testnet: Add Testnet4 difficulty adjustment rules fix

Author: fjahr

src/pow.cpp

@@ -61,7 +61,19 @@ unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nF
     // Retarget
     const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
     arith_uint256 bnNew;
-    bnNew.SetCompact(pindexLast->nBits);
+
+    // Special difficulty rule for Testnet4
+    if (params.enforce_BIP94) {
+        // Here we use the first block of the difficulty period. This way
+        // the real difficulty is always preserved in the first block as
+        // it is not allowed to use the min-difficulty exception.
+        int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
+        const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
+        bnNew.SetCompact(pindexFirst->nBits);
+    } else {
+        bnNew.SetCompact(pindexLast->nBits);
+    }
+
     bnNew *= nActualTimespan;
     bnNew /= params.nPowTargetTimespan;