1 files changed, 51 insertions, 0 deletions
diff --git a/src/pow.cpp b/src/pow.cpp
index 1414d37564..c0449cac74 100644
--- a/src/pow.cpp
+++ b/src/pow.cpp
@@ -71,6 +71,57 @@ unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nF
     return bnNew.GetCompact();
 }
 
+// Check that on difficulty adjustments, the new difficulty does not increase
+// or decrease beyond the permitted limits.
+bool PermittedDifficultyTransition(const Consensus::Params& params, int64_t height, uint32_t old_nbits, uint32_t new_nbits)
+{
+    if (params.fPowAllowMinDifficultyBlocks) return true;
+
+    if (height % params.DifficultyAdjustmentInterval() == 0) {
+        int64_t smallest_timespan = params.nPowTargetTimespan/4;
+        int64_t largest_timespan = params.nPowTargetTimespan*4;
+
+        const arith_uint256 pow_limit = UintToArith256(params.powLimit);
+        arith_uint256 observed_new_target;
+        observed_new_target.SetCompact(new_nbits);
+
+        // Calculate the largest difficulty value possible:
+        arith_uint256 largest_difficulty_target;
+        largest_difficulty_target.SetCompact(old_nbits);
+        largest_difficulty_target *= largest_timespan;
+        largest_difficulty_target /= params.nPowTargetTimespan;
+
+        if (largest_difficulty_target > pow_limit) {
+            largest_difficulty_target = pow_limit;
+        }
+
+        // Round and then compare this new calculated value to what is
+        // observed.
+        arith_uint256 maximum_new_target;
+        maximum_new_target.SetCompact(largest_difficulty_target.GetCompact());
+        if (maximum_new_target < observed_new_target) return false;
+
+        // Calculate the smallest difficulty value possible:
+        arith_uint256 smallest_difficulty_target;
+        smallest_difficulty_target.SetCompact(old_nbits);
+        smallest_difficulty_target *= smallest_timespan;
+        smallest_difficulty_target /= params.nPowTargetTimespan;
+
+        if (smallest_difficulty_target > pow_limit) {
+            smallest_difficulty_target = pow_limit;
+        }
+
+        // Round and then compare this new calculated value to what is
+        // observed.
+        arith_uint256 minimum_new_target;
+        minimum_new_target.SetCompact(smallest_difficulty_target.GetCompact());
+        if (minimum_new_target > observed_new_target) return false;
+    } else if (old_nbits != new_nbits) {
+        return false;
+    }
+    return true;
+}
+
 bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params& params)
 {
     bool fNegative;