src/chain.cpp

   1 // Copyright (c) 2009-2010 Satoshi Nakamoto
   2 // Copyright (c) 2009-2014 The Bitcoin Core developers
   3 // Distributed under the MIT software license, see the accompanying
   4 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
   5
   6 #include "chain.h"
   7
   8 using namespace std;
   9
  10 /**
  11  * CChain implementation
  12  */
  13 void CChain::SetTip(CBlockIndex *pindex) {
  14     if (pindex == NULL) {
  15         vChain.clear();
  16         return;
  17     }
  18     vChain.resize(pindex->nHeight + 1);
  19     while (pindex && vChain[pindex->nHeight] != pindex) {
  20         vChain[pindex->nHeight] = pindex;
  21         pindex = pindex->pprev;
  22     }
  23 }
  24
  25 CBlockLocator CChain::GetLocator(const CBlockIndex *pindex) const {
  26     int nStep = 1;
  27     std::vector<uint256> vHave;
  28     vHave.reserve(32);
  29
  30     if (!pindex)
  31         pindex = Tip();
  32     while (pindex) {
  33         vHave.push_back(pindex->GetBlockHash());
  34         // Stop when we have added the genesis block.
  35         if (pindex->nHeight == 0)
  36             break;
  37         // Exponentially larger steps back, plus the genesis block.
  38         int nHeight = std::max(pindex->nHeight - nStep, 0);
  39         if (Contains(pindex)) {
  40             // Use O(1) CChain index if possible.
  41             pindex = (*this)[nHeight];
  42         } else {
  43             // Otherwise, use O(log n) skiplist.
  44             pindex = pindex->GetAncestor(nHeight);
  45         }
  46         if (vHave.size() > 10)
  47             nStep *= 2;
  48     }
  49
  50     return CBlockLocator(vHave);
  51 }
  52
  53 const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {
  54     if (pindex == NULL) {
  55         return NULL;
  56     }
  57     if (pindex->nHeight > Height())
  58         pindex = pindex->GetAncestor(Height());
  59     while (pindex && !Contains(pindex))
  60         pindex = pindex->pprev;
  61     return pindex;
  62 }
  63
  64 CBlockIndex* CChain::FindLatestBefore(int64_t nTime) const
  65 {
  66     std::vector<CBlockIndex*>::const_iterator lower = std::lower_bound(vChain.begin(), vChain.end(), nTime,
  67         [](CBlockIndex* pBlock, const int64_t& time) -> bool { return pBlock->GetBlockTime() < time; });
  68     return (lower == vChain.end() ? NULL : *lower);
  69 }
  70
  71 /** Turn the lowest '1' bit in the binary representation of a number into a '0'. */
  72 int static inline InvertLowestOne(int n) { return n & (n - 1); }
  73
  74 /** Compute what height to jump back to with the CBlockIndex::pskip pointer. */
  75 int static inline GetSkipHeight(int height) {
  76     if (height < 2)
  77         return 0;
  78
  79     // Determine which height to jump back to. Any number strictly lower than height is acceptable,
  80     // but the following expression seems to perform well in simulations (max 110 steps to go back
  81     // up to 2**18 blocks).
  82     return (height & 1) ? InvertLowestOne(InvertLowestOne(height - 1)) + 1 : InvertLowestOne(height);
  83 }
  84
  85 CBlockIndex* CBlockIndex::GetAncestor(int height)
  86 {
  87     if (height > nHeight || height < 0)
  88         return NULL;
  89
  90     CBlockIndex* pindexWalk = this;
  91     int heightWalk = nHeight;
  92     while (heightWalk > height) {
  93         int heightSkip = GetSkipHeight(heightWalk);
  94         int heightSkipPrev = GetSkipHeight(heightWalk - 1);
  95         if (pindexWalk->pskip != NULL &&
  96             (heightSkip == height ||
  97              (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
  98                                        heightSkipPrev >= height)))) {
  99             // Only follow pskip if pprev->pskip isn't better than pskip->pprev.
 100             pindexWalk = pindexWalk->pskip;
 101             heightWalk = heightSkip;
 102         } else {
 103             assert(pindexWalk->pprev);
 104             pindexWalk = pindexWalk->pprev;
 105             heightWalk--;
 106         }
 107     }
 108     return pindexWalk;
 109 }
 110
 111 const CBlockIndex* CBlockIndex::GetAncestor(int height) const
 112 {
 113     return const_cast<CBlockIndex*>(this)->GetAncestor(height);
 114 }
 115
 116 void CBlockIndex::BuildSkip()
 117 {
 118     if (pprev)
 119         pskip = pprev->GetAncestor(GetSkipHeight(nHeight));
 120 }
 121
 122 arith_uint256 GetBlockProof(const CBlockIndex& block)
 123 {
 124     arith_uint256 bnTarget;
 125     bool fNegative;
 126     bool fOverflow;
 127     bnTarget.SetCompact(block.nBits, &fNegative, &fOverflow);
 128     if (fNegative || fOverflow || bnTarget == 0)
 129         return 0;
 130     // We need to compute 2**256 / (bnTarget+1), but we can't represent 2**256
 131     // as it's too large for a arith_uint256. However, as 2**256 is at least as large
 132     // as bnTarget+1, it is equal to ((2**256 - bnTarget - 1) / (bnTarget+1)) + 1,
 133     // or ~bnTarget / (nTarget+1) + 1.
 134     return (~bnTarget / (bnTarget + 1)) + 1;
 135 }
 136
 137 int64_t GetBlockProofEquivalentTime(const CBlockIndex& to, const CBlockIndex& from, const CBlockIndex& tip, const Consensus::Params& params)
 138 {
 139     arith_uint256 r;
 140     int sign = 1;
 141     if (to.nChainWork > from.nChainWork) {
 142         r = to.nChainWork - from.nChainWork;
 143     } else {
 144         r = from.nChainWork - to.nChainWork;
 145         sign = -1;
 146     }
 147     r = r * arith_uint256(params.nPowTargetSpacing) / GetBlockProof(tip);
 148     if (r.bits() > 63) {
 149         return sign * std::numeric_limits<int64_t>::max();
 150     }
 151     return sign * r.GetLow64();
 152 }