text: /* @flow */
  // Copyright (c) 2012-2013 The PPCoin 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 <boost/lexical_cast.hpp>
  
  #include "db.h"
  #include "kernel.h"
  #include "script/interpreter.h"
  #include "timedata.h"
  #include "util.h"
  
  using namespace std;
  
  bool fTestNet = false; //Params().NetworkID() == CBaseChainParams::TESTNET;
  
  // Modifier interval: time to elapse before new modifier is computed
  // Set to 3-hour for production network and 20-minute for test network
  unsigned int nModifierInterval;
  int nStakeTargetSpacing = 60 * 5; // 5 minute stake time
  unsigned int getIntervalVersion(bool fTestNet)
  {
      if (fTestNet)
          return MODIFIER_INTERVAL_TESTNET;
      else
          return MODIFIER_INTERVAL;
  }
  
  // Hard checkpoints of stake modifiers to ensure they are deterministic
  static std::map<int, unsigned int> mapStakeModifierCheckpoints =
      boost::assign::map_list_of(0, 0xfd11f4e7u);
  
  // Get time weight
  int64_t GetWeight(int64_t nIntervalBeginning, int64_t nIntervalEnd)
  {
      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);
      int64_t a = getIntervalVersion(fTestNet) * 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;
  }
  
  // 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 fModifierV2 = false;
      bool fFirstRun = true;
      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().c_str());
  
          const CBlockIndex* pindex = mapBlockIndex[item.second];
          if (fSelected && pindex->GetBlockTime() > nSelectionIntervalStop)
              break;
  
          //if the lowest block height (vSortedByTimestamp[0]) is >= switch height, use new modifier calc
          if (fFirstRun){
              fModifierV2 = pindex->nHeight >= Params().ModifierUpgradeBlock();
              fFirstRun = false;
          }
  
          if (mapSelectedBlocks.count(pindex->GetBlockHash()) > 0)
              continue;
  
          // compute the selection hash by hashing an input that is unique to that block
          uint256 hashProof;
          if(fModifierV2)
              hashProof = pindex->GetBlockHash();
          else
              hashProof = pindex->IsProofOfStake() ? 0 : pindex->GetBlockHash();
  
          CDataStream ss(SER_GETHASH, 0);
          ss << 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;
          }
      }
      if (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
      }
      if (pindexPrev->nHeight == 0) {
          //Give a stake modifier to the first block
          fGeneratedStakeModifier = true;
          nStakeModifier = uint64_t("stakemodifier");
          return true;
      }
  
      // 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");
  
      if (GetBoolArg("-printstakemodifier", false))
          LogPrintf("ComputeNextStakeModifier: prev modifier= %s time=%s\n", boost::lexical_cast<std::string>(nStakeModifier).c_str(), DateTimeStrFormat("%Y-%m-%d %H:%M:%S", nModifierTime).c_str());
  
      if (nModifierTime / getIntervalVersion(fTestNet) >= pindexPrev->GetBlockTime() / getIntervalVersion(fTestNet))
          return true;
  
      // Sort candidate blocks by timestamp
      vector<pair<int64_t, uint256> > vSortedByTimestamp;
      vSortedByTimestamp.reserve(64 * getIntervalVersion(fTestNet) / nStakeTargetSpacing);
      int64_t nSelectionInterval = GetStakeModifierSelectionInterval();
      int64_t nSelectionIntervalStart = (pindexPrev->GetBlockTime() / getIntervalVersion(fTestNet)) * getIntervalVersion(fTestNet) - 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));
          if (fDebug || GetBoolArg("-printstakemodifier", false))
              LogPrintf("ComputeNextStakeModifier: selected round %d stop=%s height=%d bit=%d\n",
                  nRound, DateTimeStrFormat("%Y-%m-%d %H:%M:%S", nSelectionIntervalStop).c_str(), pindex->nHeight, pindex->GetStakeEntropyBit());
      }
  
      // Print selection map for visualization of the selected blocks
      if (fDebug || GetBoolArg("-printstakemodifier", false)) {
          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.c_str());
      }
      if (fDebug || GetBoolArg("-printstakemodifier", false)) {
          LogPrintf("ComputeNextStakeModifier: new modifier=%s time=%s\n", boost::lexical_cast<std::string>(nStakeModifierNew).c_str(), DateTimeStrFormat("%Y-%m-%d %H:%M:%S", pindexPrev->GetBlockTime()).c_str());
      }
  
      nStakeModifier = nStakeModifierNew;
      fGeneratedStakeModifier = true;
      return true;
  }
  
  // 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
  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;
      CBlockIndex* pindexNext = chainActive[pindexFrom->nHeight + 1];
  
      // loop to find the stake modifier later by a selection interval
      while (nStakeModifierTime < pindexFrom->GetBlockTime() + nStakeModifierSelectionInterval) {
          if (!pindexNext) {
              // Should never happen
              return error("Null pindexNext\n");
          }
  
          pindex = pindexNext;
          pindexNext = chainActive[pindexNext->nHeight + 1];
          if (pindex->GeneratedStakeModifier()) {
              nStakeModifierHeight = pindex->nHeight;
              nStakeModifierTime = pindex->GetBlockTime();
          }
      }
      nStakeModifier = pindex->nStakeModifier;
      return true;
  }
  
  uint256 stakeHash(unsigned int nTimeTx, CDataStream ss, unsigned int prevoutIndex, uint256 prevoutHash, unsigned int nTimeBlockFrom)
  {
      //LasVegasCoin will hash in the transaction hash and the index number in order to make sure each hash is unique
      ss << nTimeBlockFrom << prevoutIndex << prevoutHash << nTimeTx;
      return Hash(ss.begin(), ss.end());
  }
  
  //test hash vs target
  bool stakeTargetHit(uint256 hashProofOfStake, int64_t nValueIn, uint256 bnTargetPerCoinDay)
  {
      //get the stake weight - weight is equal to coin amount
      uint256 bnCoinDayWeight = uint256(nValueIn) / 100;
  
      // Now check if proof-of-stake hash meets target protocol
      return (uint256(hashProofOfStake) < bnCoinDayWeight * bnTargetPerCoinDay);
  }
  
  //instead of looping outside and reinitializing variables many times, we will give a nTimeTx and also search interval so that we can do all the hashing here
  bool CheckStakeKernelHash(unsigned int nBits, const CBlock blockFrom, const CTransaction txPrev, const COutPoint prevout, unsigned int& nTimeTx, unsigned int nHashDrift, bool fCheck, uint256& hashProofOfStake, bool fPrintProofOfStake)
  {
      //assign new variables to make it easier to read
      int64_t nValueIn = txPrev.vout[prevout.n].nValue;
      unsigned int nTimeBlockFrom = blockFrom.GetBlockTime();
  
      if (nTimeTx < nTimeBlockFrom) // Transaction timestamp violation
          return error("CheckStakeKernelHash() : nTime violation");
  
      if (nTimeBlockFrom + nStakeMinAge > nTimeTx) // Min age requirement
          return error("CheckStakeKernelHash() : min age violation - nTimeBlockFrom=%d nStakeMinAge=%d nTimeTx=%d", nTimeBlockFrom, nStakeMinAge, nTimeTx);
  
      //grab difficulty
      uint256 bnTargetPerCoinDay;
      bnTargetPerCoinDay.SetCompact(nBits);
  
      //grab stake modifier
      uint64_t nStakeModifier = 0;
      int nStakeModifierHeight = 0;
      int64_t nStakeModifierTime = 0;
      if (!GetKernelStakeModifier(blockFrom.GetHash(), nStakeModifier, nStakeModifierHeight, nStakeModifierTime, fPrintProofOfStake)) {
          LogPrintf("CheckStakeKernelHash(): failed to get kernel stake modifier \n");
          return false;
      }
  
      //create data stream once instead of repeating it in the loop
      CDataStream ss(SER_GETHASH, 0);
      ss << nStakeModifier;
  
      //if wallet is simply checking to make sure a hash is valid
      if (fCheck) {
          hashProofOfStake = stakeHash(nTimeTx, ss, prevout.n, prevout.hash, nTimeBlockFrom);
          return stakeTargetHit(hashProofOfStake, nValueIn, bnTargetPerCoinDay);
      }
  
      bool fSuccess = false;
      unsigned int nTryTime = 0;
      unsigned int i;
      for (i = 0; i < (nHashDrift); i++) //iterate the hashing
      {
          //hash this iteration
          nTryTime = nTimeTx + nHashDrift - i;
          hashProofOfStake = stakeHash(nTryTime, ss, prevout.n, prevout.hash, nTimeBlockFrom);
  
          // if stake hash does not meet the target then continue to next iteration
          if (!stakeTargetHit(hashProofOfStake, nValueIn, bnTargetPerCoinDay))
              continue;
  
          fSuccess = true; // if we make it this far then we have successfully created a stake hash
          nTimeTx = nTryTime;
  
          if (fDebug || fPrintProofOfStake) {
              LogPrintf("CheckStakeKernelHash() : using modifier %s at height=%d timestamp=%s for block from height=%d timestamp=%s\n",
                  boost::lexical_cast<std::string>(nStakeModifier).c_str(), nStakeModifierHeight,
                  DateTimeStrFormat("%Y-%m-%d %H:%M:%S", nStakeModifierTime).c_str(),
                  mapBlockIndex[blockFrom.GetHash()]->nHeight,
                  DateTimeStrFormat("%Y-%m-%d %H:%M:%S", blockFrom.GetBlockTime()).c_str());
              LogPrintf("CheckStakeKernelHash() : pass protocol=%s modifier=%s nTimeBlockFrom=%u prevoutHash=%s nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
                  "0.3",
                  boost::lexical_cast<std::string>(nStakeModifier).c_str(),
                  nTimeBlockFrom, prevout.hash.ToString().c_str(), nTimeBlockFrom, prevout.n, nTryTime,
                  hashProofOfStake.ToString().c_str());
          }
          break;
      }
  
      mapHashedBlocks.clear();
      mapHashedBlocks[chainActive.Tip()->nHeight] = GetTime(); //store a time stamp of when we last hashed on this block
      return fSuccess;
  }
  
  // Check kernel hash target and coinstake signature
  bool CheckProofOfStake(const CBlock block, uint256& hashProofOfStake)
  {
      const CTransaction tx = block.vtx[1];
      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
      uint256 hashBlock;
      CTransaction txPrev;
      if (!GetTransaction(txin.prevout.hash, txPrev, hashBlock, true))
          return error("CheckProofOfStake() : INFO: read txPrev failed");
  
      //verify signature and script
      if (!VerifyScript(txin.scriptSig, txPrev.vout[txin.prevout.n].scriptPubKey, STANDARD_SCRIPT_VERIFY_FLAGS, TransactionSignatureChecker(&tx, 0)))
          return error("CheckProofOfStake() : VerifySignature failed on coinstake %s", tx.GetHash().ToString().c_str());
  
      CBlockIndex* pindex = NULL;
      BlockMap::iterator it = mapBlockIndex.find(hashBlock);
      if (it != mapBlockIndex.end())
          pindex = it->second;
      else
          return error("CheckProofOfStake() : read block failed");
  
      // Read block header
      CBlock blockprev;
      if (!ReadBlockFromDisk(blockprev, pindex->GetBlockPos()))
          return error("CheckProofOfStake(): INFO: failed to find block");
  
      unsigned int nInterval = 0;
      unsigned int nTime = block.nTime;
      if (!CheckStakeKernelHash(block.nBits, blockprev, txPrev, txin.prevout, nTime, nInterval, true, hashProofOfStake, fDebug))
          return error("CheckProofOfStake() : INFO: check kernel failed on coinstake %s, hashProof=%s \n", tx.GetHash().ToString().c_str(), hashProofOfStake.ToString().c_str()); // may occur during initial download or if behind on block chain sync
  
      return true;
  }
  
  // Check whether the coinstake timestamp meets protocol
  bool CheckCoinStakeTimestamp(int64_t nTimeBlock, int64_t nTimeTx)
  {
      // v0.3 protocol
      return (nTimeBlock == nTimeTx);
  }
  
  // Get stake modifier checksum
  unsigned int GetStakeModifierChecksum(const CBlockIndex* pindex)
  {
      assert(pindex->pprev || pindex->GetBlockHash() == Params().HashGenesisBlock());
      // 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;
      uint256 hashChecksum = Hash(ss.begin(), ss.end());
      hashChecksum >>= (256 - 32);
      return hashChecksum.Get64();
  }
  
  // Check stake modifier hard checkpoints
  bool CheckStakeModifierCheckpoints(int nHeight, unsigned int nStakeModifierChecksum)
  {
      if (fTestNet) return true; // Testnet has no checkpoints
      if (mapStakeModifierCheckpoints.count(nHeight)) {
          return nStakeModifierChecksum == mapStakeModifierCheckpoints[nHeight];
      }
      return true;
  }
  