go-ethereum/common/math/big.go

// Copyright 2017 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

// Package math provides integer math utilities.
package math

import (
	"math/big"
)

var (
	tt255     = BigPow(2, 255)
	tt256     = BigPow(2, 256)
	tt256m1   = new(big.Int).Sub(tt256, big.NewInt(1))
	MaxBig256 = new(big.Int).Set(tt256m1)
)

// 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) {
	if s == "" {
		return new(big.Int), true
	}
	var bigint *big.Int
	var ok bool
	if len(s) >= 2 && (s[:2] == "0x" || s[:2] == "0X") {
		bigint, ok = new(big.Int).SetString(s[2:], 16)
	} else {
		bigint, ok = new(big.Int).SetString(s, 10)
	}
	if ok && bigint.BitLen() > 256 {
		bigint, ok = nil, false
	}
	return bigint, ok
}

// MustParseBig parses s as a 256 bit big integer and panics if the string is invalid.
func MustParseBig256(s string) *big.Int {
	v, ok := ParseBig256(s)
	if !ok {
		panic("invalid 256 bit integer: " + s)
	}
	return v
}

// BigPow returns a ** b as a big integer.
func BigPow(a, b int64) *big.Int {
	r := big.NewInt(a)
	return r.Exp(r, big.NewInt(b), nil)
}

// BigMax returns the larger of x or y.
func BigMax(x, y *big.Int) *big.Int {
	if x.Cmp(y) < 0 {
		return y
	}
	return x
}

// BigMin returns the smaller of x or y.
func BigMin(x, y *big.Int) *big.Int {
	if x.Cmp(y) > 0 {
		return y
	}
	return x
}

// FirstBitSet returns the index of the first 1 bit in v, counting from LSB.
func FirstBitSet(v *big.Int) int {
	for i := 0; i < v.BitLen(); i++ {
		if v.Bit(i) > 0 {
			return i
		}
	}
	return v.BitLen()
}

// 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
	}
	ret := make([]byte, n)
	return append(ret[:len(ret)-len(bytes)], bytes...)
}

// U256 encodes as a 256 bit two's complement number. This operation is destructive.
func U256(x *big.Int) *big.Int {
	return x.And(x, tt256m1)
}

// S256 interprets x as a two's complement number.
// x must not exceed 256 bits (the result is undefined if it does) and is not modified.
//
//   S256(0)        = 0
//   S256(1)        = 1
//   S256(2**255)   = -2**255
//   S256(2**256-1) = -1
func S256(x *big.Int) *big.Int {
	if x.Cmp(tt255) < 0 {
		return x
	} else {
		return new(big.Int).Sub(x, tt256)
	}
}

// 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.
//
// 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 < wordSize; i++ {
			if word&1 == 1 {
				U256(result.Mul(result, base))
			}
			U256(base.Mul(base, base))
			word >>= 1
		}
	}
	return result
}