1// Copyright (c) 2009-2010 Satoshi Nakamoto
2// Copyright (c) 2009-present The Bitcoin Core developers
3// Distributed under the MIT software license, see the accompanying
4// file COPYING or http://www.opensource.org/licenses/mit-license.php.
5
6#include <pow.h>
7
8#include <arith_uint256.h>
9#include <chain.h>
10#include <primitives/block.h>
11#include <uint256.h>
12#include <util/check.h>
13
14unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
15{
16    assert(pindexLast != nullptr);
17    unsigned int nProofOfWorkLimit = UintToArith256(params.powLimit).GetCompact();
18
19    // Only change once per difficulty adjustment interval
20    if ((pindexLast->nHeight+1) % params.DifficultyAdjustmentInterval() != 0)
21    {
22        if (params.fPowAllowMinDifficultyBlocks)
23        {
24            // Special difficulty rule for testnet:
25            // If the new block's timestamp is more than 2* 10 minutes
26            // then it MUST be a min-difficulty block.
27            if (pblock->GetBlockTime() > pindexLast->GetBlockTime() + params.nPowTargetSpacing*2)
28                return nProofOfWorkLimit;
29            else
30            {
31                // Return the last non-special-min-difficulty-rules-block
32                const CBlockIndex* pindex = pindexLast;
33                while (pindex->pprev && pindex->nHeight % params.DifficultyAdjustmentInterval() != 0 && pindex->nBits == nProofOfWorkLimit)
34                    pindex = pindex->pprev;
35                return pindex->nBits;
36            }
37        }
38        return pindexLast->nBits;
39    }
40
41    // Go back by what we want to be 14 days worth of blocks
42    int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
43    assert(nHeightFirst >= 0);
44    const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
45    assert(pindexFirst);
46
47    return CalculateNextWorkRequired(pindexLast, pindexFirst->GetBlockTime(), params);
48}
49
50unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime, const Consensus::Params& params)
51{
52    if (params.fPowNoRetargeting)
53        return pindexLast->nBits;
54
55    // Limit adjustment step
56    int64_t nActualTimespan = pindexLast->GetBlockTime() - nFirstBlockTime;
57    if (nActualTimespan < params.nPowTargetTimespan/4)
58        nActualTimespan = params.nPowTargetTimespan/4;
59    if (nActualTimespan > params.nPowTargetTimespan*4)
60        nActualTimespan = params.nPowTargetTimespan*4;
61
62    // Retarget
63    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
64    arith_uint256 bnNew;
65
66    // Special difficulty rule for Testnet4
67    if (params.enforce_BIP94) {
68        // Here we use the first block of the difficulty period. This way
69        // the real difficulty is always preserved in the first block as
70        // it is not allowed to use the min-difficulty exception.
71        int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
72        const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
73        bnNew.SetCompact(pindexFirst->nBits);
74    } else {
75        bnNew.SetCompact(pindexLast->nBits);
76    }
77
78    bnNew *= nActualTimespan;
79    bnNew /= params.nPowTargetTimespan;
80
81    if (bnNew > bnPowLimit)
82        bnNew = bnPowLimit;
83
84    return bnNew.GetCompact();
85}
86
87// Check that on difficulty adjustments, the new difficulty does not increase
88// or decrease beyond the permitted limits.
89bool PermittedDifficultyTransition(const Consensus::Params& params, int64_t height, uint32_t old_nbits, uint32_t new_nbits)
90{
91    if (params.fPowAllowMinDifficultyBlocks) return true;
92
93    if (height % params.DifficultyAdjustmentInterval() == 0) {
94        int64_t smallest_timespan = params.nPowTargetTimespan/4;
95        int64_t largest_timespan = params.nPowTargetTimespan*4;
96
97        const arith_uint256 pow_limit = UintToArith256(params.powLimit);
98        arith_uint256 observed_new_target;
99        observed_new_target.SetCompact(new_nbits);
100
101        // Calculate the largest difficulty value possible:
102        arith_uint256 largest_difficulty_target;
103        largest_difficulty_target.SetCompact(old_nbits);
104        largest_difficulty_target *= largest_timespan;
105        largest_difficulty_target /= params.nPowTargetTimespan;
106
107        if (largest_difficulty_target > pow_limit) {
108            largest_difficulty_target = pow_limit;
109        }
110
111        // Round and then compare this new calculated value to what is
112        // observed.
113        arith_uint256 maximum_new_target;
114        maximum_new_target.SetCompact(largest_difficulty_target.GetCompact());
115        if (maximum_new_target < observed_new_target) return false;
116
117        // Calculate the smallest difficulty value possible:
118        arith_uint256 smallest_difficulty_target;
119        smallest_difficulty_target.SetCompact(old_nbits);
120        smallest_difficulty_target *= smallest_timespan;
121        smallest_difficulty_target /= params.nPowTargetTimespan;
122
123        if (smallest_difficulty_target > pow_limit) {
124            smallest_difficulty_target = pow_limit;
125        }
126
127        // Round and then compare this new calculated value to what is
128        // observed.
129        arith_uint256 minimum_new_target;
130        minimum_new_target.SetCompact(smallest_difficulty_target.GetCompact());
131        if (minimum_new_target > observed_new_target) return false;
132    } else if (old_nbits != new_nbits) {
133        return false;
134    }
135    return true;
136}
137
138// Bypasses the actual proof of work check during fuzz testing with a simplified validation checking whether
139// the most significant bit of the last byte of the hash is set.
140bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params& params)
141{
142    if (EnableFuzzDeterminism()) return (hash.data()[31] & 0x80) == 0;
143    return CheckProofOfWorkImpl(hash, nBits, params);
144}
145
146std::optional<arith_uint256> DeriveTarget(unsigned int nBits, const uint256 pow_limit)
147{
148    bool fNegative;
149    bool fOverflow;
150    arith_uint256 bnTarget;
151
152    bnTarget.SetCompact(nBits, &fNegative, &fOverflow);
153
154    // Check range
155    if (fNegative || bnTarget == 0 || fOverflow || bnTarget > UintToArith256(pow_limit))
156        return {};
157
158    return bnTarget;
159}
160
161bool CheckProofOfWorkImpl(uint256 hash, unsigned int nBits, const Consensus::Params& params)
162{
163    auto bnTarget{DeriveTarget(nBits, params.powLimit)};
164    if (!bnTarget) return false;
165
166    // Check proof of work matches claimed amount
167    if (UintToArith256(hash) > bnTarget)
168        return false;
169
170    return true;
171}
172