add parser header

lib/dune

@@ -1,4 +1,4 @@
 (library
  (name dnstoy)
- (modules types utils query network)
+ (modules types utils network query response)
  (libraries unix))

lib/query.ml

@@ -13,7 +13,7 @@ let header_to_bytes header =
     ]
 ;;
 
-let question_to_bytes question =
+let question_to_bytes (question : dns_question) =
   let buffer =
     new_buffer (2 * (get_obj_size question - 1)) [ question.type_; question.class_ ]
   in

lib/response.ml

@@ -0,0 +1,9 @@
+open Types
+open Utils
+
+let parse_header reader =
+  match List.init 6 (fun offset -> unpack_short_be reader (offset * 2)) with
+  | [ id; flags; num_questions; num_answers; num_authorities; num_additionals ] ->
+    { id; flags; num_questions; num_answers; num_authorities; num_additionals }
+  | _ -> failwith "Invalid number of fields"
+;;

lib/utils.ml

@@ -26,3 +26,9 @@ let new_buffer size list =
   if verification != size then failwith "Issue converting header to bytes";
   buffer
 ;;
+
+let unpack_short_be reader offset =
+  let msb = int_of_char (Bytes.get reader offset) in
+  let lsb = int_of_char (Bytes.get reader (offset + 1)) in
+  (msb lsl 8) + lsb
+;;