src/HOL/Data_Structures/Balance_List.thy

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/HOL/Data_Structures/Balance_List.thy	Tue Aug 09 21:18:32 2016 +0200
@@ -0,0 +1,125 @@
+(* Tobias Nipkow *)
+
+section \<open>Creating a Balanced Tree from a List\<close>
+
+theory Balance_List
+imports
+  "~~/src/HOL/Library/Tree"
+  "~~/src/HOL/Library/Float"
+begin
+
+fun bal :: "'a list \<Rightarrow> nat \<Rightarrow> 'a tree * 'a list" where
+"bal xs n = (if n=0 then (Leaf,xs) else
+ (let m = n div 2;
+      (l, ys) = bal xs m;
+      (r, zs) = bal (tl ys) (n-1-m)
+  in (Node l (hd ys) r, zs)))"
+
+declare bal.simps[simp del]
+
+definition "balance xs = fst (bal xs (length xs))"
+
+lemma bal_inorder:
+  "bal xs n = (t,ys) \<Longrightarrow> n \<le> length xs \<Longrightarrow> inorder t = take n xs \<and> ys = drop n xs"
+proof(induction xs n arbitrary: t ys rule: bal.induct)
+  case (1 xs n) show ?case
+  proof cases
+    assume "n = 0" thus ?thesis using 1 by (simp add: bal.simps)
+  next
+    assume [arith]: "n \<noteq> 0"
+    let ?n1 = "n div 2" let ?n2 = "n - 1 - ?n1"
+    from "1.prems" obtain l r xs' where
+      b1: "bal xs ?n1 = (l,xs')" and
+      b2: "bal (tl xs') ?n2 = (r,ys)" and
+      t: "t = \<langle>l, hd xs', r\<rangle>"
+      using bal.simps[of xs n] by(auto simp: Let_def split: prod.splits)
+    have IH1: "inorder l = take ?n1 xs \<and> xs' = drop ?n1 xs"
+      using b1 "1.prems" by(intro "1.IH"(1)) auto
+    have IH2: "inorder r = take ?n2 (tl xs') \<and> ys = drop ?n2 (tl xs')"
+      using b1 b2 IH1 "1.prems" by(intro "1.IH"(2)) auto
+    have "drop (n div 2) xs \<noteq> []" using "1.prems"(2) by simp
+    hence "hd (drop ?n1 xs) # take ?n2 (tl (drop ?n1 xs)) = take (?n2 + 1) (drop ?n1 xs)"
+      by (metis Suc_eq_plus1 take_Suc)
+    hence *: "inorder t = take n xs" using t IH1 IH2
+      using take_add[of ?n1 "?n2+1" xs] by(simp)
+    have "n - n div 2 + n div 2 = n" by simp
+    hence "ys = drop n xs" using IH1 IH2 by (simp add: drop_Suc[symmetric])
+    thus ?thesis using * by blast
+  qed
+qed
+
+corollary balance_inorder: "inorder(balance xs) = xs"
+using bal_inorder[of xs "length xs"]
+by (metis balance_def order_refl prod.collapse take_all)
+
+lemma bal_height: "bal xs n = (t,ys) \<Longrightarrow> height t = floorlog 2 n"
+proof(induction xs n arbitrary: t ys rule: bal.induct)
+  case (1 xs n) show ?case
+  proof cases
+    assume "n = 0" thus ?thesis
+      using "1.prems" by (simp add: floorlog_def bal.simps)
+  next
+    assume [arith]: "n \<noteq> 0"
+    from "1.prems" obtain l r xs' where
+      b1: "bal xs (n div 2) = (l,xs')" and
+      b2: "bal (tl xs') (n - 1 - n div 2) = (r,ys)" and
+      t: "t = \<langle>l, hd xs', r\<rangle>"
+      using bal.simps[of xs n] by(auto simp: Let_def split: prod.splits)
+    let ?log1 = "floorlog 2 (n div 2)"
+    let ?log2 = "floorlog 2 (n - 1 - n div 2)"
+    have IH1: "height l = ?log1" using "1.IH"(1) b1 by simp
+    have IH2: "height r = ?log2" using "1.IH"(2) b1 b2 by simp
+    have "n div 2 \<ge> n - 1 - n div 2" by arith
+    hence le: "?log2 \<le> ?log1" by(simp add:floorlog_mono)
+    have "height t = max ?log1 ?log2 + 1" by (simp add: t IH1 IH2)
+    also have "\<dots> = ?log1 + 1" using le by (simp add: max_absorb1)
+    also have "\<dots> = floorlog 2 n" by (simp add: Float.compute_floorlog)
+    finally show ?thesis .
+  qed
+qed
+
+lemma bal_min_height:
+  "bal xs n = (t,ys) \<Longrightarrow> min_height t = floorlog 2 (n + 1) - 1"
+proof(induction xs n arbitrary: t ys rule: bal.induct)
+  case (1 xs n) show ?case
+  proof cases
+    assume "n = 0" thus ?thesis
+      using "1.prems" by (simp add: floorlog_def bal.simps)
+  next
+    assume [arith]: "n \<noteq> 0"
+    from "1.prems" obtain l r xs' where
+      b1: "bal xs (n div 2) = (l,xs')" and
+      b2: "bal (tl xs') (n - 1 - n div 2) = (r,ys)" and
+      t: "t = \<langle>l, hd xs', r\<rangle>"
+      using bal.simps[of xs n] by(auto simp: Let_def split: prod.splits)
+    let ?log1 = "floorlog 2 (n div 2 + 1) - 1"
+    let ?log2 = "floorlog 2 (n - 1 - n div 2 + 1) - 1"
+    let ?log2' = "floorlog 2 (n - n div 2) - 1"
+    have "n - 1 - n div 2 + 1 = n - n div 2" by arith
+    hence IH2: "min_height r = ?log2'" using "1.IH"(2) b1 b2 by simp
+    have IH1: "min_height l = ?log1" using "1.IH"(1) b1 by simp
+    have *: "floorlog 2 (n - n div 2) \<ge> 1" by (simp add: floorlog_def)
+    have "n div 2 + 1 \<ge> n - n div 2" by arith
+    with * have le: "?log2' \<le> ?log1" by(simp add: floorlog_mono diff_le_mono)
+    have "min_height t = min ?log1 ?log2' + 1" by (simp add: t IH1 IH2)
+    also have "\<dots> = ?log2' + 1" using le by (simp add: min_absorb2)
+    also have "\<dots> = floorlog 2 (n - n div 2)" by(simp add: floorlog_def)
+    also have "n - n div 2 = (n+1) div 2" by arith
+    also have "floorlog 2 \<dots> = floorlog 2 (n+1) - 1"
+      by (simp add: Float.compute_floorlog)
+    finally show ?thesis .
+  qed
+qed
+
+lemma balanced_bal:
+  assumes "bal xs n = (t,ys)" shows "height t - min_height t \<le> 1"
+proof -
+  have "floorlog 2 n \<le> floorlog 2 (n+1)" by (rule floorlog_mono) auto
+  thus ?thesis
+    using bal_height[OF assms] bal_min_height[OF assms] by arith
+qed
+
+corollary balanced_balance: "height(balance xs) - min_height(balance xs) \<le> 1"
+by (metis balance_def balanced_bal prod.collapse)
+
+end