Theory Uprod

(* Title: HOL/Library/Uprod.thy
   Author: Andreas Lochbihler, ETH Zurich *)

section ‹Unordered pairs›

theory Uprod imports Main begin

typedef ('a, 'b) commute = "{f :: 'a  'a  'b. x y. f x y = f y x}"
  morphisms apply_commute Abs_commute
  by auto

setup_lifting type_definition_commute

lemma apply_commute_commute: "apply_commute f x y = apply_commute f y x"
by(transfer) simp

context includes lifting_syntax begin

lift_definition rel_commute :: "('a  'b  bool)  ('c  'd  bool)  ('a, 'c) commute  ('b, 'd) commute  bool"
is "λA B. A ===> A ===> B" .


definition eq_upair :: "('a × 'a)  ('a × 'a)  bool"
where "eq_upair = (λ(a, b) (c, d). a = c  b = d  a = d  b = c)"

lemma eq_upair_simps [simp]:
  "eq_upair (a, b) (c, d)  a = c  b = d  a = d  b = c"
by(simp add: eq_upair_def)

lemma equivp_eq_upair: "equivp eq_upair"
by(auto simp add: equivp_def fun_eq_iff)

quotient_type 'a uprod = "'a × 'a" / eq_upair by(rule equivp_eq_upair)

lift_definition Upair :: "'a  'a  'a uprod" is Pair parametric Pair_transfer[of "A" "A" for A] .

lemma uprod_exhaust [case_names Upair, cases type: uprod]:
  obtains a b where "x = Upair a b"
by transfer fastforce

lemma Upair_inject [simp]: "Upair a b = Upair c d  a = c  b = d  a = d  b = c"
by transfer auto

code_datatype Upair

lift_definition case_uprod :: "('a, 'b) commute  'a uprod  'b" is case_prod
  parametric case_prod_transfer[of A A for A] by auto

lemma case_uprod_simps [simp, code]: "case_uprod f (Upair x y) = apply_commute f x y"
by transfer auto

lemma uprod_split: "P (case_uprod f x)  (a b. x = Upair a b  P (apply_commute f a b))"
by transfer auto

lemma uprod_split_asm: "P (case_uprod f x)  ¬ (a b. x = Upair a b  ¬ P (apply_commute f a b))"
by transfer auto

lift_definition not_equal :: "('a, bool) commute" is "(≠)" by auto

lemma apply_not_equal [simp]: "apply_commute not_equal x y  x  y"
by transfer simp

definition proper_uprod :: "'a uprod  bool"
where "proper_uprod = case_uprod not_equal"

lemma proper_uprod_simps [simp, code]: "proper_uprod (Upair x y)  x  y"
by(simp add: proper_uprod_def)

context includes lifting_syntax begin

private lemma set_uprod_parametric':
  "(rel_prod A A ===> rel_set A) (λ(a, b). {a, b}) (λ(a, b). {a, b})"
by transfer_prover

lift_definition set_uprod :: "'a uprod  'a set" is "λ(a, b). {a, b}" 
  parametric set_uprod_parametric' by auto

lemma set_uprod_simps [simp, code]: "set_uprod (Upair x y) = {x, y}"
by transfer simp

lemma finite_set_uprod [simp]: "finite (set_uprod x)"
by(cases x) simp

private lemma map_uprod_parametric':
  "((A ===> B) ===> rel_prod A A ===> rel_prod B B) (λf. map_prod f f) (λf. map_prod f f)"
by transfer_prover

lift_definition map_uprod :: "('a  'b)  'a uprod  'b uprod" is "λf. map_prod f f"
  parametric map_uprod_parametric' by auto

lemma map_uprod_simps [simp, code]: "map_uprod f (Upair x y) = Upair (f x) (f y)"
by transfer simp

private lemma rel_uprod_transfer':
  "((A ===> B ===> (=)) ===> rel_prod A A ===> rel_prod B B ===> (=))
   (λR (a, b) (c, d). R a c  R b d  R a d  R b c) (λR (a, b) (c, d). R a c  R b d  R a d  R b c)"
by transfer_prover

lift_definition rel_uprod :: "('a  'b  bool)  'a uprod  'b uprod  bool"
  is "λR (a, b) (c, d). R a c  R b d  R a d  R b c" parametric rel_uprod_transfer'
by auto

lemma rel_uprod_simps [simp, code]:
  "rel_uprod R (Upair a b) (Upair c d)  R a c  R b d  R a d  R b c"
by transfer auto

lemma Upair_parametric [transfer_rule]: "(A ===> A ===> rel_uprod A) Upair Upair"
unfolding rel_fun_def by transfer auto

lemma case_uprod_parametric [transfer_rule]:
  "(rel_commute A B ===> rel_uprod A ===> B) case_uprod case_uprod"
unfolding rel_fun_def by transfer(force dest: rel_funD)


bnf uprod: "'a uprod" 
  map: map_uprod
  sets: set_uprod
  bd: natLeq
  rel: rel_uprod
proof -
  show "map_uprod id = id" unfolding fun_eq_iff by transfer auto
  show "map_uprod (g  f) = map_uprod g  map_uprod f" for f :: "'a  'b" and g :: "'b  'c"
    unfolding fun_eq_iff by transfer auto
  show "map_uprod f x = map_uprod g x" if "z. z  set_uprod x  f z = g z" 
    for f :: "'a  'b" and g x using that by transfer auto
  show "set_uprod  map_uprod f = (`) f  set_uprod" for f :: "'a  'b" by transfer auto
  show "card_order natLeq" by(rule natLeq_card_order)
  show "BNF_Cardinal_Arithmetic.cinfinite natLeq" by(rule natLeq_cinfinite)
  show "regularCard natLeq" by(rule regularCard_natLeq)
  show "ordLess2 (card_of (set_uprod x)) natLeq" for x :: "'a uprod"
    by (auto simp flip: finite_iff_ordLess_natLeq)
  show "rel_uprod R OO rel_uprod S  rel_uprod (R OO S)"
    for R :: "'a  'b  bool" and S :: "'b  'c  bool" by(rule predicate2I)(transfer; auto)
  show "rel_uprod R = (λx y. z. set_uprod z  {(x, y). R x y}  map_uprod fst z = x  map_uprod snd z = y)"
    for R :: "'a  'b  bool" by transfer(auto simp add: fun_eq_iff)

lemma pred_uprod_code [simp, code]: "pred_uprod P (Upair x y)  P x  P y"
by(simp add: pred_uprod_def)

instantiation uprod :: (equal) equal begin

definition equal_uprod :: "'a uprod  'a uprod  bool"
where "equal_uprod = (=)"

lemma equal_uprod_code [code]:
  "HOL.equal (Upair x y) (Upair z u)  x = z  y = u  x = u  y = z"
unfolding equal_uprod_def by simp

instance by standard(simp add: equal_uprod_def)

quickcheck_generator uprod constructors: Upair

lemma UNIV_uprod: "UNIV = (λx. Upair x x) ` UNIV  (λ(x, y). Upair x y) ` Sigma UNIV (λx. UNIV - {x})"
apply(rule set_eqI)
subgoal for x by(cases x) auto

context begin
private lift_definition upair_inv :: "'a uprod  'a"
is "λ(x, y). if x = y then x else undefined" by auto

lemma finite_UNIV_prod [simp]:
  "finite (UNIV :: 'a uprod set)  finite (UNIV :: 'a set)" (is "?lhs = ?rhs")
  assume ?lhs
  hence "finite (range (λx :: 'a. Upair x x))" by(rule finite_subset[rotated]) simp
  hence "finite (upair_inv ` range (λx :: 'a. Upair x x))" by(rule finite_imageI)
  also have "upair_inv (Upair x x) = x" for x :: 'a by transfer simp
  then have "upair_inv ` range (λx :: 'a. Upair x x) = UNIV" by(auto simp add: image_image)
  finally show ?rhs .
qed(simp add: UNIV_uprod)


lemma card_UNIV_uprod:
  "card (UNIV :: 'a uprod set) = card (UNIV :: 'a set) * (card (UNIV :: 'a set) + 1) div 2"
  (is "?UPROD = ?A * _ div _")
proof(cases "finite (UNIV :: 'a set)")
  case True
  from True obtain f :: "nat  'a" where bij: "bij_betw f {0..<?A} UNIV"
    by (blast dest: ex_bij_betw_nat_finite)
  hence [simp]: "f ` {0..<?A} = UNIV" by(rule bij_betw_imp_surj_on)
  have "UNIV = (λ(x, y). Upair (f x) (f y)) ` (SIGMA x:{0..<?A}. {..x})"
    apply(rule set_eqI)
    subgoal for x
      apply(cases x)
      subgoal for a b
        apply(cases "inv_into {0..<?A} f a  inv_into {0..<?A} f b")
        subgoal by(rule rev_image_eqI[where x="(inv_into {0..<?A} f _, inv_into {0..<?A} f _)"])
                  (auto simp add: inv_into_into[where A="{0..<?A}" and f=f, simplified] intro: f_inv_into_f[where f=f, symmetric])
          apply(simp only: not_le)
          apply(drule less_imp_le)
          apply(rule rev_image_eqI[where x="(inv_into {0..<?A} f _, inv_into {0..<?A} f _)"])
          apply(auto simp add: inv_into_into[where A="{0..<?A}" and f=f, simplified] intro: f_inv_into_f[where f=f, symmetric])
  hence "?UPROD = card " by simp
  also have " = card (SIGMA x:{0..<?A}. {..x})"
    apply(rule card_image)
    using bij[THEN bij_betw_imp_inj_on]
    by(simp add: inj_on_def Ball_def)(metis leD le_eq_less_or_eq le_less_trans)
  also have " = sum Suc {0..<?A}"
    by (subst card_SigmaI) simp_all
  also have " = sum of_nat {Suc 0..?A}"
    using sum.atLeastLessThan_reindex [symmetric, of Suc 0 ?A id]
    by (simp del: sum.op_ivl_Suc add: atLeastLessThanSuc_atLeastAtMost)
  also have " = ?A * (?A + 1) div 2"
    using gauss_sum_from_Suc_0 [of ?A, where ?'a = nat] by simp
  finally show ?thesis .
qed simp