class Stream {
  constructor(tokens) {
    this.tokens = tokens
    this.pos = 0
  }

  get next() {
    return this.tokens[this.pos++] || ''
  }

  cancel() {
    this.pos--
  }

  get done() {
    return this.pos >= this.tokens.length
  }

  get rest() {
    return this.tokens.slice(this.pos)
  }

  transaction(fn) {
    const rpos = this.pos
    const ret = fn()
    if (ret === failure) {
      this.pos = rpos
      return failure
    }
    return ret
  }
}

const failure = Symbol('failure')
const ignore = Symbol('ignore')

const eq = str => stream => stream.transaction(() => stream.next == str ? ignore : failure)

const re = regex => fn => stream => stream.transaction(() => {
  const ret = stream.next.match(regex)
  if (!ret) return failure
  return fn(ret)
})

const assembly = (...parsers) => stream => stream.transaction(() => {
  console.log('assembly')
  let result = ignore
  for (const parser of parsers) {
    result = parser(result)(stream)
    if (result == failure)
      return failure
  }
  return result
})

const chain = (...parsers) => fn => stream => stream.transaction(() => {
  const arr = []
  for (const parser of parsers) {
    const result = parser(stream)
    if (result == failure)
      return failure
    if (result != ignore)
      arr.push(result)
  }
  return fn(...arr)
})

const choice = (...parsers) => stream => {
  for (const parser of parsers) {
    const result = parser(stream)
    if (result != failure)
      return result
  }
  return failure
}

const repeatRange = parser => (low, high = Number.MAX_SAFE_INTEGER) => stream => stream.transaction(() => {
  const arr = []
  let i = 0
  for (; i < high; i++) {
    const result = parser(stream)
    if (result == failure)
      break
    if (result != ignore)
      arr.push(result)
  }
  if (i < low || i > high) return failure
  return arr
})

const repeat = parser => number => repeatRange(parser)(number, number)

const many = parser => repeatRange(parser)(1)

const zeroOrMany = parser => repeatRange(parser)(0)

const maybe = parser => repeatRange(parser)(0, 1)

const must = parser => repeatRange(parser)(1, 1)

const packer = parser => fn => stream => {
  const ret = parser(stream)
  if (ret === failure)
    return failure
  return fn(ret)
}

const $log = (...param) => stream => {
  console.log(...param, stream.rest)
  return ignore
}

const $lazy = fn => a => b => fn(a)(b)