Repository: bitcoin/bitcoin
File: src/pow.cpp (C++)

Code before:
    int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
    assert(nHeightFirst >= 0);
    const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
    assert(pindexFirst);

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

unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime, const Consensus::Params& params)
{
    if (params.fPowNoRetargeting)
        return pindexLast->nBits;

    // Limit adjustment step
    int64_t nActualTimespan = pindexLast->GetBlockTime() - nFirstBlockTime;
    if (nActualTimespan < params.nPowTargetTimespan/4)
        nActualTimespan = params.nPowTargetTimespan/4;
    if (nActualTimespan > params.nPowTargetTimespan*4)
        nActualTimespan = params.nPowTargetTimespan*4;

    // Retarget
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
    arith_uint256 bnNew;

    // Special difficulty rule for Testnet4
    if (params.fPowAllowMinDifficultyBlocks && params.hashGenesisBlock == uint256S("0x00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be350b0da8bf043")) {
        // Use the last non-special-min-difficulty-rules-block
        const CBlockIndex* pindex = pindexLast;
        const unsigned int pow_min{bnPowLimit.GetCompact()};
        while (pindex->pprev && pindex->nHeight % params.DifficultyAdjustmentInterval() != 0 && pindex->nBits == pow_min)
            pindex = pindex->pprev;
        bnNew.SetCompact(pindex->nBits);
    } else {
        bnNew.SetCompact(pindexLast->nBits);
    }

    bnNew *= nActualTimespan;
    bnNew /= params.nPowTargetTimespan;

    if (bnNew > bnPowLimit)
        bnNew = bnPowLimit;

    return bnNew.GetCompact();
}

// Check that on difficulty adjustments, the new difficulty does not increase
// or decrease beyond the permitted limits.
bool PermittedDifficultyTransition(const Consensus::Params& params, int64_t height, uint32_t old_nbits, uint32_t new_nbits)
{
    if (params.fPowAllowMinDifficultyBlocks) return true;


Review comment (suggestion):
e172a96c0781de2bbd69312905d2c16cc1745c2f: `params.fPowAllowMinDifficultyBlocks &&` is redundant. Though you could an assert.

Code after:
    int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
    assert(nHeightFirst >= 0);
    const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
    assert(pindexFirst);

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

unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime, const Consensus::Params& params)
{
    if (params.fPowNoRetargeting)
        return pindexLast->nBits;

    // Limit adjustment step
    int64_t nActualTimespan = pindexLast->GetBlockTime() - nFirstBlockTime;
    if (nActualTimespan < params.nPowTargetTimespan/4)
        nActualTimespan = params.nPowTargetTimespan/4;
    if (nActualTimespan > params.nPowTargetTimespan*4)
        nActualTimespan = params.nPowTargetTimespan*4;

    // Retarget
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
    arith_uint256 bnNew;

    // Special difficulty rule for Testnet4
    if (params.enforce_BIP94) {
        // Here we use the first block of the difficulty period. This way
        // the real difficulty is always preserved in the first block as
        // it is not allowed to use the min-difficulty exception.
        int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
        const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
        bnNew.SetCompact(pindexFirst->nBits);
    } else {
        bnNew.SetCompact(pindexLast->nBits);
    }

    bnNew *= nActualTimespan;
    bnNew /= params.nPowTargetTimespan;

    if (bnNew > bnPowLimit)
        bnNew = bnPowLimit;

    return bnNew.GetCompact();
}

// Check that on difficulty adjustments, the new difficulty does not increase
// or decrease beyond the permitted limits.
bool PermittedDifficultyTransition(const Consensus::Params& params, int64_t height, uint32_t old_nbits, uint32_t new_nbits)
{
    if (params.fPowAllowMinDifficultyBlocks) return true;