From c4d28aee9bb3ce7851e92462bb2e7fdbf88f9a9a Mon Sep 17 00:00:00 2001
From: Jeffrey Wilcke <jeffrey@ethereum.org>
Date: Fri, 30 Jun 2017 16:39:18 +0300
Subject: [PATCH] consensus/ethash: implement Metropolis EIP 100

---
 consensus/ethash/consensus.go | 48 +++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/consensus/ethash/consensus.go b/consensus/ethash/consensus.go
index dd9c81fd41..fa2d740e45 100644
--- a/consensus/ethash/consensus.go
+++ b/consensus/ethash/consensus.go
@@ -289,6 +289,8 @@ func (ethash *Ethash) verifyHeader(chain consensus.ChainReader, header, parent *
 func CalcDifficulty(config *params.ChainConfig, time uint64, parent *types.Header) *big.Int {
 	next := new(big.Int).Add(parent.Number, common.Big1)
 	switch {
+	case config.IsMetropolis(next):
+		return calcDifficultyMetropolis(time, parent)
 	case config.IsHomestead(next):
 		return calcDifficultyHomestead(time, parent)
 	default:
@@ -299,10 +301,56 @@ func CalcDifficulty(config *params.ChainConfig, time uint64, parent *types.Heade
 // Some weird constants to avoid constant memory allocs for them.
 var (
 	expDiffPeriod = big.NewInt(100000)
+	big9          = big.NewInt(9)
 	big10         = big.NewInt(10)
 	bigMinus99    = big.NewInt(-99)
 )
 
+func calcDifficultyMetropolis(time uint64, parent *types.Header) *big.Int {
+	bigTime := new(big.Int).SetUint64(time)
+	bigParentTime := new(big.Int).Set(parent.Time)
+
+	// adj_factor = max((2 if len(parent.uncles) else 1) - ((timestamp - parent.timestamp) // 9), -99)
+	var x *big.Int
+	if parent.UncleHash == types.EmptyUncleHash {
+		x = big.NewInt(1)
+	} else {
+		x = big.NewInt(2)
+	}
+	z := new(big.Int).Sub(bigTime, bigParentTime)
+	z.Div(z, big9)
+	x.Sub(x, z)
+
+	// max(1 - (block_timestamp - parent_timestamp) // 10, -99)))
+	if x.Cmp(bigMinus99) < 0 {
+		x.Set(bigMinus99)
+	}
+
+	// (parent_diff + parent_diff // 2048 * max(1 - (block_timestamp - parent_timestamp) // 10, -99))
+	y := new(big.Int).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)
+	}
+
+	// for the exponential factor
+	periodCount := new(big.Int).Add(parent.Number, common.Big1)
+	periodCount.Div(periodCount, expDiffPeriod)
+
+	// the exponential factor, commonly referred to as "the bomb"
+	// diff = diff + 2^(periodCount - 2)
+	if periodCount.Cmp(common.Big1) > 0 {
+		y.Sub(periodCount, common.Big2)
+		y.Exp(common.Big2, y, nil)
+		x.Add(x, y)
+	}
+
+	return x
+}
+
 // calcDifficultyHomestead 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 Homestead rules.