text: // 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 "kernel.h"
  #include "txdb.h"
  
  using namespace std;
  
  extern unsigned int nStakeMaxAge;
  extern unsigned int nTargetSpacing;
  
  typedef std::map<int, unsigned int> MapModifierCheckpoints;
  
  // Hard checkpoints of stake modifiers to ensure they are deterministic
  static std::map<int, unsigned int> mapStakeModifierCheckpoints =
      boost::assign::map_list_of
          ( 0, 0x0e00670b )
          ( 0, 0xbc4b99b6 )
      ;
  
  // Hard checkpoints of stake modifiers to ensure they are deterministic (testNet)
  static std::map<int, unsigned int> mapStakeModifierCheckpointsTestNet =
      boost::assign::map_list_of
          ( 0, 0xfd11f4e7 )
      ;
  
  // 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 min(nIntervalEnd - nIntervalBeginning - nStakeMinAge, (int64_t)nStakeMaxAge);
  }
  
  // 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().c_str());
          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
          uint256 hashProof = pindex->IsProofOfStake()? pindex->hashProofOfStake : 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 (fDebug && GetBoolArg("-printstakemodifier"))
          printf("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");
      if (fDebug)
      {
          printf("ComputeNextStakeModifier: prev modifier=0x%016"PRIx64" time=%s\n", nStakeModifier, DateTimeStrFormat(nModifierTime).c_str());
      }
      if (nModifierTime / nModifierInterval >= pindexPrev->GetBlockTime() / nModifierInterval)
          return true;
  
      // Sort candidate blocks by timestamp
      vector<pair<int64_t, uint256> > vSortedByTimestamp;
      vSortedByTimestamp.reserve(64 * nModifierInterval / nTargetSpacing);
      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));
          if (fDebug && GetBoolArg("-printstakemodifier"))
              printf("ComputeNextStakeModifier: selected round %d stop=%s height=%d bit=%d\n", nRound, DateTimeStrFormat(nSelectionIntervalStop).c_str(), pindex->nHeight, pindex->GetStakeEntropyBit());
      }
  
      // Print selection map for visualization of the selected blocks
      if (fDebug && GetBoolArg("-printstakemodifier"))
      {
          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");
          }
          printf("ComputeNextStakeModifier: selection height [%d, %d] map %s\n", nHeightFirstCandidate, pindexPrev->nHeight, strSelectionMap.c_str());
      }
      if (fDebug)
      {
          printf("ComputeNextStakeModifier: new modifier=0x%016"PRIx64" time=%s\n", nStakeModifierNew, DateTimeStrFormat(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
  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 (!pindex->pnext)
          {   // reached best block; may happen if node is behind on block chain
              if (fPrintProofOfStake || (pindex->GetBlockTime() + 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
                  return false;
          }
          pindex = pindex->pnext;
          if (pindex->GeneratedStakeModifier())
          {
              nStakeModifierHeight = pindex->nHeight;
              nStakeModifierTime = pindex->GetBlockTime();
          }
      }
      nStakeModifier = pindex->nStakeModifier;
      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: scrambles computation to make it very difficult to precompute
  //                  future proof-of-stake at the time of the coin's confirmation
  //   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, const CBlock& blockFrom, unsigned int nTxPrevOffset, const CTransaction& txPrev, const COutPoint& prevout, unsigned int nTimeTx, uint256& hashProofOfStake, uint256& targetProofOfStake, bool fPrintProofOfStake)
  {
      if (nTimeTx < txPrev.nTime)  // Transaction timestamp violation
          return error("CheckStakeKernelHash() : nTime violation");
  
      unsigned int nTimeBlockFrom = blockFrom.GetBlockTime();
      if (nTimeBlockFrom + nStakeMinAge > nTimeTx) // Min age requirement
          return error("CheckStakeKernelHash() : min age violation");
  
      CBigNum bnTargetPerCoinDay;
      bnTargetPerCoinDay.SetCompact(nBits);
      int64_t nValueIn = txPrev.vout[prevout.n].nValue;
  
      uint256 hashBlockFrom = blockFrom.GetHash();
  
      CBigNum bnCoinDayWeight = CBigNum(nValueIn) * GetWeight((int64_t)txPrev.nTime, (int64_t)nTimeTx) / COIN / (24 * 60 * 60);
      targetProofOfStake = (bnCoinDayWeight * bnTargetPerCoinDay).getuint256();
  
      // Calculate hash
      CDataStream ss(SER_GETHASH, 0);
      uint64_t nStakeModifier = 0;
      int nStakeModifierHeight = 0;
      int64_t nStakeModifierTime = 0;
  
      if (!GetKernelStakeModifier(hashBlockFrom, nStakeModifier, nStakeModifierHeight, nStakeModifierTime, fPrintProofOfStake))
          return false;
      ss << nStakeModifier;
  
      ss << nTimeBlockFrom << nTxPrevOffset << txPrev.nTime << prevout.n << nTimeTx;
      hashProofOfStake = Hash(ss.begin(), ss.end());
      if (fPrintProofOfStake)
      {
          printf("CheckStakeKernelHash() : using modifier 0x%016"PRIx64" at height=%d timestamp=%s for block from height=%d timestamp=%s\n",
              nStakeModifier, nStakeModifierHeight,
              DateTimeStrFormat(nStakeModifierTime).c_str(),
              mapBlockIndex[hashBlockFrom]->nHeight,
              DateTimeStrFormat(blockFrom.GetBlockTime()).c_str());
          printf("CheckStakeKernelHash() : check modifier=0x%016"PRIx64" nTimeBlockFrom=%u nTxPrevOffset=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
              nStakeModifier,
              nTimeBlockFrom, nTxPrevOffset, txPrev.nTime, prevout.n, nTimeTx,
              hashProofOfStake.ToString().c_str());
      }
  
      // Now check if proof-of-stake hash meets target protocol
      if (CBigNum(hashProofOfStake) > bnCoinDayWeight * bnTargetPerCoinDay)
          return false;
      if (fDebug && !fPrintProofOfStake)
      {
          printf("CheckStakeKernelHash() : using modifier 0x%016"PRIx64" at height=%d timestamp=%s for block from height=%d timestamp=%s\n",
              nStakeModifier, nStakeModifierHeight, 
              DateTimeStrFormat(nStakeModifierTime).c_str(),
              mapBlockIndex[hashBlockFrom]->nHeight,
              DateTimeStrFormat(blockFrom.GetBlockTime()).c_str());
          printf("CheckStakeKernelHash() : pass modifier=0x%016"PRIx64" nTimeBlockFrom=%u nTxPrevOffset=%u nTimeTxPrev=%u nPrevout=%u nTimeTx=%u hashProof=%s\n",
              nStakeModifier,
              nTimeBlockFrom, nTxPrevOffset, txPrev.nTime, prevout.n, nTimeTx,
              hashProofOfStake.ToString().c_str());
      }
      return true;
  }
  
  // Check kernel hash target and coinstake signature
  bool CheckProofOfStake(const CTransaction& tx, unsigned int nBits, uint256& hashProofOfStake, uint256& targetProofOfStake)
  {
      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
      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, 0))
          return tx.DoS(100, error("CheckProofOfStake() : VerifySignature failed on coinstake %s", tx.GetHash().ToString().c_str()));
  
      // 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
  
      if (!CheckStakeKernelHash(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().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() == (!fTestNet ? hashGenesisBlock : hashGenesisBlockTestNet));
      // 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)
  {
      MapModifierCheckpoints& checkpoints = (fTestNet ? mapStakeModifierCheckpointsTestNet : mapStakeModifierCheckpoints);
  
      if (checkpoints.count(nHeight))
          return nStakeModifierChecksum == checkpoints[nHeight];
      return true;
  }
  