diff --git a/src/chain.h b/src/chain.h
--- a/src/chain.h
+++ b/src/chain.h
@@ -353,9 +353,12 @@
};
arith_uint256 GetBlockProof(const CBlockIndex &block);
-/** Return the time it would take to redo the work difference between from and
+
+/**
+ * Return the time it would take to redo the work difference between from and
* to, assuming the current hashrate corresponds to the difficulty at tip, in
- * seconds. */
+ * seconds.
+ */
int64_t GetBlockProofEquivalentTime(const CBlockIndex &to,
const CBlockIndex &from,
const CBlockIndex &tip,
diff --git a/src/consensus/params.h b/src/consensus/params.h
--- a/src/consensus/params.h
+++ b/src/consensus/params.h
@@ -7,6 +7,7 @@
#define BITCOIN_CONSENSUS_PARAMS_H
#include "uint256.h"
+
#include <map>
#include <string>
diff --git a/src/pow.h b/src/pow.h
--- a/src/pow.h
+++ b/src/pow.h
@@ -27,4 +27,11 @@
*/
bool CheckProofOfWork(uint256 hash, uint32_t nBits, const Consensus::Params &);
+/**
+ * Bitcoin cash's difficulty adjustment mechanism.
+ */
+uint32_t GetNextCashWorkRequired(const CBlockIndex *pindexPrev,
+ const CBlockHeader *pblock,
+ const Consensus::Params ¶ms);
+
#endif // BITCOIN_POW_H
diff --git a/src/pow.cpp b/src/pow.cpp
--- a/src/pow.cpp
+++ b/src/pow.cpp
@@ -1,5 +1,6 @@
// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2016 The Bitcoin Core developers
+// Copyright (c) 2017 The Bitcoin developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
@@ -149,3 +150,124 @@
return true;
}
+
+/**
+ * Compute the a target based on the work done between 2 blocks and the time
+ * required to produce that work.
+ */
+static arith_uint256 ComputeTarget(const CBlockIndex *pindexFirst,
+ const CBlockIndex *pindexLast,
+ const Consensus::Params ¶ms) {
+ assert(pindexLast->nHeight > pindexFirst->nHeight);
+
+ /**
+ * From the total work done and the time it took to produce that much work,
+ * we can deduce how much work we expect to be produced in the targeted time
+ * between blocks.
+ */
+ arith_uint256 work = pindexLast->nChainWork - pindexFirst->nChainWork;
+ work *= params.nPowTargetSpacing;
+
+ // In order to avoid difficulty cliffs, we bound the amplitude of the
+ // adjustement we are going to do.
+ assert(pindexLast->nTime > pindexFirst->nTime);
+ int64_t nActualTimespan = pindexLast->nTime - pindexFirst->nTime;
+ if (nActualTimespan > 288 * params.nPowTargetSpacing) {
+ nActualTimespan = 288 * params.nPowTargetSpacing;
+ } else if (nActualTimespan < 72 * params.nPowTargetSpacing) {
+ nActualTimespan = 72 * params.nPowTargetSpacing;
+ }
+
+ work /= nActualTimespan;
+
+ /**
+ * We need to compute T = (2^256 / W) - 1 but 2^256 doesn't fit in 256 bits.
+ * By expressing 1 as W / W, we get (2^256 - W) / W, and we can compute
+ * 2^256 - W as the complement of W.
+ */
+ return (-work) / work;
+}
+
+/**
+ * To reduce the impact of timestamp manipulation, we select the block we are
+ * basing our computation on via a median of 3.
+ */
+static const CBlockIndex *GetSuitableBlock(const CBlockIndex *pindex) {
+ assert(pindex->nHeight >= 3);
+
+ /**
+ * In order to avoid a block is a very skewed timestamp to have too much
+ * influence, we select the median of the 3 top most blocks as a starting
+ * point.
+ */
+ const CBlockIndex *blocks[3];
+ blocks[2] = pindex;
+ blocks[1] = pindex->pprev;
+ blocks[0] = blocks[1]->pprev;
+
+ // Sorting network.
+ if (blocks[0]->nTime > blocks[2]->nTime) {
+ std::swap(blocks[0], blocks[2]);
+ }
+
+ if (blocks[0]->nTime > blocks[1]->nTime) {
+ std::swap(blocks[0], blocks[1]);
+ }
+
+ if (blocks[1]->nTime > blocks[2]->nTime) {
+ std::swap(blocks[1], blocks[2]);
+ }
+
+ // We should have our candidate in the middle now.
+ return blocks[1];
+}
+
+/**
+ * Compute the next required proof of work using a weighted average of the
+ * estimated hashrate per block.
+ *
+ * Using a weighted average ensure that the timestamp parameter cancels out in
+ * most of the calculation - except for the timestamp of the first and last
+ * block. Because timestamps are the least trustworthy information we have as
+ * input, this ensures the algorithm is more resistant to malicious inputs.
+ */
+uint32_t GetNextCashWorkRequired(const CBlockIndex *pindexPrev,
+ const CBlockHeader *pblock,
+ const Consensus::Params ¶ms) {
+ // This cannot handle the genesis block and early blocks in general.
+ assert(pindexPrev);
+
+ // 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 (params.fPowAllowMinDifficultyBlocks &&
+ (pblock->GetBlockTime() >
+ pindexPrev->GetBlockTime() + 2 * params.nPowTargetSpacing)) {
+ return UintToArith256(params.powLimit).GetCompact();
+ }
+
+ // Compute the difficulty based on the full adjustement interval.
+ const uint32_t nHeight = pindexPrev->nHeight;
+ assert(nHeight >= params.DifficultyAdjustmentInterval());
+
+ // Get the last suitable block of the difficulty interval.
+ const CBlockIndex *pindexLast = GetSuitableBlock(pindexPrev);
+ assert(pindexLast);
+
+ // Get the first suitable block of the difficulty interval.
+ uint32_t nHeightFirst = nHeight - 144;
+ const CBlockIndex *pindexFirst =
+ GetSuitableBlock(pindexPrev->GetAncestor(nHeightFirst));
+ assert(pindexFirst);
+
+ // Compute the target based on time and work done during the interval.
+ const arith_uint256 nextTarget =
+ ComputeTarget(pindexFirst, pindexLast, params);
+
+ const arith_uint256 powLimit = UintToArith256(params.powLimit);
+ if (nextTarget > powLimit) {
+ return powLimit.GetCompact();
+ }
+
+ return nextTarget.GetCompact();
+}
diff --git a/src/test/pow_tests.cpp b/src/test/pow_tests.cpp
--- a/src/test/pow_tests.cpp
+++ b/src/test/pow_tests.cpp
@@ -84,7 +84,7 @@
blocks[i].nTime = 1269211443 + i * params.nPowTargetSpacing;
blocks[i].nBits = 0x207fffff; /* target 0x7fffff000... */
blocks[i].nChainWork =
- i ? blocks[i - 1].nChainWork + GetBlockProof(blocks[i - 1])
+ i ? blocks[i - 1].nChainWork + GetBlockProof(blocks[i])
: arith_uint256(0);
}
@@ -106,6 +106,7 @@
block.nTime = pindexPrev->nTime + nTimeInterval;
block.nBits = nBits;
+ block.nChainWork = pindexPrev->nChainWork + GetBlockProof(block);
return block;
}
@@ -119,12 +120,14 @@
arith_uint256 currentPow = powLimit >> 1;
uint32_t initialBits = currentPow.GetCompact();
- // Genesis block?
+ // Genesis block.
blocks[0] = CBlockIndex();
blocks[0].nHeight = 0;
blocks[0].nTime = 1269211443;
blocks[0].nBits = initialBits;
+ blocks[0].nChainWork = GetBlockProof(blocks[0]);
+
// Pile up some blocks.
for (size_t i = 1; i < 100; i++) {
blocks[i] = GetBlockIndex(&blocks[i - 1], params.nPowTargetSpacing,
@@ -187,4 +190,181 @@
powLimit.GetCompact());
}
+BOOST_AUTO_TEST_CASE(cash_difficulty_test) {
+ SelectParams(CBaseChainParams::MAIN);
+ const Consensus::Params ¶ms = Params().GetConsensus();
+
+ std::vector<CBlockIndex> blocks(3000);
+
+ const arith_uint256 powLimit = UintToArith256(params.powLimit);
+ uint32_t powLimitBits = powLimit.GetCompact();
+ arith_uint256 currentPow = powLimit >> 4;
+ uint32_t initialBits = currentPow.GetCompact();
+
+ // Genesis block.
+ blocks[0] = CBlockIndex();
+ blocks[0].nHeight = 0;
+ blocks[0].nTime = 1269211443;
+ blocks[0].nBits = initialBits;
+
+ blocks[0].nChainWork = GetBlockProof(blocks[0]);
+
+ // Block counter.
+ size_t i;
+
+ // Pile up some blocks every 10 mins to establish some history.
+ for (i = 1; i < 2050; i++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 600, initialBits);
+ }
+
+ CBlockHeader blkHeaderDummy;
+ uint32_t nBits =
+ GetNextCashWorkRequired(&blocks[2049], &blkHeaderDummy, params);
+
+ // Difficulty stays the same as long as we produce a block every 10 mins.
+ for (size_t j = 0; j < 10; i++, j++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 600, nBits);
+ BOOST_CHECK_EQUAL(
+ GetNextCashWorkRequired(&blocks[i], &blkHeaderDummy, params),
+ nBits);
+ }
+
+ // Make sure we skip over blocks that are out of wack. To do so, we produce
+ // a block that is far in the future, and then produce a block with the
+ // expected timestamp.
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 6000, nBits);
+ BOOST_CHECK_EQUAL(
+ GetNextCashWorkRequired(&blocks[i++], &blkHeaderDummy, params), nBits);
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 2 * 600 - 6000, nBits);
+ BOOST_CHECK_EQUAL(
+ GetNextCashWorkRequired(&blocks[i++], &blkHeaderDummy, params), nBits);
+
+ // The system should continue unaffected by the block with a bogous
+ // timestamps.
+ for (size_t j = 0; j < 20; i++, j++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 600, nBits);
+ BOOST_CHECK_EQUAL(
+ GetNextCashWorkRequired(&blocks[i], &blkHeaderDummy, params),
+ nBits);
+ }
+
+ // We start emitting blocks slightly faster. The first block has no impact.
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 550, nBits);
+ BOOST_CHECK_EQUAL(
+ GetNextCashWorkRequired(&blocks[i++], &blkHeaderDummy, params), nBits);
+
+ // Now we should see difficulty increase slowly.
+ for (size_t j = 0; j < 10; i++, j++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 550, nBits);
+ const uint32_t nextBits =
+ GetNextCashWorkRequired(&blocks[i], &blkHeaderDummy, params);
+
+ arith_uint256 currentTarget;
+ currentTarget.SetCompact(nBits);
+ arith_uint256 nextTarget;
+ nextTarget.SetCompact(nextBits);
+
+ // Make sure that difficulty increases very slowly.
+ BOOST_CHECK(nextTarget < currentTarget);
+ BOOST_CHECK((currentTarget - nextTarget) < (currentTarget >> 10));
+
+ nBits = nextBits;
+ }
+
+ // Check the actual value.
+ BOOST_CHECK_EQUAL(nBits, 0x1c0fe7b1);
+
+ // If we dramatically shorten block production, difficulty increases faster.
+ for (size_t j = 0; j < 20; i++, j++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 10, nBits);
+ const uint32_t nextBits =
+ GetNextCashWorkRequired(&blocks[i], &blkHeaderDummy, params);
+
+ arith_uint256 currentTarget;
+ currentTarget.SetCompact(nBits);
+ arith_uint256 nextTarget;
+ nextTarget.SetCompact(nextBits);
+
+ // Make sure that difficulty increases faster.
+ BOOST_CHECK(nextTarget < currentTarget);
+ BOOST_CHECK((currentTarget - nextTarget) < (currentTarget >> 4));
+
+ nBits = nextBits;
+ }
+
+ // Check the actual value.
+ BOOST_CHECK_EQUAL(nBits, 0x1c0db19f);
+
+ // We start to emit blocks significantly slower. The first block has no
+ // impact.
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 6000, nBits);
+ nBits = GetNextCashWorkRequired(&blocks[i++], &blkHeaderDummy, params);
+
+ // Check the actual value.
+ BOOST_CHECK_EQUAL(nBits, 0x1c0d9222);
+
+ // If we dramatically slow down block production, difficulty decreases.
+ for (size_t j = 0; j < 93; i++, j++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 6000, nBits);
+ const uint32_t nextBits =
+ GetNextCashWorkRequired(&blocks[i], &blkHeaderDummy, params);
+
+ arith_uint256 currentTarget;
+ currentTarget.SetCompact(nBits);
+ arith_uint256 nextTarget;
+ nextTarget.SetCompact(nextBits);
+
+ // Check the difficulty decreases.
+ BOOST_CHECK(nextTarget <= powLimit);
+ BOOST_CHECK(nextTarget > currentTarget);
+ BOOST_CHECK((nextTarget - currentTarget) < (currentTarget >> 3));
+
+ nBits = nextBits;
+ }
+
+ // Check the actual value.
+ BOOST_CHECK_EQUAL(nBits, 0x1c2f13b9);
+
+ // Due to the window of time being bounded, next block's difficulty actually
+ // gets harder.
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 6000, nBits);
+ nBits = GetNextCashWorkRequired(&blocks[i++], &blkHeaderDummy, params);
+ BOOST_CHECK_EQUAL(nBits, 0x1c2ee9bf);
+
+ // And goes down again. It takes a while due to the window being bounded and
+ // the skewed block causes 2 blocks to get out of the window.
+ for (size_t j = 0; j < 192; i++, j++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 6000, nBits);
+ const uint32_t nextBits =
+ GetNextCashWorkRequired(&blocks[i], &blkHeaderDummy, params);
+
+ arith_uint256 currentTarget;
+ currentTarget.SetCompact(nBits);
+ arith_uint256 nextTarget;
+ nextTarget.SetCompact(nextBits);
+
+ // Check the difficulty decreases.
+ BOOST_CHECK(nextTarget <= powLimit);
+ BOOST_CHECK(nextTarget > currentTarget);
+ BOOST_CHECK((nextTarget - currentTarget) < (currentTarget >> 3));
+
+ nBits = nextBits;
+ }
+
+ // Check the actual value.
+ BOOST_CHECK_EQUAL(nBits, 0x1d00ffff);
+
+ // Once the difficulty reached the minimum allowed level, it doesn't get any
+ // easier.
+ for (size_t j = 0; j < 5; i++, j++) {
+ blocks[i] = GetBlockIndex(&blocks[i - 1], 6000, nBits);
+ const uint32_t nextBits =
+ GetNextCashWorkRequired(&blocks[i], &blkHeaderDummy, params);
+
+ // Check the difficulty stays constant.
+ BOOST_CHECK_EQUAL(nextBits, powLimitBits);
+ nBits = nextBits;
+ }
+}
+
BOOST_AUTO_TEST_SUITE_END()