text: // Copyright (c) 2009-2010 Satoshi Nakamoto
  // Copyright (c) 2009-2016 The Bitcoin Core developers
  // Copyright (c) 2017 The Ravencoin Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  
  #include "chain.h"
  
  /**
   * CChain implementation
   */
  void CChain::SetTip(CBlockIndex *pindex) {
      if (pindex == nullptr) {
          vChain.clear();
          return;
      }
      vChain.resize(pindex->nHeight + 1);
      while (pindex && vChain[pindex->nHeight] != pindex) {
          vChain[pindex->nHeight] = pindex;
          pindex = pindex->pprev;
      }
  }
  
  CBlockLocator CChain::GetLocator(const CBlockIndex *pindex) const {
      int nStep = 1;
      std::vector<uint256> vHave;
      vHave.reserve(32);
  
      if (!pindex)
          pindex = Tip();
      while (pindex) {
          vHave.push_back(pindex->GetBlockHash());
          // Stop when we have added the genesis block.
          if (pindex->nHeight == 0)
              break;
          // Exponentially larger steps back, plus the genesis block.
          int nHeight = std::max(pindex->nHeight - nStep, 0);
          if (Contains(pindex)) {
              // Use O(1) CChain index if possible.
              pindex = (*this)[nHeight];
          } else {
              // Otherwise, use O(log n) skiplist.
              pindex = pindex->GetAncestor(nHeight);
          }
          if (vHave.size() > 10)
              nStep *= 2;
      }
  
      return CBlockLocator(vHave);
  }
  
  const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {
      if (pindex == nullptr) {
          return nullptr;
      }
      if (pindex->nHeight > Height())
          pindex = pindex->GetAncestor(Height());
      while (pindex && !Contains(pindex))
          pindex = pindex->pprev;
      return pindex;
  }
  
  CBlockIndex* CChain::FindEarliestAtLeast(int64_t nTime) const
  {
      std::vector<CBlockIndex*>::const_iterator lower = std::lower_bound(vChain.begin(), vChain.end(), nTime,
          [](CBlockIndex* pBlock, const int64_t& time) -> bool { return pBlock->GetBlockTimeMax() < time; });
      return (lower == vChain.end() ? nullptr : *lower);
  }
  
  /** Turn the lowest '1' bit in the binary representation of a number into a '0'. */
  int static inline InvertLowestOne(int n) { return n & (n - 1); }
  
  /** Compute what height to jump back to with the CBlockIndex::pskip pointer. */
  int static inline GetSkipHeight(int height) {
      if (height < 2)
          return 0;
  
      // Determine which height to jump back to. Any number strictly lower than height is acceptable,
      // but the following expression seems to perform well in simulations (max 110 steps to go back
      // up to 2**18 blocks).
      return (height & 1) ? InvertLowestOne(InvertLowestOne(height - 1)) + 1 : InvertLowestOne(height);
  }
  
  CBlockIndex* CBlockIndex::GetAncestor(int height)
  {
      if (height > nHeight || height < 0)
          return nullptr;
  
      CBlockIndex* pindexWalk = this;
      int heightWalk = nHeight;
      while (heightWalk > height) {
          int heightSkip = GetSkipHeight(heightWalk);
          int heightSkipPrev = GetSkipHeight(heightWalk - 1);
          if (pindexWalk->pskip != nullptr &&
              (heightSkip == height ||
               (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
                                         heightSkipPrev >= height)))) {
              // Only follow pskip if pprev->pskip isn't better than pskip->pprev.
              pindexWalk = pindexWalk->pskip;
              heightWalk = heightSkip;
          } else {
              assert(pindexWalk->pprev);
              pindexWalk = pindexWalk->pprev;
              heightWalk--;
          }
      }
      return pindexWalk;
  }
  
  const CBlockIndex* CBlockIndex::GetAncestor(int height) const
  {
      return const_cast<CBlockIndex*>(this)->GetAncestor(height);
  }
  
  void CBlockIndex::BuildSkip()
  {
      if (pprev)
          pskip = pprev->GetAncestor(GetSkipHeight(nHeight));
  }
  
  arith_uint256 GetBlockProof(const CBlockIndex& block)
  {
      arith_uint256 bnTarget;
      bool fNegative;
      bool fOverflow;
      bnTarget.SetCompact(block.nBits, &fNegative, &fOverflow);
      if (fNegative || fOverflow || bnTarget == 0)
          return 0;
      // We need to compute 2**256 / (bnTarget+1), but we can't represent 2**256
      // as it's too large for an arith_uint256. However, as 2**256 is at least as large
      // as bnTarget+1, it is equal to ((2**256 - bnTarget - 1) / (bnTarget+1)) + 1,
      // or ~bnTarget / (bnTarget+1) + 1.
      return (~bnTarget / (bnTarget + 1)) + 1;
  }
  
  int64_t GetBlockProofEquivalentTime(const CBlockIndex& to, const CBlockIndex& from, const CBlockIndex& tip, const Consensus::Params& params)
  {
      arith_uint256 r;
      int sign = 1;
      if (to.nChainWork > from.nChainWork) {
          r = to.nChainWork - from.nChainWork;
      } else {
          r = from.nChainWork - to.nChainWork;
          sign = -1;
      }
      r = r * arith_uint256(params.nPowTargetSpacing) / GetBlockProof(tip);
      if (r.bits() > 63) {
          return sign * std::numeric_limits<int64_t>::max();
      }
      return sign * r.GetLow64();
  }
  
  /** Find the last common ancestor two blocks have.
   *  Both pa and pb must be non-nullptr. */
  const CBlockIndex* LastCommonAncestor(const CBlockIndex* pa, const CBlockIndex* pb) {
      if (pa->nHeight > pb->nHeight) {
          pa = pa->GetAncestor(pb->nHeight);
      } else if (pb->nHeight > pa->nHeight) {
          pb = pb->GetAncestor(pa->nHeight);
      }
  
      while (pa != pb && pa && pb) {
          pa = pa->pprev;
          pb = pb->pprev;
      }
  
      // Eventually all chain branches meet at the genesis block.
      assert(pa == pb);
      return pa;
  }
  