在 Javascript 中添加或减去 64 位整数 [英] Add or Subtract From 64bit Integer in Javascript

问题描述

是否有一种简单的方法(希望有一个小函数，而不是库)在 JavaScript 中对 64 位整数进行加减运算?

Is there a simple way (hoping for a small function, not a library) to add or subtract from a 64bit integer in JavaScript?

示例 64 位整数:291270990346989568

Example 64bit int: 291270990346989568

背景:我正在使用具有 64 位推文 ID 的 Twitter API.我想从这些 ID 中添加或减去一个，以操纵我从 Twitter 返回的结果.

Background: I am working with the Twitter API which has 64bit tweet ID's. I'd like to add or subtract one from those IDs to manipulate what results I get back from Twitter.

推荐答案

以下代码执行您所描述的操作.

The following code does what you've described.

拆分、加法或减法，然后重新合并...

Splitting, adding or subtracting, then re-joining...

此代码具有递增和递减函数，以及带有一些边缘情况的测试函数.在拆分数字字符串并对其进行数学运算时，您需要考虑前导零"会发生什么，因此有一个填充函数.

This code has both increment and decrement functions, and a test function with some edge cases. When splitting number strings and doing math on them, you need to consider what happens to "leading zeroes", so there's a padding function for that.

感谢 Justin 提供 JSFiddle 与此解决方案.

Thanks, Justin, for providing a JSFiddle with this solution.

/*
 * Prepend zeros to expand a given string to given length
 *
 * @var {String} numStr Number
 * @var {Number} len    Length to pad out to
 *
 * @returns {String}
 */
function pad0 (numStr,len) {
  while (numStr.length < len) {
    numStr = "0" + numStr;
  }
  return numStr
}

/*
 * Decrement the given (64 bit) integer.
 *
 * @var {String} int64  Postive non-zero integer, as a string
 *
 * @returns {String}
 */
function decrInt64 (int64) {
  var result = "";
  var midpt = Math.floor(int64.length/2);
  var upper = int64.substring(0,midpt);
  var lower = int64.substring(midpt);
  var upperVal = new Number(upper);
  var lowerVal = new Number(lower);

  if (lowerVal == 0) {
    if (upperVal == 0) {
      // We don't support negative numbers
      result = "*ERROR*"
    }
    else {
      // borrow 1
      result = pad0((--upperVal).toString(),upper.length) +
               (new Number("1"+lower) - 1).toString();
    }
  }
  else {
    var newLower = (lowerVal - 1).toString();
    result = upper + pad0(newLower,lower.length);
  }
  alert(result);
}

/*
 * Increment the given (64 bit) integer.
 *
 * @var {String} int64  Postive, as a string
 *
 * @returns {String}
 */
function incrInt64 (int64) {
  var result = "";
  var midpt = Math.floor(int64.length/2);
  var upper = int64.substring(0,midpt);
  var lower = int64.substring(midpt);
  var upperVal = new Number(upper);
  var lowerVal = new Number(lower);

  var newLower = (++lowerVal).toString();
  // Did we overflow?
  if (lower.length < newLower.length) {
    // Yes, carry the 1
    result = (++upperVal).toString() + newLower.substring(1);
  }
  else {
    result = upper + pad0(newLower,lower.length);
  }
  alert(result);
}

// Test function
window.displaymessage= function ()
{
  decrInt64("291270990046989568");
  incrInt64("291270990046989568");
  decrInt64("000000000000000000");
  incrInt64("000000000000000000");
  decrInt64("000000001000000000");
  incrInt64("000000001000000000");
  decrInt64("099999999999999999");
  incrInt64("099999999999999999");
  decrInt64("999999999999999999");
  incrInt64("999999999999999999");
}

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