JavaScript 大数运算

/**
 * 字符大数除法
 * @param str1
 * @param str2
 * @returns {string}
 */
const bigDivide = (str1, str2) => {
  let resultArr = []
  let remainPart = 0
  for (let i = 0; i < str1.length; i++) {
    let currentNumber = parseInt(str1.charAt(i))
    let intePart = Math.floor((remainPart * 10 + currentNumber) / str2)
    remainPart = (remainPart * 10 + currentNumber) % str2
    resultArr.push(intePart)
  }

  return resultArr.join('')
}

/**
 * 字符大数加法
 * @param str1
 * @param str2
 * @returns {string}
 */
const bigAdd = (str1, str2) => {
  let overflow = 0
  let maxLength = Math.max(str1.length, str2.length)
  let result = []
  for (let i = 0; i < maxLength; i++) {
    let number1, number2
    number1 = i <= str1.length - 1 ? parseInt(str1.charAt(str1.length - i - 1)) : 0
    number2 = i <= str2.length - 1 ? parseInt(str2.charAt(str2.length - i - 1)) : 0
    let suffix = (number1 + number2 + overflow) % 10
    result.push(suffix)
    overflow = (number1 + number2) > 10 ? 1 : 0
  }

  return result.reverse().join('')
}

/**
 * 字符大数减法
 * @param str1
 * @param str2
 * @returns {string}
 */
const bigSub = (str1, str2) => {
  let lend = 0    // 是否需要借位
  let maxLength = Math.max(str1.length, str2.length)
  let isPositive = true  // 结果是否是正数
  let isChecked = false
  let result = []

  if (str1.length < str2.length) {
    isPositive = false
  } else if (str1.length == str2.length) {
    for (let i = 0; i < str1.length; i++) {
      let number1 = parseInt(str1.charAt(i))
      let number2 = parseInt(str2.charAt(i))
      if (number1 < number2 && !isChecked) {
        isPositive = false
        isChecked = true
      }
      if (number1 > number2 && !isChecked) {
        isPositive = true
        isChecked = true
      }
    }
  }
  let remainer
  for (let i = 0; i < maxLength; i++) {
    let larger, smaller
    if (isPositive) {
      larger = i <= str1.length - 1 ? parseInt(str1.charAt(i)) : 0
      smaller = i <= str2.length - 1 ? parseInt(str2.charAt(i)) : 0
    } else {
      larger = i <= str1.length - 1 ? parseInt(str2.charAt(i)) : 0
      smaller = i <= str2.length - 1 ? parseInt(str1.charAt(i)) : 0
    }
    // positive 为 true 一定是 number1 - number2
    // positive 为 false 一定是 number2 - number1
    if (larger - smaller < 0) {
      remainer = larger + 10 - smaller - lend
      lend = 1
    } else {
      remainer = larger - smaller - lend
      lend = 0
    }
    result.push(remainer)
  }

  if (!isPositive) {
    result.push('-')
  }
  // 最后可以通过处理字符串形式抹除 0
  return result.reverse().join('')
}

/**
 * 字符大数乘法
 * @param str1
 * @param str2
 * @returns {string}
 */
const bigMulti = (str1, str2) => {
  let addObj = Object.create(null)
  for (let i = 0; i < str1.length; i++) {
    // 后位轮位置
    let number1 = parseInt(str1.charAt(str1.length - i - 1))
    let overflow = 0
    let innerResult = []
    for (let j = 0; j < str2.length; j++) {
      let number2 = parseInt(str2.charAt(str2.length - j - 1))
      let remainer = (number1 * number2 + overflow) % 10
      overflow = Math.floor((number1 * number2 + overflow) / 10)
      innerResult.push(remainer)
    }
    addObj[i] = innerResult.reverse().join('')
  }
  let target = '0'
  for (let i in addObj) {
    target = bigAdd(target, addObj[i] + produceZero(i))
  }
  return target
}

function produceZero (number) {
  let str = ''
  for (let i = 0; i < number; i++) {
    str += '0'
  }
  return str
}