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

Code before:
        }
        return pindexLast->nBits;
    }

    // Go back by what we want to be 14 days worth of blocks
    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;
    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;

    if (height % params.DifficultyAdjustmentInterval() == 0) {
        int64_t smallest_timespan = params.nPowTargetTimespan/4;
        int64_t largest_timespan = params.nPowTargetTimespan*4;

        const arith_uint256 pow_limit = UintToArith256(params.powLimit);
        arith_uint256 observed_new_target;
        observed_new_target.SetCompact(new_nbits);

Review comment (suggestion):
71ce039c20f78d6454c3f930d1510f7d3e3b8652: two more followup suggestions:
1. Document `CalculateNextWorkRequired` (including that this is only called for retarget blocks) and `GetNextWorkRequired` (called for every block)
2. Add `Assume((pindexLast->nHeight + 1) % params.DifficultyAdjustmentInterval() == 0);` at the top

Code after:
        }
        return pindexLast->nBits;
    }

    // Go back by what we want to be 14 days worth of blocks
    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