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 find_pair_and_prod items total =
  let len = List.length items in

  let rec check idx =
    let left = List.nth items (idx mod len) in
    let right = List.nth items (idx / len) in

    if left + right == total
      then left * right
      else check (idx+1)
  in check 0

let file = "day1.txt"
let channel = open_in(file)
let lines = get_lines channel
let ints = List.map int_of_string lines
let prod = find_pair_and_prod ints 2020;;
Printf.printf "%i" prod