src/HOL/Data_Structures/Binomial_Heap.thy
author Peter Lammich
Fri, 04 Dec 2020 17:54:57 +0000
changeset 72812 caf2fd14e28b
parent 72808 ba65dc3e35af
child 72910 c145be662fbd
permissions -rw-r--r--
removed session name

(* Author: Peter Lammich
           Tobias Nipkow (tuning)
*)

section \<open>Binomial Heap\<close>

theory Binomial_Heap
imports
  "HOL-Library.Pattern_Aliases"
  Complex_Main
  Priority_Queue_Specs
begin

text \<open>
  We formalize the binomial heap presentation from Okasaki's book.
  We show the functional correctness and complexity of all operations.

  The presentation is engineered for simplicity, and most
  proofs are straightforward and automatic.
\<close>

subsection \<open>Binomial Tree and Heap Datatype\<close>

datatype 'a tree = Node (rank: nat) (root: 'a) (children: "'a tree list")

type_synonym 'a heap = "'a tree list"

subsubsection \<open>Multiset of elements\<close>

fun mset_tree :: "'a::linorder tree \<Rightarrow> 'a multiset" where
  "mset_tree (Node _ a ts) = {#a#} + (\<Sum>t\<in>#mset ts. mset_tree t)"

definition mset_heap :: "'a::linorder heap \<Rightarrow> 'a multiset" where
  "mset_heap ts = (\<Sum>t\<in>#mset ts. mset_tree t)"

lemma mset_tree_simp_alt[simp]:
  "mset_tree (Node r a ts) = {#a#} + mset_heap ts"
  unfolding mset_heap_def by auto
declare mset_tree.simps[simp del]

lemma mset_tree_nonempty[simp]: "mset_tree t \<noteq> {#}"
by (cases t) auto

lemma mset_heap_Nil[simp]:
  "mset_heap [] = {#}"
by (auto simp: mset_heap_def)

lemma mset_heap_Cons[simp]: "mset_heap (t#ts) = mset_tree t + mset_heap ts"
by (auto simp: mset_heap_def)

lemma mset_heap_empty_iff[simp]: "mset_heap ts = {#} \<longleftrightarrow> ts=[]"
by (auto simp: mset_heap_def)

lemma root_in_mset[simp]: "root t \<in># mset_tree t"
by (cases t) auto

lemma mset_heap_rev_eq[simp]: "mset_heap (rev ts) = mset_heap ts"
by (auto simp: mset_heap_def)

subsubsection \<open>Invariants\<close>

text \<open>Binomial tree\<close>
fun invar_btree :: "'a::linorder tree \<Rightarrow> bool" where
"invar_btree (Node r x ts) \<longleftrightarrow>
   (\<forall>t\<in>set ts. invar_btree t) \<and> map rank ts = rev [0..<r]"

text \<open>Ordering (heap) invariant\<close>
fun invar_otree :: "'a::linorder tree \<Rightarrow> bool" where
"invar_otree (Node _ x ts) \<longleftrightarrow> (\<forall>t\<in>set ts. invar_otree t \<and> x \<le> root t)"

definition "invar_tree t \<longleftrightarrow> invar_btree t \<and> invar_otree t"

text \<open>Binomial Heap invariant\<close>
definition "invar ts \<longleftrightarrow> (\<forall>t\<in>set ts. invar_tree t) \<and> (sorted_wrt (<) (map rank ts))"


text \<open>The children of a node are a valid heap\<close>
lemma invar_children:
  "invar_tree (Node r v ts) \<Longrightarrow> invar (rev ts)"
  by (auto simp: invar_tree_def invar_def rev_map[symmetric])


subsection \<open>Operations and Their Functional Correctness\<close>

subsubsection \<open>\<open>link\<close>\<close>

context
includes pattern_aliases
begin

fun link :: "('a::linorder) tree \<Rightarrow> 'a tree \<Rightarrow> 'a tree" where
  "link (Node r x\<^sub>1 ts\<^sub>1 =: t\<^sub>1) (Node r' x\<^sub>2 ts\<^sub>2 =: t\<^sub>2) =
    (if x\<^sub>1\<le>x\<^sub>2 then Node (r+1) x\<^sub>1 (t\<^sub>2#ts\<^sub>1) else Node (r+1) x\<^sub>2 (t\<^sub>1#ts\<^sub>2))"

end

lemma invar_link:
  assumes "invar_tree t\<^sub>1"
  assumes "invar_tree t\<^sub>2"
  assumes "rank t\<^sub>1 = rank t\<^sub>2"
  shows "invar_tree (link t\<^sub>1 t\<^sub>2)"
using assms unfolding invar_tree_def
by (cases "(t\<^sub>1, t\<^sub>2)" rule: link.cases) auto

lemma rank_link[simp]: "rank (link t\<^sub>1 t\<^sub>2) = rank t\<^sub>1 + 1"
by (cases "(t\<^sub>1, t\<^sub>2)" rule: link.cases) simp

lemma mset_link[simp]: "mset_tree (link t\<^sub>1 t\<^sub>2) = mset_tree t\<^sub>1 + mset_tree t\<^sub>2"
by (cases "(t\<^sub>1, t\<^sub>2)" rule: link.cases) simp

subsubsection \<open>\<open>ins_tree\<close>\<close>

fun ins_tree :: "'a::linorder tree \<Rightarrow> 'a heap \<Rightarrow> 'a heap" where
  "ins_tree t [] = [t]"
| "ins_tree t\<^sub>1 (t\<^sub>2#ts) =
  (if rank t\<^sub>1 < rank t\<^sub>2 then t\<^sub>1#t\<^sub>2#ts else ins_tree (link t\<^sub>1 t\<^sub>2) ts)"

lemma invar_tree0[simp]: "invar_tree (Node 0 x [])"
unfolding invar_tree_def by auto

lemma invar_Cons[simp]:
  "invar (t#ts)
  \<longleftrightarrow> invar_tree t \<and> invar ts \<and> (\<forall>t'\<in>set ts. rank t < rank t')"
by (auto simp: invar_def)

lemma invar_ins_tree:
  assumes "invar_tree t"
  assumes "invar ts"
  assumes "\<forall>t'\<in>set ts. rank t \<le> rank t'"
  shows "invar (ins_tree t ts)"
using assms
by (induction t ts rule: ins_tree.induct) (auto simp: invar_link less_eq_Suc_le[symmetric])

lemma mset_heap_ins_tree[simp]:
  "mset_heap (ins_tree t ts) = mset_tree t + mset_heap ts"
by (induction t ts rule: ins_tree.induct) auto

lemma ins_tree_rank_bound:
  assumes "t' \<in> set (ins_tree t ts)"
  assumes "\<forall>t'\<in>set ts. rank t\<^sub>0 < rank t'"
  assumes "rank t\<^sub>0 < rank t"
  shows "rank t\<^sub>0 < rank t'"
using assms
by (induction t ts rule: ins_tree.induct) (auto split: if_splits)

subsubsection \<open>\<open>insert\<close>\<close>

hide_const (open) insert

definition insert :: "'a::linorder \<Rightarrow> 'a heap \<Rightarrow> 'a heap" where
"insert x ts = ins_tree (Node 0 x []) ts"

lemma invar_insert[simp]: "invar t \<Longrightarrow> invar (insert x t)"
by (auto intro!: invar_ins_tree simp: insert_def)

lemma mset_heap_insert[simp]: "mset_heap (insert x t) = {#x#} + mset_heap t"
by(auto simp: insert_def)

subsubsection \<open>\<open>merge\<close>\<close>

context
includes pattern_aliases
begin

fun merge :: "'a::linorder heap \<Rightarrow> 'a heap \<Rightarrow> 'a heap" where
  "merge ts\<^sub>1 [] = ts\<^sub>1"
| "merge [] ts\<^sub>2 = ts\<^sub>2"
| "merge (t\<^sub>1#ts\<^sub>1 =: h\<^sub>1) (t\<^sub>2#ts\<^sub>2 =: h\<^sub>2) = (
    if rank t\<^sub>1 < rank t\<^sub>2 then t\<^sub>1 # merge ts\<^sub>1 h\<^sub>2 else
    if rank t\<^sub>2 < rank t\<^sub>1 then t\<^sub>2 # merge h\<^sub>1 ts\<^sub>2
    else ins_tree (link t\<^sub>1 t\<^sub>2) (merge ts\<^sub>1 ts\<^sub>2)
  )"

end

lemma merge_simp2[simp]: "merge [] ts\<^sub>2 = ts\<^sub>2"
by (cases ts\<^sub>2) auto

lemma merge_rank_bound:
  assumes "t' \<in> set (merge ts\<^sub>1 ts\<^sub>2)"
  assumes "\<forall>t\<^sub>1\<in>set ts\<^sub>1. rank t < rank t\<^sub>1"
  assumes "\<forall>t\<^sub>2\<in>set ts\<^sub>2. rank t < rank t\<^sub>2"
  shows "rank t < rank t'"
using assms
by (induction ts\<^sub>1 ts\<^sub>2 arbitrary: t' rule: merge.induct)
   (auto split: if_splits simp: ins_tree_rank_bound)

lemma invar_merge[simp]:
  assumes "invar ts\<^sub>1"
  assumes "invar ts\<^sub>2"
  shows "invar (merge ts\<^sub>1 ts\<^sub>2)"
using assms
by (induction ts\<^sub>1 ts\<^sub>2 rule: merge.induct)
   (auto 0 3 simp: Suc_le_eq intro!: invar_ins_tree invar_link elim!: merge_rank_bound)


text \<open>Longer, more explicit proof of @{thm [source] invar_merge}, 
      to illustrate the application of the @{thm [source] merge_rank_bound} lemma.\<close>
lemma 
  assumes "invar ts\<^sub>1"
  assumes "invar ts\<^sub>2"
  shows "invar (merge ts\<^sub>1 ts\<^sub>2)"
  using assms
proof (induction ts\<^sub>1 ts\<^sub>2 rule: merge.induct)
  case (3 t\<^sub>1 ts\<^sub>1 t\<^sub>2 ts\<^sub>2)
  \<comment> \<open>Invariants of the parts can be shown automatically\<close>
  from "3.prems" have [simp]: 
    "invar_tree t\<^sub>1" "invar_tree t\<^sub>2"
    (*"invar (merge (t\<^sub>1#ts\<^sub>1) ts\<^sub>2)" 
    "invar (merge ts\<^sub>1 (t\<^sub>2#ts\<^sub>2))"
    "invar (merge ts\<^sub>1 ts\<^sub>2)"*)
    by auto

  \<comment> \<open>These are the three cases of the @{const merge} function\<close>
  consider (LT) "rank t\<^sub>1 < rank t\<^sub>2"
         | (GT) "rank t\<^sub>1 > rank t\<^sub>2"
         | (EQ) "rank t\<^sub>1 = rank t\<^sub>2"
    using antisym_conv3 by blast
  then show ?case proof cases
    case LT 
    \<comment> \<open>@{const merge} takes the first tree from the left heap\<close>
    then have "merge (t\<^sub>1 # ts\<^sub>1) (t\<^sub>2 # ts\<^sub>2) = t\<^sub>1 # merge ts\<^sub>1 (t\<^sub>2 # ts\<^sub>2)" by simp
    also have "invar \<dots>" proof (simp, intro conjI)
      \<comment> \<open>Invariant follows from induction hypothesis\<close>
      show "invar (merge ts\<^sub>1 (t\<^sub>2 # ts\<^sub>2))"
        using LT "3.IH" "3.prems" by simp

      \<comment> \<open>It remains to show that \<open>t\<^sub>1\<close> has smallest rank.\<close>
      show "\<forall>t'\<in>set (merge ts\<^sub>1 (t\<^sub>2 # ts\<^sub>2)). rank t\<^sub>1 < rank t'"
        \<comment> \<open>Which is done by auxiliary lemma @{thm [source] merge_rank_bound}\<close>
        using LT "3.prems" by (force elim!: merge_rank_bound)
    qed
    finally show ?thesis .
  next
    \<comment> \<open>@{const merge} takes the first tree from the right heap\<close>
    case GT 
    \<comment> \<open>The proof is anaologous to the \<open>LT\<close> case\<close>
    then show ?thesis using "3.prems" "3.IH" by (force elim!: merge_rank_bound)
  next
    case [simp]: EQ
    \<comment> \<open>@{const merge} links both first trees, and inserts them into the merged remaining heaps\<close>
    have "merge (t\<^sub>1 # ts\<^sub>1) (t\<^sub>2 # ts\<^sub>2) = ins_tree (link t\<^sub>1 t\<^sub>2) (merge ts\<^sub>1 ts\<^sub>2)" by simp
    also have "invar \<dots>" proof (intro invar_ins_tree invar_link) 
      \<comment> \<open>Invariant of merged remaining heaps follows by IH\<close>
      show "invar (merge ts\<^sub>1 ts\<^sub>2)"
        using EQ "3.prems" "3.IH" by auto

      \<comment> \<open>For insertion, we have to show that the rank of the linked tree is \<open>\<le>\<close> the 
          ranks in the merged remaining heaps\<close>
      show "\<forall>t'\<in>set (merge ts\<^sub>1 ts\<^sub>2). rank (link t\<^sub>1 t\<^sub>2) \<le> rank t'"
      proof -
        \<comment> \<open>Which is, again, done with the help of @{thm [source] merge_rank_bound}\<close>
        have "rank (link t\<^sub>1 t\<^sub>2) = Suc (rank t\<^sub>2)" by simp
        thus ?thesis using "3.prems" by (auto simp: Suc_le_eq elim!: merge_rank_bound)
      qed
    qed simp_all
    finally show ?thesis .
  qed
qed auto


lemma mset_heap_merge[simp]:
  "mset_heap (merge ts\<^sub>1 ts\<^sub>2) = mset_heap ts\<^sub>1 + mset_heap ts\<^sub>2"
by (induction ts\<^sub>1 ts\<^sub>2 rule: merge.induct) auto

subsubsection \<open>\<open>get_min\<close>\<close>

fun get_min :: "'a::linorder heap \<Rightarrow> 'a" where
  "get_min [t] = root t"
| "get_min (t#ts) = min (root t) (get_min ts)"

lemma invar_tree_root_min:
  assumes "invar_tree t"
  assumes "x \<in># mset_tree t"
  shows "root t \<le> x"
using assms unfolding invar_tree_def
by (induction t arbitrary: x rule: mset_tree.induct) (fastforce simp: mset_heap_def)

lemma get_min_mset:
  assumes "ts\<noteq>[]"
  assumes "invar ts"
  assumes "x \<in># mset_heap ts"
  shows "get_min ts \<le> x"
  using assms
apply (induction ts arbitrary: x rule: get_min.induct)
apply (auto
      simp: invar_tree_root_min min_def intro: order_trans;
      meson linear order_trans invar_tree_root_min
      )+
done

lemma get_min_member:
  "ts\<noteq>[] \<Longrightarrow> get_min ts \<in># mset_heap ts"
by (induction ts rule: get_min.induct) (auto simp: min_def)

lemma get_min:
  assumes "mset_heap ts \<noteq> {#}"
  assumes "invar ts"
  shows "get_min ts = Min_mset (mset_heap ts)"
using assms get_min_member get_min_mset
by (auto simp: eq_Min_iff)

subsubsection \<open>\<open>get_min_rest\<close>\<close>

fun get_min_rest :: "'a::linorder heap \<Rightarrow> 'a tree \<times> 'a heap" where
  "get_min_rest [t] = (t,[])"
| "get_min_rest (t#ts) = (let (t',ts') = get_min_rest ts
                     in if root t \<le> root t' then (t,ts) else (t',t#ts'))"

lemma get_min_rest_get_min_same_root:
  assumes "ts\<noteq>[]"
  assumes "get_min_rest ts = (t',ts')"
  shows "root t' = get_min ts"
using assms
by (induction ts arbitrary: t' ts' rule: get_min.induct) (auto simp: min_def split: prod.splits)

lemma mset_get_min_rest:
  assumes "get_min_rest ts = (t',ts')"
  assumes "ts\<noteq>[]"
  shows "mset ts = {#t'#} + mset ts'"
using assms
by (induction ts arbitrary: t' ts' rule: get_min.induct) (auto split: prod.splits if_splits)

lemma set_get_min_rest:
  assumes "get_min_rest ts = (t', ts')"
  assumes "ts\<noteq>[]"
  shows "set ts = Set.insert t' (set ts')"
using mset_get_min_rest[OF assms, THEN arg_cong[where f=set_mset]]
by auto

lemma invar_get_min_rest:
  assumes "get_min_rest ts = (t',ts')"
  assumes "ts\<noteq>[]"
  assumes "invar ts"
  shows "invar_tree t'" and "invar ts'"
proof -
  have "invar_tree t' \<and> invar ts'"
    using assms
    proof (induction ts arbitrary: t' ts' rule: get_min.induct)
      case (2 t v va)
      then show ?case
        apply (clarsimp split: prod.splits if_splits)
        apply (drule set_get_min_rest; fastforce)
        done
    qed auto
  thus "invar_tree t'" and "invar ts'" by auto
qed

subsubsection \<open>\<open>del_min\<close>\<close>

definition del_min :: "'a::linorder heap \<Rightarrow> 'a::linorder heap" where
"del_min ts = (case get_min_rest ts of
   (Node r x ts\<^sub>1, ts\<^sub>2) \<Rightarrow> merge (rev ts\<^sub>1) ts\<^sub>2)"

lemma invar_del_min[simp]:
  assumes "ts \<noteq> []"
  assumes "invar ts"
  shows "invar (del_min ts)"
using assms
unfolding del_min_def
by (auto
      split: prod.split tree.split
      intro!: invar_merge invar_children 
      dest: invar_get_min_rest
    )

lemma mset_heap_del_min:
  assumes "ts \<noteq> []"
  shows "mset_heap ts = mset_heap (del_min ts) + {# get_min ts #}"
using assms
unfolding del_min_def
apply (clarsimp split: tree.split prod.split)
apply (frule (1) get_min_rest_get_min_same_root)
apply (frule (1) mset_get_min_rest)
apply (auto simp: mset_heap_def)
done


subsubsection \<open>Instantiating the Priority Queue Locale\<close>

text \<open>Last step of functional correctness proof: combine all the above lemmas
to show that binomial heaps satisfy the specification of priority queues with merge.\<close>

interpretation binheap: Priority_Queue_Merge
  where empty = "[]" and is_empty = "(=) []" and insert = insert
  and get_min = get_min and del_min = del_min and merge = merge
  and invar = invar and mset = mset_heap
proof (unfold_locales, goal_cases)
  case 1 thus ?case by simp
next
  case 2 thus ?case by auto
next
  case 3 thus ?case by auto
next
  case (4 q)
  thus ?case using mset_heap_del_min[of q] get_min[OF _ \<open>invar q\<close>]
    by (auto simp: union_single_eq_diff)
next
  case (5 q) thus ?case using get_min[of q] by auto
next
  case 6 thus ?case by (auto simp add: invar_def)
next
  case 7 thus ?case by simp
next
  case 8 thus ?case by simp
next
  case 9 thus ?case by simp
next
  case 10 thus ?case by simp
qed


subsection \<open>Complexity\<close>

text \<open>The size of a binomial tree is determined by its rank\<close>
lemma size_mset_btree:
  assumes "invar_btree t"
  shows "size (mset_tree t) = 2^rank t"
  using assms
proof (induction t)
  case (Node r v ts)
  hence IH: "size (mset_tree t) = 2^rank t" if "t \<in> set ts" for t
    using that by auto

  from Node have COMPL: "map rank ts = rev [0..<r]" by auto

  have "size (mset_heap ts) = (\<Sum>t\<leftarrow>ts. size (mset_tree t))"
    by (induction ts) auto
  also have "\<dots> = (\<Sum>t\<leftarrow>ts. 2^rank t)" using IH
    by (auto cong: map_cong)
  also have "\<dots> = (\<Sum>r\<leftarrow>map rank ts. 2^r)"
    by (induction ts) auto
  also have "\<dots> = (\<Sum>i\<in>{0..<r}. 2^i)"
    unfolding COMPL
    by (auto simp: rev_map[symmetric] interv_sum_list_conv_sum_set_nat)
  also have "\<dots> = 2^r - 1"
    by (induction r) auto
  finally show ?case
    by (simp)
qed

lemma size_mset_tree:
  assumes "invar_tree t"
  shows "size (mset_tree t) = 2^rank t"
using assms unfolding invar_tree_def
by (simp add: size_mset_btree)

text \<open>The length of a binomial heap is bounded by the number of its elements\<close>
lemma size_mset_heap:
  assumes "invar ts"
  shows "2^length ts \<le> size (mset_heap ts) + 1"
proof -
  from \<open>invar ts\<close> have
    ASC: "sorted_wrt (<) (map rank ts)" and
    TINV: "\<forall>t\<in>set ts. invar_tree t"
    unfolding invar_def by auto

  have "(2::nat)^length ts = (\<Sum>i\<in>{0..<length ts}. 2^i) + 1"
    by (simp add: sum_power2)
  also have "\<dots> \<le> (\<Sum>t\<leftarrow>ts. 2^rank t) + 1"
    using sorted_wrt_less_sum_mono_lowerbound[OF _ ASC, of "(^) (2::nat)"]
    using power_increasing[where a="2::nat"]
    by (auto simp: o_def)
  also have "\<dots> = (\<Sum>t\<leftarrow>ts. size (mset_tree t)) + 1" using TINV
    by (auto cong: map_cong simp: size_mset_tree)
  also have "\<dots> = size (mset_heap ts) + 1"
    unfolding mset_heap_def by (induction ts) auto
  finally show ?thesis .
qed

subsubsection \<open>Timing Functions\<close>

text \<open>
  We define timing functions for each operation, and provide
  estimations of their complexity.
\<close>
definition T_link :: "'a::linorder tree \<Rightarrow> 'a tree \<Rightarrow> nat" where
[simp]: "T_link _ _ = 1"

fun T_ins_tree :: "'a::linorder tree \<Rightarrow> 'a heap \<Rightarrow> nat" where
  "T_ins_tree t [] = 1"
| "T_ins_tree t\<^sub>1 (t\<^sub>2 # rest) = (
    (if rank t\<^sub>1 < rank t\<^sub>2 then 1
     else T_link t\<^sub>1 t\<^sub>2 + T_ins_tree (link t\<^sub>1 t\<^sub>2) rest)
  )"

definition T_insert :: "'a::linorder \<Rightarrow> 'a heap \<Rightarrow> nat" where
"T_insert x ts = T_ins_tree (Node 0 x []) ts"

lemma T_ins_tree_simple_bound: "T_ins_tree t ts \<le> length ts + 1"
by (induction t ts rule: T_ins_tree.induct) auto

subsubsection \<open>\<open>T_insert\<close>\<close>

lemma T_insert_bound:
  assumes "invar ts"
  shows "T_insert x ts \<le> log 2 (size (mset_heap ts) + 1) + 1"
proof -

  have 1: "T_insert x ts \<le> length ts + 1"
    unfolding T_insert_def by (rule T_ins_tree_simple_bound)
  also have "\<dots> \<le> log 2 (2 * (size (mset_heap ts) + 1))"
  proof -
    from size_mset_heap[of ts] assms
    have "2 ^ length ts \<le> size (mset_heap ts) + 1"
      unfolding invar_def by auto
    hence "2 ^ (length ts + 1) \<le> 2 * (size (mset_heap ts) + 1)" by auto
    thus ?thesis using le_log2_of_power by blast
  qed
  finally show ?thesis
    by (simp only: log_mult of_nat_mult) auto
qed

subsubsection \<open>\<open>T_merge\<close>\<close>

context
includes pattern_aliases
begin

fun T_merge :: "'a::linorder heap \<Rightarrow> 'a heap \<Rightarrow> nat" where
  "T_merge ts\<^sub>1 [] = 1"
| "T_merge [] ts\<^sub>2 = 1"
| "T_merge (t\<^sub>1#ts\<^sub>1 =: h\<^sub>1) (t\<^sub>2#ts\<^sub>2 =: h\<^sub>2) = 1 + (
    if rank t\<^sub>1 < rank t\<^sub>2 then T_merge ts\<^sub>1 h\<^sub>2
    else if rank t\<^sub>2 < rank t\<^sub>1 then T_merge h\<^sub>1 ts\<^sub>2
    else T_ins_tree (link t\<^sub>1 t\<^sub>2) (merge ts\<^sub>1 ts\<^sub>2) + T_merge ts\<^sub>1 ts\<^sub>2
  )"

end

text \<open>A crucial idea is to estimate the time in correlation with the
  result length, as each carry reduces the length of the result.\<close>

lemma T_ins_tree_length:
  "T_ins_tree t ts + length (ins_tree t ts) = 2 + length ts"
by (induction t ts rule: ins_tree.induct) auto

lemma T_merge_length:
  "length (merge ts\<^sub>1 ts\<^sub>2) + T_merge ts\<^sub>1 ts\<^sub>2 \<le> 2 * (length ts\<^sub>1 + length ts\<^sub>2) + 1"
by (induction ts\<^sub>1 ts\<^sub>2 rule: T_merge.induct)
   (auto simp: T_ins_tree_length algebra_simps)

text \<open>Finally, we get the desired logarithmic bound\<close>
lemma T_merge_bound_aux:
  fixes ts\<^sub>1 ts\<^sub>2
  defines "n\<^sub>1 \<equiv> size (mset_heap ts\<^sub>1)"
  defines "n\<^sub>2 \<equiv> size (mset_heap ts\<^sub>2)"
  assumes BINVARS: "invar ts\<^sub>1" "invar ts\<^sub>2"
  shows "T_merge ts\<^sub>1 ts\<^sub>2 \<le> 4*log 2 (n\<^sub>1 + n\<^sub>2 + 1) + 2"
proof -
  define n where "n = n\<^sub>1 + n\<^sub>2"

  from T_merge_length[of ts\<^sub>1 ts\<^sub>2]
  have "T_merge ts\<^sub>1 ts\<^sub>2 \<le> 2 * (length ts\<^sub>1 + length ts\<^sub>2) + 1" by auto
  hence "(2::nat)^T_merge ts\<^sub>1 ts\<^sub>2 \<le> 2^(2 * (length ts\<^sub>1 + length ts\<^sub>2) + 1)"
    by (rule power_increasing) auto
  also have "\<dots> = 2*(2^length ts\<^sub>1)\<^sup>2*(2^length ts\<^sub>2)\<^sup>2"
    by (auto simp: algebra_simps power_add power_mult)
  also note BINVARS(1)[THEN size_mset_heap]
  also note BINVARS(2)[THEN size_mset_heap]
  finally have "2 ^ T_merge ts\<^sub>1 ts\<^sub>2 \<le> 2 * (n\<^sub>1 + 1)\<^sup>2 * (n\<^sub>2 + 1)\<^sup>2"
    by (auto simp: power2_nat_le_eq_le n\<^sub>1_def n\<^sub>2_def)
  from le_log2_of_power[OF this] have "T_merge ts\<^sub>1 ts\<^sub>2 \<le> log 2 \<dots>"
    by simp
  also have "\<dots> = log 2 2 + 2*log 2 (n\<^sub>1 + 1) + 2*log 2 (n\<^sub>2 + 1)"
    by (simp add: log_mult log_nat_power)
  also have "n\<^sub>2 \<le> n" by (auto simp: n_def)
  finally have "T_merge ts\<^sub>1 ts\<^sub>2 \<le> log 2 2 + 2*log 2 (n\<^sub>1 + 1) + 2*log 2 (n + 1)"
    by auto
  also have "n\<^sub>1 \<le> n" by (auto simp: n_def)
  finally have "T_merge ts\<^sub>1 ts\<^sub>2 \<le> log 2 2 + 4*log 2 (n + 1)"
    by auto
  also have "log 2 2 \<le> 2" by auto
  finally have "T_merge ts\<^sub>1 ts\<^sub>2 \<le> 4*log 2 (n + 1) + 2" by auto
  thus ?thesis unfolding n_def by (auto simp: algebra_simps)
qed

lemma T_merge_bound:
  fixes ts\<^sub>1 ts\<^sub>2
  defines "n\<^sub>1 \<equiv> size (mset_heap ts\<^sub>1)"
  defines "n\<^sub>2 \<equiv> size (mset_heap ts\<^sub>2)"
  assumes "invar ts\<^sub>1" "invar ts\<^sub>2"
  shows "T_merge ts\<^sub>1 ts\<^sub>2 \<le> 4*log 2 (n\<^sub>1 + n\<^sub>2 + 1) + 2"
using assms T_merge_bound_aux unfolding invar_def by blast

subsubsection \<open>\<open>T_get_min\<close>\<close>

fun T_get_min :: "'a::linorder heap \<Rightarrow> nat" where
  "T_get_min [t] = 1"
| "T_get_min (t#ts) = 1 + T_get_min ts"

lemma T_get_min_estimate: "ts\<noteq>[] \<Longrightarrow> T_get_min ts = length ts"
by (induction ts rule: T_get_min.induct) auto

lemma T_get_min_bound:
  assumes "invar ts"
  assumes "ts\<noteq>[]"
  shows "T_get_min ts \<le> log 2 (size (mset_heap ts) + 1)"
proof -
  have 1: "T_get_min ts = length ts" using assms T_get_min_estimate by auto
  also have "\<dots> \<le> log 2 (size (mset_heap ts) + 1)"
  proof -
    from size_mset_heap[of ts] assms have "2 ^ length ts \<le> size (mset_heap ts) + 1"
      unfolding invar_def by auto
    thus ?thesis using le_log2_of_power by blast
  qed
  finally show ?thesis by auto
qed

subsubsection \<open>\<open>T_del_min\<close>\<close>

fun T_get_min_rest :: "'a::linorder heap \<Rightarrow> nat" where
  "T_get_min_rest [t] = 1"
| "T_get_min_rest (t#ts) = 1 + T_get_min_rest ts"

lemma T_get_min_rest_estimate: "ts\<noteq>[] \<Longrightarrow> T_get_min_rest ts = length ts"
  by (induction ts rule: T_get_min_rest.induct) auto

lemma T_get_min_rest_bound_aux:
  assumes "invar ts"
  assumes "ts\<noteq>[]"
  shows "T_get_min_rest ts \<le> log 2 (size (mset_heap ts) + 1)"
proof -
  have 1: "T_get_min_rest ts = length ts" using assms T_get_min_rest_estimate by auto
  also have "\<dots> \<le> log 2 (size (mset_heap ts) + 1)"
  proof -
    from size_mset_heap[of ts] assms have "2 ^ length ts \<le> size (mset_heap ts) + 1"
      by auto
    thus ?thesis using le_log2_of_power by blast
  qed
  finally show ?thesis by auto
qed

lemma T_get_min_rest_bound:
  assumes "invar ts"
  assumes "ts\<noteq>[]"
  shows "T_get_min_rest ts \<le> log 2 (size (mset_heap ts) + 1)"
using assms T_get_min_rest_bound_aux unfolding invar_def by blast

text\<open>Note that although the definition of function \<^const>\<open>rev\<close> has quadratic complexity,
it can and is implemented (via suitable code lemmas) as a linear time function.
Thus the following definition is justified:\<close>

definition "T_rev xs = length xs + 1"

definition T_del_min :: "'a::linorder heap \<Rightarrow> nat" where
  "T_del_min ts = T_get_min_rest ts + (case get_min_rest ts of (Node _ x ts\<^sub>1, ts\<^sub>2)
                    \<Rightarrow> T_rev ts\<^sub>1 + T_merge (rev ts\<^sub>1) ts\<^sub>2
  )"

lemma T_rev_ts1_bound_aux:
  fixes ts
  defines "n \<equiv> size (mset_heap ts)"
  assumes BINVAR: "invar (rev ts)"
  shows "T_rev ts \<le> 1 + log 2 (n+1)"
proof -
  have "T_rev ts = length ts + 1" by (auto simp: T_rev_def)
  hence "2^T_rev ts = 2*2^length ts" by auto
  also have "\<dots> \<le> 2*n+2" using size_mset_heap[OF BINVAR] by (auto simp: n_def)
  finally have "2 ^ T_rev ts \<le> 2 * n + 2" .
  from le_log2_of_power[OF this] have "T_rev ts \<le> log 2 (2 * (n + 1))"
    by auto
  also have "\<dots> = 1 + log 2 (n+1)"
    by (simp only: of_nat_mult log_mult) auto
  finally show ?thesis by (auto simp: algebra_simps)
qed

lemma T_del_min_bound_aux:
  fixes ts
  defines "n \<equiv> size (mset_heap ts)"
  assumes BINVAR: "invar ts"
  assumes "ts\<noteq>[]"
  shows "T_del_min ts \<le> 6 * log 2 (n+1) + 3"
proof -
  obtain r x ts\<^sub>1 ts\<^sub>2 where GM: "get_min_rest ts = (Node r x ts\<^sub>1, ts\<^sub>2)"
    by (metis surj_pair tree.exhaust_sel)

  note BINVAR' = invar_get_min_rest[OF GM \<open>ts\<noteq>[]\<close> BINVAR]
  hence BINVAR1: "invar (rev ts\<^sub>1)" by (blast intro: invar_children)

  define n\<^sub>1 where "n\<^sub>1 = size (mset_heap ts\<^sub>1)"
  define n\<^sub>2 where "n\<^sub>2 = size (mset_heap ts\<^sub>2)"

  have T_rev_ts1_bound: "T_rev ts\<^sub>1 \<le> 1 + log 2 (n+1)"
  proof -
    note T_rev_ts1_bound_aux[OF BINVAR1, simplified, folded n\<^sub>1_def]
    also have "n\<^sub>1 \<le> n"
      unfolding n\<^sub>1_def n_def
      using mset_get_min_rest[OF GM \<open>ts\<noteq>[]\<close>]
      by (auto simp: mset_heap_def)
    finally show ?thesis by (auto simp: algebra_simps)
  qed

  have "T_del_min ts = T_get_min_rest ts + T_rev ts\<^sub>1 + T_merge (rev ts\<^sub>1) ts\<^sub>2"
    unfolding T_del_min_def by (simp add: GM)
  also have "\<dots> \<le> log 2 (n+1) + T_rev ts\<^sub>1 + T_merge (rev ts\<^sub>1) ts\<^sub>2"
    using T_get_min_rest_bound_aux[OF assms(2-)] by (auto simp: n_def)
  also have "\<dots> \<le> 2*log 2 (n+1) + T_merge (rev ts\<^sub>1) ts\<^sub>2 + 1"
    using T_rev_ts1_bound by auto
  also have "\<dots> \<le> 2*log 2 (n+1) + 4 * log 2 (n\<^sub>1 + n\<^sub>2 + 1) + 3"
    using T_merge_bound_aux[OF \<open>invar (rev ts\<^sub>1)\<close> \<open>invar ts\<^sub>2\<close>]
    by (auto simp: n\<^sub>1_def n\<^sub>2_def algebra_simps)
  also have "n\<^sub>1 + n\<^sub>2 \<le> n"
    unfolding n\<^sub>1_def n\<^sub>2_def n_def
    using mset_get_min_rest[OF GM \<open>ts\<noteq>[]\<close>]
    by (auto simp: mset_heap_def)
  finally have "T_del_min ts \<le> 6 * log 2 (n+1) + 3"
    by auto
  thus ?thesis by (simp add: algebra_simps)
qed

lemma T_del_min_bound:
  fixes ts
  defines "n \<equiv> size (mset_heap ts)"
  assumes "invar ts"
  assumes "ts\<noteq>[]"
  shows "T_del_min ts \<le> 6 * log 2 (n+1) + 3"
using assms T_del_min_bound_aux unfolding invar_def by blast

end