Theory Transitive_Closure_Table

theory Transitive_Closure_Table
imports Main
(* Author: Stefan Berghofer, Lukas Bulwahn, TU Muenchen *)

header {* A tabled implementation of the reflexive transitive closure *}

theory Transitive_Closure_Table
imports Main
begin

inductive rtrancl_path :: "('a => 'a => bool) => 'a => 'a list => 'a => bool"
for r :: "'a => 'a => bool"
where
base: "rtrancl_path r x [] x"
| step: "r x y ==> rtrancl_path r y ys z ==> rtrancl_path r x (y # ys) z"

lemma rtranclp_eq_rtrancl_path: "r** x y = (∃xs. rtrancl_path r x xs y)"
proof
assume "r** x y"
then show "∃xs. rtrancl_path r x xs y"
proof (induct rule: converse_rtranclp_induct)
case base
have "rtrancl_path r y [] y" by (rule rtrancl_path.base)
then show ?case ..
next
case (step x z)
from `∃xs. rtrancl_path r z xs y`
obtain xs where "rtrancl_path r z xs y" ..
with `r x z` have "rtrancl_path r x (z # xs) y"
by (rule rtrancl_path.step)
then show ?case ..
qed
next
assume "∃xs. rtrancl_path r x xs y"
then obtain xs where "rtrancl_path r x xs y" ..
then show "r** x y"
proof induct
case (base x)
show ?case by (rule rtranclp.rtrancl_refl)
next
case (step x y ys z)
from `r x y` `r** y z` show ?case
by (rule converse_rtranclp_into_rtranclp)
qed
qed

lemma rtrancl_path_trans:
assumes xy: "rtrancl_path r x xs y"
and yz: "rtrancl_path r y ys z"
shows "rtrancl_path r x (xs @ ys) z" using xy yz
proof (induct arbitrary: z)
case (base x)
then show ?case by simp
next
case (step x y xs)
then have "rtrancl_path r y (xs @ ys) z"
by simp
with `r x y` have "rtrancl_path r x (y # (xs @ ys)) z"
by (rule rtrancl_path.step)
then show ?case by simp
qed

lemma rtrancl_path_appendE:
assumes xz: "rtrancl_path r x (xs @ y # ys) z"
obtains "rtrancl_path r x (xs @ [y]) y" and "rtrancl_path r y ys z" using xz
proof (induct xs arbitrary: x)
case Nil
then have "rtrancl_path r x (y # ys) z" by simp
then obtain xy: "r x y" and yz: "rtrancl_path r y ys z"
by cases auto
from xy have "rtrancl_path r x [y] y"
by (rule rtrancl_path.step [OF _ rtrancl_path.base])
then have "rtrancl_path r x ([] @ [y]) y" by simp
then show ?thesis using yz by (rule Nil)
next
case (Cons a as)
then have "rtrancl_path r x (a # (as @ y # ys)) z" by simp
then obtain xa: "r x a" and az: "rtrancl_path r a (as @ y # ys) z"
by cases auto
show ?thesis
proof (rule Cons(1) [OF _ az])
assume "rtrancl_path r y ys z"
assume "rtrancl_path r a (as @ [y]) y"
with xa have "rtrancl_path r x (a # (as @ [y])) y"
by (rule rtrancl_path.step)
then have "rtrancl_path r x ((a # as) @ [y]) y"
by simp
then show ?thesis using `rtrancl_path r y ys z`
by (rule Cons(2))
qed
qed

lemma rtrancl_path_distinct:
assumes xy: "rtrancl_path r x xs y"
obtains xs' where "rtrancl_path r x xs' y" and "distinct (x # xs')" using xy
proof (induct xs rule: measure_induct_rule [of length])
case (less xs)
show ?case
proof (cases "distinct (x # xs)")
case True
with `rtrancl_path r x xs y` show ?thesis by (rule less)
next
case False
then have "∃as bs cs a. x # xs = as @ [a] @ bs @ [a] @ cs"
by (rule not_distinct_decomp)
then obtain as bs cs a where xxs: "x # xs = as @ [a] @ bs @ [a] @ cs"
by iprover
show ?thesis
proof (cases as)
case Nil
with xxs have x: "x = a" and xs: "xs = bs @ a # cs"
by auto
from x xs `rtrancl_path r x xs y` have cs: "rtrancl_path r x cs y"
by (auto elim: rtrancl_path_appendE)
from xs have "length cs < length xs" by simp
then show ?thesis
by (rule less(1)) (iprover intro: cs less(2))+
next
case (Cons d ds)
with xxs have xs: "xs = ds @ a # (bs @ [a] @ cs)"
by auto
with `rtrancl_path r x xs y` obtain xa: "rtrancl_path r x (ds @ [a]) a"
and ay: "rtrancl_path r a (bs @ a # cs) y"
by (auto elim: rtrancl_path_appendE)
from ay have "rtrancl_path r a cs y" by (auto elim: rtrancl_path_appendE)
with xa have xy: "rtrancl_path r x ((ds @ [a]) @ cs) y"
by (rule rtrancl_path_trans)
from xs have "length ((ds @ [a]) @ cs) < length xs" by simp
then show ?thesis
by (rule less(1)) (iprover intro: xy less(2))+
qed
qed
qed

inductive rtrancl_tab :: "('a => 'a => bool) => 'a list => 'a => 'a => bool"
for r :: "'a => 'a => bool"
where
base: "rtrancl_tab r xs x x"
| step: "x ∉ set xs ==> r x y ==> rtrancl_tab r (x # xs) y z ==> rtrancl_tab r xs x z"

lemma rtrancl_path_imp_rtrancl_tab:
assumes path: "rtrancl_path r x xs y"
and x: "distinct (x # xs)"
and ys: "({x} ∪ set xs) ∩ set ys = {}"
shows "rtrancl_tab r ys x y" using path x ys
proof (induct arbitrary: ys)
case base
show ?case by (rule rtrancl_tab.base)
next
case (step x y zs z)
then have "x ∉ set ys" by auto
from step have "distinct (y # zs)" by simp
moreover from step have "({y} ∪ set zs) ∩ set (x # ys) = {}"
by auto
ultimately have "rtrancl_tab r (x # ys) y z"
by (rule step)
with `x ∉ set ys` `r x y`
show ?case by (rule rtrancl_tab.step)
qed

lemma rtrancl_tab_imp_rtrancl_path:
assumes tab: "rtrancl_tab r ys x y"
obtains xs where "rtrancl_path r x xs y" using tab
proof induct
case base
from rtrancl_path.base show ?case by (rule base)
next
case step show ?case by (iprover intro: step rtrancl_path.step)
qed

lemma rtranclp_eq_rtrancl_tab_nil: "r** x y = rtrancl_tab r [] x y"
proof
assume "r** x y"
then obtain xs where "rtrancl_path r x xs y"
by (auto simp add: rtranclp_eq_rtrancl_path)
then obtain xs' where xs': "rtrancl_path r x xs' y"
and distinct: "distinct (x # xs')"
by (rule rtrancl_path_distinct)
have "({x} ∪ set xs') ∩ set [] = {}" by simp
with xs' distinct show "rtrancl_tab r [] x y"
by (rule rtrancl_path_imp_rtrancl_tab)
next
assume "rtrancl_tab r [] x y"
then obtain xs where "rtrancl_path r x xs y"
by (rule rtrancl_tab_imp_rtrancl_path)
then show "r** x y"
by (auto simp add: rtranclp_eq_rtrancl_path)
qed

declare rtranclp_rtrancl_eq[code del]
declare rtranclp_eq_rtrancl_tab_nil[THEN iffD2, code_pred_intro]

code_pred rtranclp using rtranclp_eq_rtrancl_tab_nil [THEN iffD1] by fastforce

subsection {* A simple example *}

datatype ty = A | B | C

inductive test :: "ty => ty => bool"
where
"test A B"
| "test B A"
| "test B C"

subsubsection {* Invoking with the predicate compiler and the generic code generator *}

code_pred test .

values "{x. test** A C}"
values "{x. test** C A}"
values "{x. test** A x}"
values "{x. test** x C}"

value "test** A C"
value "test** C A"

hide_type ty
hide_const test A B C

end