solved day 6 !

src/day6.ml

@@ -0,0 +1,31 @@
+open Containers
+module CharSet = Set.Make (Char)
+
+let window = 4
+
+let stream =
+  match Utils.lines_of_input 6 with
+  | [ line ] -> line
+  | _ -> failwith "more than one line in input"
+;;
+
+let find_start window =
+  let length = String.length stream in
+  let start_index =
+    List.Infix.(0 --^ (length - window))
+    |> List.drop_while (fun index ->
+      let chunk = String.take window (String.drop index stream) in
+      window <> (chunk |> String.to_list |> CharSet.of_list |> CharSet.cardinal))
+    |> List.hd
+  in
+  start_index + window
+;;
+
+let part1 () = find_start 4
+let part2 () = find_start 14
+
+let print () =
+  Printf.(
+    printf "Day 6.1: %d\n" @@ part1 ();
+    printf "Day 6.2: %d\n" @@ part2 ())
+;;