Theory Extended_Nat

(*  Title:      HOL/Library/Extended_Nat.thy
    Author:     David von Oheimb, TU Muenchen;  Florian Haftmann, TU Muenchen
    Contributions: David Trachtenherz, TU Muenchen
*)

header {* Extended natural numbers (i.e. with infinity) *}

theory Extended_Nat
imports Main
begin

class infinity =
  fixes infinity :: "'a"

notation (xsymbols)
  infinity  ("∞")

notation (HTML output)
  infinity  ("∞")

subsection {* Type definition *}

text {*
  We extend the standard natural numbers by a special value indicating
  infinity.
*}

typedef (open) enat = "UNIV :: nat option set" ..
 
definition enat :: "nat => enat" where
  "enat n = Abs_enat (Some n)"
 
instantiation enat :: infinity
begin
  definition "∞ = Abs_enat None"
  instance proof qed
end
 
rep_datatype enat "∞ :: enat"
proof -
  fix P i assume "!!j. P (enat j)" "P ∞"
  then show "P i"
  proof induct
    case (Abs_enat y) then show ?case
      by (cases y rule: option.exhaust)
         (auto simp: enat_def infinity_enat_def)
  qed
qed (auto simp add: enat_def infinity_enat_def Abs_enat_inject)

declare [[coercion "enat::nat=>enat"]]

lemma not_infinity_eq [iff]: "(x ≠ ∞) = (EX i. x = enat i)"
  by (cases x) auto

lemma not_enat_eq [iff]: "(ALL y. x ~= enat y) = (x = ∞)"
  by (cases x) auto

primrec the_enat :: "enat => nat"
  where "the_enat (enat n) = n"

subsection {* Constructors and numbers *}

instantiation enat :: "{zero, one, number}"
begin

definition
  "0 = enat 0"

definition
  [code_unfold]: "1 = enat 1"

definition
  [code_unfold, code del]: "number_of k = enat (number_of k)"

instance ..

end

definition eSuc :: "enat => enat" where
  "eSuc i = (case i of enat n => enat (Suc n) | ∞ => ∞)"

lemma enat_0: "enat 0 = 0"
  by (simp add: zero_enat_def)

lemma enat_1: "enat 1 = 1"
  by (simp add: one_enat_def)

lemma enat_number: "enat (number_of k) = number_of k"
  by (simp add: number_of_enat_def)

lemma one_eSuc: "1 = eSuc 0"
  by (simp add: zero_enat_def one_enat_def eSuc_def)

lemma infinity_ne_i0 [simp]: "(∞::enat) ≠ 0"
  by (simp add: zero_enat_def)

lemma i0_ne_infinity [simp]: "0 ≠ (∞::enat)"
  by (simp add: zero_enat_def)

lemma zero_enat_eq [simp]:
  "number_of k = (0::enat) <-> number_of k = (0::nat)"
  "(0::enat) = number_of k <-> number_of k = (0::nat)"
  unfolding zero_enat_def number_of_enat_def by simp_all

lemma one_enat_eq [simp]:
  "number_of k = (1::enat) <-> number_of k = (1::nat)"
  "(1::enat) = number_of k <-> number_of k = (1::nat)"
  unfolding one_enat_def number_of_enat_def by simp_all

lemma zero_one_enat_neq [simp]:
  "¬ 0 = (1::enat)"
  "¬ 1 = (0::enat)"
  unfolding zero_enat_def one_enat_def by simp_all

lemma infinity_ne_i1 [simp]: "(∞::enat) ≠ 1"
  by (simp add: one_enat_def)

lemma i1_ne_infinity [simp]: "1 ≠ (∞::enat)"
  by (simp add: one_enat_def)

lemma infinity_ne_number [simp]: "(∞::enat) ≠ number_of k"
  by (simp add: number_of_enat_def)

lemma number_ne_infinity [simp]: "number_of k ≠ (∞::enat)"
  by (simp add: number_of_enat_def)

lemma eSuc_enat: "eSuc (enat n) = enat (Suc n)"
  by (simp add: eSuc_def)

lemma eSuc_number_of: "eSuc (number_of k) = enat (Suc (number_of k))"
  by (simp add: eSuc_enat number_of_enat_def)

lemma eSuc_infinity [simp]: "eSuc ∞ = ∞"
  by (simp add: eSuc_def)

lemma eSuc_ne_0 [simp]: "eSuc n ≠ 0"
  by (simp add: eSuc_def zero_enat_def split: enat.splits)

lemma zero_ne_eSuc [simp]: "0 ≠ eSuc n"
  by (rule eSuc_ne_0 [symmetric])

lemma eSuc_inject [simp]: "eSuc m = eSuc n <-> m = n"
  by (simp add: eSuc_def split: enat.splits)

lemma number_of_enat_inject [simp]:
  "(number_of k :: enat) = number_of l <-> (number_of k :: nat) = number_of l"
  by (simp add: number_of_enat_def)


subsection {* Addition *}

instantiation enat :: comm_monoid_add
begin

definition [nitpick_simp]:
  "m + n = (case m of ∞ => ∞ | enat m => (case n of ∞ => ∞ | enat n => enat (m + n)))"

lemma plus_enat_simps [simp, code]:
  fixes q :: enat
  shows "enat m + enat n = enat (m + n)"
    and "∞ + q = ∞"
    and "q + ∞ = ∞"
  by (simp_all add: plus_enat_def split: enat.splits)

instance proof
  fix n m q :: enat
  show "n + m + q = n + (m + q)"
    by (cases n, auto, cases m, auto, cases q, auto)
  show "n + m = m + n"
    by (cases n, auto, cases m, auto)
  show "0 + n = n"
    by (cases n) (simp_all add: zero_enat_def)
qed

end

lemma plus_enat_0 [simp]:
  "0 + (q::enat) = q"
  "(q::enat) + 0 = q"
  by (simp_all add: plus_enat_def zero_enat_def split: enat.splits)

lemma plus_enat_number [simp]:
  "(number_of k :: enat) + number_of l = (if k < Int.Pls then number_of l
    else if l < Int.Pls then number_of k else number_of (k + l))"
  unfolding number_of_enat_def plus_enat_simps nat_arith(1) if_distrib [symmetric, of _ enat] ..

lemma eSuc_number [simp]:
  "eSuc (number_of k) = (if neg (number_of k :: int) then 1 else number_of (Int.succ k))"
  unfolding eSuc_number_of
  unfolding one_enat_def number_of_enat_def Suc_nat_number_of if_distrib [symmetric] ..

lemma eSuc_plus_1:
  "eSuc n = n + 1"
  by (cases n) (simp_all add: eSuc_enat one_enat_def)
  
lemma plus_1_eSuc:
  "1 + q = eSuc q"
  "q + 1 = eSuc q"
  by (simp_all add: eSuc_plus_1 add_ac)

lemma iadd_Suc: "eSuc m + n = eSuc (m + n)"
  by (simp_all add: eSuc_plus_1 add_ac)

lemma iadd_Suc_right: "m + eSuc n = eSuc (m + n)"
  by (simp only: add_commute[of m] iadd_Suc)

lemma iadd_is_0: "(m + n = (0::enat)) = (m = 0 ∧ n = 0)"
  by (cases m, cases n, simp_all add: zero_enat_def)

subsection {* Multiplication *}

instantiation enat :: comm_semiring_1
begin

definition times_enat_def [nitpick_simp]:
  "m * n = (case m of ∞ => if n = 0 then 0 else ∞ | enat m =>
    (case n of ∞ => if m = 0 then 0 else ∞ | enat n => enat (m * n)))"

lemma times_enat_simps [simp, code]:
  "enat m * enat n = enat (m * n)"
  "∞ * ∞ = (∞::enat)"
  "∞ * enat n = (if n = 0 then 0 else ∞)"
  "enat m * ∞ = (if m = 0 then 0 else ∞)"
  unfolding times_enat_def zero_enat_def
  by (simp_all split: enat.split)

instance proof
  fix a b c :: enat
  show "(a * b) * c = a * (b * c)"
    unfolding times_enat_def zero_enat_def
    by (simp split: enat.split)
  show "a * b = b * a"
    unfolding times_enat_def zero_enat_def
    by (simp split: enat.split)
  show "1 * a = a"
    unfolding times_enat_def zero_enat_def one_enat_def
    by (simp split: enat.split)
  show "(a + b) * c = a * c + b * c"
    unfolding times_enat_def zero_enat_def
    by (simp split: enat.split add: left_distrib)
  show "0 * a = 0"
    unfolding times_enat_def zero_enat_def
    by (simp split: enat.split)
  show "a * 0 = 0"
    unfolding times_enat_def zero_enat_def
    by (simp split: enat.split)
  show "(0::enat) ≠ 1"
    unfolding zero_enat_def one_enat_def
    by simp
qed

end

lemma mult_eSuc: "eSuc m * n = n + m * n"
  unfolding eSuc_plus_1 by (simp add: algebra_simps)

lemma mult_eSuc_right: "m * eSuc n = m + m * n"
  unfolding eSuc_plus_1 by (simp add: algebra_simps)

lemma of_nat_eq_enat: "of_nat n = enat n"
  apply (induct n)
  apply (simp add: enat_0)
  apply (simp add: plus_1_eSuc eSuc_enat)
  done

instance enat :: number_semiring
proof
  fix n show "number_of (int n) = (of_nat n :: enat)"
    unfolding number_of_enat_def number_of_int of_nat_id of_nat_eq_enat ..
qed

instance enat :: semiring_char_0 proof
  have "inj enat" by (rule injI) simp
  then show "inj (λn. of_nat n :: enat)" by (simp add: of_nat_eq_enat)
qed

lemma imult_is_0 [simp]: "((m::enat) * n = 0) = (m = 0 ∨ n = 0)"
  by (auto simp add: times_enat_def zero_enat_def split: enat.split)

lemma imult_is_infinity: "((a::enat) * b = ∞) = (a = ∞ ∧ b ≠ 0 ∨ b = ∞ ∧ a ≠ 0)"
  by (auto simp add: times_enat_def zero_enat_def split: enat.split)


subsection {* Subtraction *}

instantiation enat :: minus
begin

definition diff_enat_def:
"a - b = (case a of (enat x) => (case b of (enat y) => enat (x - y) | ∞ => 0)
          | ∞ => ∞)"

instance ..

end

lemma idiff_enat_enat [simp,code]: "enat a - enat b = enat (a - b)"
  by (simp add: diff_enat_def)

lemma idiff_infinity [simp,code]: "∞ - n = (∞::enat)"
  by (simp add: diff_enat_def)

lemma idiff_infinity_right [simp,code]: "enat a - ∞ = 0"
  by (simp add: diff_enat_def)

lemma idiff_0 [simp]: "(0::enat) - n = 0"
  by (cases n, simp_all add: zero_enat_def)

lemmas idiff_enat_0 [simp] = idiff_0 [unfolded zero_enat_def]

lemma idiff_0_right [simp]: "(n::enat) - 0 = n"
  by (cases n) (simp_all add: zero_enat_def)

lemmas idiff_enat_0_right [simp] = idiff_0_right [unfolded zero_enat_def]

lemma idiff_self [simp]: "n ≠ ∞ ==> (n::enat) - n = 0"
  by (auto simp: zero_enat_def)

lemma eSuc_minus_eSuc [simp]: "eSuc n - eSuc m = n - m"
  by (simp add: eSuc_def split: enat.split)

lemma eSuc_minus_1 [simp]: "eSuc n - 1 = n"
  by (simp add: one_enat_def eSuc_enat[symmetric] zero_enat_def[symmetric])

(*lemmas idiff_self_eq_0_enat = idiff_self_eq_0[unfolded zero_enat_def]*)

subsection {* Ordering *}

instantiation enat :: linordered_ab_semigroup_add
begin

definition [nitpick_simp]:
  "m ≤ n = (case n of enat n1 => (case m of enat m1 => m1 ≤ n1 | ∞ => False)
    | ∞ => True)"

definition [nitpick_simp]:
  "m < n = (case m of enat m1 => (case n of enat n1 => m1 < n1 | ∞ => True)
    | ∞ => False)"

lemma enat_ord_simps [simp]:
  "enat m ≤ enat n <-> m ≤ n"
  "enat m < enat n <-> m < n"
  "q ≤ (∞::enat)"
  "q < (∞::enat) <-> q ≠ ∞"
  "(∞::enat) ≤ q <-> q = ∞"
  "(∞::enat) < q <-> False"
  by (simp_all add: less_eq_enat_def less_enat_def split: enat.splits)

lemma enat_ord_code [code]:
  "enat m ≤ enat n <-> m ≤ n"
  "enat m < enat n <-> m < n"
  "q ≤ (∞::enat) <-> True"
  "enat m < ∞ <-> True"
  "∞ ≤ enat n <-> False"
  "(∞::enat) < q <-> False"
  by simp_all

instance by default
  (auto simp add: less_eq_enat_def less_enat_def plus_enat_def split: enat.splits)

end

instance enat :: ordered_comm_semiring
proof
  fix a b c :: enat
  assume "a ≤ b" and "0 ≤ c"
  thus "c * a ≤ c * b"
    unfolding times_enat_def less_eq_enat_def zero_enat_def
    by (simp split: enat.splits)
qed

lemma enat_ord_number [simp]:
  "(number_of m :: enat) ≤ number_of n <-> (number_of m :: nat) ≤ number_of n"
  "(number_of m :: enat) < number_of n <-> (number_of m :: nat) < number_of n"
  by (simp_all add: number_of_enat_def)

lemma i0_lb [simp]: "(0::enat) ≤ n"
  by (simp add: zero_enat_def less_eq_enat_def split: enat.splits)

lemma ile0_eq [simp]: "n ≤ (0::enat) <-> n = 0"
  by (simp add: zero_enat_def less_eq_enat_def split: enat.splits)

lemma infinity_ileE [elim!]: "∞ ≤ enat m ==> R"
  by (simp add: zero_enat_def less_eq_enat_def split: enat.splits)

lemma infinity_ilessE [elim!]: "∞ < enat m ==> R"
  by simp

lemma not_iless0 [simp]: "¬ n < (0::enat)"
  by (simp add: zero_enat_def less_enat_def split: enat.splits)

lemma i0_less [simp]: "(0::enat) < n <-> n ≠ 0"
  by (simp add: zero_enat_def less_enat_def split: enat.splits)

lemma eSuc_ile_mono [simp]: "eSuc n ≤ eSuc m <-> n ≤ m"
  by (simp add: eSuc_def less_eq_enat_def split: enat.splits)
 
lemma eSuc_mono [simp]: "eSuc n < eSuc m <-> n < m"
  by (simp add: eSuc_def less_enat_def split: enat.splits)

lemma ile_eSuc [simp]: "n ≤ eSuc n"
  by (simp add: eSuc_def less_eq_enat_def split: enat.splits)

lemma not_eSuc_ilei0 [simp]: "¬ eSuc n ≤ 0"
  by (simp add: zero_enat_def eSuc_def less_eq_enat_def split: enat.splits)

lemma i0_iless_eSuc [simp]: "0 < eSuc n"
  by (simp add: zero_enat_def eSuc_def less_enat_def split: enat.splits)

lemma iless_eSuc0[simp]: "(n < eSuc 0) = (n = 0)"
  by (simp add: zero_enat_def eSuc_def less_enat_def split: enat.split)

lemma ileI1: "m < n ==> eSuc m ≤ n"
  by (simp add: eSuc_def less_eq_enat_def less_enat_def split: enat.splits)

lemma Suc_ile_eq: "enat (Suc m) ≤ n <-> enat m < n"
  by (cases n) auto

lemma iless_Suc_eq [simp]: "enat m < eSuc n <-> enat m ≤ n"
  by (auto simp add: eSuc_def less_enat_def split: enat.splits)

lemma imult_infinity: "(0::enat) < n ==> ∞ * n = ∞"
  by (simp add: zero_enat_def less_enat_def split: enat.splits)

lemma imult_infinity_right: "(0::enat) < n ==> n * ∞ = ∞"
  by (simp add: zero_enat_def less_enat_def split: enat.splits)

lemma enat_0_less_mult_iff: "(0 < (m::enat) * n) = (0 < m ∧ 0 < n)"
  by (simp only: i0_less imult_is_0, simp)

lemma mono_eSuc: "mono eSuc"
  by (simp add: mono_def)


lemma min_enat_simps [simp]:
  "min (enat m) (enat n) = enat (min m n)"
  "min q 0 = 0"
  "min 0 q = 0"
  "min q (∞::enat) = q"
  "min (∞::enat) q = q"
  by (auto simp add: min_def)

lemma max_enat_simps [simp]:
  "max (enat m) (enat n) = enat (max m n)"
  "max q 0 = q"
  "max 0 q = q"
  "max q ∞ = (∞::enat)"
  "max ∞ q = (∞::enat)"
  by (simp_all add: max_def)

lemma enat_ile: "n ≤ enat m ==> ∃k. n = enat k"
  by (cases n) simp_all

lemma enat_iless: "n < enat m ==> ∃k. n = enat k"
  by (cases n) simp_all

lemma chain_incr: "∀i. ∃j. Y i < Y j ==> ∃j. enat k < Y j"
apply (induct_tac k)
 apply (simp (no_asm) only: enat_0)
 apply (fast intro: le_less_trans [OF i0_lb])
apply (erule exE)
apply (drule spec)
apply (erule exE)
apply (drule ileI1)
apply (rule eSuc_enat [THEN subst])
apply (rule exI)
apply (erule (1) le_less_trans)
done

instantiation enat :: "{bot, top}"
begin

definition bot_enat :: enat where
  "bot_enat = 0"

definition top_enat :: enat where
  "top_enat = ∞"

instance proof
qed (simp_all add: bot_enat_def top_enat_def)

end

lemma finite_enat_bounded:
  assumes le_fin: "!!y. y ∈ A ==> y ≤ enat n"
  shows "finite A"
proof (rule finite_subset)
  show "finite (enat ` {..n})" by blast

  have "A ⊆ {..enat n}" using le_fin by fastforce
  also have "… ⊆ enat ` {..n}"
    by (rule subsetI) (case_tac x, auto)
  finally show "A ⊆ enat ` {..n}" .
qed


subsection {* Well-ordering *}

lemma less_enatE:
  "[| n < enat m; !!k. n = enat k ==> k < m ==> P |] ==> P"
by (induct n) auto

lemma less_infinityE:
  "[| n < ∞; !!k. n = enat k ==> P |] ==> P"
by (induct n) auto

lemma enat_less_induct:
  assumes prem: "!!n. ∀m::enat. m < n --> P m ==> P n" shows "P n"
proof -
  have P_enat: "!!k. P (enat k)"
    apply (rule nat_less_induct)
    apply (rule prem, clarify)
    apply (erule less_enatE, simp)
    done
  show ?thesis
  proof (induct n)
    fix nat
    show "P (enat nat)" by (rule P_enat)
  next
    show "P ∞"
      apply (rule prem, clarify)
      apply (erule less_infinityE)
      apply (simp add: P_enat)
      done
  qed
qed

instance enat :: wellorder
proof
  fix P and n
  assume hyp: "(!!n::enat. (!!m::enat. m < n ==> P m) ==> P n)"
  show "P n" by (blast intro: enat_less_induct hyp)
qed

subsection {* Complete Lattice *}

instantiation enat :: complete_lattice
begin

definition inf_enat :: "enat => enat => enat" where
  "inf_enat ≡ min"

definition sup_enat :: "enat => enat => enat" where
  "sup_enat ≡ max"

definition Inf_enat :: "enat set => enat" where
  "Inf_enat A ≡ if A = {} then ∞ else (LEAST x. x ∈ A)"

definition Sup_enat :: "enat set => enat" where
  "Sup_enat A ≡ if A = {} then 0
    else if finite A then Max A
                     else ∞"
instance proof
  fix x :: "enat" and A :: "enat set"
  { assume "x ∈ A" then show "Inf A ≤ x"
      unfolding Inf_enat_def by (auto intro: Least_le) }
  { assume "!!y. y ∈ A ==> x ≤ y" then show "x ≤ Inf A"
      unfolding Inf_enat_def
      by (cases "A = {}") (auto intro: LeastI2_ex) }
  { assume "x ∈ A" then show "x ≤ Sup A"
      unfolding Sup_enat_def by (cases "finite A") auto }
  { assume "!!y. y ∈ A ==> y ≤ x" then show "Sup A ≤ x"
      unfolding Sup_enat_def using finite_enat_bounded by auto }
qed (simp_all add: inf_enat_def sup_enat_def)
end

instance enat :: complete_linorder ..

subsection {* Traditional theorem names *}

lemmas enat_defs = zero_enat_def one_enat_def number_of_enat_def eSuc_def
  plus_enat_def less_eq_enat_def less_enat_def

end