christophevg · January 21, 2011 13:43 · Jan 21, 2011
diff --git a/gistfile1.js b/gistfile1.js
@@ -0,0 +1,43 @@
+// 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.withCaching = function withCaching() {
+    if( ! this.cache ) { this.cache = {}; } // lazy initialization of cache
+    var method = this;
+    return function(input) {
+      if( typeof method.cache[input] == "undefined" ) { 
+        var result = method.apply(this, [ input ]);
+        method.cache[input] = result;
+      }
+      return method.cache[input];
+    };
+};
+
+// Part 2.
+// Use this new function to refactor the code example.
+// Some good test numbers: 524287, 9369319, 2147483647 (all primes)
+
+var isPrime = function( num ) {
+  console.log( "looking ..." );
+  var prime = num != 1;
+  // everything but 1 can be prime
+  for ( var i = 2; i < num; i++ ) { 
+    if ( num % i == 0 ) { 
+      prime = false; 
+      break; 
+    } 
+  } 
+  return prime; 
+}.withCaching();
+
+console.log( isPrime( 4 ) ); // looking ... false
+console.log( isPrime( 4 ) ); // false
+
+console.log( isPrime( 524287 ) ); // looking ... true
+console.log( isPrime( 524287 ) ); // true
+