Golang big number fromTwos bitwise operations example

bnjs_example.js

const BN = require('bn.js')

const bn = new BN(Buffer.from('fb2e', 'hex'))
const width = bn.bitLength()

console.log(bn.fromTwos(width).toString()) // -1234
console.log(bn.notn(width).iaddn(1).ineg().toString()) // -1234

from_twos_example.go

package main

import (
	"fmt"
	"math"
	"math/big"
)

func main() {
	x := []byte{0xfb, 0x2e}
	i := new(big.Int)
	i.SetBytes(x)

	width := i.BitLen()

	fmt.Println(fromTwos(i, width)) // -1234
}

func fromTwos(i *big.Int, width int) *big.Int {
	a := inotn(i, width)
	b := new(big.Int).Add(a, big.NewInt(1))
	return new(big.Int).Neg(b)
}

func inotn(n *big.Int, width int) *big.Int {
	words := n.Bits()
	var bytesNeeded = int(math.Ceil(float64(width) / float64(26)))
	var bitsLeft = width % 26
	var length = n.BitLen()

	// extend the buffer with leading zeroes
	for length < bytesNeeded {
		words[length] = 0
		length = length + 1
	}

	if bitsLeft > 0 {
		bytesNeeded--
	}

	var i int
	// handle complete words
	for i = 0; i < bytesNeeded; i++ {
		words[i] = big.Word(^int(words[i]) & 0x3ffffff)
	}

	// handle the residue
	if bitsLeft > 0 {
		words[i] = big.Word(^int(words[i]) & (0x3ffffff >> (26 - uint(bitsLeft))))
	}

	ret := new(big.Int)
	ret.SetBits(words)
	return ret
}