dfa.js

const T = {
  INT: 'INT',
  SIGN: 'SIGN',
  DOT: 'DOT',
  EXPO: 'EXPO',
  SPACE: 'SPACE',
  INVALID: 'INVALID',
  EMPTY: 'EMPTY',
}

let currentState = 'EMPTY'
let last = null
let excludes = []

const trans = (event) => {
  // console.log( '\ttrans:', event )

  switch(currentState){

    // 第一次進入
    case 'EMPTY':

      switch(event){
        case 'EXPO':
          currentState = 'PENDING'
          break

        default:
          currentState = event
          last = event
      }

      break

    // 只要遇到一次 invalid，後面就都無效了
    case 'SPACE':
    case 'INVALID':
      currentState = 'INVALID'
      break

    // 現在是數字的話
    case 'INT':
      switch(event){
        case 'INT':
        case 'DOT':

        // debugger //
          if(excludes.includes(event)){
            currentState = 'INVALID'
          }else{
            currentState = event
          }
          break
        case 'EXPO':
          currentState = 'PENDING'
          break

        default:
          currentState = 'INVALID'

      }
      last = event
      break

    // 現在是 SIGN
    case 'SIGN':
      switch(event){
        case 'INT':
        case 'EXPO':
          currentState = event
          break
        default:
          currentState = 'INVALID'

      }
      last = event
      break

    // 現在是 DOT
    case 'DOT':
      switch(event){
        case 'INT':
        case 'EXPO':
          currentState = event
          break
        default:
          currentState = 'INVALID'

      }
      last = event
      break

    // 現在是 expo
    case 'EXPO':
      switch(event){
        case 'INT':
        case 'SIGN':
          currentState = event
          break
        default:
          currentState = 'PENDING'
      }
      last = event
      break

    case 'PENDING':
      switch(event){
        case 'INT':
        case 'SIGN':
          // e3 已有 e，後面又出現 3，要檢查 e 前方是否有數字才算合格
          currentState = last == 'INT' || last == 'EXPO' ? event : 'INVALID'
          break
        default:
          currentState = 'INVALID'

      }
      last = event
      break

    default:
      debugger
  }

  if(currentState === 'PENDING' || currentState === 'EXPO'){
    excludes.push('DOT')
  }
}

// 判斷字元的型別
const inputType = c => {
  // console.log( '\tc=', c )
  if(isNaN(+c) === false){
    return 'INT'
  } else if (c === '+' || c === '-') {
    return 'SIGN'
  } else if (c === '.') {
    return 'DOT'
  } else if (c === 'e') {
    return 'EXPO'
  } else if (c === ' ') {
    return 'SPACE'
  } else{
    return 'INVALID'
  }
}

// 進入點
const isNumber = input => {
  input
  .trim()
  .split('')
  .forEach(c => {
    // console.log( '\nbefore: ', currentState )
    trans(inputType(c))
    // console.log( 'after: ', currentState )
  })

  return currentState
}

// 跑一堆測試，要全通過
const tests = [
  '9e2.5',
  '1e',
  '1e3',
  'e3',
  '0',
  ' 0.1',
  'abc',
  '1 a',
  '2e10',
  '-90e3',
  ' 6e-1',
  ' 99e.25',
  ' 99e2.5',
  '53.5e93',
  ' --6',
  '-+3',
  '95a54e53',
  '12e+-34',
  '1234',
  '12+34',
  '12-34',
  '12e+34',
  '12e34',
]
.forEach( str => {
  currentState = 'EMPTY'
  last = null
  excludes = []
  const res = isNumber(str)
  const res2 = res == 'INVALID' || res == 'PENDING'
  console.log( isNaN(Number(str)) == res2, str, Number(str), res  )
})