let rec skipWhile p xs =
    match xs with
        | [] -> []
        | x :: xr -> if p x then skipWhile p xr else xs

let has_ng w x history =
    match List.exists (fun e -> x = e) history with
        | true -> true
        | false ->
            let lcands = [x-1 .. -1 .. 0]
            let rcands = [x+1 .. 1 .. w]
            let has_ng' es = Seq.zip history es |> Seq.exists (fun (e0,e1) -> e0 = e1)
            has_ng' lcands || has_ng' rcands

let solve w h =
    let allxs = List.init w id
    let rec add_line y xs tmp =
        if y >= h
        then
            seq {yield tmp}
        else
            match skipWhile (fun x -> has_ng w x tmp) xs with
                | x :: xr -> seq {yield! add_line (y+1) allxs (x :: tmp);
                                  yield! add_line y xr tmp}
                | [] -> Seq.empty
    add_line 0 allxs []



// visualize :: [int] -> string
let draw size n i =
    match i with
        | _ when i = n -> Some('*', i+1)
        | _ when i > size -> None
        | _ -> Some('-',i+1)

let seq_to_string s = System.String (Seq.toArray s)

let visualize board =
    let draw_line = board |> Seq.length |> draw
    let generate_line n = 0 |> Seq.unfold (draw_line n) |> seq_to_string in
    board |> Seq.map generate_line |> String.concat "\n"

(*
board = [0;1;2]

rendering:
  * - -
  - * -
  - - *

borad = [1;2;0]
  - * -
  - - *
  * - -
*)

[<EntryPoint>]
let main args =
    solve 8 8
    |> Seq.map visualize
    |> printf "%A"
    0