(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
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(* This file develops the theory of finite graphs represented by an "edge"    *)
(* relation over a finType T; this mainly amounts to the theory of the        *)
(* transitive closure of such relations.                                      *)
(*   For g : T -> seq T, e : rel T and f : T -> T we define:                  *)
(*         grel g == the adjacency relation y \in g x of the graph g.         *)
(*       rgraph e == the graph (x |-> enum (e x)) of the relation e.          *)
(*    dfs g n v x == the list of points traversed by a depth-first search of  *)
(*                   the g, at depth n, starting from x, and avoiding v.      *)
(* dfs_path g v x y <-> there is a path from x to y in g \ v.                 *)
(*      connect e == the transitive closure of e (computed by dfs).           *)
(*  connect_sym e <-> connect e is symmetric, hence an equivalence relation.  *)
(*       root e x == a representative of connect e x, which is the component  *)
(*                   of x in the transitive closure of e.                     *)
(*        roots e == the codomain predicate of root e.                        *)
(*     n_comp e a == the number of e-connected components of a, when a is     *)
(*                   e-closed and connect e is symmetric.                     *)
(*                   equivalence classes of connect e if connect_sym e holds. *)
(*     closed e a == the collective predicate a is e-invariant.               *)
(*    closure e a == the e-closure of a (the image of a under connect e).     *)
(* rel_adjunction h e e' a <-> in the e-closed domain a, h is the left part   *)
(*                   of an adjunction from e to another relation e'.          *)
(*     fconnect f == connect (frel f), i.e., "connected under f iteration".   *)
(*      froot f x == root (frel f) x, the root of the orbit of x under f.     *)
(*       froots f == roots (frel f) == orbit representatives for f.           *)
(*      orbit f x == lists the f-orbit of x.                                  *)
(*   findex f x y == index of y in the f-orbit of x.                          *)
(*      order f x == size (cardinal) of the f-orbit of x.                     *)
(*  order_set f n == elements of f-order n.                                   *)
(*         finv f == the inverse of f, if f is injective.                     *)
(*                := finv f x := iter (order x).-1 f x.                       *)
(*      fcard f a == number of orbits of f in a, provided a is f-invariant    *)
(*                   f is one-to-one.                                         *)
(*    fclosed f a == the collective predicate a is f-invariant.               *)
(*   fclosure f a == the closure of a under f iteration.                      *)
(* fun_adjunction == rel_adjunction (frel f).                                 *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Definition grel (T : eqType) (g : T -> seq T) := [rel x y | y \in g x].
(* Decidable connectivity in finite types.                                  *)
Section Connect.
Variable T : finType.
Section Dfs.
Variable g : T -> seq T.
Implicit Type v w a : seq T.
Fixpoint dfs n v x :=
if x \in v then v else
if n is n'.+1 then foldl (dfs n') (x :: v) (g x) else v.

Lemma dfsP x y :
reflect (exists2 p, path (grel g) x p & y = last x p) (y \in dfs #|T| [::] x).