litecoin/src/pow.cpp

// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2014 The Bitcoin developers
// Distributed under the MIT/X11 software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include "pow.h"

#include "chainparams.h"
#include "core.h"
#include "main.h"
#include "uint256.h"

unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock)
{
    unsigned int nProofOfWorkLimit = Params().ProofOfWorkLimit().GetCompact();

    // Genesis block
    if (pindexLast == NULL)
        return nProofOfWorkLimit;

    // Only change once per interval
    if ((pindexLast->nHeight+1) % Params().Interval() != 0)
    {
        if (Params().AllowMinDifficultyBlocks())
        {
            // 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().TargetSpacing()*2)
                return nProofOfWorkLimit;
            else
            {
                // Return the last non-special-min-difficulty-rules-block
                const CBlockIndex* pindex = pindexLast;
                while (pindex->pprev && pindex->nHeight % Params().Interval() != 0 && pindex->nBits == nProofOfWorkLimit)
                    pindex = pindex->pprev;
                return pindex->nBits;
            }
        }
        return pindexLast->nBits;
    }

    // Go back by what we want to be 14 days worth of blocks
    const CBlockIndex* pindexFirst = pindexLast;
    for (int i = 0; pindexFirst && i < Params().Interval()-1; i++)
        pindexFirst = pindexFirst->pprev;
    assert(pindexFirst);

    // Limit adjustment step
    int64_t nActualTimespan = pindexLast->GetBlockTime() - pindexFirst->GetBlockTime();
    LogPrintf("  nActualTimespan = %d  before bounds\n", nActualTimespan);
    if (nActualTimespan < Params().TargetTimespan()/4)
        nActualTimespan = Params().TargetTimespan()/4;
    if (nActualTimespan > Params().TargetTimespan()*4)
        nActualTimespan = Params().TargetTimespan()*4;

    // Retarget
    uint256 bnNew;
    uint256 bnOld;
    bnNew.SetCompact(pindexLast->nBits);
    bnOld = bnNew;
    bnNew *= nActualTimespan;
    bnNew /= Params().TargetTimespan();

    if (bnNew > Params().ProofOfWorkLimit())
        bnNew = Params().ProofOfWorkLimit();

    /// debug print
    LogPrintf("GetNextWorkRequired RETARGET\n");
    LogPrintf("Params().TargetTimespan() = %d    nActualTimespan = %d\n", Params().TargetTimespan(), 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)
{
    bool fNegative;
    bool fOverflow;
    uint256 bnTarget;
    bnTarget.SetCompact(nBits, &fNegative, &fOverflow);

    // Check range
    if (fNegative || bnTarget == 0 || fOverflow || bnTarget > Params().ProofOfWorkLimit())
        return error("CheckProofOfWork() : nBits below minimum work");

    // Check proof of work matches claimed amount
    if (hash > bnTarget)
        return error("CheckProofOfWork() : hash doesn't match nBits");

    return true;
}

//
// minimum amount of work that could possibly be required nTime after
// minimum work required was nBase
//
unsigned int ComputeMinWork(unsigned int nBase, int64_t nTime)
{
    const uint256 &bnLimit = Params().ProofOfWorkLimit();
    // Testnet has min-difficulty blocks
    // after Params().TargetSpacing()*2 time between blocks:
    if (Params().AllowMinDifficultyBlocks() && nTime > Params().TargetSpacing()*2)
        return bnLimit.GetCompact();

    uint256 bnResult;
    bnResult.SetCompact(nBase);
    while (nTime > 0 && bnResult < bnLimit)
    {
        // Maximum 400% adjustment...
        bnResult *= 4;
        // ... in best-case exactly 4-times-normal target time
        nTime -= Params().TargetTimespan()*4;
    }
    if (bnResult > bnLimit)
        bnResult = bnLimit;
    return bnResult.GetCompact();
}