Convert between bigint and buffer

bigint.test.ts

import { expect, use } from 'chai'
import chaiAsPromised from 'chai-as-promised'
import { buffer2bigint, bigint2buffer } from '../src/bigint'
use(chaiAsPromised)

describe('Test Utils', () => {

    before(async () => {
    })

    it('test 0', async () => {
      const value = 0
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })

    it('test -1', async () => {
      const value = -1
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test 1', async () => {
      const value = 1
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint 0n', async () => {
      const value = 0
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint -1n', async () => {
      const value = -1n
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint 1n', async () => {
      const value = 1n
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint 0x7f', async () => {
      const value = 0x7f
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint 0x7f+1', async () => {
      const value = 0x7f+1
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint 0x7f-1', async () => {
      const value = 0x7f-1
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint -0x7f', async () => {
      const value = -0x7f
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint 0xffffff', async () => {
      const value = 0xffffff
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint -0xffffff', async () => {
      const value = -0xffffff
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint max n', async () => {
      const value = 1200n*10000n*10n**18n
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
    it('test bigint neg n', async () => {
      const value = -1200n*10000n*10n**18n
      const buf = bigint2buffer(value)
      console.log(buf.toString('hex'))
      const result = buffer2bigint(buf)
      console.log(result)
      await expect(result).is.eq(BigInt(value))
    })
})

bigint.ts

// Converts a bigint into a sign-magnitude representation
export function bigint2buffer(value: number | bigint): Buffer {
  let hex = value < 0 ? (-value).toString(16) : value.toString(16);
  if (hex.length % 2 !== 0) hex = "0" + hex;

  const m = parseInt("0x" + hex.slice(0, 2), 16);
  if (m >= 0x7f) {
    hex = "00" + hex;
  }
  
  let buffer = Buffer.from(hex, "hex");

  if (value >= 0) {
    return buffer;
  }

  for (let i = 0; i < buffer.length; i++) {
    buffer[i] = ~buffer[i]!;
  }

  for (let i = buffer.length - 1; i >= 0; i--) {
    if (buffer[i] === 0xff) {
      buffer[i] = 0x00;
    } else {
      buffer[i]++;
      break;
    }
  }

  return buffer;
}

export function buffer2bigint(buffer: Buffer): bigint {
  const isNegative = buffer[0]! & 0x80;
  if (isNegative) {
    let carry = true;
    for (let i = buffer.length - 1; i >= 0; i--) {
      buffer[i] = ~buffer[i]! & 0xff;
      if (carry) {
        carry = buffer[i] === 0xff;
        buffer[i]++;
      }
    }
  }

  return (
    BigInt("0x" + buffer.toString("hex")) *
    (isNegative ? BigInt(-1) : BigInt(1))
  );
}