--- a/2023/day03.ml	Sun Dec 03 12:39:11 2023 +0100
+++ b/2023/day03.ml	Sun Dec 03 15:11:01 2023 +0100
@@ -95,14 +95,17 @@
     let pr = parse_line line lineno in
     Sexp.to_string_hum (List.sexp_of_t sexp_of_item pr)
 
+  (* fold over string lines and parse each line into `item list` *)
   let lines_folder (count, lines) line =
     let parsed = parse_line line count in
     (count + 1, parsed :: lines)
 
+  (* read all lines from ch *)
   let read_lines ch =
     let _, all_lines = In_channel.fold_lines ch ~init:(0, []) ~f:lines_folder in
     all_lines
 
+  (* build a hashmap of position -> symbol for all symbols in lines *)
   let build_symbol_map all_lines =
     let symmap = create_position_tbl () in
     let f = function
@@ -114,11 +117,13 @@
     List.iter ~f:ff all_lines;
     symmap
 
+  (* return all number items from item list list (list of parsed lines) *)
   let all_numbers all_lines =
     let f = function Number _ -> true | _ -> false in
     let ff = List.filter ~f in
     List.concat (List.map ~f:ff all_lines)
 
+  (* print symbol map for lines received on ch *)
   let _debug_symbol_map ch =
     let all_lines = read_lines ch in
     let symmap = build_symbol_map all_lines in
@@ -128,6 +133,7 @@
          (fun (a, b) -> List [ sexp_of_position a; Atom (Char.to_string b) ])
          alist)
 
+  (* create a counting sequence *)
   let count_seq (from : int) (upto : int) : int Sequence.t =
     let f c =
       if Int.equal c upto then None
@@ -135,6 +141,7 @@
     in
     Sequence.unfold ~init:from ~f
 
+  (* generate adjacent positions for number at x, y with number of digits. *)
   let adjacent_positions { x; y } digits =
     let corners =
       [
@@ -156,6 +163,7 @@
     in
     List.concat [ corners; upper; lower ]
 
+  (* check if there is a symbol at any adjacent position in the symbol map *)
   let rec check_adjacent_symbol symmap = function
     | [] -> false
     | (x, y) :: poss ->