Add symmetric SHA256D RandomX difficulty adjustment

src/pow.cpp

@@ -192,22 +192,132 @@ static unsigned int ApplyFastRiseProtection(
     return fastTarget.GetCompact();
 }
 
-unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params, int algo)
+static unsigned int CalculateMultiAlgoWorkRequired(
+    const CBlockIndex* pindexLast,
+    const Consensus::Params& params,
+    int algo)
 {
-    if (algo == ALGO_RANDOMX) {
+    if (pindexLast == nullptr) {
+        return InitialDifficulty(params);
+    }
+
     const CBlockIndex* pindexPrevAlgo =
-            GetLastBlockIndexForAlgoFast(pindexLast, ALGO_RANDOMX);
+        GetLastBlockIndexForAlgoFast(pindexLast, algo);
 
     if (pindexPrevAlgo == nullptr) {
         return InitialDifficulty(params);
     }
 
-        // RandomX will get its own per-algo retarget logic here.
-        // For now, preserve the last RandomX difficulty.
+    if (params.fPowNoRetargeting || params.fEasyPow) {
         return pindexPrevAlgo->nBits;
     }
 
-    if (algo != ALGO_SHA256D) {
+    // With two algorithms and a 10-minute global block target, each
+    // algorithm is expected to appear approximately once every 20 minutes.
+    // Use 10 samples per algorithm => 20 global blocks.
+    const int64_t averagingBlocks =
+        params.difficultyAveragingWindow * NUM_ALGOS;
+
+    const CBlockIndex* pindexFirst = pindexLast;
+    for (int64_t i = 0; pindexFirst && i < averagingBlocks; ++i) {
+        pindexFirst = pindexFirst->pprev;
+    }
+
+    if (pindexFirst == nullptr) {
+        return pindexPrevAlgo->nBits;
+    }
+
+    const int64_t targetTimespan =
+        averagingBlocks * params.nPowTargetSpacing;
+
+    int64_t actualTimespan =
+        pindexLast->GetMedianTimePast() -
+        pindexFirst->GetMedianTimePast();
+
+    // Preserve the existing Vexta 1/4 damping.
+    actualTimespan =
+        targetTimespan + (actualTimespan - targetTimespan) / 4;
+
+    // Preserve the existing 92% / 116% adjustment bounds, scaled to
+    // the longer multi-algo averaging window.
+    const int64_t minTimespan =
+        targetTimespan * params.difficultyMinActualTimespan /
+        params.difficultyTargetTimespan;
+
+    const int64_t maxTimespan =
+        targetTimespan * params.difficultyMaxActualTimespan /
+        params.difficultyTargetTimespan;
+
+    if (actualTimespan < minTimespan) {
+        actualTimespan = minTimespan;
+    }
+    if (actualTimespan > maxTimespan) {
+        actualTimespan = maxTimespan;
+    }
+
+    arith_uint256 bnNew;
+    bnNew.SetCompact(pindexPrevAlgo->nBits);
+
+    // Global chain-speed adjustment.
+    bnNew *= actualTimespan;
+    bnNew /= targetTimespan;
+
+    // Local per-algo balancing.
+    //
+    // For two algorithms:
+    //   ideal next algo: adjustment = 0
+    //   same algo twice: adjustment = +1 => 4% harder
+    //   algo skipped once: adjustment = -1 => 4% easier
+    const int nAdjustments =
+        pindexPrevAlgo->nHeight + NUM_ALGOS - 1 - pindexLast->nHeight;
+
+    static constexpr int64_t LOCAL_TARGET_ADJUSTMENT = 4;
+
+    if (nAdjustments > 0) {
+        for (int i = 0; i < nAdjustments; ++i) {
+            bnNew *= 100;
+            bnNew /= 100 + LOCAL_TARGET_ADJUSTMENT;
+        }
+    } else if (nAdjustments < 0) {
+        for (int i = 0; i < -nAdjustments; ++i) {
+            bnNew *= 100 + LOCAL_TARGET_ADJUSTMENT;
+            bnNew /= 100;
+
+            if (bnNew > UintToArith256(params.powLimit)) {
+                bnNew = UintToArith256(params.powLimit);
+                break;
+            }
+        }
+    }
+
+    if (bnNew == 0) {
+        bnNew = 1;
+    }
+
+    if (bnNew > UintToArith256(params.powLimit)) {
+        bnNew = UintToArith256(params.powLimit);
+    }
+
+    return bnNew.GetCompact();
+}
+
+unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params, int algo)
+{
+    if (algo != ALGO_SHA256D && algo != ALGO_RANDOMX) {
+        return InitialDifficulty(params);
+    }
+
+    // Before RandomX activation, preserve the existing SHA256D consensus
+    // exactly. RandomX remains unavailable through validation.
+    //
+    // From activation onward both algorithms use the same symmetric
+    // two-algorithm DAA.
+    if (pindexLast != nullptr &&
+        pindexLast->nHeight + 1 >= params.randomXActivationHeight) {
+        return CalculateMultiAlgoWorkRequired(pindexLast, params, algo);
+    }
+
+    if (algo == ALGO_RANDOMX) {
         return InitialDifficulty(params);
     }
 

src/test/pow_tests.cpp

@@ -532,6 +532,175 @@ BOOST_AUTO_TEST_CASE(FastRise_zero_and_negative_intervals_test)
 
 
 
+
+
+BOOST_AUTO_TEST_CASE(MultiAlgo_DAA_activation_boundary_test)
+{
+    auto chainParams = CreateChainParams(*m_node.args, CBaseChainParams::MAIN);
+    auto consensus = chainParams->GetConsensus();
+
+    consensus.randomXActivationHeight = 40;
+
+    std::vector<CBlockIndex> blocks(40);
+
+    arith_uint256 baseTarget = UintToArith256(consensus.powLimit);
+    baseTarget >>= 8;
+    const unsigned int baseBits = baseTarget.GetCompact();
+    const int64_t startTime = 1800000000;
+
+    CBlockIndex* latest[NUM_ALGOS_IMPL]{};
+
+    for (int i = 0; i < 40; ++i) {
+        blocks[i].pprev = i ? &blocks[i - 1] : nullptr;
+        blocks[i].nHeight = i;
+        blocks[i].nTime = startTime + i * consensus.nPowTargetSpacing;
+        blocks[i].nBits = baseBits;
+        blocks[i].nVersion =
+            BLOCK_VERSION_DEFAULT | BLOCK_VERSION_SHA256D;
+
+        for (int algo = 0; algo < NUM_ALGOS_IMPL; ++algo) {
+            blocks[i].lastAlgoBlocks[algo] = latest[algo];
+        }
+
+        latest[ALGO_SHA256D] = &blocks[i];
+    }
+
+    // Height 39 -> next block is height 40, exactly the multi-algo
+    // activation boundary.
+    //
+    // SHA256D is being requested directly after another SHA256D block,
+    // so the local balancing rule must make it exactly one 4% step harder.
+    const unsigned int firstMultiAlgoShaBits =
+        GetNextWorkRequired(&blocks[39], nullptr, consensus, ALGO_SHA256D);
+
+    arith_uint256 compactBaseTarget;
+    compactBaseTarget.SetCompact(baseBits);
+
+    arith_uint256 expectedShaTarget = compactBaseTarget;
+    expectedShaTarget *= 100;
+    expectedShaTarget /= 104;
+
+    BOOST_CHECK_EQUAL(
+        firstMultiAlgoShaBits,
+        expectedShaTarget.GetCompact());
+
+    // No RandomX block exists yet. For now this deliberately documents
+    // the current fallback to InitialDifficulty(). This is NOT the final
+    // RandomX launch calibration and will be replaced by an explicit
+    // RandomX initial target before activation is enabled.
+    const unsigned int firstRandomXBits =
+        GetNextWorkRequired(&blocks[39], nullptr, consensus, ALGO_RANDOMX);
+
+    BOOST_CHECK_EQUAL(
+        firstRandomXBits,
+        InitialDifficulty(consensus));
+}
+
+
+BOOST_AUTO_TEST_CASE(MultiAlgo_DAA_local_balance_test)
+{
+    auto chainParams = CreateChainParams(*m_node.args, CBaseChainParams::MAIN);
+    auto consensus = chainParams->GetConsensus();
+
+    // Test-only activation. Mainnet RandomX remains disabled in chainparams.
+    consensus.randomXActivationHeight = 20;
+
+    std::vector<CBlockIndex> blocks(40);
+
+    arith_uint256 baseTarget = UintToArith256(consensus.powLimit);
+    baseTarget >>= 8;
+    const unsigned int baseBits = baseTarget.GetCompact();
+    const int64_t startTime = 1800000000;
+
+    for (int i = 0; i < 40; ++i) {
+        blocks[i].pprev = i ? &blocks[i - 1] : nullptr;
+        blocks[i].nHeight = i;
+        blocks[i].nTime = startTime + i * consensus.nPowTargetSpacing;
+        blocks[i].nBits = baseBits;
+
+        // Before activation the chain is SHA256D-only.
+        // From activation onward, build an ideal alternating sequence.
+        if (i >= consensus.randomXActivationHeight && (i & 1)) {
+            blocks[i].nVersion =
+                BLOCK_VERSION_DEFAULT | BLOCK_VERSION_RANDOMX;
+        } else {
+            blocks[i].nVersion =
+                BLOCK_VERSION_DEFAULT | BLOCK_VERSION_SHA256D;
+        }
+    }
+
+    auto rebuildAlgoLinks = [&]() {
+        CBlockIndex* latest[NUM_ALGOS_IMPL]{};
+
+        for (auto& block : blocks) {
+            for (int algo = 0; algo < NUM_ALGOS_IMPL; ++algo) {
+                block.lastAlgoBlocks[algo] = latest[algo];
+            }
+
+            const int algo = block.GetAlgo();
+            BOOST_REQUIRE(algo >= 0);
+            BOOST_REQUIRE(algo < NUM_ALGOS_IMPL);
+            latest[algo] = &block;
+        }
+    };
+
+    rebuildAlgoLinks();
+
+    // Height 39 is RandomX. Therefore SHA256D is the ideal next algo.
+    // With perfect 10-minute global spacing its target must stay unchanged.
+    const unsigned int balancedShaBits =
+        GetNextWorkRequired(&blocks[39], nullptr, consensus, ALGO_SHA256D);
+
+    BOOST_CHECK_EQUAL(balancedShaBits, baseBits);
+
+    // RandomX was just mined at height 39. Mining RandomX again immediately
+    // must make that algo 4% harder.
+    const unsigned int repeatedRandomXBits =
+        GetNextWorkRequired(&blocks[39], nullptr, consensus, ALGO_RANDOMX);
+
+    arith_uint256 compactBaseTarget;
+    compactBaseTarget.SetCompact(baseBits);
+
+    arith_uint256 expectedRepeatedRandomXTarget = compactBaseTarget;
+    expectedRepeatedRandomXTarget *= 100;
+    expectedRepeatedRandomXTarget /= 104;
+
+    BOOST_CHECK_EQUAL(
+        repeatedRandomXBits,
+        expectedRepeatedRandomXTarget.GetCompact());
+
+    // Now simulate SHA256D also being mined at height 39, directly after
+    // SHA256D at height 38. SHA must become harder while the missing RandomX
+    // side becomes easier.
+    blocks[39].nVersion =
+        BLOCK_VERSION_DEFAULT | BLOCK_VERSION_SHA256D;
+
+    rebuildAlgoLinks();
+
+    const unsigned int repeatedShaBits =
+        GetNextWorkRequired(&blocks[39], nullptr, consensus, ALGO_SHA256D);
+
+    const unsigned int missingRandomXBits =
+        GetNextWorkRequired(&blocks[39], nullptr, consensus, ALGO_RANDOMX);
+
+    arith_uint256 expectedRepeatedShaTarget = compactBaseTarget;
+    expectedRepeatedShaTarget *= 100;
+    expectedRepeatedShaTarget /= 104;
+
+    arith_uint256 expectedMissingRandomXTarget = compactBaseTarget;
+    expectedMissingRandomXTarget *= 104;
+    expectedMissingRandomXTarget /= 100;
+
+    BOOST_CHECK_EQUAL(
+        repeatedShaBits,
+        expectedRepeatedShaTarget.GetCompact());
+
+    BOOST_CHECK_EQUAL(
+        missingRandomXBits,
+        expectedMissingRandomXTarget.GetCompact());
+}
+
+
 BOOST_AUTO_TEST_CASE(RandomX_seed_height_test)
 {
     auto chainParams = CreateChainParams(*m_node.args, CBaseChainParams::MAIN);