Source file path.ml

(**************************************************************************)
(*                                                                        *)
(*  Ocamlgraph: a generic graph library for OCaml                         *)
(*  Copyright (C) 2004-2010                                               *)
(*  Sylvain Conchon, Jean-Christophe Filliatre and Julien Signoles        *)
(*                                                                        *)
(*  This software is free software; you can redistribute it and/or        *)
(*  modify it under the terms of the GNU Library General Public           *)
(*  License version 2.1, with the special exception on linking            *)
(*  described in file LICENSE.                                            *)
(*                                                                        *)
(*  This software is distributed in the hope that it will be useful,      *)
(*  but WITHOUT ANY WARRANTY; without even the implied warranty of        *)
(*  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.                  *)
(*                                                                        *)
(**************************************************************************)

(* $Id: path.ml,v 1.6 2005-07-18 07:10:35 filliatr Exp $ *)

module type G = sig
  type t
  module V : Sig.COMPARABLE
  module E : sig
    type t
    type label
    val label : t -> label
    val src : t -> V.t
    val dst : t -> V.t
    val create : V.t -> label -> V.t -> t
  end
  val iter_vertex : (V.t -> unit) -> t -> unit
  val fold_vertex : (V.t -> 'a -> 'a) -> t  -> 'a -> 'a
  val iter_succ : (V.t -> unit) -> t -> V.t -> unit
  val iter_succ_e : (E.t -> unit) -> t -> V.t -> unit
  val fold_edges_e : (E.t -> 'a -> 'a) -> t -> 'a -> 'a
  val nb_vertex : t -> int
end

(** Weight signature for Johnson's algorithm. *)
module type WJ = sig
  include Sig.WEIGHT
  val sub : t -> t -> t
  (** Subtraction of weights. *)
end

module Dijkstra
    (G: G)
    (W: Sig.WEIGHT with type edge = G.E.t) =
struct

  open G.E

  module H =  Hashtbl.Make(G.V)

  module Elt = struct
    type t = W.t * G.V.t * G.E.t list

    (* weights are compared first, and minimal weights come first in the
       queue *)
    let compare (w1,v1,_) (w2,v2,_) =
      let cw = W.compare w2 w1 in
      if cw != 0 then cw else G.V.compare v1 v2
  end

  module PQ = Heap.Imperative(Elt)

  let shortest_path g v1 v2 =
    let visited = H.create 97 in
    let dist = H.create 97 in
    let q = PQ.create 17 in
    let rec loop () =
      if PQ.is_empty q then raise Not_found;
      let (w,v,p) = PQ.pop_maximum q in
      if G.V.compare v v2 = 0 then
        List.rev p, w
      else begin
        if not (H.mem visited v) then begin
          H.add visited v ();
          G.iter_succ_e
            (fun e ->
               let ev = dst e in
               if not (H.mem visited ev) then begin
                 let dev = W.add w (W.weight e) in
                 let improvement =
                   try W.compare dev (H.find dist ev) < 0 with Not_found -> true
                 in
                 if improvement then begin
                   H.replace dist ev dev;
                   PQ.add q (dev, ev, e :: p)
                 end
               end)
            g v
        end;
        loop ()
      end
    in
    PQ.add q (W.zero, v1, []);
    H.add dist v1 W.zero;
    loop ()

end

(* The following module is a contribution of Yuto Takei (University of Tokyo) *)

module BellmanFord
    (G: G)
    (W: Sig.WEIGHT with type edge = G.E.t) =
struct

  open G.E

  module H = Hashtbl.Make(G.V)

  exception NegativeCycle of G.E.t list

  let all_shortest_paths g vs =
    let dist = H.create 97 in
    H.add dist vs W.zero;
    let admissible = H.create 97 in
    let build_cycle_from x0 =
      let rec traverse_parent x ret =
        let e = H.find admissible x in
        let s = src e in
        if G.V.equal s x0 then e :: ret else traverse_parent s (e :: ret)
      in
      traverse_parent x0 []
    in
    let find_cycle x0 =
      let visited = H.create 97 in
      let rec visit x =
        if H.mem visited x then
          build_cycle_from x
        else begin
          H.add visited x ();
          let e = H.find admissible x in
          visit (src e)
        end
      in
      visit x0
    in
    let rec relax i =
      let update = G.fold_edges_e
          (fun e x ->
             let ev1 = src e in
             let ev2 = dst e in
             try begin
               let dev1 = H.find dist ev1 in
               let dev2 = W.add dev1 (W.weight e) in
               let improvement =
                 try W.compare dev2 (H.find dist ev2) < 0
                 with Not_found -> true
               in
               if improvement then begin
                 H.replace dist ev2 dev2;
                 H.replace admissible ev2 e;
                 Some ev2
               end else x
             end with Not_found -> x) g None in
      match update with
      | Some x ->
        if i == G.nb_vertex g then raise (NegativeCycle (find_cycle x))
        else relax (i + 1)
      | None -> dist
    in
    relax 0

  let find_negative_cycle_from g vs =
    try let _ = all_shortest_paths g vs in raise Not_found
    with NegativeCycle l -> l


  module Comp = Components.Make(G)

  (* This is rather inefficient implementation. Indeed, for each
     strongly connected component, we run a full Bellman-Ford
     algorithm using one of its vertex as source, taking all edges
     into consideration.  Instead, we could limit ourselves to the
     edges of the component. *)
  let find_negative_cycle g =
    let rec iter = function
      | [] ->
        raise Not_found
      | (x :: _) :: cl ->
        begin try find_negative_cycle_from g x with Not_found -> iter cl end
      | [] :: _ ->
        assert false (* a component is not empty *)
    in
    iter (Comp.scc_list g)

end

module Johnson
    (G: G)
    (W: WJ with type edge = G.E.t) =
struct

  module HVV = Hashtbl.Make(Util.HTProduct(G.V)(G.V))

  module G' = struct
    type t = G.t
    module V = struct
      type t = New | Old of G.V.t
      let compare v u = match v, u with
        | New, New -> 0
        | New, Old _ -> -1
        | Old _, New -> 1
        | Old v, Old u -> G.V.compare v u
      let hash v = match v with
        | Old v -> G.V.hash v
        | New -> 42
      let equal v u = match v, u with
        | New, New -> true
        | New, Old _ | Old _, New -> false
        | Old v, Old u -> G.V.equal v u
    end
    module E = struct
      type label = G.E.label
      type t = NewE of V.t | OldE of G.E.t
      let src e = match e with
        | NewE _ -> V.New
        | OldE e -> V.Old (G.E.src e)
      let dst e = match e with
        | NewE v -> v
        | OldE e -> V.Old (G.E.dst e)
      let label e = match e with
        | NewE _ -> assert false
        | OldE e -> G.E.label e
      let create v l u = match v, u with
        | V.New, V.Old u -> NewE (V.Old u)
        | V.Old v, V.Old u -> OldE (G.E.create v l u)
        | _, _ -> assert false
    end
    let iter_vertex f g = f V.New; G.iter_vertex (fun v -> f (V.Old v)) g
    let fold_vertex f g acc =
      let acc' = f V.New acc in
      G.fold_vertex (fun v a -> f (V.Old v) a) g acc'
    let iter_succ f g v = match v with
      | V.New -> G.iter_vertex (fun u -> f (V.Old u)) g
      | V.Old v -> G.iter_succ (fun u -> f (V.Old u)) g v
    let iter_succ_e f g v = match v with
      | V.New ->
        G.iter_vertex (fun u -> f (E.NewE (V.Old u))) g
      | V.Old v -> G.iter_succ_e (fun e -> f (E.OldE e)) g v
    let fold_edges_e f g acc =
      let acc' =
        G.fold_vertex (fun x _ -> f (E.NewE (V.Old x)) acc) g acc
      in
      G.fold_edges_e (fun edg ->
          let v1 = G.E.src edg in
          let v2 = G.E.dst edg in
          let l = G.E.label edg in
          f (E.create (V.Old v1) l (V.Old v2))) g acc'
    let nb_vertex g = G.nb_vertex g + 1
  end

  module W' = struct
    open G'.E

    type edge = G'.E.t
    type t = W.t
    let zero = W.zero
    let weight e = match e with
      | NewE _ -> zero
      | OldE e -> W.weight e
    let compare = W.compare
    let add = W.add
  end

  module BF = BellmanFord(G')(W')

  let all_pairs_shortest_paths g =
    let pairs_dist = HVV.create 97 in
    let bf_res = BF.all_shortest_paths g G'.V.New in
    let module W'' = struct
      type edge = W.edge
      type t = W.t
      let add = W.add
      let sub = W.sub
      let weight e =
        let v1 = G.E.src e in
        let v2 = G.E.dst e in
        add (W.weight e)
          (W.sub (BF.H.find bf_res (G'.V.Old v1))
             (BF.H.find bf_res (G'.V.Old v2)))
      let compare = W.compare
      let zero = W.zero
    end
    in
    let module D = Dijkstra(G)(W'') in
    G.iter_vertex
      (fun v ->
         G.iter_vertex
           (fun u ->
              try
                let (_, d) = D.shortest_path g v u in
                HVV.add pairs_dist (v, u)
                  (W''.add d
                     (W''.sub (BF.H.find bf_res (G'.V.Old u))
                        (BF.H.find bf_res (G'.V.Old v))
                     ))
              with Not_found -> () ) g) g;
    pairs_dist

end

module Check
    (G :
     sig
       type t
       module V : Sig.COMPARABLE
       val iter_succ : (V.t -> unit) -> t -> V.t -> unit
     end) =
struct

  module HV = Hashtbl.Make(G.V)
  module HVV = Hashtbl.Make(Util.HTProduct(G.V)(G.V))

  (* the cache contains the path tests already computed *)
  type path_checker = { cache : bool HVV.t; graph : G.t }

  let create g = { cache = HVV.create 97; graph = g }

  let check_path pc v1 v2 =
    try
      HVV.find pc.cache (v1, v2)
    with Not_found ->
      (* the path is not in cache; we check it with a BFS *)
      let visited = HV.create 97 in
      let q = Queue.create () in
      (* [visited] contains exactly the vertices that have been added to [q] *)
      let push v =
        if not (HV.mem visited v) then (HV.add visited v (); Queue.add v q) in
      let rec loop () =
        if Queue.is_empty q then begin
          HVV.add pc.cache (v1, v2) false;
          false
        end else begin
          let v = Queue.pop q in
          HVV.add pc.cache (v1, v) true;
          G.V.compare v v2 = 0 ||
          match HVV.find_opt pc.cache (v, v2) with
          | Some true -> HVV.add pc.cache (v1, v2) true; true
          | _ -> G.iter_succ push pc.graph v; loop ()
        end
      in
      push v1;
      loop ()

end

(** 0-1 BFS

    When edge weights are limited to 0 or 1, this is more efficient than
    running Dijkstra's algorithm. *)

module Bfs01(G: sig
  type t
  module V: Sig.COMPARABLE
  module E: sig type t val dst : t -> V.t end
  val iter_succ_e : (E.t -> unit) -> t -> V.t -> unit
end) = struct

  module H = Hashtbl.Make(G.V)

  let iter f g ~zero s =
    let visited = H.create 16 in
    let d = Deque.create () in
    Deque.push_front d (s, 0); H.add visited s ();
    while Deque.length d > 0 do
      let v, n = Deque.pop_front d in
      f v n;
      G.iter_succ_e (fun e ->
          let w = G.E.dst e in
          if not (H.mem visited w) then (
            H.add visited w ();
            if zero e then Deque.push_front d (w, n  )
                      else Deque.push_back  d (w, n+1)
          )
        ) g v
    done

end