replace parse_line with CC map_same and take_drop

thank u https://doc.sherlocode.com

src/day3.ml

@@ -10,22 +10,16 @@ let priority char =
   | _ -> failwith @@ Printf.sprintf "'%c' is not a valid item" char
 ;;
 
-
-let parse_line line =
-  let half_len = String.length line / 2 in
-  let first_half = String.sub line 0 half_len in
-  let second_half = String.sub line half_len half_len in
-  set_of_string first_half, set_of_string second_half
-;;
-
 let part1 lines =
   List.fold_left
     (fun acc line ->
-      let first, second = parse_line line in
+      let half_len = String.length line / 2 in
+      let first, second = Pair.map_same set_of_string (String.take_drop half_len line) in
       let item =
         match CharSet.elements @@ CharSet.inter first second with
         | [ item ] -> item
-        | _ -> failwith "sad"
+        | _ ->
+          failwith "first and second half of each line should only have 1 intersection"
       in
       acc + priority item)
     0
@@ -33,9 +27,9 @@ let part1 lines =
 ;;
 
 (*
-x thank you https://doc.sherlocode.com/ for helping me find Containers and its
+   shout out https://doc.sherlocode.com/ for helping me find Containers and its
-x List.chunks and List.reduce functions
+   List.chunks and List.reduce functions
-x*)
+*)
 let part2 lines =
   let groups = List.chunks 3 (List.map set_of_string lines) in
   List.fold_left
@@ -43,7 +37,7 @@ let part2 lines =
       let item =
         match CharSet.elements @@ List.reduce_exn CharSet.inter group with
         | [ item ] -> item
-        | _ -> failwith "sad"
+        | _ -> failwith "each group of 3 lines should only have 1 intersection"
       in
       acc + priority item)
     0