solved day 4 !

and updated main.ml lol

src/day4.ml

@@ -0,0 +1,40 @@
+open Containers
+
+let lines = Utils.lines_of_input 4
+
+let parse_line =
+  function%pcre
+  | {|(?<a>\d+)-(?<b>\d+),(?<c>\d+)-(?<d>\d+)|} ->
+    (int_of_string a, int_of_string b), (int_of_string c, int_of_string d)
+  | line -> failwith @@ Printf.sprintf "cannot parse line '%s'" line
+;;
+
+let count_overlaps is_overlapping =
+  lines
+  |> List.fold_left
+       (fun acc line ->
+         match is_overlapping @@ parse_line line with
+         | true -> acc + 1
+         | false -> acc)
+       0
+;;
+
+let part1 =
+  count_overlaps (fun pairs ->
+    match pairs with
+    | (l1, r1), (l2, r2) when (l1 <= l2 && r1 >= r2) || (l2 <= l1 && r2 >= r1) -> true
+    | _ -> false)
+;;
+
+let part2 =
+  count_overlaps (fun pairs ->
+    match pairs with
+    | (l1, r1), (l2, r2) when r1 < l2 || r2 < l1 -> false
+    | _ -> true)
+;;
+
+let print () =
+  Printf.(
+    printf "Day 4.1: %d\n" part1;
+    printf "Day 4.2: %d\n" part2)
+;;