Bitcoin Output Descriptor Checksum algorithm in JavaScript

descriptorChecksum.js

/*
 * input: "wpkh([d34db33f/84h/0h/0h]0279be667ef9dcbbac55a06295Ce870b07029Bfcdb2dce28d959f2815b16f81798)"
 * output: "wpkh([d34db33f/84h/0h/0h]0279be667ef9dcbbac55a06295Ce870b07029Bfcdb2dce28d959f2815b16f81798)#qwlqgth7"
 * (This has been checked to match bitcoin-core)
 */
function descriptorChecksum(desc) {
  if (!(typeof desc === 'string' || desc instanceof String)) throw new Error('desc must be string')

  const descParts = desc.match(/^(.*?)(?:#([qpzry9x8gf2tvdw0s3jn54khce6mua7l]{8}))?$/);
  if (descParts[1] === '') throw new Error('desc string must not be empty')

  const INPUT_CHARSET = '0123456789()[],\'/*abcdefgh@:$%{}IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~ijklmnopqrstuvwxyzABCDEFGH`#"\\ ';
  const CHECKSUM_CHARSET = 'qpzry9x8gf2tvdw0s3jn54khce6mua7l';
  const MOD_CONSTS = [
    0xf5dee51989,
    0xa9fdca3312,
    0x1bab10e32d,
    0x3706b1677a,
    0x644d626ffd,
  ];
  const BIT35 = 0x800000000; // Math.pow(2, 35)
  const BIT31 = 0x80000000; // Math.pow(2, 31)
  const BIT5 = 0x20; // Math.pow(2, 5)

  function polyMod(c, val) {
    const c0 = Math.floor(c / BIT35);
    let ret = xor5Byte((c % BIT35) * BIT5, val)
    if (c0 & 1) ret = xor5Byte(ret, MOD_CONSTS[0])
    if (c0 & 2) ret = xor5Byte(ret, MOD_CONSTS[1])
    if (c0 & 4) ret = xor5Byte(ret, MOD_CONSTS[2])
    if (c0 & 8) ret = xor5Byte(ret, MOD_CONSTS[3])
    if (c0 & 16) ret = xor5Byte(ret, MOD_CONSTS[4])
    return ret
  }

  function xor5Byte(a, b) {
    const a1 = Math.floor(a / BIT31);
    const a2 = a % BIT31
    const b1 = Math.floor(b / BIT31);
    const b2 = b % BIT31
    return (a1 ^ b1) * BIT31 + (a2 ^ b2)
  }

  let c = 1
  let cls = 0
  let clscount = 0
  for (const ch of descParts[1]) {
    const pos = INPUT_CHARSET.indexOf(ch)
    if (pos === -1) return ''
    c = polyMod(c, pos & 31)
    cls = cls * 3 + (pos >> 5)
    clscount++
    if (clscount === 3) {
      c = polyMod(c, cls)
      cls = 0
      clscount = 0
    }
  }
  if (clscount > 0) {
    c = polyMod(c, cls)
  }
  for (let i = 0; i < 8; i++) {
    c = polyMod(c, 0)
  }
  c = xor5Byte(c, 1)

  const arr = []
  for (let i = 0; i < 8; i++) {
    arr.push(CHECKSUM_CHARSET.charAt(Math.floor(c / Math.pow(2, (5 * (7 - i)))) % BIT5))
  }
  const checksum = arr.join('')
  if (descParts[2] !== undefined && descParts[2] !== checksum) throw new Error('Checksum Mismatch')

  return `${descParts[1]}#${checksum}`
}