Small performance cache improvement

package.json

@@ -1,6 +1,6 @@
 {"name": "porter-stemmer",
  "description": "Martin Porter's stemmer wrapped in CommonJS for use in node.js",
- "version": "0.9.1", 
+ "version": "0.9.3", 
 
  "authors": ["Jed Parsons <jed@jedparsons.com> (http://jedparsons.com)",
              "Christoper McKenzie <cmckenzie@iizuu.com> (http://qaa.ath.cx/porter_js_demo.html",

porter.js

@@ -187,10 +187,14 @@
   }
 
   // memoize at the module level
-  var memo = {};
+  var memo = Object.create(null);
   var memoizingStemmer = function(w) {
     if (!memo[w]) {
       memo[w] = stemmer(w);
+      // the stemmed output should stem to itself, cache this too for a small performance boost
+      if (w !== memo[w]) {
+        memo[memo[w]] = memo[w];
+      }
     }
     return memo[w];
   }

test/input.txt

@@ -4355,6 +4355,7 @@ constrains
 constraint
 constring
 construction
+constructor
 construe
 consul
 consuls

test/memoizing_stemmer_vows.js

@@ -25,6 +25,14 @@ vows.describe('The memoizing stemmer')
         assert(results[0] === results[1]);
       }
     }
+  },
+  "passes Dr Porter's test": function() {
+    var vocabulary = fs.readFileSync('./input.txt').toString().trim().split('\n');
+    var expected = fs.readFileSync('./output.txt').toString().trim().split('\n');
+
+    for (var i=0; i<vocabulary.length; i++) {
+      assert(memoizingStemmer(vocabulary[i]) === expected[i]);
+    }
   }
 })
 

test/output.txt

@@ -4355,6 +4355,7 @@ constrain
 constraint
 constr
 construct
+constructor
 constru
 consul
 consul