add some comments on variables

pecos/core/ann/hnsw.hpp

@@ -248,8 +248,13 @@ namespace ann {
 
         ProductQuantizer4Bits quantizer;
         index_type num_node;
+        // code_dimension is number of 4 bits code used to encode a data point in GraphPQ4Bits
+        // code_dimension can be different from parameter num_local_codebooks in quantizer
+        // as we might adjust code_dimension to make it divisble by 4. More details can be
+        // found in pad_parameters function of ann/quantizer_impl/x86.hpp
         size_t code_dimension;
-        size_t code_offset;
+        // code_offset helps to locate memory position containing neighboring codes
+        size_t code_offset;  
         size_t node_mem_size;
         index_type max_degree;
         std::vector<uint64_t> mem_start_of_node;
@@ -1025,8 +1030,8 @@ namespace ann {
         index_type efC;    // size of priority queue for construction time
         index_type max_level;
         index_type init_node;
-        index_type subspace_dimension;
-        index_type sub_sample_points;
+        index_type subspace_dimension;  // dimension of each subspace in Product Quantization
+        index_type sub_sample_points;   // number of sub-sampled points used to build quantizer subspace centors. 
 
         GraphL0<feat_vec_t> feature_vec;           // feature vectors only
         GraphL1 graph_l1;                       // neighborhood graphs from level 1 and above

pecos/core/ann/quantizer_impl/common.hpp

@@ -22,8 +22,14 @@ namespace ann {
 
 
     struct ProductQuantizer4BitsBase {
+        // num_of_local_centroids denotes number of cluster centers used in quantization
+        // In 4 Bit case, it's a fixed to be 16
         const size_t num_of_local_centroids = 16;
+        // num_local_codebooks denotes number of local codebooks we have or in other words,
+        // number of subspace we have in Product Quantization.
+        // Supposedly, num_local_codebooks * local_dimension equals dimension of original data vector
         index_type num_local_codebooks;
+        // local dimension denotes the dimensionality of subspace in Product Quantization
         int local_dimension;
         std::vector<float> global_centroid;
         std::vector<float> local_codebooks;