js 大數計算

原理

JavaScript 安全整數 是 -2^53-1 ~ 2^53-1 ，即: -9007199254740991 ~ 9007199254740991; 換句話說，整數超過這個范圍就會丟失精度，那超過這個范圍的基本運算也別指望有多精確了;

換一種思路，用字符串表示數字，比如 "9007199254740999"，不存在四舍五入精度丟失的問題，這樣不管數值多大都沒影響;

用數字表示字符串之后，那么數字的運算就轉換成字符串的運算;

big number add

從最后一個數字開始向前依次相加，逢10進1，下一位數字相加的時候加上次計算的進位；

const add = (num1, num2) => {
  // 獲取最大長度
  const len = Math.max(num1.length, num2.length);
  // 對齊
  num1 = num1.padStart(len, 0);
  num2 = num2.padStart(len, 0);

  let flag = 0；
  let result = ``;
  let temp = 0;
  for(let i=len-1; i>=0;  i--){
    temp = flag + parseInt(num1[i]) + parseInt(num2[i])
    result = (temp%10) + result 
    flag = parseInt(temp/10)
  }
  // 判斷是否進位
  return result = (flag === 1 ? '1' : '') + result;
}

const n1 = "9007199254740990"
const n2 = "1229007199254740993443"

add(n1, n2);
 // "1229016206453995734433"

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MathML

Mathematical Markup Language (MathML) Version 3.0 2nd Edition

W3C Recommendation 10 April 2014

https://www.w3.org/TR/MathML/

HTML var tag

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/var

<table width="100%">
  <tbody>
      <tr>
        <td id="eqnoc2"><var>E</var> = <var>m</var><var>c</var><sup>2</sup></td>
      </tr>
  </tbody>
</table>

E = mc²

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/kbd

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/code

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/samp

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/sup

<p><var>a<sup>2</sup></var> + <var>b<sup>2</sup></var> = <var>c<sup>2</sup></var></p>

a² + b² = c²

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/sub

H<sub>2</sub>O

H₂O

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/wbr

MathJax

xml for math

https://mathjax.github.io/MathJax-demos-web/tex-chtml.html

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