// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2018 The Bitcoin 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 <bignum.h>
#include <chain.h>
#include <primitives/block.h>
#include <uint256.h>
#include <logging.h>

unsigned int static DarkGravityWave(const CBlockIndex* pindexLast, const Consensus::Params& params) {
    /* current difficulty formula, dash - DarkGravity v3, written by Evan Duffield - evan@dash.org */
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
    int64_t nPastBlocks = 24;

    // make sure we have at least (nPastBlocks + 1) blocks, otherwise just return powLimit
    if (!pindexLast || pindexLast->nHeight < nPastBlocks) {
        return bnPowLimit.GetCompact();
    }

    const CBlockIndex *pindex = pindexLast;
    arith_uint256 bnPastTargetAvg;

    for (unsigned int nCountBlocks = 1; nCountBlocks <= nPastBlocks; nCountBlocks++) {
        arith_uint256 bnTarget = arith_uint256().SetCompact(pindex->nBits);
        if (nCountBlocks == 1) {
            bnPastTargetAvg = bnTarget;
        } else {
            // NOTE: that's not an average really...
            bnPastTargetAvg = (bnPastTargetAvg * nCountBlocks + bnTarget) / (nCountBlocks + 1);
        }

        if(nCountBlocks != nPastBlocks) {
            assert(pindex->pprev); // should never fail
            pindex = pindex->pprev;
        }
    }

    arith_uint256 bnNew(bnPastTargetAvg);

    int64_t nActualTimespan = pindexLast->GetBlockTime() - pindex->GetBlockTime();
    // NOTE: is this accurate? nActualTimespan counts it for (nPastBlocks - 1) blocks only...
    int64_t nTargetTimespan = nPastBlocks * params.nPowTargetSpacing;

    if (nActualTimespan < nTargetTimespan/3)
        nActualTimespan = nTargetTimespan/3;
    if (nActualTimespan > nTargetTimespan*3)
        nActualTimespan = nTargetTimespan*3;

    // Retarget
    bnNew *= nActualTimespan;
    bnNew /= nTargetTimespan;

    if (bnNew > bnPowLimit) {
        bnNew = bnPowLimit;
    }

    return bnNew.GetCompact();
}

unsigned int GetNextWorkRequiredBTC(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
{
    static CBigNum bnProofOfWorkLimit(params.powLimit);

    // Genesis block
    if (pindexLast == NULL)
        return bnProofOfWorkLimit.GetCompact();

    // Only change once per difficulty adjustment interval
    if ((pindexLast->nHeight+1) % params.DifficultyAdjustmentInterval() != 0)
    {
        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() > pindexLast->GetBlockTime() + params.nPowTargetSpacing*2)
                return bnProofOfWorkLimit.GetCompact();
            else
            {
                // Return the last non-special-min-difficulty-rules-block
                const CBlockIndex* pindex = pindexLast;
                while (pindex->pprev && pindex->nHeight % params.DifficultyAdjustmentInterval() != 0 && pindex->nBits == bnProofOfWorkLimit.GetCompact())
                    pindex = pindex->pprev;
                return pindex->nBits;
            }
        }
        return pindexLast->nBits;
    }

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

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

    // Retarget
    CBigNum bnNew;
    bnNew.SetCompact(pindexLast->nBits);
    bnNew *= nActualTimespan;
    bnNew /= params.nPowTargetTimespan;

    if (bnNew > bnProofOfWorkLimit)
        bnNew = bnProofOfWorkLimit;

    /// debug print
    LogPrintf("GetNextWorkRequired RETARGET\n");
    LogPrintf("params.nPowTargetTimespan = %d    nActualTimespan = %d\n", params.nPowTargetTimespan, nActualTimespan);
    return bnNew.GetCompact();

}

unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
{
    assert(pindexLast != nullptr);
    assert(pblock != nullptr);
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);

    if (pindexLast->nHeight + 1 < 894500) {
        return GetNextWorkRequiredBTC(pindexLast, pblock, params);
    }

    return DarkGravityWave(pindexLast, 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
    arith_uint256 bnNew;
    arith_uint256 bnOld;
    bnNew.SetCompact(pindexLast->nBits);
    bnOld = bnNew;
    // Fairbrix: intermediate uint256 can overflow by 1 bit
    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
    bool fShift = bnNew.bits() > bnPowLimit.bits() - 1;
    if (fShift)
        bnNew >>= 1;
    bnNew *= nActualTimespan;
    bnNew /= params.nPowTargetTimespan;
    if (fShift)
        bnNew <<= 1;

    if (bnNew > bnPowLimit)
        bnNew = bnPowLimit;

    /// debug print
    LogPrintf("GetNextWorkRequired RETARGET\n");
    LogPrintf("params.nPowTargetTimespan = %d    nActualTimespan = %d\n", params.nPowTargetTimespan, nActualTimespan);
    LogPrintf("Before: %08x  %s\n", pindexLast->nBits, bnOld.ToString());
    LogPrintf("After:  %08x  %s\n", bnNew.GetCompact(), bnNew.ToString());
    return bnNew.GetCompact();
}

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;
}