author | nipkow |
Sat, 21 Apr 2018 11:13:35 +0200 | |
changeset 68021 | b91a043c0dcb |
parent 68020 | 6aade817bee5 |
child 68413 | b56ed5010e69 |
permissions | -rw-r--r-- |
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(* Author: Tobias Nipkow *) |
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section \<open>Leftist Heap\<close> |
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theory Leftist_Heap |
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imports |
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Base_FDS |
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Tree2 |
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Priority_Queue |
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Complex_Main |
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begin |
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fun mset_tree :: "('a,'b) tree \<Rightarrow> 'a multiset" where |
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"mset_tree Leaf = {#}" | |
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"mset_tree (Node _ l a r) = {#a#} + mset_tree l + mset_tree r" |
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type_synonym 'a lheap = "('a,nat)tree" |
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fun rank :: "'a lheap \<Rightarrow> nat" where |
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"rank Leaf = 0" | |
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"rank (Node _ _ _ r) = rank r + 1" |
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fun rk :: "'a lheap \<Rightarrow> nat" where |
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"rk Leaf = 0" | |
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"rk (Node n _ _ _) = n" |
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text\<open>The invariants:\<close> |
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fun (in linorder) heap :: "('a,'b) tree \<Rightarrow> bool" where |
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"heap Leaf = True" | |
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"heap (Node _ l m r) = |
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(heap l \<and> heap r \<and> (\<forall>x \<in> set_mset(mset_tree l + mset_tree r). m \<le> x))" |
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fun ltree :: "'a lheap \<Rightarrow> bool" where |
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"ltree Leaf = True" | |
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"ltree (Node n l a r) = |
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(n = rank r + 1 \<and> rank l \<ge> rank r \<and> ltree l & ltree r)" |
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definition node :: "'a lheap \<Rightarrow> 'a \<Rightarrow> 'a lheap \<Rightarrow> 'a lheap" where |
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"node l a r = |
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(let rl = rk l; rr = rk r |
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in if rl \<ge> rr then Node (rr+1) l a r else Node (rl+1) r a l)" |
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fun get_min :: "'a lheap \<Rightarrow> 'a" where |
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"get_min(Node n l a r) = a" |
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text \<open>For function \<open>merge\<close>:\<close> |
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unbundle pattern_aliases |
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declare size_prod_measure[measure_function] |
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fun merge :: "'a::ord lheap \<Rightarrow> 'a lheap \<Rightarrow> 'a lheap" where |
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"merge Leaf t2 = t2" | |
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"merge t1 Leaf = t1" | |
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"merge (Node n1 l1 a1 r1 =: t1) (Node n2 l2 a2 r2 =: t2) = |
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(if a1 \<le> a2 then node l1 a1 (merge r1 t2) |
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else node l2 a2 (merge r2 t1))" |
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lemma merge_code: "merge t1 t2 = (case (t1,t2) of |
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(Leaf, _) \<Rightarrow> t2 | |
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(_, Leaf) \<Rightarrow> t1 | |
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(Node n1 l1 a1 r1, Node n2 l2 a2 r2) \<Rightarrow> |
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if a1 \<le> a2 then node l1 a1 (merge r1 t2) else node l2 a2 (merge r2 t1))" |
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by(induction t1 t2 rule: merge.induct) (simp_all split: tree.split) |
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hide_const (open) insert |
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definition insert :: "'a::ord \<Rightarrow> 'a lheap \<Rightarrow> 'a lheap" where |
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"insert x t = merge (Node 1 Leaf x Leaf) t" |
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fun del_min :: "'a::ord lheap \<Rightarrow> 'a lheap" where |
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"del_min Leaf = Leaf" | |
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"del_min (Node n l x r) = merge l r" |
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subsection "Lemmas" |
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lemma mset_tree_empty: "mset_tree t = {#} \<longleftrightarrow> t = Leaf" |
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by(cases t) auto |
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lemma rk_eq_rank[simp]: "ltree t \<Longrightarrow> rk t = rank t" |
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by(cases t) auto |
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lemma ltree_node: "ltree (node l a r) \<longleftrightarrow> ltree l \<and> ltree r" |
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by(auto simp add: node_def) |
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lemma heap_node: "heap (node l a r) \<longleftrightarrow> |
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heap l \<and> heap r \<and> (\<forall>x \<in> set_mset(mset_tree l + mset_tree r). a \<le> x)" |
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by(auto simp add: node_def) |
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subsection "Functional Correctness" |
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lemma mset_merge: "mset_tree (merge h1 h2) = mset_tree h1 + mset_tree h2" |
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by (induction h1 h2 rule: merge.induct) (auto simp add: node_def ac_simps) |
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lemma mset_insert: "mset_tree (insert x t) = mset_tree t + {#x#}" |
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by (auto simp add: insert_def mset_merge) |
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lemma get_min: "\<lbrakk> heap h; h \<noteq> Leaf \<rbrakk> \<Longrightarrow> get_min h = Min_mset (mset_tree h)" |
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by (induction h) (auto simp add: eq_Min_iff) |
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lemma mset_del_min: "mset_tree (del_min h) = mset_tree h - {# get_min h #}" |
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by (cases h) (auto simp: mset_merge) |
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lemma ltree_merge: "\<lbrakk> ltree l; ltree r \<rbrakk> \<Longrightarrow> ltree (merge l r)" |
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proof(induction l r rule: merge.induct) |
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case (3 n1 l1 a1 r1 n2 l2 a2 r2) |
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show ?case (is "ltree(merge ?t1 ?t2)") |
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proof cases |
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assume "a1 \<le> a2" |
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hence "ltree (merge ?t1 ?t2) = ltree (node l1 a1 (merge r1 ?t2))" by simp |
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also have "\<dots> = (ltree l1 \<and> ltree(merge r1 ?t2))" |
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by(simp add: ltree_node) |
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also have "..." using "3.prems" "3.IH"(1)[OF \<open>a1 \<le> a2\<close>] by (simp) |
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finally show ?thesis . |
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next (* analogous but automatic *) |
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assume "\<not> a1 \<le> a2" |
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thus ?thesis using 3 by(simp)(auto simp: ltree_node) |
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qed |
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qed simp_all |
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lemma heap_merge: "\<lbrakk> heap l; heap r \<rbrakk> \<Longrightarrow> heap (merge l r)" |
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proof(induction l r rule: merge.induct) |
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case 3 thus ?case by(auto simp: heap_node mset_merge ball_Un) |
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qed simp_all |
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lemma ltree_insert: "ltree t \<Longrightarrow> ltree(insert x t)" |
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by(simp add: insert_def ltree_merge del: merge.simps split: tree.split) |
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lemma heap_insert: "heap t \<Longrightarrow> heap(insert x t)" |
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by(simp add: insert_def heap_merge del: merge.simps split: tree.split) |
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lemma ltree_del_min: "ltree t \<Longrightarrow> ltree(del_min t)" |
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by(cases t)(auto simp add: ltree_merge simp del: merge.simps) |
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lemma heap_del_min: "heap t \<Longrightarrow> heap(del_min t)" |
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by(cases t)(auto simp add: heap_merge simp del: merge.simps) |
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text \<open>Last step of functional correctness proof: combine all the above lemmas |
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to show that leftist heaps satisfy the specification of priority queues with merge.\<close> |
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interpretation lheap: Priority_Queue_Merge |
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where empty = Leaf and is_empty = "\<lambda>h. h = Leaf" |
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and insert = insert and del_min = del_min |
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and get_min = get_min and merge = merge |
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and invar = "\<lambda>h. heap h \<and> ltree h" and mset = mset_tree |
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proof(standard, goal_cases) |
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case 1 show ?case by simp |
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next |
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case (2 q) show ?case by (cases q) auto |
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next |
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case 3 show ?case by(rule mset_insert) |
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next |
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case 4 show ?case by(rule mset_del_min) |
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next |
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case 5 thus ?case by(simp add: get_min mset_tree_empty) |
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next |
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case 6 thus ?case by(simp) |
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next |
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case 7 thus ?case by(simp add: heap_insert ltree_insert) |
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next |
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case 8 thus ?case by(simp add: heap_del_min ltree_del_min) |
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next |
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case 9 thus ?case by (simp add: mset_merge) |
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next |
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case 10 thus ?case by (simp add: heap_merge ltree_merge) |
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qed |
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subsection "Complexity" |
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lemma pow2_rank_size1: "ltree t \<Longrightarrow> 2 ^ rank t \<le> size1 t" |
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proof(induction t) |
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case Leaf show ?case by simp |
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next |
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case (Node n l a r) |
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hence "rank r \<le> rank l" by simp |
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hence *: "(2::nat) ^ rank r \<le> 2 ^ rank l" by simp |
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have "(2::nat) ^ rank \<langle>n, l, a, r\<rangle> = 2 ^ rank r + 2 ^ rank r" |
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by(simp add: mult_2) |
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also have "\<dots> \<le> size1 l + size1 r" |
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using Node * by (simp del: power_increasing_iff) |
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also have "\<dots> = size1 \<langle>n, l, a, r\<rangle>" by simp |
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finally show ?case . |
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qed |
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text\<open>Explicit termination argument: sum of sizes\<close> |
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fun t_merge :: "'a::ord lheap \<Rightarrow> 'a lheap \<Rightarrow> nat" where |
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"t_merge Leaf t2 = 1" | |
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"t_merge t2 Leaf = 1" | |
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"t_merge (Node n1 l1 a1 r1 =: t1) (Node n2 l2 a2 r2 =: t2) = |
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(if a1 \<le> a2 then 1 + t_merge r1 t2 |
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else 1 + t_merge r2 t1)" |
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definition t_insert :: "'a::ord \<Rightarrow> 'a lheap \<Rightarrow> nat" where |
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"t_insert x t = t_merge (Node 1 Leaf x Leaf) t" |
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fun t_del_min :: "'a::ord lheap \<Rightarrow> nat" where |
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"t_del_min Leaf = 1" | |
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"t_del_min (Node n l a r) = t_merge l r" |
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lemma t_merge_rank: "t_merge l r \<le> rank l + rank r + 1" |
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proof(induction l r rule: merge.induct) |
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case 3 thus ?case |
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by(simp)(fastforce split: tree.splits simp del: t_merge.simps) |
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qed simp_all |
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corollary t_merge_log: assumes "ltree l" "ltree r" |
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shows "t_merge l r \<le> log 2 (size1 l) + log 2 (size1 r) + 1" |
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using le_log2_of_power[OF pow2_rank_size1[OF assms(1)]] |
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le_log2_of_power[OF pow2_rank_size1[OF assms(2)]] t_merge_rank[of l r] |
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by linarith |
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corollary t_insert_log: "ltree t \<Longrightarrow> t_insert x t \<le> log 2 (size1 t) + 2" |
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using t_merge_log[of "Node 1 Leaf x Leaf" t] |
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by(simp add: t_insert_def split: tree.split) |
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(* FIXME mv ? *) |
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lemma ld_ld_1_less: |
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assumes "x > 0" "y > 0" shows "log 2 x + log 2 y + 1 < 2 * log 2 (x+y)" |
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proof - |
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have "2 powr (log 2 x + log 2 y + 1) = 2*x*y" |
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using assms by(simp add: powr_add) |
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also have "\<dots> < (x+y)^2" using assms |
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by(simp add: numeral_eq_Suc algebra_simps add_pos_pos) |
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also have "\<dots> = 2 powr (2 * log 2 (x+y))" |
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using assms by(simp add: powr_add log_powr[symmetric]) |
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finally show ?thesis by simp |
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qed |
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corollary t_del_min_log: assumes "ltree t" |
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shows "t_del_min t \<le> 2 * log 2 (size1 t) + 1" |
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proof(cases t) |
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case Leaf thus ?thesis using assms by simp |
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next |
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case [simp]: (Node _ t1 _ t2) |
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have "t_del_min t = t_merge t1 t2" by simp |
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also have "\<dots> \<le> log 2 (size1 t1) + log 2 (size1 t2) + 1" |
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using \<open>ltree t\<close> by (auto simp: t_merge_log simp del: t_merge.simps) |
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also have "\<dots> \<le> 2 * log 2 (size1 t) + 1" |
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using ld_ld_1_less[of "size1 t1" "size1 t2"] by (simp) |
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finally show ?thesis . |
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qed |
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end |