Defuse difficulty bomb before block 3M

+ New difficulty calculation will be activated on CLOHF1

consensus/ethash/consensus.go

@@ -306,6 +306,8 @@ func (ethash *Ethash) CalcDifficulty(chain consensus.ChainReader, time uint64, p
 func CalcDifficulty(config *params.ChainConfig, time uint64, parent *types.Header) *big.Int {
 	next := new(big.Int).Add(parent.Number, big1)
 	switch {
+	case config.IsCLOHF1(next):
+		return calcDifficultyCLOHF1(time, parent)
 	case config.IsBombDisposal(next):
 		return calcDifficultyBombDisposal(time, parent)
 	case config.IsByzantium(next):
@@ -330,6 +332,47 @@ var (
 	big2999999    = big.NewInt(2999999)
 )
 
+// calcDifficultyCLOHF1 is the difficulty adjustment algorithm. It returns
+// the difficulty that a new block should have when created at time given the
+// parent block's time and difficulty. The calculation uses the CLOHF1 rules.
+func calcDifficultyCLOHF1(time uint64, parent *types.Header) *big.Int {
+	// https://github.com/ethereum/EIPs/issues/100.
+	// algorithm:
+	// diff = (parent_diff +
+	//         (parent_diff / 2048 * max((2 if len(parent.uncles) else 1) - ((timestamp - parent.timestamp) // 9), -99))
+	//        )
+
+	bigTime := new(big.Int).SetUint64(time)
+	bigParentTime := new(big.Int).Set(parent.Time)
+
+	// holds intermediate values to make the algo easier to read & audit
+	x := new(big.Int)
+	y := new(big.Int)
+
+	// (2 if len(parent_uncles) else 1) - (block_timestamp - parent_timestamp) // 9
+	x.Sub(bigTime, bigParentTime)
+	x.Div(x, big9)
+	if parent.UncleHash == types.EmptyUncleHash {
+		x.Sub(big1, x)
+	} else {
+		x.Sub(big2, x)
+	}
+	// max((2 if len(parent_uncles) else 1) - (block_timestamp - parent_timestamp) // 9, -99)
+	if x.Cmp(bigMinus99) < 0 {
+		x.Set(bigMinus99)
+	}
+	// parent_diff + (parent_diff / 2048 * max((2 if len(parent.uncles) else 1) - ((timestamp - parent.timestamp) // 9), -99))
+	y.Div(parent.Difficulty, params.DifficultyBoundDivisor)
+	x.Mul(y, x)
+	x.Add(parent.Difficulty, x)
+
+	// minimum difficulty can ever be (before exponential factor)
+	if x.Cmp(params.MinimumDifficulty) < 0 {
+		x.Set(params.MinimumDifficulty)
+	}
+	return x
+}
+
 // calcDifficultyByzantium is the difficulty adjustment algorithm. It returns
 // the difficulty that a new block should have when created at time given the
 // parent block's time and difficulty. The calculation uses the Byzantium rules.