consensus: don't use arith_uint256 in consensus.h

Requiring arith_uint256 at such a base level is not good for modularity.

src/chainparams.cpp

@@ -105,7 +105,7 @@ public:
         consensus.nMajorityEnforceBlockUpgrade = 750;
         consensus.nMajorityRejectBlockOutdated = 950;
         consensus.nMajorityWindow = 1000;
-        consensus.powLimit = ~arith_uint256(0) >> 32;
+        consensus.powLimit = uint256S("00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
         consensus.nPowTargetTimespan = 14 * 24 * 60 * 60; // two weeks
         consensus.nPowTargetSpacing = 10 * 60;
         consensus.fPowAllowMinDifficultyBlocks = false;
@@ -245,7 +245,7 @@ public:
         consensus.nMajorityEnforceBlockUpgrade = 750;
         consensus.nMajorityRejectBlockOutdated = 950;
         consensus.nMajorityWindow = 1000;
-        consensus.powLimit = ~arith_uint256(0) >> 1;
+        consensus.powLimit = uint256S("7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
         pchMessageStart[0] = 0xfa;
         pchMessageStart[1] = 0xbf;
         pchMessageStart[2] = 0xb5;

src/chainparams.h

@@ -6,7 +6,6 @@
 #ifndef BITCOIN_CHAINPARAMS_H
 #define BITCOIN_CHAINPARAMS_H
 
-#include "arith_uint256.h"
 #include "chainparamsbase.h"
 #include "checkpoints.h"
 #include "consensus/params.h"
@@ -45,7 +44,7 @@ public:
     const CMessageHeader::MessageStartChars& MessageStart() const { return pchMessageStart; }
     const std::vector<unsigned char>& AlertKey() const { return vAlertPubKey; }
     int GetDefaultPort() const { return nDefaultPort; }
-    const arith_uint256& ProofOfWorkLimit() const { return consensus.powLimit; }
+    const uint256& ProofOfWorkLimit() const { return consensus.powLimit; }
     int SubsidyHalvingInterval() const { return consensus.nSubsidyHalvingInterval; }
     int EnforceBlockUpgradeMajority() const { return consensus.nMajorityEnforceBlockUpgrade; }
     int RejectBlockOutdatedMajority() const { return consensus.nMajorityRejectBlockOutdated; }

src/consensus/params.h

@@ -6,7 +6,6 @@
 #ifndef BITCOIN_CONSENSUS_CONSENSUS_PARAMS_H
 #define BITCOIN_CONSENSUS_CONSENSUS_PARAMS_H
 
-#include "arith_uint256.h"
 #include "uint256.h"
 
 namespace Consensus {
@@ -21,7 +20,7 @@ struct Params {
     int nMajorityRejectBlockOutdated;
     int nMajorityWindow;
     /** Proof of work parameters */
-    arith_uint256 powLimit;
+    uint256 powLimit;
     bool fPowAllowMinDifficultyBlocks;
     int64_t nPowTargetSpacing;
     int64_t nPowTargetTimespan;

src/pow.cpp

@@ -13,7 +13,7 @@
 
 unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
 {
-    unsigned int nProofOfWorkLimit = params.powLimit.GetCompact();
+    unsigned int nProofOfWorkLimit = UintToArith256(params.powLimit).GetCompact();
 
     // Genesis block
     if (pindexLast == NULL)
@@ -61,6 +61,7 @@ unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nF
         nActualTimespan = params.nPowTargetTimespan*4;
 
     // Retarget
+    const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
     arith_uint256 bnNew;
     arith_uint256 bnOld;
     bnNew.SetCompact(pindexLast->nBits);
@@ -68,8 +69,8 @@ unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nF
     bnNew *= nActualTimespan;
     bnNew /= params.nPowTargetTimespan;
 
-    if (bnNew > params.powLimit)
-        bnNew = params.powLimit;
+    if (bnNew > bnPowLimit)
+        bnNew = bnPowLimit;
 
     /// debug print
     LogPrintf("GetNextWorkRequired RETARGET\n");
@@ -89,7 +90,7 @@ bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params&
     bnTarget.SetCompact(nBits, &fNegative, &fOverflow);
 
     // Check range
-    if (fNegative || bnTarget == 0 || fOverflow || bnTarget > params.powLimit)
+    if (fNegative || bnTarget == 0 || fOverflow || bnTarget > UintToArith256(params.powLimit))
         return error("CheckProofOfWork(): nBits below minimum work");
 
     // Check proof of work matches claimed amount