how to convert javascript BigInt to Uint8Array with negative values considered

Question

I have a java backend that's reading/writing arbitrary precision numbers with javascript front-end using bytes.

On Java side I just do:

new BigInteger("123").toByteArray()

The doc says:

Returns a byte array containing the two's-complement 
representation of this BigInteger.

On javascript side I need to read the bytes into javascript's BigInt, and vise versa.

I have the javascript reading the bytes into BigInt as bellow:

const hex = Buffer.from(bytes).toString('hex')
let big = BigInt('0x' + hex)
if (a[0] & 0x80) {
  const negative = BigInt('0x1' + '0'.repeat(hex.length))
  big -= negative
}

But converting BigInt to bytes seems to be very tricky. I looked around and found that other solutions all only cover positive numbers but not negative numbers

This code doesn't let the caller specify if they want the BigInt value represented in little-endian or big-endian byte-order. — Dai, Jul 23 '20 at 00:22
@dai sorry for not writing the question properly. It was very late of that night or very early in the morning :D I just didn't write it up properly. But I went through it for a long time and was hoping someone else might find it useful — nemo, Aug 19 '20 at 10:25

score 0 · Answer 1 · answered Aug 19 '20 at 10:24

After digging around and understanding two's-complement representation, I'm answering my own question:

const big0 = BigInt(0)
const big1 = BigInt(1)
const big8 = BigInt(8)  

function bigToUint8Array(big: bigint) {
  if (big < big0) {
    // work out how long is the big int in bits and add 1
    const bits: bigint = (BigInt(big.toString(2).length) / big8 + big1) * big8  
    // create a BigInt that's 100000... of length of big + 1
    const prefix1: bigint = big1 << bits
    big += prefix1
  }
  let hex = big.toString(16)
  if (hex.length % 2) {
    hex = '0' + hex
  }
  const len = hex.length / 2
  const u8 = new Uint8Array(len)
  var i = 0
  var j = 0
  while (i < len) {
    u8[i] = parseInt(hex.slice(j, j + 2), 16)
    i += 1
    j += 2
  }
  return u8
}

how to convert javascript BigInt to Uint8Array with negative values considered

1 Answers1