Add smoothing constant to IDF formula in BM25 to prevent negative scores (#5696)

Co-authored-by: prayas7102 <[email protected]>
Co-authored-by: Alex Klymenko <[email protected]>

Author: 3 people

src/main/java/com/thealgorithms/searches/BM25InvertedIndex.java

@@ -215,6 +215,6 @@ private double computeBM25Score(int termFrequency, double docLength, double idf)
      */
     private double computeIDF(int docFrequency) {
         // Total number of documents in the index
-        return Math.log((totalDocuments - docFrequency + 0.5) / (docFrequency + 0.5));
+        return Math.log((totalDocuments - docFrequency + 0.5) / (docFrequency + 0.5) + 1);
     }
 }

src/test/java/com/thealgorithms/searches/BM25InvertedIndexTest.java

@@ -50,13 +50,15 @@ void testSearchRanking() {
         // Perform search for the term "good"
         List<SearchResult> results = index.search("good");
         assertFalse(results.isEmpty());
-
+        for (SearchResult result : results) {
+            System.out.println(result);
+        }
         // Validate the ranking based on the provided relevance scores
-        assertEquals(6, results.get(0).getDocId()); // It's a Wonderful Life should be ranked 1st
-        assertEquals(7, results.get(1).getDocId()); // The Pursuit of Happyness should be ranked 2nd
+        assertEquals(1, results.get(0).getDocId()); // The Shawshank Redemption should be ranked 1st
+        assertEquals(8, results.get(1).getDocId()); // A Few Good Men should be ranked 2nd
         assertEquals(5, results.get(2).getDocId()); // Good Will Hunting should be ranked 3rd
-        assertEquals(8, results.get(3).getDocId()); // A Few Good Men should be ranked 4th
-        assertEquals(1, results.get(4).getDocId()); // The Shawshank Redemption should be ranked 5th
+        assertEquals(7, results.get(3).getDocId()); // The Pursuit of Happyness should be ranked 4th
+        assertEquals(6, results.get(4).getDocId()); // It's a Wonderful Life should be ranked 5th
 
         // Ensure the relevance scores are in descending order
         for (int i = 0; i < results.size() - 1; i++) {