module StringMap = Map.Make (String)

exception Parse_error of string
exception Type_error of string

type typ =
  | TInt
  | TBool
  | TVoid
  | TStruct of string
  | TArray of typ

type binop =
  | Add
  | Sub
  | Mul
  | Div
  | Mod
  | And
  | Or
  | Eq
  | Ne
  | Lt
  | Le
  | Gt
  | Ge

type unop = Neg | Not

type expr =
  | IntLit of int
  | BoolLit of bool
  | Var of string
  | Binop of binop * expr * expr
  | Unop of unop * expr
  | Assign of expr * expr
  | Call of expr * expr list
  | ArrayGet of expr * expr
  | StructGet of expr * string

type stmt =
  | VarDecl of typ * string * expr option
  | Expr of expr
  | If of expr * stmt list * stmt list
  | ForEach of typ * string * expr * stmt list
  | Return of expr option
  | Block of stmt list

type func = {
  name : string;
  params : (typ * string) list;
  ret : typ;
  body : stmt list;
}

type struct_def = {
  sname : string;
  fields : (typ * string) list;
}

type top =
  | TopStruct of struct_def
  | TopFunc of func
  | TopGlobalVar of typ * string * expr option

type program = top list

let string_of_typ =
  let rec go = function
    | TInt -> "int"
    | TBool -> "bool"
    | TVoid -> "void"
    | TStruct n -> "struct " ^ n
    | TArray t -> go t ^ "[]"
  in
  go

let rec equal_typ a b =
  match (a, b) with
  | TInt, TInt | TBool, TBool | TVoid, TVoid -> true
  | TStruct x, TStruct y -> String.equal x y
  | TArray x, TArray y -> equal_typ x y
  | _ -> false

type token =
  | TIntKw
  | TBoolKw
  | TVoidKw
  | TStructKw
  | TIf
  | TElse
  | TFor
  | TEach
  | TForEach
  | TIn
  | TReturn
  | TTrue
  | TFalse
  | TIdent of string
  | TIntLit of int
  | TLParen
  | TRParen
  | TLBrace
  | TRBrace
  | TLBracket
  | TRBracket
  | TSemicolon
  | TComma
  | TDot
  | TAssign
  | TPlus
  | TMinus
  | TStar
  | TSlash
  | TPercent
  | TAndAnd
  | TOrOr
  | TBang
  | TEqEq
  | TNe
  | TLt
  | TLe
  | TGt
  | TGe
  | TEOF

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

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

let is_ident_start c =
  (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c = '_'

let is_ident_char c = is_ident_start c || is_digit c

let keyword_or_ident s =
  match s with
  | "int" -> TIntKw
  | "bool" -> TBoolKw
  | "void" -> TVoidKw
  | "struct" -> TStructKw
  | "if" -> TIf
  | "else" -> TElse
  | "for" -> TFor
  | "each" -> TEach
  | "foreach" -> TForEach
  | "in" -> TIn
  | "return" -> TReturn
  | "true" -> TTrue
  | "false" -> TFalse
  | _ -> TIdent s

let lex (src : string) : token list =
  let n = String.length src in
  let rec skip_line_comment i =
    if i >= n then i
    else if src.[i] = '\n' then i + 1
    else skip_line_comment (i + 1)
  in
  let rec skip_block_comment i =
    if i + 1 >= n then raise (Parse_error "unterminated block comment")
    else if src.[i] = '*' && src.[i + 1] = '/' then i + 2
    else skip_block_comment (i + 1)
  in
  let rec read_number i j =
    if j < n && is_digit src.[j] then read_number i (j + 1)
    else
      let s = String.sub src i (j - i) in
      (TIntLit (int_of_string s), j)
  in
  let rec read_ident i j =
    if j < n && is_ident_char src.[j] then read_ident i (j + 1)
    else
      let s = String.sub src i (j - i) in
      (keyword_or_ident s, j)
  in
  let rec loop i acc =
    if i >= n then List.rev (TEOF :: acc)
    else if is_space src.[i] then loop (i + 1) acc
    else
      match src.[i] with
      | '/' when i + 1 < n && src.[i + 1] = '/' -> loop (skip_line_comment (i + 2)) acc
      | '/' when i + 1 < n && src.[i + 1] = '*' -> loop (skip_block_comment (i + 2)) acc
      | '(' -> loop (i + 1) (TLParen :: acc)
      | ')' -> loop (i + 1) (TRParen :: acc)
      | '{' -> loop (i + 1) (TLBrace :: acc)
      | '}' -> loop (i + 1) (TRBrace :: acc)
      | '[' -> loop (i + 1) (TLBracket :: acc)
      | ']' -> loop (i + 1) (TRBracket :: acc)
      | ';' -> loop (i + 1) (TSemicolon :: acc)
      | ',' -> loop (i + 1) (TComma :: acc)
      | '.' -> loop (i + 1) (TDot :: acc)
      | '+' -> loop (i + 1) (TPlus :: acc)
      | '-' -> loop (i + 1) (TMinus :: acc)
      | '*' -> loop (i + 1) (TStar :: acc)
      | '%' -> loop (i + 1) (TPercent :: acc)
      | '/' -> loop (i + 1) (TSlash :: acc)
      | '!' when i + 1 < n && src.[i + 1] = '=' -> loop (i + 2) (TNe :: acc)
      | '!' -> loop (i + 1) (TBang :: acc)
      | '=' when i + 1 < n && src.[i + 1] = '=' -> loop (i + 2) (TEqEq :: acc)
      | '=' -> loop (i + 1) (TAssign :: acc)
      | '&' when i + 1 < n && src.[i + 1] = '&' -> loop (i + 2) (TAndAnd :: acc)
      | '|' when i + 1 < n && src.[i + 1] = '|' -> loop (i + 2) (TOrOr :: acc)
      | '<' when i + 1 < n && src.[i + 1] = '=' -> loop (i + 2) (TLe :: acc)
      | '<' -> loop (i + 1) (TLt :: acc)
      | '>' when i + 1 < n && src.[i + 1] = '=' -> loop (i + 2) (TGe :: acc)
      | '>' -> loop (i + 1) (TGt :: acc)
      | c when is_digit c ->
          let tok, j = read_number i (i + 1) in
          loop j (tok :: acc)
      | c when is_ident_start c ->
          let tok, j = read_ident i (i + 1) in
          loop j (tok :: acc)
      | c ->
          let msg = Printf.sprintf "unexpected character: %c" c in
          raise (Parse_error msg)
  in
  loop 0 []

type parser_state = {
  toks : token array;
  mutable i : int;
}

let mk_state toks = { toks = Array.of_list toks; i = 0 }

let peek st = if st.i < Array.length st.toks then st.toks.(st.i) else TEOF

let consume st =
  let t = peek st in
  st.i <- st.i + 1;
  t

let expect st tok =
  match (tok, consume st) with
  | TLParen, TLParen
  | TRParen, TRParen
  | TLBrace, TLBrace
  | TRBrace, TRBrace
  | TLBracket, TLBracket
  | TRBracket, TRBracket
  | TSemicolon, TSemicolon
  | TComma, TComma
  | TDot, TDot
  | TAssign, TAssign
  | TPlus, TPlus
  | TMinus, TMinus
  | TStar, TStar
  | TSlash, TSlash
  | TPercent, TPercent
  | TAndAnd, TAndAnd
  | TOrOr, TOrOr
  | TBang, TBang
  | TEqEq, TEqEq
  | TNe, TNe
  | TLt, TLt
  | TLe, TLe
  | TGt, TGt
  | TGe, TGe
  | TIf, TIf
  | TElse, TElse
  | TForEach, TForEach
  | TFor, TFor
  | TEach, TEach
  | TIn, TIn
  | TReturn, TReturn
  | TIntKw, TIntKw
  | TBoolKw, TBoolKw
  | TVoidKw, TVoidKw
  | TStructKw, TStructKw
  | TEOF, TEOF -> ()
  | _ -> raise (Parse_error "unexpected token")

let expect_ident st =
  match consume st with
  | TIdent s -> s
  | _ -> raise (Parse_error "expected identifier")

let starts_type = function TIntKw | TBoolKw | TVoidKw | TStructKw -> true | _ -> false

let rec parse_type st =
  let base =
    match consume st with
    | TIntKw -> TInt
    | TBoolKw -> TBool
    | TVoidKw -> TVoid
    | TStructKw -> TStruct (expect_ident st)
    | _ -> raise (Parse_error "expected type")
  in
  let rec arrays t =
    match peek st with
    | TLBracket ->
        expect st TLBracket;
        expect st TRBracket;
        arrays (TArray t)
    | _ -> t
  in
  arrays base

let rec parse_program st =
  let rec loop acc =
    match peek st with
    | TEOF -> List.rev acc
    | _ -> loop (parse_top st :: acc)
  in
  loop []

and parse_top st =
  match peek st with
  | TStructKw ->
      expect st TStructKw;
      let sname = expect_ident st in
      (match peek st with
      | TLBrace ->
          expect st TLBrace;
          let rec fields acc =
            match peek st with
            | TRBrace -> List.rev acc
            | _ ->
                let t = parse_type st in
                let n = expect_ident st in
                expect st TSemicolon;
                fields ((t, n) :: acc)
          in
          let fs = fields [] in
          expect st TRBrace;
          expect st TSemicolon;
          TopStruct { sname; fields = fs }
      | _ ->
          let ty = TStruct sname in
          parse_top_after_type st ty)
  | _ ->
      let ty = parse_type st in
      parse_top_after_type st ty

and parse_top_after_type st ty =
  let name = expect_ident st in
  match peek st with
  | TLParen ->
      expect st TLParen;
      let params = parse_params st in
      expect st TRParen;
      let body = parse_stmt_as_block st in
      TopFunc { name; params; ret = ty; body }
  | _ ->
      let init =
        match peek st with
        | TAssign ->
            expect st TAssign;
            Some (parse_expr st)
        | _ -> None
      in
      expect st TSemicolon;
      TopGlobalVar (ty, name, init)

and parse_params st =
  match peek st with
  | TRParen -> []
  | _ ->
      let rec loop acc =
        let t = parse_type st in
        let n = expect_ident st in
        match peek st with
        | TComma ->
            expect st TComma;
            loop ((t, n) :: acc)
        | _ -> List.rev ((t, n) :: acc)
      in
      loop []

and parse_stmt_as_block st =
  match peek st with
  | TLBrace ->
      expect st TLBrace;
      let rec loop acc =
        match peek st with
        | TRBrace ->
            expect st TRBrace;
            List.rev acc
        | _ -> loop (parse_stmt st :: acc)
      in
      loop []
  | _ -> [ parse_stmt st ]

and parse_stmt st =
  match peek st with
  | TLBrace -> Block (parse_stmt_as_block st)
  | TIf ->
      expect st TIf;
      expect st TLParen;
      let cond = parse_expr st in
      expect st TRParen;
      let then_body = parse_stmt_as_block st in
      let else_body =
        match peek st with
        | TElse ->
            expect st TElse;
            parse_stmt_as_block st
        | _ -> []
      in
      If (cond, then_body, else_body)
  | TForEach | TFor ->
      (match peek st with
      | TForEach -> expect st TForEach
      | TFor ->
          expect st TFor;
          expect st TEach
      | _ -> ());
      expect st TLParen;
      let it_t = parse_type st in
      let it_name = expect_ident st in
      expect st TIn;
      let iterable = parse_expr st in
      expect st TRParen;
      let body = parse_stmt_as_block st in
      ForEach (it_t, it_name, iterable, body)
  | TReturn ->
      expect st TReturn;
      let v =
        match peek st with
        | TSemicolon -> None
        | _ -> Some (parse_expr st)
      in
      expect st TSemicolon;
      Return v
  | t when starts_type t ->
      let ty = parse_type st in
      let n = expect_ident st in
      let init =
        match peek st with
        | TAssign ->
            expect st TAssign;
            Some (parse_expr st)
        | _ -> None
      in
      expect st TSemicolon;
      VarDecl (ty, n, init)
  | _ ->
      let e = parse_expr st in
      expect st TSemicolon;
      Expr e

and parse_expr st = parse_assignment st

and parse_assignment st =
  let lhs = parse_or st in
  match peek st with
  | TAssign ->
      expect st TAssign;
      let rhs = parse_assignment st in
      Assign (lhs, rhs)
  | _ -> lhs

and parse_or st =
  let rec loop left =
    match peek st with
    | TOrOr ->
        expect st TOrOr;
        loop (Binop (Or, left, parse_and st))
    | _ -> left
  in
  loop (parse_and st)

and parse_and st =
  let rec loop left =
    match peek st with
    | TAndAnd ->
        expect st TAndAnd;
        loop (Binop (And, left, parse_equality st))
    | _ -> left
  in
  loop (parse_equality st)

and parse_equality st =
  let rec loop left =
    match peek st with
    | TEqEq ->
        expect st TEqEq;
        loop (Binop (Eq, left, parse_rel st))
    | TNe ->
        expect st TNe;
        loop (Binop (Ne, left, parse_rel st))
    | _ -> left
  in
  loop (parse_rel st)

and parse_rel st =
  let rec loop left =
    match peek st with
    | TLt ->
        expect st TLt;
        loop (Binop (Lt, left, parse_add st))
    | TLe ->
        expect st TLe;
        loop (Binop (Le, left, parse_add st))
    | TGt ->
        expect st TGt;
        loop (Binop (Gt, left, parse_add st))
    | TGe ->
        expect st TGe;
        loop (Binop (Ge, left, parse_add st))
    | _ -> left
  in
  loop (parse_add st)

and parse_add st =
  let rec loop left =
    match peek st with
    | TPlus ->
        expect st TPlus;
        loop (Binop (Add, left, parse_mul st))
    | TMinus ->
        expect st TMinus;
        loop (Binop (Sub, left, parse_mul st))
    | _ -> left
  in
  loop (parse_mul st)

and parse_mul st =
  let rec loop left =
    match peek st with
    | TStar ->
        expect st TStar;
        loop (Binop (Mul, left, parse_unary st))
    | TSlash ->
        expect st TSlash;
        loop (Binop (Div, left, parse_unary st))
    | TPercent ->
        expect st TPercent;
        loop (Binop (Mod, left, parse_unary st))
    | _ -> left
  in
  loop (parse_unary st)

and parse_unary st =
  match peek st with
  | TMinus ->
      expect st TMinus;
      Unop (Neg, parse_unary st)
  | TBang ->
      expect st TBang;
      Unop (Not, parse_unary st)
  | _ -> parse_postfix st

and parse_postfix st =
  let rec loop e =
    match peek st with
    | TLParen ->
        expect st TLParen;
        let args = parse_args st in
        expect st TRParen;
        loop (Call (e, args))
    | TLBracket ->
        expect st TLBracket;
        let idx = parse_expr st in
        expect st TRBracket;
        loop (ArrayGet (e, idx))
    | TDot ->
        expect st TDot;
        let fld = expect_ident st in
        loop (StructGet (e, fld))
    | _ -> e
  in
  loop (parse_primary st)

and parse_args st =
  match peek st with
  | TRParen -> []
  | _ ->
      let rec loop acc =
        let e = parse_expr st in
        match peek st with
        | TComma ->
            expect st TComma;
            loop (e :: acc)
        | _ -> List.rev (e :: acc)
      in
      loop []

and parse_primary st =
  match consume st with
  | TIntLit n -> IntLit n
  | TTrue -> BoolLit true
  | TFalse -> BoolLit false
  | TIdent s -> Var s
  | TLParen ->
      let e = parse_expr st in
      expect st TRParen;
      e
  | _ -> raise (Parse_error "expected expression")

let parse_string src =
  try
    let st = mk_state (lex src) in
    Ok (parse_program st)
  with Parse_error msg -> Error msg

type func_sig = {
  fparams : typ list;
  fret : typ;
}

type tc_ctx = {
  structs : (typ StringMap.t) StringMap.t;
  funcs : func_sig StringMap.t;
  globals : typ StringMap.t;
}

let fail_type msg = raise (Type_error msg)

let expect_type got want =
  if not (equal_typ got want) then
    fail_type (Printf.sprintf "type mismatch: got %s, expected %s" (string_of_typ got) (string_of_typ want))

let rec validate_type (structs : (typ StringMap.t) StringMap.t) (allow_void : bool) = function
  | TVoid when allow_void -> ()
  | TVoid -> fail_type "void is not a valid variable type"
  | TStruct n ->
      if not (StringMap.mem n structs) then fail_type ("unknown struct type: " ^ n)
  | TArray t ->
      if equal_typ t TVoid then fail_type "array element type cannot be void";
      validate_type structs false t
  | TInt | TBool -> ()

let collect_ctx (prog : program) : tc_ctx =
  let rec collect tops structs funcs globals =
    match tops with
    | [] -> { structs; funcs; globals }
    | TopStruct s :: tl ->
        if StringMap.mem s.sname structs then fail_type ("duplicate struct: " ^ s.sname);
        let fields =
          List.fold_left
            (fun acc (t, n) ->
              if StringMap.mem n acc then fail_type ("duplicate field " ^ n ^ " in struct " ^ s.sname);
              StringMap.add n t acc)
            StringMap.empty s.fields
        in
        collect tl (StringMap.add s.sname fields structs) funcs globals
    | TopFunc f :: tl ->
        if StringMap.mem f.name funcs then fail_type ("duplicate function: " ^ f.name);
        let sig_ = { fparams = List.map fst f.params; fret = f.ret } in
        collect tl structs (StringMap.add f.name sig_ funcs) globals
    | TopGlobalVar (t, n, _) :: tl ->
        if StringMap.mem n globals then fail_type ("duplicate global variable: " ^ n);
        collect tl structs funcs (StringMap.add n t globals)
  in
  collect prog StringMap.empty StringMap.empty StringMap.empty

let lookup_var env x =
  match StringMap.find_opt x env with
  | Some t -> t
  | None -> fail_type ("unknown variable: " ^ x)

let lookup_struct_field ctx sname fname =
  match StringMap.find_opt sname ctx.structs with
  | None -> fail_type ("unknown struct: " ^ sname)
  | Some fields ->
      (match StringMap.find_opt fname fields with
      | None -> fail_type ("unknown field " ^ fname ^ " on struct " ^ sname)
      | Some t -> t)

let rec infer_expr ctx env = function
  | IntLit _ -> TInt
  | BoolLit _ -> TBool
  | Var x -> lookup_var env x
  | Unop (Neg, e) ->
      expect_type (infer_expr ctx env e) TInt;
      TInt
  | Unop (Not, e) ->
      expect_type (infer_expr ctx env e) TBool;
      TBool
  | Binop (op, a, b) ->
      let ta = infer_expr ctx env a in
      let tb = infer_expr ctx env b in
      (match op with
      | Add | Sub | Mul | Div | Mod ->
          expect_type ta TInt;
          expect_type tb TInt;
          TInt
      | And | Or ->
          expect_type ta TBool;
          expect_type tb TBool;
          TBool
      | Lt | Le | Gt | Ge ->
          expect_type ta TInt;
          expect_type tb TInt;
          TBool
      | Eq | Ne ->
          if not (equal_typ ta tb) then fail_type "equality operands must have same type";
          TBool)
  | Assign (lhs, rhs) ->
      (match lhs with Var _ | ArrayGet _ | StructGet _ -> () | _ -> fail_type "left side of assignment is not assignable");
      let tl = infer_expr ctx env lhs in
      let tr = infer_expr ctx env rhs in
      expect_type tr tl;
      tl
  | ArrayGet (arr, idx) ->
      expect_type (infer_expr ctx env idx) TInt;
      (match infer_expr ctx env arr with
      | TArray t -> t
      | t -> fail_type ("indexing requires array, got " ^ string_of_typ t))
  | StructGet (obj, fld) ->
      (match infer_expr ctx env obj with
      | TStruct sname -> lookup_struct_field ctx sname fld
      | t -> fail_type ("field access requires struct, got " ^ string_of_typ t))
  | Call (callee, args) ->
      let fname =
        match callee with
        | Var n -> n
        | _ -> fail_type "only direct function calls are supported"
      in
      let sig_ =
        match StringMap.find_opt fname ctx.funcs with
        | Some s -> s
        | None -> fail_type ("unknown function: " ^ fname)
      in
      if List.length args <> List.length sig_.fparams then
        fail_type
          (Printf.sprintf "function %s expects %d arguments, got %d" fname (List.length sig_.fparams)
             (List.length args));
      List.iter2 (fun arg pty -> expect_type (infer_expr ctx env arg) pty) args sig_.fparams;
      sig_.fret

let rec check_stmt ctx ret env = function
  | VarDecl (t, n, init) ->
      validate_type ctx.structs false t;
      (match init with None -> () | Some e -> expect_type (infer_expr ctx env e) t);
      StringMap.add n t env
  | Expr e ->
      ignore (infer_expr ctx env e);
      env
  | If (cond, tbranch, ebranch) ->
      expect_type (infer_expr ctx env cond) TBool;
      ignore (check_block ctx ret env tbranch);
      ignore (check_block ctx ret env ebranch);
      env
  | ForEach (it_t, it_name, iterable, body) ->
      validate_type ctx.structs false it_t;
      (match infer_expr ctx env iterable with
      | TArray elem_t -> expect_type elem_t it_t
      | t -> fail_type ("foreach expects array iterable, got " ^ string_of_typ t));
      let env' = StringMap.add it_name it_t env in
      ignore (check_block ctx ret env' body);
      env
  | Return None ->
      expect_type TVoid ret;
      env
  | Return (Some e) ->
      expect_type (infer_expr ctx env e) ret;
      env
  | Block stmts ->
      ignore (check_block ctx ret env stmts);
      env

and check_block ctx ret env stmts = List.fold_left (check_stmt ctx ret) env stmts

let rec has_return_stmt = function
  | Return _ -> true
  | If (_, t, e) -> List.exists has_return_stmt t || List.exists has_return_stmt e
  | ForEach (_, _, _, body) | Block body -> List.exists has_return_stmt body
  | VarDecl _ | Expr _ -> false

let check_program (prog : program) =
  let ctx = collect_ctx prog in
  StringMap.iter (fun _ t -> validate_type ctx.structs false t) ctx.globals;
  StringMap.iter
    (fun _ sig_ ->
      List.iter (validate_type ctx.structs false) sig_.fparams;
      validate_type ctx.structs true sig_.fret)
    ctx.funcs;
  List.iter
    (function
      | TopStruct s ->
          List.iter (fun (t, _) -> validate_type ctx.structs false t) s.fields
      | TopGlobalVar (t, _, init) ->
          validate_type ctx.structs false t;
          let env = ctx.globals in
          (match init with None -> () | Some e -> expect_type (infer_expr ctx env e) t)
      | TopFunc f ->
          let env_with_globals = ctx.globals in
          let env =
            List.fold_left
              (fun acc (t, n) ->
                validate_type ctx.structs false t;
                if StringMap.mem n acc then fail_type ("duplicate parameter name: " ^ n);
                StringMap.add n t acc)
              env_with_globals f.params
          in
          ignore (check_block ctx f.ret env f.body);
          if (not (equal_typ f.ret TVoid)) && not (List.exists has_return_stmt f.body) then
            fail_type ("non-void function " ^ f.name ^ " must return a value"))
    prog

let type_check (prog : program) =
  try
    check_program prog;
    Ok ()
  with Type_error msg -> Error msg

let parse_and_type_check src =
  match parse_string src with
  | Error e -> Error ("Parse error: " ^ e)
  | Ok prog ->
      (match type_check prog with
      | Error e -> Error ("Type error: " ^ e)
      | Ok () -> Ok prog)