Theory HOL-Data_Structures.AVL_Set
subsection ‹Invariant›
theory AVL_Set
imports
AVL_Set_Code
"HOL-Number_Theory.Fib"
begin
fun avl :: "'a tree_ht ⇒ bool" where
"avl Leaf = True" |
"avl (Node l (a,n) r) =
(abs(int(height l) - int(height r)) ≤ 1 ∧
n = max (height l) (height r) + 1 ∧ avl l ∧ avl r)"
subsubsection ‹Insertion maintains AVL balance›
declare Let_def [simp]
lemma ht_height[simp]: "avl t ⟹ ht t = height t"
by (cases t rule: tree2_cases) simp_all
text ‹First, a fast but relatively manual proof with many lemmas:›
lemma height_balL:
"⟦ avl l; avl r; height l = height r + 2 ⟧ ⟹
height (balL l a r) ∈ {height r + 2, height r + 3}"
by (auto simp:node_def balL_def split:tree.split)
lemma height_balR:
"⟦ avl l; avl r; height r = height l + 2 ⟧ ⟹
height (balR l a r) : {height l + 2, height l + 3}"
by(auto simp add:node_def balR_def split:tree.split)
lemma height_node[simp]: "height(node l a r) = max (height l) (height r) + 1"
by (simp add: node_def)
lemma height_balL2:
"⟦ avl l; avl r; height l ≠ height r + 2 ⟧ ⟹
height (balL l a r) = 1 + max (height l) (height r)"
by (simp_all add: balL_def)
lemma height_balR2:
"⟦ avl l; avl r; height r ≠ height l + 2 ⟧ ⟹
height (balR l a r) = 1 + max (height l) (height r)"
by (simp_all add: balR_def)
lemma avl_balL:
"⟦ avl l; avl r; height r - 1 ≤ height l ∧ height l ≤ height r + 2 ⟧ ⟹ avl(balL l a r)"
by(auto simp: balL_def node_def split!: if_split tree.split)
lemma avl_balR:
"⟦ avl l; avl r; height l - 1 ≤ height r ∧ height r ≤ height l + 2 ⟧ ⟹ avl(balR l a r)"
by(auto simp: balR_def node_def split!: if_split tree.split)
text‹Insertion maintains the AVL property. Requires simultaneous proof.›
theorem avl_insert:
"avl t ⟹ avl(insert x t)"
"avl t ⟹ height (insert x t) ∈ {height t, height t + 1}"
proof (induction t rule: tree2_induct)
case (Node l a _ r)
case 1
show ?case
proof(cases "x = a")
case True with 1 show ?thesis by simp
next
case False
show ?thesis
proof(cases "x<a")
case True with 1 Node(1,2) show ?thesis by (auto intro!:avl_balL)
next
case False with 1 Node(3,4) ‹x≠a› show ?thesis by (auto intro!:avl_balR)
qed
qed
case 2
show ?case
proof(cases "x = a")
case True with 2 show ?thesis by simp
next
case False
show ?thesis
proof(cases "x<a")
case True
show ?thesis
proof(cases "height (insert x l) = height r + 2")
case False with 2 Node(1,2) ‹x < a› show ?thesis by (auto simp: height_balL2)
next
case True
hence "(height (balL (insert x l) a r) = height r + 2) ∨
(height (balL (insert x l) a r) = height r + 3)" (is "?A ∨ ?B")
using 2 Node(1,2) height_balL[OF _ _ True] by simp
thus ?thesis
proof
assume ?A with 2 ‹x < a› show ?thesis by (auto)
next
assume ?B with 2 Node(2) True ‹x < a› show ?thesis by (simp) arith
qed
qed
next
case False
show ?thesis
proof(cases "height (insert x r) = height l + 2")
case False with 2 Node(3,4) ‹¬x < a› show ?thesis by (auto simp: height_balR2)
next
case True
hence "(height (balR l a (insert x r)) = height l + 2) ∨
(height (balR l a (insert x r)) = height l + 3)" (is "?A ∨ ?B")
using 2 Node(3) height_balR[OF _ _ True] by simp
thus ?thesis
proof
assume ?A with 2 ‹¬x < a› show ?thesis by (auto)
next
assume ?B with 2 Node(4) True ‹¬x < a› show ?thesis by (simp) arith
qed
qed
qed
qed
qed simp_all
text ‹Now an automatic proof without lemmas:›
theorem avl_insert_auto: "avl t ⟹
avl(insert x t) ∧ height (insert x t) ∈ {height t, height t + 1}"
apply (induction t rule: tree2_induct)
apply (auto simp: balL_def balR_def node_def max_absorb2 split!: if_split tree.split)
done
subsubsection ‹Deletion maintains AVL balance›
lemma avl_split_max:
"⟦ avl t; t ≠ Leaf ⟧ ⟹
avl (fst (split_max t)) ∧
height t ∈ {height(fst (split_max t)), height(fst (split_max t)) + 1}"
by(induct t rule: split_max_induct)
(auto simp: balL_def node_def max_absorb2 split!: prod.split if_split tree.split)
text‹Deletion maintains the AVL property:›
theorem avl_delete:
"avl t ⟹ avl(delete x t)"
"avl t ⟹ height t ∈ {height (delete x t), height (delete x t) + 1}"
proof (induct t rule: tree2_induct)
case (Node l a n r)
case 1
show ?case
proof(cases "x = a")
case True thus ?thesis
using 1 avl_split_max[of l] by (auto intro!: avl_balR split: prod.split)
next
case False thus ?thesis
using Node 1 by (auto intro!: avl_balL avl_balR)
qed
case 2
show ?case
proof(cases "x = a")
case True thus ?thesis using 2 avl_split_max[of l]
by(auto simp: balR_def max_absorb2 split!: if_splits prod.split tree.split)
next
case False
show ?thesis
proof(cases "x<a")
case True
show ?thesis
proof(cases "height r = height (delete x l) + 2")
case False
thus ?thesis using 2 Node(1,2) ‹x < a› by(auto simp: balR_def)
next
case True
thus ?thesis using height_balR[OF _ _ True, of a] 2 Node(1,2) ‹x < a› by simp linarith
qed
next
case False
show ?thesis
proof(cases "height l = height (delete x r) + 2")
case False
thus ?thesis using 2 Node(3,4) ‹¬x < a› ‹x ≠ a› by(auto simp: balL_def)
next
case True
thus ?thesis
using height_balL[OF _ _ True, of a] 2 Node(3,4) ‹¬x < a› ‹x ≠ a› by simp linarith
qed
qed
qed
qed simp_all
text ‹A more automatic proof.
Complete automation as for insertion seems hard due to resource requirements.›
theorem avl_delete_auto:
"avl t ⟹ avl(delete x t)"
"avl t ⟹ height t ∈ {height (delete x t), height (delete x t) + 1}"
proof (induct t rule: tree2_induct)
case (Node l a n r)
case 1
thus ?case
using Node avl_split_max[of l] by (auto intro!: avl_balL avl_balR split: prod.split)
case 2
show ?case
using 2 Node avl_split_max[of l]
by auto
(auto simp: balL_def balR_def max_absorb1 max_absorb2 split!: tree.splits prod.splits if_splits)
qed simp_all
subsection "Overall correctness"
interpretation S: Set_by_Ordered
where empty = empty and isin = isin and insert = insert and delete = delete
and inorder = inorder and inv = avl
proof (standard, goal_cases)
case 1 show ?case by (simp add: empty_def)
next
case 2 thus ?case by(simp add: isin_set_inorder)
next
case 3 thus ?case by(simp add: inorder_insert)
next
case 4 thus ?case by(simp add: inorder_delete)
next
case 5 thus ?case by (simp add: empty_def)
next
case 6 thus ?case by (simp add: avl_insert(1))
next
case 7 thus ?case by (simp add: avl_delete(1))
qed
subsection ‹Height-Size Relation›
text ‹Any AVL tree of height ‹n› has at least ‹fib (n+2)› leaves:›
theorem avl_fib_bound:
"avl t ⟹ fib(height t + 2) ≤ size1 t"
proof (induction rule: tree2_induct)
case (Node l a h r)
have 1: "height l + 1 ≤ height r + 2" and 2: "height r + 1 ≤ height l + 2"
using Node.prems by auto
have "fib (max (height l) (height r) + 3) ≤ size1 l + size1 r"
proof cases
assume "height l ≥ height r"
hence "fib (max (height l) (height r) + 3) = fib (height l + 3)"
by(simp add: max_absorb1)
also have "… = fib (height l + 2) + fib (height l + 1)"
by (simp add: numeral_eq_Suc)
also have "… ≤ size1 l + fib (height l + 1)"
using Node by (simp)
also have "… ≤ size1 r + size1 l"
using Node fib_mono[OF 1] by auto
also have "… = size1 (Node l (a,h) r)"
by simp
finally show ?thesis
by (simp)
next
assume "¬ height l ≥ height r"
hence "fib (max (height l) (height r) + 3) = fib (height r + 3)"
by(simp add: max_absorb1)
also have "… = fib (height r + 2) + fib (height r + 1)"
by (simp add: numeral_eq_Suc)
also have "… ≤ size1 r + fib (height r + 1)"
using Node by (simp)
also have "… ≤ size1 r + size1 l"
using Node fib_mono[OF 2] by auto
also have "… = size1 (Node l (a,h) r)"
by simp
finally show ?thesis
by (simp)
qed
also have "… = size1 (Node l (a,h) r)"
by simp
finally show ?case by (simp del: fib.simps add: numeral_eq_Suc)
qed auto
lemma avl_fib_bound_auto: "avl t ⟹ fib (height t + 2) ≤ size1 t"
proof (induction t rule: tree2_induct)
case Leaf thus ?case by (simp)
next
case (Node l a h r)
have 1: "height l + 1 ≤ height r + 2" and 2: "height r + 1 ≤ height l + 2"
using Node.prems by auto
have left: "height l ≥ height r ⟹ ?case" (is "?asm ⟹ _")
using Node fib_mono[OF 1] by (simp add: max.absorb1)
have right: "height l ≤ height r ⟹ ?case"
using Node fib_mono[OF 2] by (simp add: max.absorb2)
show ?case using left right using Node.prems by simp linarith
qed
text ‹An exponential lower bound for \<^const>‹fib›:›
lemma fib_lowerbound:
defines "φ ≡ (1 + sqrt 5) / 2"
shows "real (fib(n+2)) ≥ φ ^ n"
proof (induction n rule: fib.induct)
case 1
then show ?case by simp
next
case 2
then show ?case by (simp add: φ_def real_le_lsqrt)
next
case (3 n)
have "φ ^ Suc (Suc n) = φ ^ 2 * φ ^ n"
by (simp add: field_simps power2_eq_square)
also have "… = (φ + 1) * φ ^ n"
by (simp_all add: φ_def power2_eq_square field_simps)
also have "… = φ ^ Suc n + φ ^ n"
by (simp add: field_simps)
also have "… ≤ real (fib (Suc n + 2)) + real (fib (n + 2))"
by (intro add_mono "3.IH")
finally show ?case by simp
qed
text ‹The size of an AVL tree is (at least) exponential in its height:›
lemma avl_size_lowerbound:
defines "φ ≡ (1 + sqrt 5) / 2"
assumes "avl t"
shows "φ ^ (height t) ≤ size1 t"
proof -
have "φ ^ height t ≤ fib (height t + 2)"
unfolding φ_def by(rule fib_lowerbound)
also have "… ≤ size1 t"
using avl_fib_bound[of t] assms by simp
finally show ?thesis .
qed
text ‹The height of an AVL tree is most \<^term>‹(1/log 2 φ)› ‹≈ 1.44› times worse
than \<^term>‹log 2 (size1 t)›:›
lemma avl_height_upperbound:
defines "φ ≡ (1 + sqrt 5) / 2"
assumes "avl t"
shows "height t ≤ (1/log 2 φ) * log 2 (size1 t)"
proof -
have "φ > 0" "φ > 1" by(auto simp: φ_def pos_add_strict)
hence "height t = log φ (φ ^ height t)" by(simp add: log_nat_power)
also have "… ≤ log φ (size1 t)"
using avl_size_lowerbound[OF assms(2), folded φ_def] ‹1 < φ›
by (simp add: le_log_of_power)
also have "… = (1/log 2 φ) * log 2 (size1 t)"
by(simp add: log_base_change[of 2 φ])
finally show ?thesis .
qed
end