provide distance convert between distance and cosine similarity

Author: Qi Chen

AnnService/inc/Core/Common/DistanceUtils.h

@@ -102,17 +102,21 @@ namespace SPTAG
                 return func(p1, p2, length);
             }
 
+            template<typename T>
             static inline float ConvertCosineSimilarityToDistance(float cs)
             {
                 // Cosine similarity is in [-1, 1], the higher the value, the closer are the two vectors. 
                 // However, the tree is built and searched based on "distance" between two vectors, that's >=0. The smaller the value, the closer are the two vectors.
                 // So we do a linear conversion from a cosine similarity to a distance value.
-                return 1 - cs; //[1, 3]
+                int base = Utils::GetBase<T>();
+                return (1 - cs) * (base * base);
             }
 
+            template<typename T>
             static inline float ConvertDistanceBackToCosineSimilarity(float d)
             {
-                return 1 - d;
+                int base = Utils::GetBase<T>();
+                return 1 - d / (base * base);
             }
         };
         template<typename T>