(* Baseline: n-queens count for n=6.
   We count placements of n queens on an n×n board such that no two
   share a row, column, or diagonal. *)

let safe q queens =
  let rec go qs offset =
    match qs with
    | [] -> true
    | h :: t ->
      if h = q then false
      else if h - q = offset then false
      else if q - h = offset then false
      else go t (offset + 1)
  in
  go queens 1
;;

let rec range a b =
  if a > b then [] else a :: range (a + 1) b
;;

let rec solve n queens row =
  if row > n then 1
  else
    List.fold_left
      (fun acc col ->
        if safe col queens then
          acc + solve n (col :: queens) (row + 1)
        else
          acc)
      0
      (range 1 n)
;;

solve 4 [] 1