kevacoin/src/pow.cpp

// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2014 The Bitcoin Core developers
// Copyright (c) 2014-2015 The Dogecoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <pow.h>

#include <arith_uint256.h>
#include <chain.h>
#include <primitives/block.h>
#include <uint256.h>
#include <util.h>

// Copy and modified from CalculateDogecoinNextWorkRequired (dogecoin.cpp)
unsigned int CalculateDigishieldNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime, const Consensus::Params& params)
{
    if (params.fPowNoRetargeting)
        return pindexLast->nBits;

    const int64_t retargetTimespan = params.nPowTargetTimespan;
    const int64_t nActualTimespan = pindexLast->GetBlockTime() - nFirstBlockTime;
    int64_t nModulatedTimespan = nActualTimespan;
    int64_t nMaxTimespan;
    int64_t nMinTimespan;

    // amplitude filter - thanks to daft27 for this code
    nModulatedTimespan = retargetTimespan + (nModulatedTimespan - retargetTimespan) / 8;

    nMinTimespan = retargetTimespan - (retargetTimespan / 4);
    nMaxTimespan = retargetTimespan + (retargetTimespan / 2);

    // Limit adjustment step
    if (nModulatedTimespan < nMinTimespan)
        nModulatedTimespan = nMinTimespan;
    else if (nModulatedTimespan > nMaxTimespan)
        nModulatedTimespan = nMaxTimespan;

    // Retarget
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
    arith_uint256 bnNew;
    arith_uint256 bnOld;
    bnNew.SetCompact(pindexLast->nBits);
    bnOld = bnNew;
    bnNew *= nModulatedTimespan;
    bnNew /= retargetTimespan;

    if (bnNew > bnPowLimit)
        bnNew = bnPowLimit;

    return bnNew.GetCompact();
}

unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
{
    assert(pindexLast != nullptr);
    // Go back by what we want to be 14 days worth of blocks
    // Kevacoin: This fixes an issue where a 51% attack can change difficulty at will.
    // Go back the full period unless it's the first retarget after genesis. Code courtesy of Art Forz
    int blockstogoback = params.DifficultyAdjustmentInterval()-1;
    if ((pindexLast->nHeight+1) != params.DifficultyAdjustmentInterval())
        blockstogoback = params.DifficultyAdjustmentInterval();

    // Go back by what we want to be 14 days worth of blocks
    const CBlockIndex* pindexFirst = pindexLast;
    for (int i = 0; pindexFirst && i < blockstogoback; i++)
        pindexFirst = pindexFirst->pprev;

    assert(pindexFirst);

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

bool CheckProofOfWork(uint256 hash, unsigned int 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;
}