consensus: add DeriveTarget() to pow.h

Split CheckProofOfWorkImpl() to introduce a helper function
DeriveTarget() which converts the nBits value to the target.

The function takes pow_limit as an argument so later commits can
avoid having to pass ChainstateManager through the call stack.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/pow.cpp

@@ -143,7 +143,7 @@ bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params&
     return CheckProofOfWorkImpl(hash, nBits, params);
 }
 
-bool CheckProofOfWorkImpl(uint256 hash, unsigned int nBits, const Consensus::Params& params)
+std::optional<arith_uint256> DeriveTarget(unsigned int nBits, const uint256 pow_limit)
 {
     bool fNegative;
     bool fOverflow;
@@ -152,8 +152,16 @@ bool CheckProofOfWorkImpl(uint256 hash, unsigned int nBits, const Consensus::Par
     bnTarget.SetCompact(nBits, &fNegative, &fOverflow);
 
     // Check range
-    if (fNegative || bnTarget == 0 || fOverflow || bnTarget > UintToArith256(params.powLimit))
+    if (fNegative || bnTarget == 0 || fOverflow || bnTarget > UintToArith256(pow_limit))
-        return false;
+        return {};
+
+    return bnTarget;
+}
+
+bool CheckProofOfWorkImpl(uint256 hash, unsigned int nBits, const Consensus::Params& params)
+{
+    auto bnTarget{DeriveTarget(nBits, params.powLimit)};
+    if (!bnTarget) return false;
 
     // Check proof of work matches claimed amount
     if (UintToArith256(hash) > bnTarget)

src/pow.h

@@ -13,6 +13,18 @@
 class CBlockHeader;
 class CBlockIndex;
 class uint256;
+class arith_uint256;
+
+/**
+ * Convert nBits value to target.
+ *
+ * @param[in] nBits     compact representation of the target
+ * @param[in] pow_limit PoW limit (consensus parameter)
+ *
+ * @return              the proof-of-work target or nullopt if the nBits value
+ *                      is invalid (due to overflow or exceeding pow_limit)
+ */
+std::optional<arith_uint256> DeriveTarget(unsigned int nBits, const uint256 pow_limit);
 
 unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params&);
 unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime, const Consensus::Params&);