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diff --git a/2024/ocaml/lib/day_05/part_01.ml b/2024/ocaml/lib/day_05/part_01.ml
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+++ b/2024/ocaml/lib/day_05/part_01.ml
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+(* Day 5: Part 1 *)
+
+module StringMap = Map.Make (String)
+
+let inverse_page_rule page_rule =
+  String.split_on_char '|' page_rule |> fun lst ->
+  match lst with [] -> "" | _ :: _ -> String.concat "|" (List.rev lst)
+
+(** [parse_aux acc page_rules] is a map of all invalid transitions as defined by
+    [page_rules] *)
+let rec get_invalid_transitions_aux acc page_rules : unit StringMap.t =
+  match page_rules with
+  | [] -> acc
+  | h :: t ->
+      get_invalid_transitions_aux
+        (acc |> StringMap.add (inverse_page_rule h) ())
+        t
+
+let get_invalid_transitions page_rules =
+  get_invalid_transitions_aux StringMap.empty page_rules
+
+(** [solve page_rules updates] is the solution for this puzzle, given the
+    [page_rules] and a list of [updates].
+
+    BELOW DOESN'T WORK: no guarantee that second page comes immediately after
+    the first
+    - pass in updates to solve
+    - make sliding window of 2 elements
+    - invert element order and concat
+    - check if key is in map from [parse page_rules],
+    - if yes, then it is an illegal transition
+    - if no, then add the list to a list for further processing *)
+let solve page_rules _ =
+  let invalid_transitions = get_invalid_transitions page_rules in
+  ()
+
+let _ =
+  solve
+    [
+      "47|53";
+      "97|13";
+      "97|61";
+      "97|47";
+      "75|29";
+      "61|13";
+      "75|53";
+      "29|13";
+      "97|29";
+      "53|29";
+      "61|53";
+      "97|53";
+      "61|29";
+      "47|13";
+      "75|47";
+      "97|75";
+      "47|61";
+      "75|61";
+      "47|29";
+      "75|13";
+      "53|13";
+    ]
+    []