tighten up day 13

List.compare is neat!

src/day13.ml

@@ -11,33 +11,25 @@ let packet_of_str line =
     take_while1 (function
       | '0' .. '9' -> true
       | _ -> false)
-    >>= fun s -> return (I (int_of_string s)) <?> "int"
+    >>= fun s -> return @@ I (int_of_string s) <?> "int"
   in
   let packet =
     fix (fun packet ->
-      let list =
-        char '[' *> sep_by (char ',') packet
-        <* char ']'
-        >>= fun s -> return (L s) <?> "list"
-      in
+      let lb, rb = char '[', char ']' in
+      let delim = sep_by (char ',') in
+      let list = lb *> delim packet <* rb >>= fun p -> return @@ L p <?> "list" in
       choice [ int; list ])
     <?> "packet"
   in
   Result.get_or_failwith @@ parse_string ~consume:All packet line
 ;;
 
-let rec is_ordered = function
-  | I a, I b when a = b -> None
-  | I a, I b -> Some (a < b)
-  | (I _ as a), (L _ as b) -> is_ordered (L [ a ], b)
-  | (L _ as a), (I _ as b) -> is_ordered (a, L [ b ])
-  | L [], L [] -> None
-  | L [], _ -> Some true
-  | _, L [] -> Some false
-  | L (a :: a_tl), L (b :: b_tl) ->
-    (match is_ordered (a, b) with
-     | None -> is_ordered (L a_tl, L b_tl)
-     | result -> result)
+let rec is_ordered a b =
+  match a, b with
+  | I a, I b -> Int.compare a b
+  | L a, L b -> List.compare is_ordered a b
+  | (I _ as a), (L _ as b) -> is_ordered (L [ a ]) b
+  | (L _ as a), (I _ as b) -> is_ordered a (L [ b ])
 ;;
 
 let%expect_test "Day 13.1" =
@@ -48,14 +40,9 @@ let%expect_test "Day 13.1" =
     |> List.chunks 2
     |> List.mapi (fun i packets ->
       match packets with
-      | [ a; b ] -> i + 1, is_ordered (a, b)
-      | _ -> failwith "List.chunks didnt work lol")
-    |> List.fold_left
-         (fun result item ->
-           match item with
-           | i, Some true -> result + i
-           | _, _ -> result)
-         0
+      | [ a; b ] when is_ordered a b < 0 -> i + 1
+      | _ -> 0)
+    |> List.reduce_exn ( + )
   in
   Printf.printf "%i" result;
   [%expect {| 6272 |}]
@@ -65,20 +52,11 @@ let%expect_test "Day 13.2" =
   let result =
     Utils.lines_of_input 13
     |> List.filter Fun.(not % String.is_empty)
-    |> List.append [ "[[2]]"; "[[6]]" ]
-    |> List.map (fun line -> line, packet_of_str line)
-    |> List.sort (fun (_, a) (_, b) ->
-      match is_ordered (a, b) with
-      | None -> 0
-      | Some true -> -1
-      | Some false -> 1)
-    |> List.mapi (fun i (line, _) -> i + 1, line)
-    |> List.fold_left
-         (fun result (i, line) ->
-           match line with
-           | "[[2]]" | "[[6]]" -> result * i
-           | _ -> result)
-         1
+    |> List.map (fun line -> false, packet_of_str line)
+    |> List.append [ true, packet_of_str "[[2]]"; true, packet_of_str "[[6]]" ]
+    |> List.sort (fun (_, a) (_, b) -> is_ordered a b)
+    |> List.mapi (fun i (is_divider, _) -> if is_divider then i + 1 else 1)
+    |> List.reduce_exn ( * )
   in
   Printf.printf "%i" result;
   [%expect {| 22288 |}]