From 3285a0fda37207ca1b79ac28e2c12c6f5efff89b Mon Sep 17 00:00:00 2001
From: Martin Holst Swende <martin@swende.se>
Date: Sun, 28 May 2017 23:39:33 +0200
Subject: core/vm, common/math: Add fast getByte for bigints, improve opByte

---
 common/math/big.go      | 28 +++++++++++++++
 common/math/big_test.go | 93 ++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 120 insertions(+), 1 deletion(-)

(limited to 'common')

diff --git a/common/math/big.go b/common/math/big.go
index fd0174b36..48ad90216 100644
--- a/common/math/big.go
+++ b/common/math/big.go
@@ -130,6 +130,34 @@ 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 {
+	words := bigint.Bits()
+	// Check word-bucket the byte will reside in
+	i := n / wordBytes
+	if i >= len(words) {
+		return byte(0)
+	}
+	word := words[i]
+	// Offset of the byte
+	shift := 8 * uint(n%wordBytes)
+
+	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 {
+		return byte(0)
+	}
+	return LittleEndianByteAt(bigint, 31-n)
+}
+
 // ReadBits encodes the absolute value of bigint as big-endian bytes. Callers must ensure
 // that buf has enough space. If buf is too short the result will be incomplete.
 func ReadBits(bigint *big.Int, buf []byte) {
diff --git a/common/math/big_test.go b/common/math/big_test.go
index e789bd18e..d4de7b8c3 100644
--- a/common/math/big_test.go
+++ b/common/math/big_test.go
@@ -21,6 +21,8 @@ import (
 	"encoding/hex"
 	"math/big"
 	"testing"
+
+	"github.com/ethereum/go-ethereum/common"
 )
 
 func TestHexOrDecimal256(t *testing.T) {
@@ -133,8 +135,40 @@ func TestPaddedBigBytes(t *testing.T) {
 	}
 }
 
-func BenchmarkPaddedBigBytes(b *testing.B) {
+func BenchmarkPaddedBigBytesLargePadding(b *testing.B) {
 	bigint := MustParseBig256("123456789123456789123456789123456789")
+	for i := 0; i < b.N; i++ {
+		PaddedBigBytes(bigint, 200)
+	}
+}
+func BenchmarkPaddedBigBytesSmallPadding(b *testing.B) {
+	bigint := MustParseBig256("0x18F8F8F1000111000110011100222004330052300000000000000000FEFCF3CC")
+	for i := 0; i < b.N; i++ {
+		PaddedBigBytes(bigint, 5)
+	}
+}
+
+func BenchmarkPaddedBigBytesSmallOnePadding(b *testing.B) {
+	bigint := MustParseBig256("0x18F8F8F1000111000110011100222004330052300000000000000000FEFCF3CC")
+	for i := 0; i < b.N; i++ {
+		PaddedBigBytes(bigint, 32)
+	}
+}
+func BenchmarkByteAtBrandNew(b *testing.B) {
+	bigint := MustParseBig256("0x18F8F8F1000111000110011100222004330052300000000000000000FEFCF3CC")
+	for i := 0; i < b.N; i++ {
+		BigEndian32ByteAt(bigint, 15)
+	}
+}
+func BenchmarkByteAt(b *testing.B) {
+	bigint := MustParseBig256("0x18F8F8F1000111000110011100222004330052300000000000000000FEFCF3CC")
+	for i := 0; i < b.N; i++ {
+		BigEndian32ByteAt(bigint, 15)
+	}
+}
+func BenchmarkByteAtOld(b *testing.B) {
+
+	bigint := MustParseBig256("0x18F8F8F1000111000110011100222004330052300000000000000000FEFCF3CC")
 	for i := 0; i < b.N; i++ {
 		PaddedBigBytes(bigint, 32)
 	}
@@ -173,7 +207,64 @@ func TestU256(t *testing.T) {
 		}
 	}
 }
+func TestLittleEndianByteAt(t *testing.T) {
+
+	tests := []struct {
+		x   string
+		y   int
+		exp byte
+	}{
+		{"0", 0, 0x00},
+		{"1", 1, 0x00},
+		{"0", 1, 0x00},
+		//{"1", 0, 0x01},
+		{"0000000000000000000000000000000000000000000000000000000000102030", 0, 0x30},
+		{"0000000000000000000000000000000000000000000000000000000000102030", 1, 0x20},
+		{"ABCDEF0908070605040302010000000000000000000000000000000000000000", 31, 0xAB},
+		{"ABCDEF0908070605040302010000000000000000000000000000000000000000", 32, 0x00},
+	}
+	for _, test := range tests {
+		v := new(big.Int).SetBytes(common.Hex2Bytes(test.x))
+		actual := LittleEndianByteAt(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)
+		}
+
+	}
+}
+func TestBigEndianByteAt(t *testing.T) {
 
+	tests := []struct {
+		x   string
+		y   int
+		exp byte
+	}{
+		{"0", 0, 0x00},
+		{"1", 1, 0x00},
+		{"0", 1, 0x00},
+		{"1", 0, 0x00},
+		{"0000000000000000000000000000000000000000000000000000000000102030", 0, 0x00},
+		{"0000000000000000000000000000000000000000000000000000000000102030", 1, 0x00},
+		{"ABCDEF0908070605040302010000000000000000000000000000000000000000", 31, 0x00},
+		{"ABCDEF0908070605040302010000000000000000000000000000000000000000", 32, 0x00},
+		{"ABCDEF0908070605040302010000000000000000000000000000000000000000", 0, 0xAB},
+		{"ABCDEF0908070605040302010000000000000000000000000000000000000000", 1, 0xCD},
+		{"00CDEF090807060504030201ffffffffffffffffffffffffffffffffffffffff", 0, 0x00},
+		{"00CDEF090807060504030201ffffffffffffffffffffffffffffffffffffffff", 1, 0xCD},
+		{"0000000000000000000000000000000000000000000000000000000000102030", 31, 0x30},
+		{"0000000000000000000000000000000000000000000000000000000000102030", 30, 0x20},
+		{"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 32, 0x0},
+		{"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 0xFFFFFFFF, 0x0},
+	}
+	for _, test := range tests {
+		v := new(big.Int).SetBytes(common.Hex2Bytes(test.x))
+		actual := BigEndian32ByteAt(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)
+		}
+
+	}
+}
 func TestS256(t *testing.T) {
 	tests := []struct{ x, y *big.Int }{
 		{x: big.NewInt(0), y: big.NewInt(0)},
-- 
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