// 解決浮動運算問題，避免小數點後產生多位數和計算精度損失。

class Precision {
  // 乘法
  times (num1, num2, ...others) {
    if (others.length > 0) {
      return this.times(this.times(num1, num2), ...others)
    }
    const num1Changed = this.float2Fixed(num1)
    const num2Changed = this.float2Fixed(num2)
    // 把兩個數的小數位數相加
    
 
const baseNum = this.digitLength(num1) + this.digitLength(num2)
    const leftValue = num1Changed * num2Changed
    this.checkBoundary(leftValue)
    return leftValue / Math.pow(10, baseNum)
  }
  // 精確加法
  plus (num1, num2, ...others) {
    if (others.length > 0) {
      return this.plus(this.plus(num1, num2), ...others)
    }
    const 
 
baseNum = Math.pow(10, Math.max(this.digitLength(num1), this.digitLength(num2)))
    return (this.times(num1, baseNum) + this.times(num2, baseNum)) / baseNum
  }
  // 精確減法
  minus (num1, num2, ...others) {
    if (others.length > 0) {
      return this.minus(this.minus(num1, num2), ...others)
    }
    const 
 
baseNum = Math.pow(10, Math.max(this.digitLength(num1), this.digitLength(num2)))
    return (this.times(num1, baseNum) - this.times(num2, baseNum)) / baseNum
  }
  // 精確除法
  divide (num1, num2, ...others) {
    if (others.length > 0) {
      return this.divide(this.divide(num1, num2), ...others)
    }
    const num1Change = this.float2Fixed(num1)
    const num2Change = this.float2Fixed(num2)
    return this.times(num1Change / num2Change, Math.pow(10, this.digitLength(num2) - this.digitLength(num1)))
  }
  // 四捨五入，且保留小數
  round (num, ratio) {
    const base = Math.pow(10, ratio)
    return this.divide(Math.round(this.times(num, base)), base)
  }
  // 把小數轉成整數，支援科學計數法。如果是小數則放大成整數
  float2Fixed (num) {
    if (~num.toString().indexOf('e')) {
      return Number(num.toString().replace('.', ''))
    }
    const dlen = this.digitLength(num)
    return dlen > 0 ? num * Math.pow(10, dlen) : num
  }
  // 獲取當前數小數位的長度(處理科學計數法，本質上處理e-n的情況)
  digitLength (num) {
    const eSplit = num.toString().split(/[eE]/)
    const len = (eSplit[0].split('.')[1]|| '').length - (+ eSplit[1] || 0)
    return len > 0 ? len : 0
  }
  //  檢測數字是否越界，如果越界給出提示
  checkBoundary(num) {
    if (num > Number.MAX_SAFE_INTEGER || num < Number.MIN_SAFE_INTEGER) {
      console.log(`${num} is beyond boundary when transfer to integer, the results may not be accurate`)
    }
  }
}
let precision = new Precision()
console.log(precision.times(0.1, 0.2, 0.3))
console.log(precision.plus(0.1, 0.2))
console.log(precision.minus(0.1, 0.2))
console.log(precision.divide(0.1, 0.2))