From b9923b46f68717de24629f8f689125680ea1b460 Mon Sep 17 00:00:00 2001
From: Jeffrey Wilcke <jeffrey@ethereum.org>
Date: Thu, 20 Oct 2016 15:00:53 +0200
Subject: [PATCH] common/math, core/vm: implement fast EXP.

Courtesy @chfast & @karalabe
---
 common/math/exp.go      | 34 ++++++++++++++++++++++++++++++++++
 core/vm/instructions.go |  5 +++--
 2 files changed, 37 insertions(+), 2 deletions(-)
 create mode 100644 common/math/exp.go

diff --git a/common/math/exp.go b/common/math/exp.go
new file mode 100644
index 0000000000..6301bc590a
--- /dev/null
+++ b/common/math/exp.go
@@ -0,0 +1,34 @@
+package math
+
+import (
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/common"
+)
+
+const (
+	// Perverted constants straight out of big.Int
+	// https://golang.org/src/math/big/arith.go
+	_m    = ^big.Word(0)
+	_logS = _m>>8&1 + _m>>16&1 + _m>>32&1
+	_S    = 1 << _logS
+	_W    = _S << 3 // word size in bits
+)
+
+// Exp implement exponentiation by squaring algorithm.
+//
+// Courtesy @karalabe and @chfast
+func Exp(base, exponent *big.Int) *big.Int {
+	result := big.NewInt(1)
+
+	for _, word := range exponent.Bits() {
+		for i := 0; i < _W; i++ {
+			if word&1 == 1 {
+				common.U256(result.Mul(result, base))
+			}
+			common.U256(base.Mul(base, base))
+			word >>= 1
+		}
+	}
+	return result
+}
diff --git a/core/vm/instructions.go b/core/vm/instructions.go
index 3352877c1f..94a8c6348c 100644
--- a/core/vm/instructions.go
+++ b/core/vm/instructions.go
@@ -21,6 +21,7 @@ import (
 	"math/big"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/params"
 )
@@ -192,8 +193,8 @@ func opSmod(instr instruction, pc *uint64, env Environment, contract *Contract,
 }
 
 func opExp(instr instruction, pc *uint64, env Environment, contract *Contract, memory *Memory, stack *Stack) {
-	x, y := stack.pop(), stack.pop()
-	stack.push(U256(x.Exp(x, y, Pow256)))
+	base, exponent := stack.pop(), stack.pop()
+	stack.push(math.Exp(base, exponent))
 }
 
 func opSignExtend(instr instruction, pc *uint64, env Environment, contract *Contract, memory *Memory, stack *Stack) {