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->nHeight > Height())
  55         pindex = pindex->GetAncestor(Height());
  56     while (pindex && !Contains(pindex))
  57         pindex = pindex->pprev;
  58     return pindex;
  59 }
  60
  61 /** Turn the lowest '1' bit in the binary representation of a number into a '0'. */
  62 int static inline InvertLowestOne(int n) { return n & (n - 1); }
  63
  64 /** Compute what height to jump back to with the CBlockIndex::pskip pointer. */
  65 int static inline GetSkipHeight(int height) {
  66     if (height < 2)
  67         return 0;
  68
  69     // Determine which height to jump back to. Any number strictly lower than height is acceptable,
  70     // but the following expression seems to perform well in simulations (max 110 steps to go back
  71     // up to 2**18 blocks).
  72     return (height & 1) ? InvertLowestOne(InvertLowestOne(height - 1)) + 1 : InvertLowestOne(height);
  73 }
  74
  75 CBlockIndex* CBlockIndex::GetAncestor(int height)
  76 {
  77     if (height > nHeight || height < 0)
  78         return NULL;
  79
  80     CBlockIndex* pindexWalk = this;
  81     int heightWalk = nHeight;
  82     while (heightWalk > height) {
  83         int heightSkip = GetSkipHeight(heightWalk);
  84         int heightSkipPrev = GetSkipHeight(heightWalk - 1);
  85         if (pindexWalk->pskip != NULL &&
  86             (heightSkip == height ||
  87              (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
  88                                        heightSkipPrev >= height)))) {
  89             // Only follow pskip if pprev->pskip isn't better than pskip->pprev.
  90             pindexWalk = pindexWalk->pskip;
  91             heightWalk = heightSkip;
  92         } else {
  93             pindexWalk = pindexWalk->pprev;
  94             heightWalk--;
  95         }
  96     }
  97     return pindexWalk;
  98 }
  99
 100 const CBlockIndex* CBlockIndex::GetAncestor(int height) const
 101 {
 102     return const_cast<CBlockIndex*>(this)->GetAncestor(height);
 103 }
 104
 105 void CBlockIndex::BuildSkip()
 106 {
 107     if (pprev)
 108         pskip = pprev->GetAncestor(GetSkipHeight(nHeight));
 109 }