(** S-expression parser.

    Recursive descent over a string, producing [Sx_types.value list].
    Supports: lists, dicts, symbols, keywords, strings (with escapes),
    numbers, booleans, nil, comments, quote/quasiquote/unquote sugar. *)

open Sx_types

type state = {
  src : string;
  len : int;
  mutable pos : int;
}

let make_state src = { src; len = String.length src; pos = 0 }

let peek s = if s.pos < s.len then Some s.src.[s.pos] else None
let advance s = s.pos <- s.pos + 1
let at_end s = s.pos >= s.len

let skip_whitespace_and_comments s =
  let rec go () =
    if at_end s then ()
    else match s.src.[s.pos] with
    | ' ' | '\t' | '\n' | '\r' -> advance s; go ()
    | ';' ->
      while s.pos < s.len && s.src.[s.pos] <> '\n' do advance s done;
      if s.pos < s.len then advance s;
      go ()
    | _ -> ()
  in go ()

(* Character classification — matches spec/parser.sx ident-start/ident-char.
   ident-start: a-z A-Z _ ~ * + - > < = / ! ? &
   ident-char:  ident-start plus 0-9 . : / # , *)
let is_ident_start = function
  | 'a'..'z' | 'A'..'Z' | '_' | '~' | '*' | '+' | '-'
  | '>' | '<' | '=' | '/' | '!' | '?' | '&' -> true
  | _ -> false

let is_ident_char = function
  | c when is_ident_start c -> true
  | '0'..'9' | '.' | ':' | '#' | ',' -> true
  | _ -> false

(* Symbol reading uses ident_char; first char must be ident_start or digit/colon *)
let is_symbol_char = is_ident_char

let read_string s =
  (* s.pos is on the opening quote *)
  advance s;
  let buf = Buffer.create 64 in
  let rec go () =
    if at_end s then raise (Parse_error "Unterminated string");
    let c = s.src.[s.pos] in
    advance s;
    if c = '"' then Buffer.contents buf
    else if c = '\\' then begin
      if at_end s then raise (Parse_error "Unterminated string escape");
      let esc = s.src.[s.pos] in
      advance s;
      (match esc with
       | 'n' -> Buffer.add_char buf '\n'
       | 't' -> Buffer.add_char buf '\t'
       | 'r' -> Buffer.add_char buf '\r'
       | '"' -> Buffer.add_char buf '"'
       | '\\' -> Buffer.add_char buf '\\'
       | 'u' ->
         (* \uXXXX — read 4 hex digits, encode as UTF-8 *)
         if s.pos + 4 > s.len then raise (Parse_error "Incomplete \\u escape");
         let hex = String.sub s.src s.pos 4 in
         s.pos <- s.pos + 4;
         let code = int_of_string ("0x" ^ hex) in
         let ubuf = Buffer.create 4 in
         Buffer.add_utf_8_uchar ubuf (Uchar.of_int code);
         Buffer.add_string buf (Buffer.contents ubuf)
       | '`' -> Buffer.add_char buf '`'
       | _ -> Buffer.add_char buf '\\'; Buffer.add_char buf esc);
      go ()
    end else begin
      Buffer.add_char buf c;
      go ()
    end
  in go ()

let read_symbol s =
  let start = s.pos in
  while s.pos < s.len && is_symbol_char s.src.[s.pos] do advance s done;
  String.sub s.src start (s.pos - start)

let try_number str =
  match float_of_string_opt str with
  | Some n -> Some (Number n)
  | None -> None

let rec read_value s : value =
  skip_whitespace_and_comments s;
  if at_end s then raise (Parse_error "Unexpected end of input");
  match s.src.[s.pos] with
  | '(' -> read_list s ')'
  | '[' -> read_list s ']'
  | '{' -> read_dict s
  | '"' -> String (read_string s)
  | '\'' -> advance s; List [Symbol "quote"; read_value s]
  | '`' -> advance s; List [Symbol "quasiquote"; read_value s]
  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = ';' ->
    (* Datum comment: #; discards next expression *)
    advance s; advance s;
    ignore (read_value s);
    read_value s
  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = '\'' ->
    (* Quote shorthand: #'expr -> (quote expr) *)
    advance s; advance s;
    List [Symbol "quote"; read_value s]
  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = '|' ->
    (* Raw string: #|...|  — ends at next | *)
    advance s; advance s;
    let buf = Buffer.create 64 in
    let rec go () =
      if at_end s then raise (Parse_error "Unterminated raw string");
      let c = s.src.[s.pos] in
      advance s;
      if c = '|' then
        String (Buffer.contents buf)
      else begin
        Buffer.add_char buf c;
        go ()
      end
    in go ()
  | ',' ->
    (* Unquote / splice-unquote — matches spec: , always triggers unquote *)
    advance s;
    if s.pos < s.len && s.src.[s.pos] = '@' then begin
      advance s;
      List [Symbol "splice-unquote"; read_value s]
    end else
      List [Symbol "unquote"; read_value s]
  | _ ->
    begin
      (* Symbol, keyword, number, or boolean *)
      let token = read_symbol s in
      if token = "" then raise (Parse_error ("Unexpected char: " ^ String.make 1 s.src.[s.pos]));
      match token with
      | "true" -> Bool true
      | "false" -> Bool false
      | "nil" -> Nil
      | _ when token.[0] = ':' ->
        Keyword (String.sub token 1 (String.length token - 1))
      | _ ->
        match try_number token with
        | Some n -> n
        | None -> Symbol token
    end

and read_list s close_char =
  advance s; (* skip opening paren/bracket *)
  let items = ref [] in
  let rec go () =
    skip_whitespace_and_comments s;
    if at_end s then raise (Parse_error "Unterminated list");
    if s.src.[s.pos] = close_char then begin
      advance s;
      List (List.rev !items)
    end else begin
      items := read_value s :: !items;
      go ()
    end
  in go ()

and read_dict s =
  advance s; (* skip { *)
  let d = make_dict () in
  let rec go () =
    skip_whitespace_and_comments s;
    if at_end s then raise (Parse_error "Unterminated dict");
    if s.src.[s.pos] = '}' then begin
      advance s;
      Dict d
    end else begin
      let key = read_value s in
      let key_str = match key with
        | Keyword k -> k
        | String k -> k
        | Symbol k -> k
        | _ -> raise (Parse_error "Dict key must be keyword, string, or symbol")
      in
      let v = read_value s in
      dict_set d key_str v;
      go ()
    end
  in go ()


(** Parse a string into a list of SX values. *)
let parse_all src =
  let s = make_state src in
  let results = ref [] in
  let rec go () =
    skip_whitespace_and_comments s;
    if at_end s then List.rev !results
    else begin
      results := read_value s :: !results;
      go ()
    end
  in go ()

(** Parse a file into a list of SX values. *)
let parse_file path =
  let ic = open_in path in
  let n = in_channel_length ic in
  let src = really_input_string ic n in
  close_in ic;
  parse_all src