src/HOL/Library/Permutation.thy
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(*  Title:      HOL/Library/Permutation.thy
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    Author:     Lawrence C Paulson and Thomas M Rasmussen and Norbert Voelker
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*)
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header {* Permutations *}
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theory Permutation
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imports Multiset
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begin
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consts
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  perm :: "('a list * 'a list) set"
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abbreviation
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  perm_rel :: "'a list => 'a list => bool"    ("_ <~~> _"  [50, 50] 50)
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  "x <~~> y == (x, y) \<in> perm"
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inductive perm
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  intros
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    Nil  [intro!]: "[] <~~> []"
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    swap [intro!]: "y # x # l <~~> x # y # l"
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    Cons [intro!]: "xs <~~> ys ==> z # xs <~~> z # ys"
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    trans [intro]: "xs <~~> ys ==> ys <~~> zs ==> xs <~~> zs"
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lemma perm_refl [iff]: "l <~~> l"
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  by (induct l) auto
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subsection {* Some examples of rule induction on permutations *}
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lemma xperm_empty_imp_aux: "xs <~~> ys ==> xs = [] --> ys = []"
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    -- {*the form of the premise lets the induction bind @{term xs}
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         and @{term ys} *}
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  apply (erule perm.induct)
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     apply (simp_all (no_asm_simp))
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  done
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lemma xperm_empty_imp: "[] <~~> ys ==> ys = []"
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  using xperm_empty_imp_aux by blast
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text {*
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  \medskip This more general theorem is easier to understand!
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  *}
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lemma perm_length: "xs <~~> ys ==> length xs = length ys"
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  by (erule perm.induct) simp_all
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lemma perm_empty_imp: "[] <~~> xs ==> xs = []"
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  by (drule perm_length) auto
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lemma perm_sym: "xs <~~> ys ==> ys <~~> xs"
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  by (erule perm.induct) auto
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lemma perm_mem [rule_format]: "xs <~~> ys ==> x mem xs --> x mem ys"
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  by (erule perm.induct) auto
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subsection {* Ways of making new permutations *}
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text {*
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  We can insert the head anywhere in the list.
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*}
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lemma perm_append_Cons: "a # xs @ ys <~~> xs @ a # ys"
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  by (induct xs) auto
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lemma perm_append_swap: "xs @ ys <~~> ys @ xs"
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  apply (induct xs)
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    apply simp_all
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  apply (blast intro: perm_append_Cons)
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  done
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lemma perm_append_single: "a # xs <~~> xs @ [a]"
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  by (rule perm.trans [OF _ perm_append_swap]) simp
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lemma perm_rev: "rev xs <~~> xs"
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  apply (induct xs)
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   apply simp_all
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  apply (blast intro!: perm_append_single intro: perm_sym)
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  done
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lemma perm_append1: "xs <~~> ys ==> l @ xs <~~> l @ ys"
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  by (induct l) auto
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lemma perm_append2: "xs <~~> ys ==> xs @ l <~~> ys @ l"
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  by (blast intro!: perm_append_swap perm_append1)
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subsection {* Further results *}
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lemma perm_empty [iff]: "([] <~~> xs) = (xs = [])"
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  by (blast intro: perm_empty_imp)
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lemma perm_empty2 [iff]: "(xs <~~> []) = (xs = [])"
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  apply auto
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  apply (erule perm_sym [THEN perm_empty_imp])
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  done
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lemma perm_sing_imp [rule_format]: "ys <~~> xs ==> xs = [y] --> ys = [y]"
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  by (erule perm.induct) auto
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lemma perm_sing_eq [iff]: "(ys <~~> [y]) = (ys = [y])"
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  by (blast intro: perm_sing_imp)
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lemma perm_sing_eq2 [iff]: "([y] <~~> ys) = (ys = [y])"
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  by (blast dest: perm_sym)
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subsection {* Removing elements *}
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consts
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  remove :: "'a => 'a list => 'a list"
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primrec
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  "remove x [] = []"
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  "remove x (y # ys) = (if x = y then ys else y # remove x ys)"
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lemma perm_remove: "x \<in> set ys ==> ys <~~> x # remove x ys"
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  by (induct ys) auto
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lemma remove_commute: "remove x (remove y l) = remove y (remove x l)"
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  by (induct l) auto
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lemma multiset_of_remove[simp]:
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    "multiset_of (remove a x) = multiset_of x - {#a#}"
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  apply (induct x)
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   apply (auto simp: multiset_eq_conv_count_eq)
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  done
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text {* \medskip Congruence rule *}
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lemma perm_remove_perm: "xs <~~> ys ==> remove z xs <~~> remove z ys"
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  by (erule perm.induct) auto
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lemma remove_hd [simp]: "remove z (z # xs) = xs"
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  by auto
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lemma cons_perm_imp_perm: "z # xs <~~> z # ys ==> xs <~~> ys"
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  by (drule_tac z = z in perm_remove_perm) auto
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lemma cons_perm_eq [iff]: "(z#xs <~~> z#ys) = (xs <~~> ys)"
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  by (blast intro: cons_perm_imp_perm)
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lemma append_perm_imp_perm: "!!xs ys. zs @ xs <~~> zs @ ys ==> xs <~~> ys"
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  apply (induct zs rule: rev_induct)
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   apply (simp_all (no_asm_use))
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  apply blast
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  done
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lemma perm_append1_eq [iff]: "(zs @ xs <~~> zs @ ys) = (xs <~~> ys)"
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  by (blast intro: append_perm_imp_perm perm_append1)
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lemma perm_append2_eq [iff]: "(xs @ zs <~~> ys @ zs) = (xs <~~> ys)"
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  apply (safe intro!: perm_append2)
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  apply (rule append_perm_imp_perm)
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  apply (rule perm_append_swap [THEN perm.trans])
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    -- {* the previous step helps this @{text blast} call succeed quickly *}
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  apply (blast intro: perm_append_swap)
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  done
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lemma multiset_of_eq_perm: "(multiset_of xs = multiset_of ys) = (xs <~~> ys) "
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  apply (rule iffI)
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  apply (erule_tac [2] perm.induct, simp_all add: union_ac)
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  apply (erule rev_mp, rule_tac x=ys in spec)
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  apply (induct_tac xs, auto)
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  apply (erule_tac x = "remove a x" in allE, drule sym, simp)
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  apply (subgoal_tac "a \<in> set x")
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  apply (drule_tac z=a in perm.Cons)
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  apply (erule perm.trans, rule perm_sym, erule perm_remove)
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  apply (drule_tac f=set_of in arg_cong, simp)
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  done
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lemma multiset_of_le_perm_append:
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    "(multiset_of xs \<le># multiset_of ys) = (\<exists>zs. xs @ zs <~~> ys)";
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  apply (auto simp: multiset_of_eq_perm[THEN sym] mset_le_exists_conv)
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  apply (insert surj_multiset_of, drule surjD)
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  apply (blast intro: sym)+
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  done
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end