From 1496b3aff6a537a1c5597ec55b8d3b25afded26c Mon Sep 17 00:00:00 2001
From: Martin Holst Swende <martin@swende.se>
Date: Tue, 6 Jun 2017 18:38:38 +0200
Subject: [PATCH] common/math, core/vm: Un-expose bigEndianByteAt, use correct
 terms for endianness

---
 common/math/big.go      | 22 +++++++++++-----------
 common/math/big_test.go | 11 ++++++-----
 core/vm/instructions.go |  4 ++--
 3 files changed, 19 insertions(+), 18 deletions(-)

diff --git a/common/math/big.go b/common/math/big.go
index 48ad902168..c49d751fae 100644
--- a/common/math/big.go
+++ b/common/math/big.go
@@ -130,10 +130,10 @@ func PaddedBigBytes(bigint *big.Int, n int) []byte {
 	return ret
 }
 
-// LittleEndianByteAt returns the byte at position n,
-// if bigint is considered little-endian.
-// So n==0 gives the least significant byte
-func LittleEndianByteAt(bigint *big.Int, n int) byte {
+// bigEndianByteAt returns the byte at position n,
+// if bigint is considered big-endian.
+// So n==0 returns the least significant byte
+func bigEndianByteAt(bigint *big.Int, n int) byte {
 	words := bigint.Bits()
 	// Check word-bucket the byte will reside in
 	i := n / wordBytes
@@ -147,15 +147,15 @@ func LittleEndianByteAt(bigint *big.Int, n int) byte {
 	return byte(word >> shift)
 }
 
-// BigEndian32ByteAt returns the byte at position n,
-// if bigint is considered big-endian.
-// So n==0 gives the most significant byte
-// WARNING: Only works for bigints in 32-byte range
-func BigEndian32ByteAt(bigint *big.Int, n int) byte {
-	if n > 31 {
+// Byte returns the byte at position n,
+// if bigint is considered little-endian with the supplied padlength.
+// n==0 returns the most significant byte
+// bigint '5', padlength 32, n=31 => 5
+func Byte(bigint *big.Int, padlength, n int) byte {
+	if n >= padlength {
 		return byte(0)
 	}
-	return LittleEndianByteAt(bigint, 31-n)
+	return bigEndianByteAt(bigint, padlength-1-n)
 }
 
 // ReadBits encodes the absolute value of bigint as big-endian bytes. Callers must ensure
diff --git a/common/math/big_test.go b/common/math/big_test.go
index d4de7b8c3d..7cce7c212b 100644
--- a/common/math/big_test.go
+++ b/common/math/big_test.go
@@ -157,13 +157,13 @@ func BenchmarkPaddedBigBytesSmallOnePadding(b *testing.B) {
 func BenchmarkByteAtBrandNew(b *testing.B) {
 	bigint := MustParseBig256("0x18F8F8F1000111000110011100222004330052300000000000000000FEFCF3CC")
 	for i := 0; i < b.N; i++ {
-		BigEndian32ByteAt(bigint, 15)
+		bigEndianByteAt(bigint, 15)
 	}
 }
 func BenchmarkByteAt(b *testing.B) {
 	bigint := MustParseBig256("0x18F8F8F1000111000110011100222004330052300000000000000000FEFCF3CC")
 	for i := 0; i < b.N; i++ {
-		BigEndian32ByteAt(bigint, 15)
+		bigEndianByteAt(bigint, 15)
 	}
 }
 func BenchmarkByteAtOld(b *testing.B) {
@@ -225,7 +225,7 @@ func TestLittleEndianByteAt(t *testing.T) {
 	}
 	for _, test := range tests {
 		v := new(big.Int).SetBytes(common.Hex2Bytes(test.x))
-		actual := LittleEndianByteAt(v, test.y)
+		actual := bigEndianByteAt(v, test.y)
 		if actual != test.exp {
 			t.Fatalf("Expected  [%v] %v:th byte to be %v, was %v.", test.x, test.y, test.exp, actual)
 		}
@@ -254,11 +254,12 @@ func TestBigEndianByteAt(t *testing.T) {
 		{"0000000000000000000000000000000000000000000000000000000000102030", 31, 0x30},
 		{"0000000000000000000000000000000000000000000000000000000000102030", 30, 0x20},
 		{"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 32, 0x0},
-		{"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 0xFFFFFFFF, 0x0},
+		{"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 31, 0xFF},
+		{"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 0xFFFF, 0x0},
 	}
 	for _, test := range tests {
 		v := new(big.Int).SetBytes(common.Hex2Bytes(test.x))
-		actual := BigEndian32ByteAt(v, test.y)
+		actual := Byte(v, 32, test.y)
 		if actual != test.exp {
 			t.Fatalf("Expected  [%v] %v:th byte to be %v, was %v.", test.x, test.y, test.exp, actual)
 		}
diff --git a/core/vm/instructions.go b/core/vm/instructions.go
index bcaf18e8a3..f5164fcdd4 100644
--- a/core/vm/instructions.go
+++ b/core/vm/instructions.go
@@ -258,8 +258,8 @@ func opXor(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stac
 func opByte(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
 	th, val := stack.pop(), stack.peek()
 	if th.Cmp(common.Big32) < 0 {
-		b := math.BigEndian32ByteAt(val, int(th.Int64()))
-		val.SetInt64(int64(b))
+		b := math.Byte(val, 32, int(th.Int64()))
+		val.SetUint64(uint64(b))
 	} else {
 		val.SetUint64(0)
 	}