text: // Copyright (c) 2012-2013 The PPCoin developers
  // Copyright (c) 2014 The BlackCoin developers
  // Copyright (c) 2016-2018 The SantaCoin developers
  // Distributed under the MIT/X11 software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  
  #include <boost/assign/list_of.hpp>
  
  #include "kernel.h"
  #include "database/txdb.h"
  
  using namespace std;
  
  // Get time weight
  int64_t GetWeight(int64_t nIntervalBeginning, int64_t nIntervalEnd)
  {
      // 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
  
      return nIntervalEnd - nIntervalBeginning - nStakeMinAge;
  }
  
  // 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;
  }
  
  // Get selection interval section (in seconds)
  static int64_t GetStakeModifierSelectionIntervalSection(int nSection)
  {
      assert (nSection >= 0 && nSection < 64);
      return (nModifierInterval * 63 / (63 + ((63 - nSection) * (MODIFIER_INTERVAL_RATIO - 1))));
  }
  
  // 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;
  }
  
  // select a block from the candidate blocks in vSortedByTimestamp, excluding
  // already selected blocks in vSelectedBlocks, and with timestamp up to
  // nSelectionIntervalStop.
  static bool SelectBlockFromCandidates(vector<pair<int64_t, uint256> >& vSortedByTimestamp, map<uint256, const CBlockIndex*>& mapSelectedBlocks,
      int64_t nSelectionIntervalStop, uint64_t nStakeModifierPrev, const CBlockIndex** pindexSelected)
  {
      bool fSelected = false;
      uint256 hashBest = 0;
      *pindexSelected = (const CBlockIndex*) 0;
      BOOST_FOREACH(const PAIRTYPE(int64_t, uint256)& item, vSortedByTimestamp)
      {
          if (!mapBlockIndex.count(item.second))
              return error("SelectBlockFromCandidates: failed to find block index for candidate block %s", item.second.ToString());
          const CBlockIndex* pindex = mapBlockIndex[item.second];
          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
          CDataStream ss(SER_GETHASH, 0);
          ss << pindex->hashProof << nStakeModifierPrev;
          uint256 hashSelection = Hash(ss.begin(), ss.end());
          // 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;
          }
      }
      LogPrint("stakemodifier", "SelectBlockFromCandidates: selection hash=%s\n", hashBest.ToString());
      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("stakemodifier", "ComputeNextStakeModifier: prev modifier=0x%016x time=%s\n", nStakeModifier, DateTimeStrFormat(nModifierTime));
      if (nModifierTime / nModifierInterval >= pindexPrev->GetBlockTime() / nModifierInterval)
          return true;
  
      // Sort candidate blocks by timestamp
      vector<pair<int64_t, uint256> > vSortedByTimestamp;
      vSortedByTimestamp.reserve(64 * nModifierInterval / Params().TargetSpacing());
      int64_t nSelectionInterval = GetStakeModifierSelectionInterval();
      int64_t nSelectionIntervalStart = (pindexPrev->GetBlockTime() / nModifierInterval) * nModifierInterval - nSelectionInterval;
      const CBlockIndex* pindex = pindexPrev;
      while (pindex && pindex->GetBlockTime() >= nSelectionIntervalStart)
      {
          vSortedByTimestamp.push_back(make_pair(pindex->GetBlockTime(), pindex->GetBlockHash()));
          pindex = pindex->pprev;
      }
      int nHeightFirstCandidate = pindex ? (pindex->nHeight + 1) : 0;
      reverse(vSortedByTimestamp.begin(), vSortedByTimestamp.end());
      sort(vSortedByTimestamp.begin(), vSortedByTimestamp.end());
  
      // Select 64 blocks from candidate blocks to generate stake modifier
      uint64_t nStakeModifierNew = 0;
      int64_t nSelectionIntervalStop = nSelectionIntervalStart;
      map<uint256, const CBlockIndex*> mapSelectedBlocks;
      for (int nRound=0; nRound<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(make_pair(pindex->GetBlockHash(), pindex));
          LogPrint("stakemodifier", "ComputeNextStakeModifier: selected round %d stop=%s height=%d bit=%d\n", nRound, DateTimeStrFormat(nSelectionIntervalStop), pindex->nHeight, pindex->GetStakeEntropyBit());
      }
  
      // Print selection map for visualization of the selected blocks
      if (LogAcceptCategory("stakemodifier"))
      {
          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;
          }
          BOOST_FOREACH(const PAIRTYPE(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");
          }
          LogPrintf("ComputeNextStakeModifier: selection height [%d, %d] map %s\n", nHeightFirstCandidate, pindexPrev->nHeight, strSelectionMap);
      }
      LogPrint("stakemodifier", "ComputeNextStakeModifier: new modifier=0x%016x time=%s\n", nStakeModifierNew, DateTimeStrFormat(pindexPrev->GetBlockTime()));
  
      nStakeModifier = nStakeModifierNew;
      fGeneratedStakeModifier = true;
      return true;
  }
  
  // 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.
  uint256 ComputeStakeModifierV2(const CBlockIndex* pindexPrev, const uint256& kernel)
  {
      if (!pindexPrev)
          return 0;  // genesis block's modifier is 0
  
      CDataStream ss(SER_GETHASH, 0);
      ss << kernel << pindexPrev->bnStakeModifierV2;
      return Hash(ss.begin(), ss.end());
  }
  
  // SantaCoin kernel protocol
  // coinstake must meet hash target according to the protocol:
  // kernel (input 0) must meet the formula
  //     hash(nStakeModifier + txPrev.block.nTime + txPrev.nTime + txPrev.vout.hash + txPrev.vout.n + nTime) < bnTarget * nWeight
  // this ensures that the chance of getting a coinstake is proportional to the
  // amount of coins one owns.
  // The reason this hash is chosen is the following:
  //   nStakeModifier: scrambles computation to make it very difficult to precompute
  //                   future proof-of-stake
  //   txPrev.block.nTime: prevent nodes from guessing a good timestamp to
  //                       generate transaction for future advantage,
  //                       obsolete since v3
  //   txPrev.nTime: slightly scrambles computation
  //   txPrev.vout.hash: hash of txPrev, to 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
  //   nTime: current timestamp
  //   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(CBlockIndex* pindexPrev, unsigned int nBits, const CBlock& blockFrom, unsigned int nTxPrevOffset, const CTransaction& txPrev, const COutPoint& prevout, unsigned int nTimeTx, uint256& hashProofOfStake, uint256& targetProofOfStake, bool fPrintProofOfStake)
  {
      unsigned int nTimeBlockFrom = blockFrom.GetBlockTime();
  
      if (nTimeTx < txPrev.nTime)  // Transaction timestamp violation
          return error("CheckStakeKernelHash() : nTime violation");
  
      // Base target
      CBigNum bnTarget;
      bnTarget.SetCompact(nBits);
  
      // Weighted target
      int64_t nValueIn = txPrev.vout[prevout.n].nValue;
      CBigNum bnWeight = CBigNum(nValueIn);
      bnTarget *= bnWeight;
  
      targetProofOfStake = bnTarget.getuint256();
  
      uint64_t nStakeModifier = pindexPrev->nStakeModifier;
      uint256 bnStakeModifierV2 = pindexPrev->bnStakeModifierV2;
      int nStakeModifierHeight = pindexPrev->nHeight;
      int64_t nStakeModifierTime = pindexPrev->nTime;
  
      // Calculate hash
      CDataStream ss(SER_GETHASH, 0);
  
      if (IsProtocolV3(nTimeTx))
          ss << bnStakeModifierV2;
      else
          ss << nStakeModifier << nTimeBlockFrom;
  
      ss << txPrev.nTime << prevout.hash << prevout.n << nTimeTx;
      hashProofOfStake = Hash(ss.begin(), ss.end());
  
      if (fPrintProofOfStake)
      {
          LogPrintf("CheckStakeKernelHash() : using modifier 0x%016x at height=%d timestamp=%s for block from timestamp=%s\n",
              nStakeModifier, nStakeModifierHeight,
              DateTimeStrFormat(nStakeModifierTime),
              DateTimeStrFormat(nTimeBlockFrom));
          LogPrintf("CheckStakeKernelHash() : check modifier=0x%016x nTimeBlockFrom=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
              nStakeModifier,
              nTimeBlockFrom, txPrev.nTime, prevout.n, nTimeTx,
              hashProofOfStake.ToString());
      }
  
      // Now check if proof-of-stake hash meets target protocol
      if (CBigNum(hashProofOfStake) > bnTarget)
          return false;
  
      if (fDebug && !fPrintProofOfStake)
      {
          LogPrintf("CheckStakeKernelHash() : using modifier 0x%016x at height=%d timestamp=%s for block from timestamp=%s\n",
              nStakeModifier, nStakeModifierHeight,
              DateTimeStrFormat(nStakeModifierTime),
              DateTimeStrFormat(nTimeBlockFrom));
          LogPrintf("CheckStakeKernelHash() : pass modifier=0x%016x nTimeBlockFrom=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
              nStakeModifier,
              nTimeBlockFrom, txPrev.nTime, prevout.n, nTimeTx,
              hashProofOfStake.ToString());
      }
  
      return true;
  }
  
  // Check kernel hash target and coinstake signature
  bool CheckProofOfStake(CBlockIndex* pindexPrev, const CTransaction& tx, unsigned int nBits, uint256& hashProofOfStake, uint256& targetProofOfStake)
  {
      if (!tx.IsCoinStake())
          return error("CheckProofOfStake() : called on non-coinstake %s", tx.GetHash().ToString());
  
      // 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
      CTxDB txdb("r");
      CTransaction txPrev;
      CTxIndex txindex;
      if (!txPrev.ReadFromDisk(txdb, txin.prevout, txindex))
          return tx.DoS(1, error("CheckProofOfStake() : INFO: read txPrev failed"));  // previous transaction not in main chain, may occur during initial download
  
      // Verify signature
      if (!VerifySignature(txPrev, tx, 0, SCRIPT_VERIFY_NONE, 0))
          return tx.DoS(100, error("CheckProofOfStake() : VerifySignature failed on coinstake %s", tx.GetHash().ToString()));
  
      // Read block header
      CBlock block;
      if (!block.ReadFromDisk(txindex.pos.nFile, txindex.pos.nBlockPos, false))
          return fDebug? error("CheckProofOfStake() : read block failed") : false; // unable to read block of previous transaction
  
      // Min age requirement
      if (IsProtocolV3(tx.nTime))
      {
          int nDepth;
          if (IsConfirmedInNPrevBlocks(txindex, pindexPrev, nStakeMinConfirmations - 1, nDepth))
              return tx.DoS(100, error("CheckProofOfStake() : tried to stake at depth %d", nDepth + 1));
      }
      else
      {
          unsigned int nTimeBlockFrom = block.GetBlockTime();
          if (nTimeBlockFrom + nStakeMinAge > tx.nTime)
              return error("CheckProofOfStake() : min age violation");
      }
  
      if (!CheckStakeKernelHash(pindexPrev, nBits, block, txindex.pos.nTxPos - txindex.pos.nBlockPos, txPrev, txin.prevout, tx.nTime, hashProofOfStake, targetProofOfStake, fDebug))
          return tx.DoS(1, error("CheckProofOfStake() : INFO: check kernel failed on coinstake %s, hashProof=%s", tx.GetHash().ToString(), hashProofOfStake.ToString())); // may occur during initial download or if behind on block chain sync
  
      return true;
  }
  
  // Check whether the coinstake timestamp meets protocol
  bool CheckCoinStakeTimestamp(int nHeight, int64_t nTimeBlock, int64_t nTimeTx)
  {
      return (nTimeBlock == nTimeTx) && ((nTimeTx & STAKE_TIMESTAMP_MASK) == 0);
  }
  
  bool CheckKernel(CBlockIndex* pindexPrev, unsigned int nBits, int64_t nTime, const COutPoint& prevout, int64_t* pBlockTime)
  {
      uint256 hashProofOfStake, targetProofOfStake;
  
      CTxDB txdb("r");
      CTransaction txPrev;
      CTxIndex txindex;
      if (!txPrev.ReadFromDisk(txdb, prevout, txindex))
          return false;
  
      // Read block header
      CBlock block;
      if (!block.ReadFromDisk(txindex.pos.nFile, txindex.pos.nBlockPos, false))
          return false;
  
      if (IsProtocolV3(nTime))
      {
          int nDepth;
          if (IsConfirmedInNPrevBlocks(txindex, pindexPrev, nStakeMinConfirmations - 1, nDepth))
              return false;
      }
      else
      {
          if (block.GetBlockTime() + nStakeMinAge > nTime)
              return false; // only count coins meeting min age requirement
      }
  
      if (pBlockTime)
          *pBlockTime = block.GetBlockTime();
  
      return CheckStakeKernelHash(pindexPrev, nBits, block, txindex.pos.nTxPos - txindex.pos.nBlockPos, txPrev, prevout, nTime, hashProofOfStake, targetProofOfStake);
  }
  