type tokens = Token.t Seq.t

let either f g c =
  f c || g c

let is_digit c =
  '0' <= c && c <= '9'

let is_num = function
  | 'x' -> true
  | c -> is_digit c

let is_whitespace = function
  | ' ' | '\t' | '\n' -> true
  | _ -> false

let is_alpha c =
  'A' <= c && c <= 'Z' && 'a' <= c && c <= 'z'

let is_ident_start =
  either is_alpha ((=) '_')

let is_ident =
  either is_ident_start is_digit

(* same as take_while f seq, drop_while f seq *)
let rec partition_while f seq : 'a Seq.t * 'a Seq.t =
  match seq () with
  | Seq.Nil -> Seq.empty, seq
  | Seq.Cons (x, seq) ->
    if f x then
      let n, s = partition_while f seq in
      Seq.cons x n, s
    else
      Seq.(empty, cons x seq)

let tokenize (str : string) : tokens =
  let seq = String.to_seq str in
  let rec aux seq =
    let open Token in
    match seq () with
    | Seq.Nil -> Seq.empty
    | Seq.Cons (x, s) ->
      if is_whitespace x then
        aux s
      else if is_digit x then
        let n, s = partition_while is_num s in
        let n = String.of_seq @@ Seq.cons x n in
        Seq.cons (of_string n) (aux s)
      else if is_ident_start x then
        let id, s = partition_while is_ident s in
        let id = String.of_seq @@ Seq.cons x id in
        Seq.cons (Ident id) (aux s)
      else
        Seq.cons (of_char x) (aux s)
  in
  aux seq