# Theory Nat_Bijection

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theory Nat_Bijection
imports Parity
`(*  Title:      HOL/Library/Nat_Bijection.thy    Author:     Brian Huffman    Author:     Florian Haftmann    Author:     Stefan Richter    Author:     Tobias Nipkow    Author:     Alexander Krauss*)header {* Bijections between natural numbers and other types *}theory Nat_Bijectionimports Main Paritybeginsubsection {* Type @{typ "nat × nat"} *}text "Triangle numbers: 0, 1, 3, 6, 10, 15, ..."definition  triangle :: "nat => nat"where  "triangle n = n * Suc n div 2"lemma triangle_0 [simp]: "triangle 0 = 0"unfolding triangle_def by simplemma triangle_Suc [simp]: "triangle (Suc n) = triangle n + Suc n"unfolding triangle_def by simpdefinition  prod_encode :: "nat × nat => nat"where  "prod_encode = (λ(m, n). triangle (m + n) + m)"text {* In this auxiliary function, @{term "triangle k + m"} is an invariant. *}fun  prod_decode_aux :: "nat => nat => nat × nat"where  "prod_decode_aux k m =    (if m ≤ k then (m, k - m) else prod_decode_aux (Suc k) (m - Suc k))"declare prod_decode_aux.simps [simp del]definition  prod_decode :: "nat => nat × nat"where  "prod_decode = prod_decode_aux 0"lemma prod_encode_prod_decode_aux:  "prod_encode (prod_decode_aux k m) = triangle k + m"apply (induct k m rule: prod_decode_aux.induct)apply (subst prod_decode_aux.simps)apply (simp add: prod_encode_def)donelemma prod_decode_inverse [simp]: "prod_encode (prod_decode n) = n"unfolding prod_decode_def by (simp add: prod_encode_prod_decode_aux)lemma prod_decode_triangle_add:  "prod_decode (triangle k + m) = prod_decode_aux k m"apply (induct k arbitrary: m)apply (simp add: prod_decode_def)apply (simp only: triangle_Suc add_assoc)apply (subst prod_decode_aux.simps, simp)donelemma prod_encode_inverse [simp]: "prod_decode (prod_encode x) = x"unfolding prod_encode_defapply (induct x)apply (simp add: prod_decode_triangle_add)apply (subst prod_decode_aux.simps, simp)donelemma inj_prod_encode: "inj_on prod_encode A"by (rule inj_on_inverseI, rule prod_encode_inverse)lemma inj_prod_decode: "inj_on prod_decode A"by (rule inj_on_inverseI, rule prod_decode_inverse)lemma surj_prod_encode: "surj prod_encode"by (rule surjI, rule prod_decode_inverse)lemma surj_prod_decode: "surj prod_decode"by (rule surjI, rule prod_encode_inverse)lemma bij_prod_encode: "bij prod_encode"by (rule bijI [OF inj_prod_encode surj_prod_encode])lemma bij_prod_decode: "bij prod_decode"by (rule bijI [OF inj_prod_decode surj_prod_decode])lemma prod_encode_eq: "prod_encode x = prod_encode y <-> x = y"by (rule inj_prod_encode [THEN inj_eq])lemma prod_decode_eq: "prod_decode x = prod_decode y <-> x = y"by (rule inj_prod_decode [THEN inj_eq])text {* Ordering properties *}lemma le_prod_encode_1: "a ≤ prod_encode (a, b)"unfolding prod_encode_def by simplemma le_prod_encode_2: "b ≤ prod_encode (a, b)"unfolding prod_encode_def by (induct b, simp_all)subsection {* Type @{typ "nat + nat"} *}definition  sum_encode  :: "nat + nat => nat"where  "sum_encode x = (case x of Inl a => 2 * a | Inr b => Suc (2 * b))"definition  sum_decode  :: "nat => nat + nat"where  "sum_decode n = (if even n then Inl (n div 2) else Inr (n div 2))"lemma sum_encode_inverse [simp]: "sum_decode (sum_encode x) = x"unfolding sum_decode_def sum_encode_defby (induct x) simp_alllemma sum_decode_inverse [simp]: "sum_encode (sum_decode n) = n"unfolding sum_decode_def sum_encode_def numeral_2_eq_2by (simp add: even_nat_div_two_times_two odd_nat_div_two_times_two_plus_one         del: mult_Suc)lemma inj_sum_encode: "inj_on sum_encode A"by (rule inj_on_inverseI, rule sum_encode_inverse)lemma inj_sum_decode: "inj_on sum_decode A"by (rule inj_on_inverseI, rule sum_decode_inverse)lemma surj_sum_encode: "surj sum_encode"by (rule surjI, rule sum_decode_inverse)lemma surj_sum_decode: "surj sum_decode"by (rule surjI, rule sum_encode_inverse)lemma bij_sum_encode: "bij sum_encode"by (rule bijI [OF inj_sum_encode surj_sum_encode])lemma bij_sum_decode: "bij sum_decode"by (rule bijI [OF inj_sum_decode surj_sum_decode])lemma sum_encode_eq: "sum_encode x = sum_encode y <-> x = y"by (rule inj_sum_encode [THEN inj_eq])lemma sum_decode_eq: "sum_decode x = sum_decode y <-> x = y"by (rule inj_sum_decode [THEN inj_eq])subsection {* Type @{typ "int"} *}definition  int_encode :: "int => nat"where  "int_encode i = sum_encode (if 0 ≤ i then Inl (nat i) else Inr (nat (- i - 1)))"definition  int_decode :: "nat => int"where  "int_decode n = (case sum_decode n of Inl a => int a | Inr b => - int b - 1)"lemma int_encode_inverse [simp]: "int_decode (int_encode x) = x"unfolding int_decode_def int_encode_def by simplemma int_decode_inverse [simp]: "int_encode (int_decode n) = n"unfolding int_decode_def int_encode_def using sum_decode_inverse [of n]by (cases "sum_decode n", simp_all)lemma inj_int_encode: "inj_on int_encode A"by (rule inj_on_inverseI, rule int_encode_inverse)lemma inj_int_decode: "inj_on int_decode A"by (rule inj_on_inverseI, rule int_decode_inverse)lemma surj_int_encode: "surj int_encode"by (rule surjI, rule int_decode_inverse)lemma surj_int_decode: "surj int_decode"by (rule surjI, rule int_encode_inverse)lemma bij_int_encode: "bij int_encode"by (rule bijI [OF inj_int_encode surj_int_encode])lemma bij_int_decode: "bij int_decode"by (rule bijI [OF inj_int_decode surj_int_decode])lemma int_encode_eq: "int_encode x = int_encode y <-> x = y"by (rule inj_int_encode [THEN inj_eq])lemma int_decode_eq: "int_decode x = int_decode y <-> x = y"by (rule inj_int_decode [THEN inj_eq])subsection {* Type @{typ "nat list"} *}fun  list_encode :: "nat list => nat"where  "list_encode [] = 0"| "list_encode (x # xs) = Suc (prod_encode (x, list_encode xs))"function  list_decode :: "nat => nat list"where  "list_decode 0 = []"| "list_decode (Suc n) = (case prod_decode n of (x, y) => x # list_decode y)"by pat_completeness autotermination list_decodeapply (relation "measure id", simp_all)apply (drule arg_cong [where f="prod_encode"])apply (drule sym)apply (simp add: le_imp_less_Suc le_prod_encode_2)donelemma list_encode_inverse [simp]: "list_decode (list_encode x) = x"by (induct x rule: list_encode.induct) simp_alllemma list_decode_inverse [simp]: "list_encode (list_decode n) = n"apply (induct n rule: list_decode.induct, simp)apply (simp split: prod.split)apply (simp add: prod_decode_eq [symmetric])donelemma inj_list_encode: "inj_on list_encode A"by (rule inj_on_inverseI, rule list_encode_inverse)lemma inj_list_decode: "inj_on list_decode A"by (rule inj_on_inverseI, rule list_decode_inverse)lemma surj_list_encode: "surj list_encode"by (rule surjI, rule list_decode_inverse)lemma surj_list_decode: "surj list_decode"by (rule surjI, rule list_encode_inverse)lemma bij_list_encode: "bij list_encode"by (rule bijI [OF inj_list_encode surj_list_encode])lemma bij_list_decode: "bij list_decode"by (rule bijI [OF inj_list_decode surj_list_decode])lemma list_encode_eq: "list_encode x = list_encode y <-> x = y"by (rule inj_list_encode [THEN inj_eq])lemma list_decode_eq: "list_decode x = list_decode y <-> x = y"by (rule inj_list_decode [THEN inj_eq])subsection {* Finite sets of naturals *}subsubsection {* Preliminaries *}lemma finite_vimage_Suc_iff: "finite (Suc -` F) <-> finite F"apply (safe intro!: finite_vimageI inj_Suc)apply (rule finite_subset [where B="insert 0 (Suc ` Suc -` F)"])apply (rule subsetI, case_tac x, simp, simp)apply (rule finite_insert [THEN iffD2])apply (erule finite_imageI)donelemma vimage_Suc_insert_0: "Suc -` insert 0 A = Suc -` A"by autolemma vimage_Suc_insert_Suc:  "Suc -` insert (Suc n) A = insert n (Suc -` A)"by autolemma even_nat_Suc_div_2: "even x ==> Suc x div 2 = x div 2"by (simp only: numeral_2_eq_2 even_nat_plus_one_div_two)lemma div2_even_ext_nat:  "[|x div 2 = y div 2; even x = even y|] ==> (x::nat) = y"apply (rule mod_div_equality [of x 2, THEN subst])apply (rule mod_div_equality [of y 2, THEN subst])apply (case_tac "even x")apply (simp add: numeral_2_eq_2 even_nat_equiv_def)apply (simp add: numeral_2_eq_2 odd_nat_equiv_def)donesubsubsection {* From sets to naturals *}definition  set_encode :: "nat set => nat"where  "set_encode = setsum (op ^ 2)"lemma set_encode_empty [simp]: "set_encode {} = 0"by (simp add: set_encode_def)lemma set_encode_insert [simp]:  "[|finite A; n ∉ A|] ==> set_encode (insert n A) = 2^n + set_encode A"by (simp add: set_encode_def)lemma even_set_encode_iff: "finite A ==> even (set_encode A) <-> 0 ∉ A"unfolding set_encode_def by (induct set: finite, auto)lemma set_encode_vimage_Suc: "set_encode (Suc -` A) = set_encode A div 2"apply (cases "finite A")apply (erule finite_induct, simp)apply (case_tac x)apply (simp add: even_nat_Suc_div_2 even_set_encode_iff vimage_Suc_insert_0)apply (simp add: finite_vimageI add_commute vimage_Suc_insert_Suc)apply (simp add: set_encode_def finite_vimage_Suc_iff)donelemmas set_encode_div_2 = set_encode_vimage_Suc [symmetric]subsubsection {* From naturals to sets *}definition  set_decode :: "nat => nat set"where  "set_decode x = {n. odd (x div 2 ^ n)}"lemma set_decode_0 [simp]: "0 ∈ set_decode x <-> odd x"by (simp add: set_decode_def)lemma set_decode_Suc [simp]:  "Suc n ∈ set_decode x <-> n ∈ set_decode (x div 2)"by (simp add: set_decode_def div_mult2_eq)lemma set_decode_zero [simp]: "set_decode 0 = {}"by (simp add: set_decode_def)lemma set_decode_div_2: "set_decode (x div 2) = Suc -` set_decode x"by autolemma set_decode_plus_power_2:  "n ∉ set_decode z ==> set_decode (2 ^ n + z) = insert n (set_decode z)" apply (induct n arbitrary: z, simp_all)  apply (rule set_eqI, induct_tac x, simp, simp add: even_nat_Suc_div_2) apply (rule set_eqI, induct_tac x, simp, simp add: add_commute)donelemma finite_set_decode [simp]: "finite (set_decode n)"apply (induct n rule: nat_less_induct)apply (case_tac "n = 0", simp)apply (drule_tac x="n div 2" in spec, simp)apply (simp add: set_decode_div_2)apply (simp add: finite_vimage_Suc_iff)donesubsubsection {* Proof of isomorphism *}lemma set_decode_inverse [simp]: "set_encode (set_decode n) = n"apply (induct n rule: nat_less_induct)apply (case_tac "n = 0", simp)apply (drule_tac x="n div 2" in spec, simp)apply (simp add: set_decode_div_2 set_encode_vimage_Suc)apply (erule div2_even_ext_nat)apply (simp add: even_set_encode_iff)donelemma set_encode_inverse [simp]: "finite A ==> set_decode (set_encode A) = A"apply (erule finite_induct, simp_all)apply (simp add: set_decode_plus_power_2)donelemma inj_on_set_encode: "inj_on set_encode (Collect finite)"by (rule inj_on_inverseI [where g="set_decode"], simp)lemma set_encode_eq:  "[|finite A; finite B|] ==> set_encode A = set_encode B <-> A = B"by (rule iffI, simp add: inj_onD [OF inj_on_set_encode], simp)end`