chain.cpp

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// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2016 The Bitcoin 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"
#include "validation.h"
 
using namespace std;
 
/* Moved here from the header, because we need auxpow and the logic
   becomes more involved.  */
CBlockHeader CBlockIndex::GetBlockHeader(const Consensus::Params& consensusParams, bool fCheckPOW) const
{
    CBlockHeader block;
 
    block.nVersion       = nVersion;
 
    /* The CBlockIndex object's block header is missing the auxpow.
       So if this is an auxpow block, read it from disk instead.  We only
       have to read the actual *header*, not the full block.  */
    if (block.IsAuxpow())
    {
        ReadBlockHeaderFromDisk(block, this, consensusParams, fCheckPOW);
        return block;
    }
 
    if (pprev)
        block.hashPrevBlock = pprev->GetBlockHash();
    block.hashMerkleRoot = hashMerkleRoot;
    block.nTime          = nTime;
    block.nBits          = nBits;
    block.nNonce         = nNonce;
    return block;
}
 
void CChain::SetTip(CBlockIndex *pindex) {
    if (pindex == NULL) {
        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 == NULL) {
        return NULL;
    }
    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() ? NULL : *lower);
}
 
int static inline InvertLowestOne(int n) { return n & (n - 1); }
 
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 NULL;
 
    CBlockIndex* pindexWalk = this;
    int heightWalk = nHeight;
    while (heightWalk > height) {
        int heightSkip = GetSkipHeight(heightWalk);
        int heightSkipPrev = GetSkipHeight(heightWalk - 1);
        if (pindexWalk->pskip != NULL &&
            (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 a 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 / (nTarget+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();
}