(* Common util. Should probably put this in a module or something.. *)
let get_one_line file =
  try Some (input_line file) with
    End_of_file -> None

let get_lines file =
  let rec input lines =
    match get_one_line file with
      Some line -> input (line :: lines)
    | None -> List.rev lines
  in input []

let read_file file =
  let channel = open_in(file) in
  get_lines channel

let explode s = (* convert a string to a list of chars *)
  let rec exp i l =
   if i < 0 then l else exp (i - 1) (s.[i] :: l) in
  exp (String.length s - 1) []

let implode cl = String.concat "" (List.map (String.make 1) cl)

let (<<) f g x = f(g(x)) (* how is 'compose' not in the standard library.. *)
let (>>) f g x = g(f(x))

let string_of_char = String.make 1

(* --- *)

let fix_lines glue_char lines = (* puts each group on 1 line *)
  let glue s = if String.equal s "" then "\n" else glue_char in (* which char to concat the lines *)
  let fix_line acc cur = acc ^ (glue cur) ^ cur in
  List.fold_left fix_line "" lines

let dedup =
  explode >> (* string -> char list *)
  List.fold_left (fun acc x -> if String.contains acc x then acc else acc ^ (string_of_char x)) ""

let intersection group = (* find the chars common to all individuals in a group *)
  List.fold_left (^) "" group (* *all* of the chars among individuals *)
    |> dedup (* deduped *)
    |> explode (* string to char list *)
    |> List.filter (fun c -> List.for_all (fun g -> String.contains g c) group) (* check each char among each individual for membership *)
    |> implode (* turn it back into a string *)

let () =
  let file = read_file "day6.txt" in

  let any_sum = file
    |> fix_lines "" (* put each group on one line *)
    |> String.split_on_char '\n' (* convert to list *)
    |> List.map dedup (* remove dups *)
    |> List.map String.length (* make list of lengths *)
    |> List.fold_left (+) 0 (* sum list *)
  in

  let all_sum = file
    |> fix_lines " " (* keep spaces between individuals *)
    |> String.split_on_char '\n'
    |> List.map String.trim
    |> List.map (String.split_on_char ' ') (* each line now is a list of groups *)
    |> List.map intersection (* list of strings with intersecting chars *)
    |> List.map String.length (* list of lengths *)
    |> List.fold_left (+) 0 (* sum list *)
  in

  Printf.printf "any_sum %i\n" any_sum;
  Printf.printf "all_sum %i\n" all_sum;