(*
Copyright (c) 2013, Simon Cruanes
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(** {1 Bijective Serializer/Deserializer} *)

(** This module helps writing serialization/deserialization code in
    a type-safe way. It uses GADTs, and as such requires OCaml >= 4.00.1.

    Conceptually, a value of type ['a] {! t} describes the (persistent) structure
    of the type ['a]. Combinators, listed in the next section (e.g., {!list_}
    or {!pair}), are used to describe complicated structures from simpler
    ones.
    
    For instance, to serialize a value of type [(int * string) list]:

{[let bij = Bij.(list_ (pair int_ string_));;

let l = [(1, "foo"); (2, "bar")];;

Bij.TrBencode.to_string ~bij l;;
- : string = "lli1e3:fooeli2e3:baree"
]}

  Some types may not be directly describable, for instance records or
  algebraic types. For those, more subtle combinators exist:

  - {!map} is a bijection between two types, and should be typically used to
    map records to tuples (for which combinators exist)

  - {!switch} is a case disjunction. Each case can map to a different type,
    thank to the power of GADT, and a {b key} needs to be provided for
    each case, so that de-serialization can know which type to read.

  - {!fix} allows to describe recursive encodings. The user provides a function
    which, given a ['a t lazy_t], builds a ['a t], and return its fixpoint.

  For instance, let's take a simple symbolic expressions structure (can
  be found in the corresponding test file "tests/test_bij.ml"):

{[
type term =
  | Const of string
  | Int of int
  | App of term list;;

let bij_term =
  Bij.(fix
    (fun bij ->
    switch
    ~inject:(function
      | Const s -> "const", BranchTo (string_, s)
      | Int i -> "int", BranchTo (int_, i)
      | App l -> "app", BranchTo (list_ (Lazy.force bij), l))
    ~extract:(function
      | "const" -> BranchFrom (string_, fun x -> Const x)
      | "int" -> BranchFrom (int_, fun x -> Int x)
      | "app" -> BranchFrom (list_ (Lazy.force bij), fun l -> App l)
      | _ -> raise (DecodingError "unexpected case switch")))
    )
]}

  A bijection could be used for many things, but here our focus is on
  serialization and de-serialization. The idea is that we can map a value
  [x : 'a] to some general-purpose serialization format
  (json, XML, B-encode, etc.) that we can then write to the disk or network;
  the reverse operation is also possible (and bijectivity is enforced
  by the fact that we use a single datatype ['a t] to describe both mappings).

  For now, only a bijection to B-encode (see {!Bencode} and {!Bij.TrBencode})
  is provided. The code is quite straightforward and could be extended
  to XML or Json without hassle.
*)

type _ t = private
  | Unit : unit t
  | String : string t
  | Int : int t
  | Bool : bool t
  | Float : float t
  | List : 'a t -> 'a list t
  | Many : 'a t -> 'a list t
  | Opt : 'a t -> 'a option t
  | Pair : 'a t * 'b t -> ('a * 'b) t
  | Triple : 'a t * 'b t * 'c t -> ('a * 'b * 'c) t
  | Quad : 'a t * 'b t * 'c t * 'd t -> ('a * 'b * 'c * 'd) t
  | Quint : 'a t * 'b t * 'c t * 'd t * 'e t -> ('a * 'b * 'c * 'd * 'e) t
  | Guard : ('a -> bool) * 'a t -> 'a t
  | Map : ('a -> 'b) * ('b -> 'a) * 'b t -> 'a t
  | Switch :  ('a -> string * 'a inject_branch) *
              (string-> 'a extract_branch) -> 'a t
and _ inject_branch =
  | BranchTo : 'b t * 'b -> 'a inject_branch
and _ extract_branch =
  | BranchFrom : 'b t * ('b -> 'a) -> 'a extract_branch

(** {2 Bijection description} *)

val unit_ : unit t
val string_ : string t
val int_ : int t
val bool_ : bool t
val float_ : float t

val list_ : 'a t -> 'a list t
val many : 'a t -> 'a list t  (* non empty *)
val opt : 'a t -> 'a option t
val pair : 'a t -> 'b t -> ('a * 'b) t
val triple : 'a t -> 'b t -> 'c t -> ('a * 'b * 'c) t
val quad : 'a t -> 'b t -> 'c t -> 'd t -> ('a * 'b * 'c * 'd) t
val quint : 'a t -> 'b t -> 'c t -> 'd t -> 'e t -> ('a * 'b * 'c * 'd * 'e) t
val guard : ('a -> bool) -> 'a t -> 'a t
  (** Validate values at encoding and decoding *)

val map : inject:('a -> 'b) -> extract:('b -> 'a) -> 'b t -> 'a t

val switch : inject:('a -> string * 'a inject_branch) ->
             extract:(string -> 'a extract_branch) -> 'a t
  (** Discriminates unions based on the next character.
      [inject] must give a unique key for each branch, as well as mapping to another
      type (the argument of the algebraic constructor);
      [extract] retrieves which type to parse based on the key. *)

val fix : ('a t lazy_t -> 'a t) -> 'a t
  (** Helper for recursive encodings. The parameter is the recursive bijection
      itself. It must be lazy. *)

(** {2 Helpers} *)

val with_version : string -> 'a t -> 'a t
  (** Guards the values with a given version. Only values encoded with
      the same version will fit. *)

val array_ : 'a t -> 'a array t

val hashtbl : 'a t -> 'b t -> ('a, 'b) Hashtbl.t t

(** {2 Exceptions} *)

exception EncodingError of string
  (** Raised when encoding is impossible *)

exception DecodingError of string
  (** Raised when decoding is impossible *)