BCH工作量证明源代码分析

uint32_t GetNextCashWorkRequired(const CBlockIndex *pindexPrev,
        const CBlockHeader *pblock, const Consensus::Params &params) {
    // This cannot handle the genesis block and early blocks in general.
    assert(pindexPrev);

    // Special difficulty rule for testnet:
    // If the new block's timestamp is more than 2* 10 minutes then allow
    // mining of a min-difficulty block.  //
    if (params.fPowAllowMinDifficultyBlocks &&
        (pblock->GetBlockTime() >
         pindexPrev->GetBlockTime() + 2 * params.nPowTargetSpacing)) {
        return UintToArith256(params.powLimit).GetCompact();
    }

    // Compute the difficulty based on the full adjustement interval.
    const uint32_t nHeight = pindexPrev->nHeight;
    assert(nHeight >= params.DifficultyAdjustmentInterval());

    // Get the last suitable block of the difficulty interval.
    const CBlockIndex *pindexLast = GetSuitableBlock(pindexPrev);
    assert(pindexLast);

    // Get the first suitable block of the difficulty interval.
    uint32_t nHeightFirst = nHeight - 144;
    const CBlockIndex *pindexFirst =
        GetSuitableBlock(pindexPrev->GetAncestor(nHeightFirst));
    assert(pindexFirst);

    // Compute the target based on time and work done during the interval.
    const arith_uint256 nextTarget =
        ComputeTarget(pindexFirst, pindexLast, params);

    const arith_uint256 powLimit = UintToArith256(params.powLimit);
    if (nextTarget > powLimit) {
        return powLimit.GetCompact();
    }

    return nextTarget.GetCompact()
}

/**
 * Compute the a target based on the work done between 2 blocks and the time
 * required to produce that work.
 */
static arith_uint256 ComputeTarget(const CBlockIndex *pindexFirst,
                                   const CBlockIndex *pindexLast,
                                   const Consensus::Params &params) {
    assert(pindexLast->nHeight > pindexFirst->nHeight);

    /**
     * From the total work done and the time it took to produce that much work,
     * we can deduce how much work we expect to be produced in the targeted time
     * between blocks.
     */
std::cout << "pindexLast->height : " << pindexLast->nHeight << ", pindexLast->nChainWork : " << pindexLast->nChainWork.GetCompact() <<
          ", pindexFirst->nHeight : " << pindexFirst->nHeight << ", pindexFirst->nChainWork : " << pindexFirst->nChainWork.GetCompact() << std::endl;
    arith_uint256 work = pindexLast->nChainWork - pindexFirst->nChainWork;
    work *= params.nPowTargetSpacing;

    // In order to avoid difficulty cliffs, we bound the amplitude of the
    // adjustement we are going to do.
    assert(pindexLast->nTime > pindexFirst->nTime);
    int64_t nActualTimespan = pindexLast->nTime - pindexFirst->nTime;
    if (nActualTimespan > 288 * params.nPowTargetSpacing) {
        nActualTimespan = 288 * params.nPowTargetSpacing;
    } else if (nActualTimespan < 72 * params.nPowTargetSpacing) {
        nActualTimespan = 72 * params.nPowTargetSpacing;
    }

    work /= nActualTimespan;

    /**
     * We need to compute T = (2^256 / W) - 1 but 2^256 doesn't fit in 256 bits.
     * By expressing 1 as W / W, we get (2^256 - W) / W, and we can compute
     * 2^256 - W as the complement of W.
     */
    return (-work) / work;
}

uint32_t GetNextEDAWorkRequired(const CBlockIndex *pindexPrev,
    const CBlockHeader *pblock, const Consensus::Params &params) {
    // Only change once per difficulty adjustment interval
    uint32_t nHeight = pindexPrev->nHeight + 1;
    if (nHeight % params.DifficultyAdjustmentInterval() == 0) {
        // Go back by what we want to be 14 days worth of blocks
        assert(nHeight >= params.DifficultyAdjustmentInterval());
        uint32_t nHeightFirst = nHeight - params.DifficultyAdjustmentInterval();
        const CBlockIndex *pindexFirst = pindexPrev->GetAncestor(nHeightFirst);
        assert(pindexFirst);

        return CalculateNextWorkRequired(pindexPrev,
                                         pindexFirst->GetBlockTime(), params);
    }

    const uint32_t nProofOfWorkLimit =
        UintToArith256(params.powLimit).GetCompact();

    if (params.fPowAllowMinDifficultyBlocks) {
        // Special difficulty rule for testnet:
        // If the new block's timestamp is more than 2* 10 minutes then allow
        // mining of a min-difficulty block.
        if (pblock->GetBlockTime() >
            pindexPrev->GetBlockTime() + 2 * params.nPowTargetSpacing) {
            return nProofOfWorkLimit;
        }

        // Return the last non-special-min-difficulty-rules-block
        const CBlockIndex *pindex = pindexPrev;
        while (pindex->pprev &&
               pindex->nHeight % params.DifficultyAdjustmentInterval() != 0 &&
               pindex->nBits == nProofOfWorkLimit) {
            pindex = pindex->pprev;
        }

        return pindex->nBits;
    }
    // We can't go bellow the minimum, so early bail.
    uint32_t nBits = pindexPrev->nBits;
    if (nBits == nProofOfWorkLimit) {
        return nProofOfWorkLimit;
    }

    // If producing the last 6 block took less than 12h, we keep the same
    // difficulty.
    const CBlockIndex *pindex6 = pindexPrev->GetAncestor(nHeight - 7);
    assert(pindex6);
    int64_t mtp6blocks =
        pindexPrev->GetMedianTimePast() - pindex6->GetMedianTimePast();
    if (mtp6blocks < 12 * 3600) {
        return nBits;
    }

    // If producing the last 6 block took more than 12h, increase the difficulty
    // target by 1/4 (which reduces the difficulty by 20%). This ensure the
    // chain do not get stuck in case we lose hashrate abruptly.
    arith_uint256 nPow;
    nPow.SetCompact(nBits);

    nPow += (nPow >> 2);

    // Make sure we do not go bellow allowed values.
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
    if (nPow > bnPowLimit) nPow = bnPowLimit;

    return nPow.GetCompact();
    ......
}

uint32_t CalculateNextWorkRequired(const CBlockIndex *pindexPrev,
        if (params.fPowNoRetargeting) {
        return pindexPrev->nBits;
    }
    // Limit adjustment step
    int64_t nActualTimespan = pindexPrev->GetBlockTime() - nFirstBlockTime;
    if (nActualTimespan < params.nPowTargetTimespan / 4) {
        nActualTimespan = params.nPowTargetTimespan / 4;
    }
    if (nActualTimespan > params.nPowTargetTimespan * 4) {
        nActualTimespan = params.nPowTargetTimespan * 4;
    }
    std::cout << "nActualTimespan : " << nActualTimespan << std::endl;

    // Retarget
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
    arith_uint256 bnNew;
    bnNew.SetCompact(pindexPrev->nBits);
    bnNew *= nActualTimespan;
    bnNew /= params.nPowTargetTimespan;

    if (bnNew > bnPowLimit) bnNew = bnPowLimit;

    return bnNew.GetCompact();
}

bool CheckProofOfWork(uint256 hash, uint32_t nBits,
                      const Consensus::Params &params) {
    bool fNegative;
    bool fOverflow;
    arith_uint256 bnTarget;

    bnTarget.SetCompact(nBits, &fNegative, &fOverflow);

    // Check range
    if (fNegative || bnTarget == 0 || fOverflow ||
        bnTarget > UintToArith256(params.powLimit)) {
        return false;
    }

    // Check proof of work matches claimed amount
    if (UintToArith256(hash) > bnTarget) {
        return false;
    }

    return true;
}