Add TF-IDF based ranking

Author: singhpratyush

ranking/tf_idf_ranker.py

@@ -0,0 +1,17 @@
+import operator
+
+from ranking import Ranker
+
+
+class TfIdfRanker(Ranker):
+
+    @staticmethod
+    def get_top_docs(index, tokens):
+        documents = {}
+        for token in tokens:
+            relevant_docs = index.get_docs_for_token(token)
+            for doc_id, freq in relevant_docs:
+                if doc_id not in documents:
+                    documents[doc_id] = 0.
+                documents[doc_id] += freq * index.idf(token)
+        return sorted(documents.items(), key=operator.itemgetter(1), reverse=True)