javascriptmasterclass.com - Homework 2 (isPrime)

com.javascriptmasterclass.homework-2.js

// Part 1.
// Implement a function prototype extension that caches function results for
// the same input arguments of a function with one parameter.
//
// For example:
// Make sin(1) have the result of Math.sin(1), but use a cached value
// for future calls.

Function.prototype.cached = function ()
{
	var self = this;
	self.cache = self.cache || {};
	
	return function () {
		var cache = self.cache,
		    key = Array.prototype.slice.call(arguments).join('');

		return key in cache ? 
			cache[key] 
			:
			(cache[key] = self.apply(self, arguments));
	};
}

// Part 2.
// Use this new function to refactor the code example.
// Some good test numbers: 524287, 9369319, 2147483647 (all primes)
//
// In other words, if you have this code:
//
//  Function.prototype.cached = function(){
//    ? (what goes here?)
//  }
//  
//  // For the isPrime function

function isPrime( num ) { 
  // everything but 1 can be prime
  var prime = num != 1;
  for ( var i = 2; i < num; i++ ) { 
    if ( num % i == 0 ) { 
      prime = false; 
      break; 
    } 
  } 

  return prime; 
} 

  var cachedIsPrime = isPrime.cached();
  isPrime(11); // --> true (cache miss, calls original isPrime(1) function, stores return value in cache)
  isPrime(11); // --> true (cache hit, directly returns value)
//  
//  // ..but it short work on anything that takes just one (numeric) argument:
//  
  var cachedSin = Math.sin.cached();
//  
  cachedSin(1); // --> 0.8414709848078965 (cache miss, calls original Math.sin(1) function, stores return value in cache)
  cachedSin(1); // --> 0.8414709848078965 (cache hit, directly returns value)