Theory Sublist_Order

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theory Sublist_Order
imports Main

(*  Title:      HOL/Library/Sublist_Order.thy
    Authors:    Peter Lammich, Uni Muenster <peter.lammich@uni-muenster.de>
                Florian Haftmann, TU Muenchen
*)

header {* Sublist Ordering *}

theory Sublist_Order
imports Main
begin

text {*
  This theory defines sublist ordering on lists.
  A list @{text ys} is a sublist of a list @{text xs},
  iff one obtains @{text ys} by erasing some elements from @{text xs}.
*}

subsection {* Definitions and basic lemmas *}

instantiation list :: (type) order
begin

inductive less_eq_list where
  empty [simp, intro!]: "[] ≤ xs"
  | drop: "ys ≤ xs ==> ys ≤ x # xs"
  | take: "ys ≤ xs ==> x # ys ≤ x # xs"

lemmas ileq_empty = empty
lemmas ileq_drop = drop
lemmas ileq_take = take

lemma ileq_cases [cases set, case_names empty drop take]:
  assumes "xs ≤ ys"
    and "xs = [] ==> P"
    and "!!z zs. ys = z # zs ==> xs ≤ zs ==> P"
    and "!!x zs ws. xs = x # zs ==> ys = x # ws ==> zs ≤ ws ==> P"
  shows P
  using assms by (blast elim: less_eq_list.cases)

lemma ileq_induct [induct set, case_names empty drop take]:
  assumes "xs ≤ ys"
    and "!!zs. P [] zs"
    and "!!z zs ws. ws ≤ zs ==>  P ws zs ==> P ws (z # zs)"
    and "!!z zs ws. ws ≤ zs ==> P ws zs ==> P (z # ws) (z # zs)"
  shows "P xs ys" 
  using assms by (induct rule: less_eq_list.induct) blast+

definition
  [code del]: "(xs :: 'a list) < ys <-> xs ≤ ys ∧ ¬ ys ≤ xs"

lemma ileq_length: "xs ≤ ys ==> length xs ≤ length ys"
  by (induct rule: ileq_induct) auto
lemma ileq_below_empty [simp]: "xs ≤ [] <-> xs = []"
  by (auto dest: ileq_length)

instance proof
  fix xs ys :: "'a list"
  show "xs < ys <-> xs ≤ ys ∧ ¬ ys ≤ xs" unfolding less_list_def .. 
next
  fix xs :: "'a list"
  show "xs ≤ xs" by (induct xs) (auto intro!: ileq_empty ileq_drop ileq_take)
next
  fix xs ys :: "'a list"
  (* TODO: Is there a simpler proof ? *)
  { fix n
    have "!!l l'. [|l≤l'; l'≤l; n=length l + length l'|] ==> l=l'"
    proof (induct n rule: nat_less_induct)
      case (1 n l l') from "1.prems"(1) show ?case proof (cases rule: ileq_cases)
        case empty with "1.prems"(2) show ?thesis by auto 
      next
        case (drop a l2') with "1.prems"(2) have "length l'≤length l" "length l ≤ length l2'" "1+length l2' = length l'" by (auto dest: ileq_length)
        hence False by simp thus ?thesis ..
      next
        case (take a l1' l2') hence LEN': "length l1' + length l2' < length l + length l'" by simp
        from "1.prems" have LEN: "length l' = length l" by (auto dest!: ileq_length)
        from "1.prems"(2) show ?thesis proof (cases rule: ileq_cases[case_names empty' drop' take'])
          case empty' with take LEN show ?thesis by simp 
        next
          case (drop' ah l2h) with take LEN have "length l1' ≤ length l2h" "1+length l2h = length l2'" "length l2' = length l1'" by (auto dest: ileq_length)
          hence False by simp thus ?thesis ..
        next
          case (take' ah l1h l2h)
          with take have 2: "ah=a" "l1h=l2'" "l2h=l1'" "l1' ≤ l2'" "l2' ≤ l1'" by auto
          with LEN' "1.hyps" "1.prems"(3) have "l1'=l2'" by blast
          with take 2 show ?thesis by simp
        qed
      qed
    qed
  }
  moreover assume "xs ≤ ys" "ys ≤ xs"
  ultimately show "xs = ys" by blast
next
  fix xs ys zs :: "'a list"
  {
    fix n
    have "!!x y z. [|x ≤ y; y ≤ z; n=length x + length y + length z|] ==> x ≤ z" 
    proof (induct rule: nat_less_induct[case_names I])
      case (I n x y z)
      from I.prems(2) show ?case proof (cases rule: ileq_cases)
        case empty with I.prems(1) show ?thesis by auto
      next
        case (drop a z') hence "length x + length y + length z' < length x + length y + length z" by simp
        with I.hyps I.prems(3,1) drop(2) have "x≤z'" by blast
        with drop(1) show ?thesis by (auto intro: ileq_drop)
      next
        case (take a y' z') from I.prems(1) show ?thesis proof (cases rule: ileq_cases[case_names empty' drop' take'])
          case empty' thus ?thesis by auto
        next
          case (drop' ah y'h) with take have "x≤y'" "y'≤z'" "length x + length y' + length z' < length x + length y + length z" by auto
          with I.hyps I.prems(3) have "x≤z'" by (blast)
          with take(2) show ?thesis  by (auto intro: ileq_drop)
        next
          case (take' ah x' y'h) with take have 2: "x=a#x'" "x'≤y'" "y'≤z'" "length x' + length y' + length z' < length x + length y + length z" by auto
          with I.hyps I.prems(3) have "x'≤z'" by blast
          with 2 take(2) show ?thesis by (auto intro: ileq_take)
        qed
      qed
    qed
  }
  moreover assume "xs ≤ ys" "ys ≤ zs"
  ultimately show "xs ≤ zs" by blast
qed

end

lemmas ileq_intros = ileq_empty ileq_drop ileq_take

lemma ileq_drop_many: "xs ≤ ys ==> xs ≤ zs @ ys"
  by (induct zs) (auto intro: ileq_drop)
lemma ileq_take_many: "xs ≤ ys ==> zs @ xs ≤ zs @ ys"
  by (induct zs) (auto intro: ileq_take)

lemma ileq_same_length: "xs ≤ ys ==> length xs = length ys ==> xs = ys"
  by (induct rule: ileq_induct) (auto dest: ileq_length)
lemma ileq_same_append [simp]: "x # xs ≤ xs <-> False"
  by (auto dest: ileq_length)

lemma ilt_length [intro]:
  assumes "xs < ys"
  shows "length xs < length ys"
proof -
  from assms have "xs ≤ ys" and "xs ≠ ys" by (simp_all add: less_le)
  moreover with ileq_length have "length xs ≤ length ys" by auto
  ultimately show ?thesis by (auto intro: ileq_same_length)
qed

lemma ilt_empty [simp]: "[] < xs <-> xs ≠ []"
  by (unfold less_list_def, auto)
lemma ilt_emptyI: "xs ≠ [] ==> [] < xs"
  by (unfold less_list_def, auto)
lemma ilt_emptyD: "[] < xs ==> xs ≠ []"
  by (unfold less_list_def, auto)
lemma ilt_below_empty[simp]: "xs < [] ==> False"
  by (auto dest: ilt_length)
lemma ilt_drop: "xs < ys ==> xs < x # ys"
  by (unfold less_le) (auto intro: ileq_intros)
lemma ilt_take: "xs < ys ==> x # xs < x # ys"
  by (unfold less_le) (auto intro: ileq_intros)
lemma ilt_drop_many: "xs < ys ==> xs < zs @ ys"
  by (induct zs) (auto intro: ilt_drop)
lemma ilt_take_many: "xs < ys ==> zs @ xs < zs @ ys"
  by (induct zs) (auto intro: ilt_take)


subsection {* Appending elements *}

lemma ileq_rev_take: "xs ≤ ys ==> xs @ [x] ≤ ys @ [x]"
  by (induct rule: ileq_induct) (auto intro: ileq_intros ileq_drop_many)
lemma ilt_rev_take: "xs < ys ==> xs @ [x] < ys @ [x]"
  by (unfold less_le) (auto dest: ileq_rev_take)
lemma ileq_rev_drop: "xs ≤ ys ==> xs ≤ ys @ [x]"
  by (induct rule: ileq_induct) (auto intro: ileq_intros)
lemma ileq_rev_drop_many: "xs ≤ ys ==> xs ≤ ys @ zs"
  by (induct zs rule: rev_induct) (auto dest: ileq_rev_drop)


subsection {* Relation to standard list operations *}

lemma ileq_map: "xs ≤ ys ==> map f xs ≤ map f ys"
  by (induct rule: ileq_induct) (auto intro: ileq_intros)
lemma ileq_filter_left[simp]: "filter f xs ≤ xs"
  by (induct xs) (auto intro: ileq_intros)
lemma ileq_filter: "xs ≤ ys ==> filter f xs ≤ filter f ys"
  by (induct rule: ileq_induct) (auto intro: ileq_intros) 

end