LWMA2.cpp

// LWMA for BTC clones
// Copyright (c) 2017-2018 The Bitcoin Gold developers
// Copyright (c) 2018 Zawy (M.I.T license continued)
// Algorithm by zawy, a modification of WT-144 by Tom Harding
// Code by h4x3rotab of BTC Gold, modified/updated by zawy
// Updated to LWMA2 by iamstenman
// https://github.com/zawy12/difficulty-algorithms/issues/3#issuecomment-388386175

unsigned int Lwma2CalculateNextWorkRequired(const CBlockIndex* pindexLast, const Consensus::Params& params)
{
    const int64_t T = params.nPowTargetSpacing;
    const int64_t N = params.lwma2AveragingWindow;
    const int64_t k = N * (N + 1) * T / 2;
    const int height = pindexLast->nHeight;
    assert(height > N);

    arith_uint256 sum_target, previous_diff, next_target;
    int64_t t = 0, j = 0, solvetime, solvetime_sum;

    // Loop through N most recent blocks. 
    for (int i = height - N + 1; i <= height; i++) {
        const CBlockIndex* block = pindexLast->GetAncestor(i);
        const CBlockIndex* block_Prev = block->GetAncestor(i - 1);
        solvetime = block->GetBlockTime() - block_Prev->GetBlockTime();
        solvetime = std::max(-6*T, std::min(solvetime, 6 * T));
        j++;
        t += solvetime * j; // Weighted solvetime sum.
        arith_uint256 target;
        target.SetCompact(block->nBits);
        sum_target += target / (k * N);

        if (i > height - 3) {
            solvetime_sum += solvetime;
        }
        
        if (i == height) {
            previous_diff = target.SetCompact(block->nBits); // Latest block target
        }
    }
    
    next_target = t * sum_target;
    
    // The following limits are the generous max that should reasonably occur.
    next_target = std::max((previous_diff * 67) / 100, std::min(next_target, (previous_diff * 150) / 100));

    if (solvetime_sum < (8 * T) / 10) {
        next_target = previous_diff * 100 / 106;
    }

    return next_target.GetCompact();
}