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-rw-r--r--common/math/big.go33
-rw-r--r--common/math/big_test.go23
2 files changed, 48 insertions, 8 deletions
diff --git a/common/math/big.go b/common/math/big.go
index c0508c102..704ca40a9 100644
--- a/common/math/big.go
+++ b/common/math/big.go
@@ -28,6 +28,13 @@ var (
MaxBig256 = new(big.Int).Set(tt256m1)
)
+const (
+ // number of bits in a big.Word
+ wordBits = 32 << (uint64(^big.Word(0)) >> 63)
+ // number of bytes in a big.Word
+ wordBytes = wordBits / 8
+)
+
// ParseBig256 parses s as a 256 bit integer in decimal or hexadecimal syntax.
// Leading zeros are accepted. The empty string parses as zero.
func ParseBig256(s string) (*big.Int, bool) {
@@ -91,12 +98,25 @@ func FirstBitSet(v *big.Int) int {
// PaddedBigBytes encodes a big integer as a big-endian byte slice. The length
// of the slice is at least n bytes.
func PaddedBigBytes(bigint *big.Int, n int) []byte {
- bytes := bigint.Bytes()
- if len(bytes) >= n {
- return bytes
+ if bigint.BitLen()/8 >= n {
+ return bigint.Bytes()
}
ret := make([]byte, n)
- return append(ret[:len(ret)-len(bytes)], bytes...)
+ ReadBits(bigint, ret)
+ return ret
+}
+
+// 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) {
+ i := len(buf)
+ for _, d := range bigint.Bits() {
+ for j := 0; j < wordBytes && i > 0; j++ {
+ i--
+ buf[i] = byte(d)
+ d >>= 8
+ }
+ }
}
// U256 encodes as a 256 bit two's complement number. This operation is destructive.
@@ -119,9 +139,6 @@ func S256(x *big.Int) *big.Int {
}
}
-// wordSize is the size number of bits in a big.Word.
-const wordSize = 32 << (uint64(^big.Word(0)) >> 63)
-
// Exp implements exponentiation by squaring.
// Exp returns a newly-allocated big integer and does not change
// base or exponent. The result is truncated to 256 bits.
@@ -131,7 +148,7 @@ func Exp(base, exponent *big.Int) *big.Int {
result := big.NewInt(1)
for _, word := range exponent.Bits() {
- for i := 0; i < wordSize; i++ {
+ for i := 0; i < wordBits; i++ {
if word&1 == 1 {
U256(result.Mul(result, base))
}
diff --git a/common/math/big_test.go b/common/math/big_test.go
index a0f48a8eb..6eb13f4f1 100644
--- a/common/math/big_test.go
+++ b/common/math/big_test.go
@@ -18,6 +18,7 @@ package math
import (
"bytes"
+ "encoding/hex"
"math/big"
"testing"
)
@@ -131,6 +132,28 @@ func TestPaddedBigBytes(t *testing.T) {
}
}
+func BenchmarkPaddedBigBytes(b *testing.B) {
+ bigint := MustParseBig256("123456789123456789123456789123456789")
+ for i := 0; i < b.N; i++ {
+ PaddedBigBytes(bigint, 32)
+ }
+}
+
+func TestReadBits(t *testing.T) {
+ check := func(input string) {
+ want, _ := hex.DecodeString(input)
+ int, _ := new(big.Int).SetString(input, 16)
+ buf := make([]byte, len(want))
+ ReadBits(int, buf)
+ if !bytes.Equal(buf, want) {
+ t.Errorf("have: %x\nwant: %x", buf, want)
+ }
+ }
+ check("000000000000000000000000000000000000000000000000000000FEFCF3F8F0")
+ check("0000000000012345000000000000000000000000000000000000FEFCF3F8F0")
+ check("18F8F8F1000111000110011100222004330052300000000000000000FEFCF3F8F0")
+}
+
func TestU256(t *testing.T) {
tests := []struct{ x, y *big.Int }{
{x: big.NewInt(0), y: big.NewInt(0)},