[fix] division by zero in idf

src/main/java/org/gradoop/famer/linking/similarityMeasuring/methods/TFIDF.java

@@ -50,7 +50,7 @@ public class TFIDF implements SimilarityComputation<String> {
   }
 
   /**
-   * Calculate the inverse document frequency after  log(|D| / |{d:ti ∈ d}|)
+   * Calculate the inverse document frequency after  log(|D| / 1 + |{d:ti ∈ d}|)
    *
    * @param term the term
    * @param doc1 the first document
@@ -69,7 +69,7 @@ public class TFIDF implements SimilarityComputation<String> {
     if (df == 0) {
       return 0D;
     }
-    return Math.log(2 / (float) df);
+    return Math.log(2 / (float) 1 + df);
   }
 
   /**