let (++) f g x = f (g x);;
let id x = x;;
let rec fold_nat f x n = if n = 0 then x else f (fold_nat f x (n-1)) n ;;

let print_hline = Console.print_hline;;

open Pure

type var = int;;
type t =
 | V of var
 | A of t * t
 | L of (t * t list (*garbage*))
 | C (* constant *)
;;

let delta = L(A(V 0, V 0),[]);;

let rec is_stuck = function
 | C -> true
 | A(t,_) -> is_stuck t
 | _ -> false
;;

let eta_eq' =
 let rec aux l1 l2 t1 t2 = match t1, t2 with
  | _, _ when is_stuck t1 || is_stuck t2 -> true
  | L t1, L t2 -> aux l1 l2 (fst t1) (fst t2)
  | L t1, t2 -> aux l1 (l2+1) (fst t1) t2
  | t1, L t2 -> aux (l1+1) l2 t1 (fst t2)
  | V a, V b -> a + l1 = b + l2
  | A(t1,t2), A(u1,u2) -> aux l1 l2 t1 u1 && aux l1 l2 t2 u2
  | _, _ -> false
 in aux ;;
let eta_eq = eta_eq' 0 0;;

(* is arg1 eta-subterm of arg2 ? *)
let eta_subterm u =
 let rec aux lev t = if t = C then false else (eta_eq' lev 0 u t || match t with
 | L(t,g) -> List.exists (aux (lev+1)) (t::g)
 | A(t1, t2) -> aux lev t1 || aux lev t2
 | _ -> false) in
 aux 0
;;

(* does NOT lift the argument *)
let mk_lams = fold_nat (fun x _ -> L(x,[])) ;;

let string_of_t =
  let string_of_bvar =
   let bound_vars = ["x"; "y"; "z"; "w"; "q"] in
   let bvarsno = List.length bound_vars in
   fun nn -> if nn < bvarsno then List.nth bound_vars nn else "x" ^ (string_of_int (nn - bvarsno + 1)) in
  let rec string_of_term_w_pars level = function
    | V v -> if v >= level then "`" ^ string_of_int (v-level) else
       string_of_bvar (level - v-1)
    | C -> "C"
    | A _
    | L _ as t -> "(" ^ string_of_term_no_pars level t ^ ")"
  and string_of_term_no_pars_app level = function
    | A(t1,t2) -> string_of_term_no_pars_app level t1 ^ " " ^ string_of_term_w_pars level t2
    | _ as t -> string_of_term_w_pars level t
  and string_of_term_no_pars level = function
    | L(t,g) -> "λ" ^ string_of_bvar level ^ ". " ^ string_of_term_no_pars (level+1) t
       ^ (if g = [] then "" else String.concat ", " ("" :: List.map (string_of_term_w_pars level) g))
    | _ as t -> string_of_term_no_pars_app level t
  in string_of_term_no_pars 0
;;

type problem = {
   orig_freshno: int
 ; freshno : int
 ; div : t
 ; conv : t
 ; sigma : (var * t) list (* substitutions *)
}

let string_of_problem p =
 let lines = [
  "[DV] " ^ string_of_t p.div;
  "[CV] " ^ string_of_t p.conv;
 ] in
 String.concat "\n" lines
;;

exception B;;
exception Done of (var * t) list (* substitution *);;
exception Fail of int * string;;

let problem_fail p reason =
 print_endline "!!!!!!!!!!!!!!! FAIL !!!!!!!!!!!!!!!";
 print_endline (string_of_problem p);
 raise (Fail (-1, reason))
;;

let freshvar ({freshno} as p) =
 {p with freshno=freshno+1}, freshno+1
;;

(* CSC: rename? is an applied C an inert?
   is_inert and get_inert work inconsistently *)
let rec is_inert =
 function
 | A(t,_) -> is_inert t
 | V _ -> true
 | C
 | L _ -> false
;;

let rec is_constant =
 function
    C -> true
  | V _ -> false
  | A(t,_)
  | L(t,_) -> is_constant t
;;

let rec get_inert = function
 | V _ | C as t -> (t,0)
 | A(t, _) -> let hd,args = get_inert t in hd,args+1
 | _ -> assert false
;;

let args_of_inert =
 let rec aux acc =
  function
   | V _ | C -> acc
   | A(t, a) -> aux (a::acc) t
   | _ -> assert false
 in
  aux []
;;

(* precomputes the number of leading lambdas in a term,
   after replacing _v_ w/ a term starting with n lambdas *)
let rec no_leading_lambdas v n = function
 | L(t,_) -> 1 + no_leading_lambdas (v+1) n t
 | A _ as t -> let v', m = get_inert t in if V v = v' then max 0 (n - m) else 0
 | V v' -> if v = v' then n else 0
 | C -> 0
;;

let rec subst level delift sub =
 function
 | V v -> (if v = level + fst sub then lift level (snd sub) else V (if delift && v > level then v-1 else v)), []
 | L x -> let t, g = subst_in_lam (level+1) delift sub x in L(t, g), []
 | A (t1,t2) ->
  let t1, g1 = subst level delift sub t1 in
  let t2, g2 = subst level delift sub t2 in
  let t3, g3 = mk_app t1 t2 in
  t3, g1 @ g2 @ g3
 | C -> C, []
and subst_in_lam level delift sub (t, g) =
  let t', g' = subst level delift sub t in
  let g'' = List.fold_left
   (fun xs t ->
     let x,y = subst level delift sub t in
     (x :: y @ xs)) g' g in t', g''
and mk_app t1 t2 = if t1 = delta && t2 = delta then raise B
 else match t1 with
 | L x -> subst_in_lam 0 true (0, t2) x
 | _ -> A (t1, t2), []
and lift n =
 let rec aux lev =
  function
  | V m -> V (if m >= lev then m + n else m)
  | L(t,g) -> L (aux (lev+1) t, List.map (aux (lev+1)) g)
  | A (t1, t2) -> A (aux lev t1, aux lev t2)
  | C -> C
 in aux 0
;;
let subst = subst 0 false;;

let subst_in_problem ((v, t) as sub) p =
print_endline ("-- SUBST " ^ string_of_t (V v) ^ " |-> " ^ string_of_t t);
 let sigma = sub :: p.sigma in
 let div, g = try subst sub p.div with B -> raise (Done sigma) in
 assert (g = []);
 let conv, f = try subst sub p.conv with B -> raise (Fail(-1, "p.conv diverged")) in
 assert (g = []);
 {p with div; conv; sigma}
;;

let get_subterms_with_head hd_var =
 let rec aux lev inert_done g = function
 | L(t,g') -> List.fold_left (aux (lev+1) false) g (t::g')
 | C | V _ -> g
 | A(t1,t2) as t ->
   let hd_var', n_args' = get_inert t1 in
   if not inert_done && hd_var' = V (hd_var + lev)
    then lift ~-lev t :: aux lev false (aux lev true g t1) t2
    else                 aux lev false (aux lev true g t1) t2
 in aux 0 false []
;;

let purify =
 let rec aux = function
 | L(t,g) ->
    let t = aux (lift (List.length g) t) in
    let t = List.fold_left (fun t g -> Pure.A(Pure.L t, aux g)) t g in
    Pure.L t
 | A (t1,t2) -> Pure.A (aux t1, aux t2)
 | V n -> Pure.V (n)
 | C -> Pure.V (min_int/2)
 in aux
;;

let check p sigma =
 print_endline "Checking...";
 let div = purify p.div in
 let conv = purify p.conv in
 let sigma = List.map (fun (v,t) -> v, purify t) sigma in
 let freshno = List.fold_right (max ++ fst) sigma 0 in
 let env = Pure.env_of_sigma freshno sigma in
 assert (Pure.diverged (Pure.mwhd (env,div,[])));
 print_endline " D diverged.";
 assert (not (Pure.diverged (Pure.mwhd (env,conv,[]))));
 print_endline " C converged.";
 ()
;;

let sanity p =
 print_endline (string_of_problem p); (* non cancellare *)
 if not (is_inert p.div) then problem_fail p "p.div converged";
 (* Trailing constant args can be removed because do not contribute to eta-diff *)
 let rec remove_trailing_constant_args = function
 | A(t1, t2) when is_constant t2 -> remove_trailing_constant_args t1
 | _ as t -> t in
 let p = {p with div=remove_trailing_constant_args p.div} in
 p
;;

(* drops the arguments of t after the n-th *)
let inert_cut_at n t =
 let rec aux t =
  match t with
  | V _ as t -> 0, t
  | A(t1,_) as t ->
    let k', t' = aux t1 in
     if k' = n then n, t'
      else k'+1, t
  | _ -> assert false
 in snd (aux t)
;;

(* return the index of the first argument with a difference
   (the first argument is 0) *)
let find_eta_difference p t =
 let divargs = args_of_inert p.div in
 let conargs = args_of_inert t in
 let rec aux k divargs conargs =
  match divargs,conargs with
     [],_ -> []
   | _::_,[] -> [k]
   | t1::divargs,t2::conargs ->
      (if not (eta_eq t1 t2) then [k] else []) @ aux (k+1) divargs conargs
 in
  aux 0 divargs conargs
;;

let compute_max_lambdas_at hd_var j =
 let rec aux hd = function
 | A(t1,t2) ->
    (if get_inert t1 = (V hd, j)
      then max ( (*FIXME*)
       if is_inert t2 && let hd', j' = get_inert t2 in hd' = V hd
        then let hd', j' = get_inert t2 in j - j'
        else no_leading_lambdas hd_var j t2)
      else id) (max (aux hd t1) (aux hd t2))
 | L(t,_) -> aux (hd+1) t
 | V _ | C -> 0
 in aux hd_var
;;

let print_cmd s1 s2 = print_endline (">> " ^ s1 ^ " " ^ s2);;

(* returns Some i if i is the smallest integer s.t. p holds for the i-th
   element of the list in input *)
let smallest_such_that p =
 let rec aux i =
  function
     [] -> None
   | hd::_ when (print_endline (string_of_t hd) ; p hd) -> Some i
   | _::tl -> aux (i+1) tl
 in
  aux 0
;;

(* step on the head of div, on the k-th argument, with n fresh vars *)
let step k n p =
 let hd, _ = get_inert p.div in
 match hd with
 | C | L _ | A _  -> assert false
 | V var ->
print_cmd "STEP" ("on " ^ string_of_t (V var) ^ " (on " ^ string_of_int (k+1) ^ "th)");
 let p, t = (* apply fresh vars *)
  fold_nat (fun (p, t) _ ->
    let p, v = freshvar p in
    p, A(t, V (v + k + 1))
  ) (p, V 0) n in
 let t = (* apply unused bound variables V_{k-1}..V_1 *)
  fold_nat (fun t m -> A(t, V (k-m+1))) t k in
 let t = mk_lams t (k+1) in (* make leading lambdas *)
 let subst = var, t in
 let p = subst_in_problem subst p in
 sanity p
;;

let finish p =
 (* one-step version of eat *)
 let compute_max_arity =
   let rec aux n = function
   | A(t1,t2) -> max (aux (n+1) t1) (aux 0 t2)
   | L(t,g) -> List.fold_right (max ++ (aux 0)) (t::g) 0
   | _ -> n
 in aux 0 in
print_cmd "FINISH" "";
 (* First, a step on the last argument of the divergent.
    Because of the sanity check, it will never be a constant term. *)
 let div_hd, div_nargs = get_inert p.div in
 let div_hd = match div_hd with V n -> n | _ -> assert false in
 let j = div_nargs - 1 in
 let arity = compute_max_arity p.conv in
 let n = 1 + arity + max
  (compute_max_lambdas_at div_hd j p.div)
  (compute_max_lambdas_at div_hd j p.conv) in
 let p = step j n p in
 (* Now, find first argument of div that is a variable never applied anywhere.
 It must exist because of some invariant, since we just did a step,
 and because of the arity of the divergent *)
 let div_hd, div_nargs = get_inert p.div in
 let div_hd = match div_hd with V n -> n | _ -> assert false in
 let rec aux m = function
 | A(t, V delta_var) ->
   if delta_var <> div_hd && get_subterms_with_head delta_var p.conv = []
   then m, delta_var
   else aux (m-1) t
 | A(t,_) -> aux (m-1) t
 | _ -> assert false in
 let m, delta_var = aux div_nargs p.div in
 let p = subst_in_problem (delta_var, delta) p in
 let p = subst_in_problem (div_hd, mk_lams delta (m-1)) p in
 sanity p
;;

let auto p =
 let rec aux p =
 let hd, n_args = get_inert p.div in
 match hd with
 | C | L _ | A _  -> assert false
 | V hd_var ->
 let tms = get_subterms_with_head hd_var p.conv in
 if List.exists (fun t -> snd (get_inert t) >= n_args) tms
  then (
    (* let tms = List.sort (fun t1 t2 -> - compare (snd (get_inert t1)) (snd (get_inert t2))) tms in *)
    List.iter (fun t -> try
      let js = find_eta_difference p t in
      (* print_endline (String.concat ", " (List.map string_of_int js)); *)
      if js = [] then problem_fail p "no eta difference found (div subterm of conv?)";
      let js = List.rev js in
       List.iter
        (fun j ->
         try
          let k = 1 + max
           (compute_max_lambdas_at hd_var j p.div)
            (compute_max_lambdas_at hd_var j p.conv) in
          ignore (aux (step j k p))
         with Fail(_, s) ->
          print_endline ("Backtracking (eta_diff) because: " ^ s)) js;
       raise (Fail(-1, "no eta difference"))
      with Fail(_, s) ->
       print_endline ("Backtracking (get_subterms) because: " ^ s)) tms;
     raise (Fail(-1, "no similar terms"))
   )
  else
    problem_fail (finish p) "Finish did not complete the problem"
  in
  try
   aux p
  with Done sigma -> sigma
;;

let problem_of (label, div, convs, ps, var_names) =
 print_hline ();
 let rec aux lev = function
 | `Lam(_, t) -> L (aux (lev+1) t, [])
 | `I (v, args) -> Listx.fold_left (fun x y -> fst (mk_app x (aux lev y))) (aux lev (`Var v)) args
 | `Var(v,_) -> if v >= lev && List.nth var_names (v-lev) = "C" then C else V v
 | `N _ | `Match _ -> assert false in
 assert (List.length ps = 0);
 let convs = List.rev convs in
 let conv = List.fold_left (fun x y -> fst (mk_app x (aux 0 (y :> Num.nf)))) (V (List.length var_names)) convs in
 let var_names = "@" :: var_names in
 let div = match div with
 | Some div -> aux 0 (div :> Num.nf)
 | None -> assert false in
 let varno = List.length var_names in
 {orig_freshno=varno; freshno=1+varno; div; conv; sigma=[]}
;;

let solve p =
 if is_constant p.div
  then print_endline "!!! div is stuck. Problem was not run !!!"
 else if eta_subterm p.div p.conv
  then print_endline "!!! div is subterm of conv. Problem was not run !!!"
  else let p = sanity p (* initial sanity check *) in check p (auto p)
;;

Problems.main (solve ++ problem_of);

(* Example usage of interactive: *)

(* let interactive div conv cmds =
 let p = problem_of div conv in
 try (
 let p = List.fold_left (|>) p cmds in
 let rec f p cmds =
  let nth spl n = int_of_string (List.nth spl n) in
  let read_cmd () =
   let s = read_line () in
   let spl = Str.split (Str.regexp " +") s in
   s, let uno = List.hd spl in
    try if uno = "eat" then eat
    else if uno = "step" then step (nth spl 1) (nth spl 2)
    else failwith "Wrong input."
    with Failure s -> print_endline s; (fun x -> x) in
  let str, cmd = read_cmd () in
  let cmds = (" " ^ str ^ ";")::cmds in
  try
   let p = cmd p in f p cmds
  with
  | Done _ -> print_endline "Done! Commands history: "; List.iter print_endline (List.rev cmds)
 in f p []
 ) with Done _ -> ()
;; *)

(* interactive "x y"
 "@ (x x) (y x) (y z)" [step 0 1; step 0 2; eat]
;; *)