parseLengthPrefixed4.js

import { Buffer } from 'buffer';

const digits = [...'0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'].map(c => c.charCodeAt(0))

export const parseLengthPrefixed4 = (str1, {
  open = '['.charCodeAt(0),
  close = ']'.charCodeAt(0),
} = {}) => {
  const parents = []
  let parent = {subvalues: []}, buffer = [], affix = []
  const str = Buffer.from(str1, 'utf8')

  let i = 0
  for (; i < str.length; ++i) {
    const c = str[i]

    if (digits.includes(c)) buffer.push(c)
    else if (c === open) {
      if (buffer.length > 0) {
        const length = Number.parseInt(String.fromCharCode(...buffer), 36)
        buffer = []
        ++i
        affix = str.slice(i, i + length)
        i += length - 1
      }
      const value = {subvalues: []}
      parent.subvalues.push({prefix: affix, value})
      parents.push(parent)
      parent = value
    } else if (c === close) {
      if (buffer.length > 0) {
        const length = Number.parseInt(String.fromCharCode(...buffer), 36)
        buffer = []
        ++i
        affix = str.slice(i, i + length)
        i += length - 1
      }
      if (parents.length === 0) {
        if (i !== str.length - 1) throw Error('Unexpected end!')
        else break
      }
      parent.suffix = affix
      if (parents.length < 1) throw Error('Unexpected close!')
      parent = parents.pop()
    } else throw Error(`unexpected byte ${c}`)
  }
  if (buffer.length > 0 || parents.length > 0) throw Error('Unexpected end!')
  parent.suffix = affix
  parent.open = open
  parent.close = close
  return parent
}