js 大整数加法和乘法

export default function Big() {
  "use strict";

  function intToDigits(n) {
    return n
      .split("")
      .reverse()
      .map((d) => +d);
  }

  function doubleToDidits(n) {
    return [n % 10, (n / 10) >> 0];
  }

  function isBigInt(n) {
    return typeof n === "string" && !n.includes(".");
  }

  function add(n0, n) {
    if (!isBigInt(n0) || !isBigInt(n)) {
      throw Error("Please enter a int");
    }

    n0 = intToDigits(n0);
    n = intToDigits(n);
    let max = Math.max(n0.length, n.length),
      _high = 0,
      digits = [];
    for (let i = 0; i < max; i++) {
      const [low, high] = doubleToDidits((n0[i] || 0) + (n[i] || 0) + _high);
      digits.push(low);
      _high = high;
    }
    _high && digits.push(_high);

    return digits.reverse().join("");
  }

  function mul(n0, n) {
    if (!isBigInt(n0) || !isBigInt(n)) {
      throw Error("Please enter a int");
    }

    n0 = intToDigits(n0);
    n = intToDigits(n);
    const D1 = n0.reduce(function (D1, d1, i) {
      let _high = 0;
      let D2 = n.reduce(function (D2, d2) {
        const [low, high] = doubleToDidits(d1 * d2 + _high);
        D2.push(low);
        _high = high;
        return D2;
      }, new Array(i).fill(0));
      _high && D2.push(_high);
      return add(D1, D2.reverse().join(""));
    }, "");

    return D1;
  }

  this.add = add;
  this.mul = mul;
}