perf: improve creating tfIdf with cache (#433)

Author: vivekjoshi556

lib/search.js

@@ -88,11 +88,15 @@ let getIdf = (word) => {
 
 let createTfIdf = () => {
   corpus.tfidf = {};
+  let idfCache = {};
+
   Object.keys(corpus.fileWords).forEach((file) => {
     corpus.tfidf[file] = {};
     Object.keys(corpus.fileWords[file]).forEach((word) => {
-      let tfidf = getTf(word, file) * getIdf(word);
-      corpus.tfidf[file][word] = tfidf;
+      if(!(word in idfCache)) {
+        idfCache[word] = getIdf(word);
+      }
+      corpus.tfidf[file][word] = getTf(word, file) * idfCache[word];
     });
   });
 };