(* Title: HOL/UNITY/Reach.thy
ID: $Id$
Author: Lawrence C Paulson, Cambridge University Computer Laboratory
Copyright 1998 University of Cambridge
Reachability in Directed Graphs. From Chandy and Misra, section 6.4.
[this example took only four days!]
*)
theory Reach imports UNITY_Main begin
typedecl vertex
types state = "vertex=>bool"
consts
init :: "vertex"
edges :: "(vertex*vertex) set"
constdefs
asgt :: "[vertex,vertex] => (state*state) set"
"asgt u v == {(s,s'). s' = s(v:= s u | s v)}"
Rprg :: "state program"
"Rprg == mk_total_program ({%v. v=init}, \<Union>(u,v)\<in>edges. {asgt u v}, UNIV)"
reach_invariant :: "state set"
"reach_invariant == {s. (\<forall>v. s v --> (init, v) \<in> edges^*) & s init}"
fixedpoint :: "state set"
"fixedpoint == {s. \<forall>(u,v)\<in>edges. s u --> s v}"
metric :: "state => nat"
"metric s == card {v. ~ s v}"
text{**We assume that the set of vertices is finite*}
axioms
finite_graph: "finite (UNIV :: vertex set)"
(*TO SIMPDATA.ML?? FOR CLASET?? *)
lemma ifE [elim!]:
"[| if P then Q else R;
[| P; Q |] ==> S;
[| ~ P; R |] ==> S |] ==> S"
by (simp split: split_if_asm)
declare Rprg_def [THEN def_prg_Init, simp]
declare asgt_def [THEN def_act_simp,simp]
text{*All vertex sets are finite*}
declare finite_subset [OF subset_UNIV finite_graph, iff]
declare reach_invariant_def [THEN def_set_simp, simp]
lemma reach_invariant: "Rprg \<in> Always reach_invariant"
apply (rule AlwaysI, force)
apply (unfold Rprg_def, safety)
apply (blast intro: rtrancl_trans)
done
(*** Fixedpoint ***)
(*If it reaches a fixedpoint, it has found a solution*)
lemma fixedpoint_invariant_correct:
"fixedpoint \<inter> reach_invariant = { %v. (init, v) \<in> edges^* }"
apply (unfold fixedpoint_def)
apply (rule equalityI)
apply (auto intro!: ext)
apply (erule rtrancl_induct, auto)
done
lemma lemma1:
"FP Rprg \<subseteq> fixedpoint"
apply (simp add: FP_def fixedpoint_def Rprg_def mk_total_program_def)
apply (auto simp add: stable_def constrains_def)
apply (drule bspec, assumption)
apply (simp add: Image_singleton image_iff)
apply (drule fun_cong, auto)
done
lemma lemma2:
"fixedpoint \<subseteq> FP Rprg"
apply (simp add: FP_def fixedpoint_def Rprg_def mk_total_program_def)
apply (auto intro!: ext simp add: stable_def constrains_def)
done
lemma FP_fixedpoint: "FP Rprg = fixedpoint"
by (rule equalityI [OF lemma1 lemma2])
(*If we haven't reached a fixedpoint then there is some edge for which u but
not v holds. Progress will be proved via an ENSURES assertion that the
metric will decrease for each suitable edge. A union over all edges proves
a LEADSTO assertion that the metric decreases if we are not at a fixedpoint.
*)
lemma Compl_fixedpoint: "- fixedpoint = (\<Union>(u,v)\<in>edges. {s. s u & ~ s v})"
apply (simp add: FP_fixedpoint [symmetric] Rprg_def mk_total_program_def)
apply (auto simp add: Compl_FP UN_UN_flatten)
apply (rule fun_upd_idem, force)
apply (force intro!: rev_bexI simp add: fun_upd_idem_iff)
done
lemma Diff_fixedpoint:
"A - fixedpoint = (\<Union>(u,v)\<in>edges. A \<inter> {s. s u & ~ s v})"
by (simp add: Diff_eq Compl_fixedpoint, blast)
(*** Progress ***)
lemma Suc_metric: "~ s x ==> Suc (metric (s(x:=True))) = metric s"
apply (unfold metric_def)
apply (subgoal_tac "{v. ~ (s (x:=True)) v} = {v. ~ s v} - {x}")
prefer 2 apply force
apply (simp add: card_Suc_Diff1)
done
lemma metric_less [intro!]: "~ s x ==> metric (s(x:=True)) < metric s"
by (erule Suc_metric [THEN subst], blast)
lemma metric_le: "metric (s(y:=s x | s y)) \<le> metric s"
apply (case_tac "s x --> s y")
apply (auto intro: less_imp_le simp add: fun_upd_idem)
done
lemma LeadsTo_Diff_fixedpoint:
"Rprg \<in> ((metric-`{m}) - fixedpoint) LeadsTo (metric-`(lessThan m))"
apply (simp (no_asm) add: Diff_fixedpoint Rprg_def)
apply (rule LeadsTo_UN, auto)
apply (ensures_tac "asgt a b")
prefer 2 apply blast
apply (simp (no_asm_use) add: linorder_not_less)
apply (drule metric_le [THEN order_antisym])
apply (auto elim: less_not_refl3 [THEN [2] rev_notE])
done
lemma LeadsTo_Un_fixedpoint:
"Rprg \<in> (metric-`{m}) LeadsTo (metric-`(lessThan m) \<union> fixedpoint)"
apply (rule LeadsTo_Diff [OF LeadsTo_Diff_fixedpoint [THEN LeadsTo_weaken_R]
subset_imp_LeadsTo], auto)
done
(*Execution in any state leads to a fixedpoint (i.e. can terminate)*)
lemma LeadsTo_fixedpoint: "Rprg \<in> UNIV LeadsTo fixedpoint"
apply (rule LessThan_induct, auto)
apply (rule LeadsTo_Un_fixedpoint)
done
lemma LeadsTo_correct: "Rprg \<in> UNIV LeadsTo { %v. (init, v) \<in> edges^* }"
apply (subst fixedpoint_invariant_correct [symmetric])
apply (rule Always_LeadsTo_weaken [OF reach_invariant LeadsTo_fixedpoint], auto)
done
end