NONE, false, &txdata);
+        if (pvChecks) {
+            pvChecks->push_back(CScriptCheck());
+            check.swap(pvChecks->back());
+        }
+        else if (!check())
+            return error("CheckProofOfStake() : script-verify-failed %s", ScriptErrorString(check.GetScriptError()));
+    }
+
+    CBlockIndex* pblockindex = mapBlockIndex[hashPrevBlock];
+    CBlock blockPrev;
+    CDiskBlockPos blockPos = pblockindex->GetBlockPos();
+
+    if (!ReadBlockFromDisk(blockPrev, blockPos, Params().GetConsensus()))
+        return error("CheckProofOfStake() : read block failed");
+
+    int prevTxOffsetInBlock = blockPos.nPos + GetSerializeSize(CBlock(), SER_DISK, CLIENT_VERSION) - (2 * GetSizeOfCompactSize(0)) + GetSizeOfCompactSize(blockPrev.vtx.size());
+    for (auto& i : blockPrev.vtx) {
+        if (i->GetHash() == txPrev->GetHash())
+            break;
+        prevTxOffsetInBlock += GetSerializeSize(i, SER_DISK, CLIENT_VERSION);
+    }
+
+    if (!CheckStakeKernelHash(nBits, pblockindex, prevTxOffsetInBlock - blockPos.nPos, txPrev, txin.prevout, tx->nTime, hashProofOfStake))
+        return error("CheckProofOfStake() : INFO: check kernel failed on coinstake %s, hashProof=%s", tx->GetHash().ToString().c_str(), hashProofOfStake.ToString().c_str()); // may occur during initial download or if behind on block chain sync
+
+    return true;
+}
 
-}
\ No newline at end of file
diff --git a/src/validation.cpp b/src/validation.cpp
index 90dcfb254d..5d21593b57 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -3212,6 +3212,12 @@ CBlockIndex* BlockManager::AddToBlockIndex(const CBlockHeader& block)
 
     setDirtyBlockIndex.insert(pindexNew);
 
+	if (pindexNew->IsProofOfStake()) {
+        if (!mapProofOfStake.count(hash))
+            error("AddToBlockIndex() : hashProofOfStake not found in map");
+        pindexNew->hashProofOfStake = UintToArith256(mapProofOfStake[hash]);
+    }
+
     return pindexNew;
 }
 
@@ -3354,6 +3360,11 @@ bool CheckBlock(const CBlock& block, BlockValidationState& state, const Consensu
     if (block.fChecked)
         return true;
 
+    if (block.IsProofOfWork())
+        if (!CheckBlockHeader(block, state, consensusParams, false))
+            if (!CheckBlockHeader(block, state, consensusParams, false))
+                return false;
+
     // Check that the header is valid (particularly PoW).  This is mostly
     // redundant with the call in AcceptBlockHeader.
     if (!CheckBlockHeader(block, state, consensusParams, fCheckPOW))
@@ -4156,6 +4167,12 @@ bool BlockManager::LoadBlockIndex(
             pindex->BuildSkip();
         if (pindex->IsValid(BLOCK_VALID_TREE) && (pindexBestHeader == nullptr || CBlockIndexWorkComparator()(pindexBestHeader, pindex)))
             pindexBestHeader = pindex;
+
+		// ppcoin: calculate stake modifier checksum
+        pindex->nStakeModifierChecksum = GetStakeModifierChecksum(pindex);
+        if (!CheckStakeModifierCheckpoints(pindex->nHeight, pindex->nStakeModifierChecksum))
+            return error("CChainState::LoadBlockIndex() : Failed stake modifier checkpoint height=%d, modifier=0x%016x", pindex->nHeight, pindex->nStakeModifier);
+
     }
 
     return true;
@@ -4660,6 +4677,8 @@ bool CChainState::LoadGenesisBlock(const CChainParams& chainparams)
 
     try {
         const CBlock& block = chainparams.GenesisBlock();
+        CBlockIndex* pindex = AddToBlockIndex(block);
+
         FlatFilePos blockPos = SaveBlockToDisk(block, 0, chainparams, nullptr);
         if (blockPos.IsNull())
             return error("%s: writing genesis block to disk failed", __func__);
diff --git a/src/validation.h b/src/validation.h
index ffb038ad75..04e25d1152 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -72,7 +72,7 @@ static const int MAX_SCRIPTCHECK_THREADS = 15;
 static const int DEFAULT_SCRIPTCHECK_THREADS = 0;
 static const int64_t DEFAULT_MAX_TIP_AGE = 24 * 60 * 60;
 static const bool DEFAULT_CHECKPOINTS_ENABLED = true;
-static const bool DEFAULT_TXINDEX = false;
+static const bool DEFAULT_TXINDEX = true;
 static const char* const DEFAULT_BLOCKFILTERINDEX = "0";
 /** Default for -persistmempool */
 static const bool DEFAULT_PERSIST_MEMPOOL = true;