(*
Copyright (c) 2013, Simon Cruanes
All rights reserved.

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modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this
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*)

(** {2 Hypergraph Representation}

Generalized Hypergraphs. Objects are either constants, or hyperedges that
connect [n] other objets together (a [n]-tuple). Each hyperedge can contain
additional data.
*)

module type S = sig
  type const
    (** Constants. Those are what can annotate hyperedges or make single,
        leaf, nodes. *)

  type t
    (** An hypergraph. It stores a set of edges, and possibly inherits from
        another graph. *)

  type edge
    (** A single edge of the hypergraph. *)

  val self : t -> edge
    (** The edge that represents (reifies) the hypergraph itself *)

  val eq : edge -> edge -> bool
    (** Equality of the two edges. *)

  val arity : edge -> int
    (** Number of sub-elements of the edge (how many other edges it connects
        together) *)

  val nth : edge -> int -> edge
    (** [nth x i] accesses the [i]-th sub-node of [x].
        @raise Invalid_argument if [i >= arity x]. *)

  val make_graph : ?parent:t -> unit -> t
    (** New graph, possibly inheriting from another graph. *)

  val make_edge : t -> edge array -> edge
    (** Create a new hyperedge from an ordered tuple of sub-edges.
        The edge belongs to the given graph.
        The array must not be used afterwards and must not be empty.
        @raise Invalid_argument if the array is empty *)

  val make_const : t -> const -> edge
    (** Constant edge, without sub-edges *)

  val fresh : t -> edge
    (** Fresh edge, without constant. It is equal to no other edge. *)

  module EdgeTbl : Hashtbl.S with type key = edge

  val pp : ?printed:unit EdgeTbl.t ->
           Buffer.t -> edge -> unit
    (** Print the edge on the buffer. @param printed: sub-edges already
        printed. *)
end

module type PARAM = sig
  type const

  val eq : const -> const -> bool
  val hash : const -> int
  val to_string : const -> string  (* for printing *)
end

module Make(P : PARAM) = struct
  type const = P.const

  type edge =
    | Fresh of int
    | Const of const
    | Edge of edge array

  let rec eq e1 e2 = match e1, e2 with
    | Fresh _, Fresh _ -> e1 == e2
    | Const c1, Const c2 -> P.eq c1 c2
    | Edge a1, Edge a2 ->
      Array.length a1 = Array.length a2 &&
      begin try
        for i = 0 to Array.length a1 - 1 do
          if not (eq (Array.unsafe_get a1 i) (Array.unsafe_get a2 i))
            then raise Exit;
        done; true
      with Exit -> false
      end
    | _ -> false

  let rec hash e = match e with
    | Fresh i -> i
    | Const c -> P.hash c
    | Edge a ->
      let h = ref 0 in
      for i = 0 to Array.length a - 1 do
        h := max_int land (!h * 65599 + (hash (Array.unsafe_get a i)))
      done;
      !h

  (* hashtable on edges *)
  module EdgeTbl = Hashtbl.Make(struct
    type t = edge
    let equal = eq
    let hash = hash
  end)

  (* hashtable on edges * int *)
  module BackTbl = Hashtbl.Make(struct
    type t = edge * int
    let equal (e1, i1) (e2, i2) = i1 = i2 && eq e1 e2
    let hash (e, i) = i * 65599 + hash e
  end)

  (** Hypergraph: set of edges. We map each edge to other edges that point
      to it (knowing which ones it points to is trivial) *)
  type t = {
    edges : unit EdgeTbl.t;
    backref : edge BackTbl.t; 
    parent : t option;
    mutable count : int;  (* used for Fresh nodes *)
    self : edge;
  }

  let arity e = match e with
    | Fresh _
    | Const _ -> 0
    | Edge a -> Array.length a

  let nth e i = match e with
    | Fresh _
    | Const _ -> raise (Invalid_argument"HGraph.nth")
    | Edge a -> a.(i)

  let self g = g.self

  let make_graph ?parent () =
    let g = { 
      parent;
      edges = EdgeTbl.create 15;
      backref = BackTbl.create 15;
      count = 1;
      self = Fresh 0;
    } in
    g

  (* add a backref from [e]'s sub-edges to [e] *)
  let _add_backrefs g e = match e with
    | Fresh _
    | Const _ -> assert false
    | Edge a ->
      for i = 0 to Array.length a - 1 do
        BackTbl.add g.backref (Array.unsafe_get a i, i) e
      done

  let make_edge g sub =
    if Array.length sub = 0 then raise (Invalid_argument "HGraph.make_edge");
    let e = Edge sub in
    (* add edge if not already present *)
    if not (EdgeTbl.mem g.edges e) then begin
      EdgeTbl.add g.edges e ();
      _add_backrefs g e
    end;
    e

  let make_const g c =
    let e = Const c in
    if not (EdgeTbl.mem g.edges e) then
      EdgeTbl.add g.edges e ();
    e

  let fresh g =
    let e = Fresh g.count in
    g.count <- g.count + 1;
    (* always new! *)
    EdgeTbl.add g.edges e ();
    e

  let pp ?(printed=EdgeTbl.create 7) buf e =
    let rec pp buf e = match e with
    | Fresh i -> Printf.bprintf buf "_e%d" i
    | Const c -> Buffer.add_string buf (P.to_string c)
    | Edge a ->
      if not (EdgeTbl.mem printed e) then begin
        EdgeTbl.add printed e ();
        Buffer.add_char buf '[';
        for i = 0 to Array.length a - 1 do
          if i > 0 then Buffer.add_char buf ' ';
          pp buf a.(i)
        done
      end
    in
    pp buf e
end

(** {2 Useful default} *)

module DefaultParam = struct
  type const =
    | S of string
    | I of int

  type data = unit

  let eq c1 c2 = match c1, c2 with
    | S s1, S s2 -> s1 = s2
    | I i1, I i2 -> i1 = i2
    | _ -> false

  let hash = function
    | S s -> Hashtbl.hash s
    | I i -> i

  let to_string = function
    | S s -> s
    | I i -> string_of_int i

  let i i = I i
  let s s = S s
end

module Default = Make(DefaultParam)