open Ast.Value

exception Invalid_character of char
exception Expected of char

let invalid_char c = raise @@ Invalid_character c
let expected c = raise @@ Expected c

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

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

let is_num = function
  | 'x' -> true
  | '.' -> 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

let expect_char c seq =
  match seq () with
  | Seq.Nil -> expected c
  | Seq.Cons (x, seq) ->
    if x = c then seq else expected c

let expect_token str tok seq =
  let rec aux ts seq =
    match ts (), seq () with
    | Seq.Nil, _ -> Some seq
    | Seq.Cons _, Seq.Nil -> None
    | Seq.Cons (a, ts), Seq.Cons (b, seq) ->
      if a = b then aux ts seq else None
  in
  let str = String.to_seq str in
  aux str seq |> Option.map (fun s -> tok, s)

let find_token seq =
  !Token.tokens |> List.find_map
    (fun (s, t) -> expect_token s t seq)

(* same as take_while f seq, drop_while f seq *)
let rec partition_while f seq =
  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 =
  let seq = String.to_seq str in
  let rec aux seq =
    let open Token in
    let open Seq in
    match seq () with
    | Nil -> empty
    | Cons (x, seq) ->
      (* skip whitespace *)
      if is_whitespace x then
        aux seq

      (* string *)
      else if x = '"' then
        let str, seq = partition_while ((<>) '"') seq in
        let str = String (String.of_seq str) in
        let seq = expect_char '"' seq in
        cons (Value str) (aux seq)

      (* number (int, float) *)
      else if is_digit x then
        let n, seq = partition_while is_num seq in
        let n = String.of_seq @@ cons x n in
        let n =
          if String.contains n '.' (* float *)
          then Float (float_of_string n)
          else Int (int_of_string n)
        in
        cons (Value n) (aux seq)

      (* idents *)
      else if is_ident_start x then
        let id, seq = partition_while is_ident seq in
        let id = String.of_seq @@ cons x id in
        cons (Ident id) (aux seq)

      (* tokens *)
      else
        match find_token @@ cons x seq with
        | None -> invalid_char x
        | Some (t, seq) -> cons t (aux seq)
  in
  aux seq