javascript memoization

最新推荐文章于 2024-08-24 14:18:30 发布

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本文介绍了JavaScript中一种称为缓存的技术，通过缓存已计算的结果来避免重复计算，从而提高程序运行效率。文章提供了实现缓存的具体代码示例，并通过单元测试验证了其正确性和有效性。

I am using the word, memoization, because it is cool and it sounds like a buzzword, and author of the javascript ninja also called it so.

so, what is memoization. " Memoization is the process of building a function which is capable of remembering its previously computed answers. "

so let 's see some sample implementation that realize the memoization.

/**************************************
*@Summary
*  this is the js file that demonstrate the use of the memoization (memorization), I called it memoization as it is like some buzzword as Ninja is calling it so.
*
* Memorization is a very common technique that you can save some compute values as a cache, and you can shortcut some long operation if a value has already computed
*
* @Todo:
*   this file is not created, will come back to it with implementation

***************************************/


/**
* @Summary
*   memoized
* @Usage: 
*  isPrime.memoized(5)...
*  isPrime._values[5]
*/
Function.prototype.memoized = function (key) {
  this._values = this._values || {};  // this is a common technique to initialize something that if it does not have a default value, assign one 
  return this._values[key] !== undefined ?
      this._values[key] :
      this._values[key] = this.apply(this, arguments); // the reason why not to store this a fn and so some manipulation is because 'this' we are just using the same function
};

/**
* @Summary
*   memoize
* @Usage: 
*  var isPrime = (function(num) { 
     .. 
}).memoize();
* isPrime(5)
*/
Function.prototype.memoize = function () {
  var fn = this;
  return function () {
    return fn.memoized.apply(fn, arguments);
  };
};




/**
* @Summary
*   isPrime
* @Comment: this is the function that we test for memoization
*/
function isPrime(num) {
  var prime = num != 1;
  for (var i = 2; i < num; i++) {
    if (num % i == 0) {
      prime = false;
      break;
    }
  }
  return prime;
}

and the following code that test the memoizaion.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script type ="text/javascript" src="memoization.js"></script>
    <script type="text/javascript" src="../unit.js"></script>
    <script type="text/javascript">

      window.onload = function () {
        test("memoization test", function () {

          var isPrime_moized = isPrime.memoize();

          assert(isPrime_moized(5), "Make sure the function works, 5 is prime.");
          assert(isPrime._values[5], "Make sure the answer is cached.");

          assert(isPrime.memoized(5), "Make sure the function works, 5 is prime.");
          assert(isPrime._values[5], "Make sure the answer is cached.");
        });
      };
    </script>

    <style type="text/css" >
      #results li.pass { color: Green }
      #results li.fail { color: Red }
    </style>

</head>
<body>
<ul id="results" />
</body>
</html>

It is believed that with the memoization, you can achieve faster process (trade off spaces for time.)