Theory Int

theory Int
imports Quotient Fun_Def
(*  Title:      HOL/Int.thy
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Author:     Tobias Nipkow, Florian Haftmann, TU Muenchen
*)

section ‹The Integers as Equivalence Classes over Pairs of Natural Numbers›

theory Int
  imports Equiv_Relations Power Quotient Fun_Def
begin

subsection ‹Definition of integers as a quotient type›

definition intrel :: "(nat × nat) ⇒ (nat × nat) ⇒ bool"
  where "intrel = (λ(x, y) (u, v). x + v = u + y)"

lemma intrel_iff [simp]: "intrel (x, y) (u, v) ⟷ x + v = u + y"
  by (simp add: intrel_def)

quotient_type int = "nat × nat" / "intrel"
  morphisms Rep_Integ Abs_Integ
proof (rule equivpI)
  show "reflp intrel" by (auto simp: reflp_def)
  show "symp intrel" by (auto simp: symp_def)
  show "transp intrel" by (auto simp: transp_def)
qed

lemma eq_Abs_Integ [case_names Abs_Integ, cases type: int]:
  "(⋀x y. z = Abs_Integ (x, y) ⟹ P) ⟹ P"
  by (induct z) auto


subsection ‹Integers form a commutative ring›

instantiation int :: comm_ring_1
begin

lift_definition zero_int :: "int" is "(0, 0)" .

lift_definition one_int :: "int" is "(1, 0)" .

lift_definition plus_int :: "int ⇒ int ⇒ int"
  is "λ(x, y) (u, v). (x + u, y + v)"
  by clarsimp

lift_definition uminus_int :: "int ⇒ int"
  is "λ(x, y). (y, x)"
  by clarsimp

lift_definition minus_int :: "int ⇒ int ⇒ int"
  is "λ(x, y) (u, v). (x + v, y + u)"
  by clarsimp

lift_definition times_int :: "int ⇒ int ⇒ int"
  is "λ(x, y) (u, v). (x*u + y*v, x*v + y*u)"
proof (clarsimp)
  fix s t u v w x y z :: nat
  assume "s + v = u + t" and "w + z = y + x"
  then have "(s + v) * w + (u + t) * x + u * (w + z) + v * (y + x) =
    (u + t) * w + (s + v) * x + u * (y + x) + v * (w + z)"
    by simp
  then show "(s * w + t * x) + (u * z + v * y) = (u * y + v * z) + (s * x + t * w)"
    by (simp add: algebra_simps)
qed

instance
  by standard (transfer; clarsimp simp: algebra_simps)+

end

abbreviation int :: "nat ⇒ int"
  where "int ≡ of_nat"

lemma int_def: "int n = Abs_Integ (n, 0)"
  by (induct n) (simp add: zero_int.abs_eq, simp add: one_int.abs_eq plus_int.abs_eq)

lemma int_transfer [transfer_rule]: "(rel_fun (=) pcr_int) (λn. (n, 0)) int"
  by (simp add: rel_fun_def int.pcr_cr_eq cr_int_def int_def)

lemma int_diff_cases: obtains (diff) m n where "z = int m - int n"
  by transfer clarsimp


subsection ‹Integers are totally ordered›

instantiation int :: linorder
begin

lift_definition less_eq_int :: "int ⇒ int ⇒ bool"
  is "λ(x, y) (u, v). x + v ≤ u + y"
  by auto

lift_definition less_int :: "int ⇒ int ⇒ bool"
  is "λ(x, y) (u, v). x + v < u + y"
  by auto

instance
  by standard (transfer, force)+

end

instantiation int :: distrib_lattice
begin

definition "(inf :: int ⇒ int ⇒ int) = min"

definition "(sup :: int ⇒ int ⇒ int) = max"

instance
  by standard (auto simp add: inf_int_def sup_int_def max_min_distrib2)

end

subsection ‹Ordering properties of arithmetic operations›

instance int :: ordered_cancel_ab_semigroup_add
proof
  fix i j k :: int
  show "i ≤ j ⟹ k + i ≤ k + j"
    by transfer clarsimp
qed

text ‹Strict Monotonicity of Multiplication.›

text ‹Strict, in 1st argument; proof is by induction on ‹k > 0›.›
lemma zmult_zless_mono2_lemma: "i < j ⟹ 0 < k ⟹ int k * i < int k * j"
  for i j :: int
proof (induct k)
  case 0
  then show ?case by simp
next
  case (Suc k)
  then show ?case
    by (cases "k = 0") (simp_all add: distrib_right add_strict_mono)
qed

lemma zero_le_imp_eq_int: "0 ≤ k ⟹ ∃n. k = int n"
  for k :: int
  apply transfer
  apply clarsimp
  apply (rule_tac x="a - b" in exI)
  apply simp
  done

lemma zero_less_imp_eq_int: "0 < k ⟹ ∃n>0. k = int n"
  for k :: int
  apply transfer
  apply clarsimp
  apply (rule_tac x="a - b" in exI)
  apply simp
  done

lemma zmult_zless_mono2: "i < j ⟹ 0 < k ⟹ k * i < k * j"
  for i j k :: int
  by (drule zero_less_imp_eq_int) (auto simp add: zmult_zless_mono2_lemma)


text ‹The integers form an ordered integral domain.›

instantiation int :: linordered_idom
begin

definition zabs_def: "¦i::int¦ = (if i < 0 then - i else i)"

definition zsgn_def: "sgn (i::int) = (if i = 0 then 0 else if 0 < i then 1 else - 1)"

instance
proof
  fix i j k :: int
  show "i < j ⟹ 0 < k ⟹ k * i < k * j"
    by (rule zmult_zless_mono2)
  show "¦i¦ = (if i < 0 then -i else i)"
    by (simp only: zabs_def)
  show "sgn (i::int) = (if i=0 then 0 else if 0<i then 1 else - 1)"
    by (simp only: zsgn_def)
qed

end

lemma zless_imp_add1_zle: "w < z ⟹ w + 1 ≤ z"
  for w z :: int
  by transfer clarsimp

lemma zless_iff_Suc_zadd: "w < z ⟷ (∃n. z = w + int (Suc n))"
  for w z :: int
  apply transfer
  apply auto
  apply (rename_tac a b c d)
  apply (rule_tac x="c+b - Suc(a+d)" in exI)
  apply arith
  done

lemma zabs_less_one_iff [simp]: "¦z¦ < 1 ⟷ z = 0" (is "?lhs ⟷ ?rhs")
  for z :: int
proof
  assume ?rhs
  then show ?lhs by simp
next
  assume ?lhs
  with zless_imp_add1_zle [of "¦z¦" 1] have "¦z¦ + 1 ≤ 1" by simp
  then have "¦z¦ ≤ 0" by simp
  then show ?rhs by simp
qed


subsection ‹Embedding of the Integers into any ‹ring_1›: ‹of_int››

context ring_1
begin

lift_definition of_int :: "int ⇒ 'a"
  is "λ(i, j). of_nat i - of_nat j"
  by (clarsimp simp add: diff_eq_eq eq_diff_eq diff_add_eq
      of_nat_add [symmetric] simp del: of_nat_add)

lemma of_int_0 [simp]: "of_int 0 = 0"
  by transfer simp

lemma of_int_1 [simp]: "of_int 1 = 1"
  by transfer simp

lemma of_int_add [simp]: "of_int (w + z) = of_int w + of_int z"
  by transfer (clarsimp simp add: algebra_simps)

lemma of_int_minus [simp]: "of_int (- z) = - (of_int z)"
  by (transfer fixing: uminus) clarsimp

lemma of_int_diff [simp]: "of_int (w - z) = of_int w - of_int z"
  using of_int_add [of w "- z"] by simp

lemma of_int_mult [simp]: "of_int (w*z) = of_int w * of_int z"
  by (transfer fixing: times) (clarsimp simp add: algebra_simps)

lemma mult_of_int_commute: "of_int x * y = y * of_int x"
  by (transfer fixing: times) (auto simp: algebra_simps mult_of_nat_commute)

text ‹Collapse nested embeddings.›
lemma of_int_of_nat_eq [simp]: "of_int (int n) = of_nat n"
  by (induct n) auto

lemma of_int_numeral [simp, code_post]: "of_int (numeral k) = numeral k"
  by (simp add: of_nat_numeral [symmetric] of_int_of_nat_eq [symmetric])

lemma of_int_neg_numeral [code_post]: "of_int (- numeral k) = - numeral k"
  by simp

lemma of_int_power [simp]: "of_int (z ^ n) = of_int z ^ n"
  by (induct n) simp_all

lemma of_int_of_bool [simp]:
  "of_int (of_bool P) = of_bool P"
  by auto

end

context ring_char_0
begin

lemma of_int_eq_iff [simp]: "of_int w = of_int z ⟷ w = z"
  by transfer (clarsimp simp add: algebra_simps of_nat_add [symmetric] simp del: of_nat_add)

text ‹Special cases where either operand is zero.›
lemma of_int_eq_0_iff [simp]: "of_int z = 0 ⟷ z = 0"
  using of_int_eq_iff [of z 0] by simp

lemma of_int_0_eq_iff [simp]: "0 = of_int z ⟷ z = 0"
  using of_int_eq_iff [of 0 z] by simp

lemma of_int_eq_1_iff [iff]: "of_int z = 1 ⟷ z = 1"
  using of_int_eq_iff [of z 1] by simp

lemma numeral_power_eq_of_int_cancel_iff [simp]:
  "numeral x ^ n = of_int y ⟷ numeral x ^ n = y"
  using of_int_eq_iff[of "numeral x ^ n" y, unfolded of_int_numeral of_int_power] .

lemma of_int_eq_numeral_power_cancel_iff [simp]:
  "of_int y = numeral x ^ n ⟷ y = numeral x ^ n"
  using numeral_power_eq_of_int_cancel_iff [of x n y] by (metis (mono_tags))

lemma neg_numeral_power_eq_of_int_cancel_iff [simp]:
  "(- numeral x) ^ n = of_int y ⟷ (- numeral x) ^ n = y"
  using of_int_eq_iff[of "(- numeral x) ^ n" y]
  by simp

lemma of_int_eq_neg_numeral_power_cancel_iff [simp]:
  "of_int y = (- numeral x) ^ n ⟷ y = (- numeral x) ^ n"
  using neg_numeral_power_eq_of_int_cancel_iff[of x n y] by (metis (mono_tags))

lemma of_int_eq_of_int_power_cancel_iff[simp]: "(of_int b) ^ w = of_int x ⟷ b ^ w = x"
  by (metis of_int_power of_int_eq_iff)

lemma of_int_power_eq_of_int_cancel_iff[simp]: "of_int x = (of_int b) ^ w ⟷ x = b ^ w"
  by (metis of_int_eq_of_int_power_cancel_iff)

end

context linordered_idom
begin

text ‹Every ‹linordered_idom› has characteristic zero.›
subclass ring_char_0 ..

lemma of_int_le_iff [simp]: "of_int w ≤ of_int z ⟷ w ≤ z"
  by (transfer fixing: less_eq)
    (clarsimp simp add: algebra_simps of_nat_add [symmetric] simp del: of_nat_add)

lemma of_int_less_iff [simp]: "of_int w < of_int z ⟷ w < z"
  by (simp add: less_le order_less_le)

lemma of_int_0_le_iff [simp]: "0 ≤ of_int z ⟷ 0 ≤ z"
  using of_int_le_iff [of 0 z] by simp

lemma of_int_le_0_iff [simp]: "of_int z ≤ 0 ⟷ z ≤ 0"
  using of_int_le_iff [of z 0] by simp

lemma of_int_0_less_iff [simp]: "0 < of_int z ⟷ 0 < z"
  using of_int_less_iff [of 0 z] by simp

lemma of_int_less_0_iff [simp]: "of_int z < 0 ⟷ z < 0"
  using of_int_less_iff [of z 0] by simp

lemma of_int_1_le_iff [simp]: "1 ≤ of_int z ⟷ 1 ≤ z"
  using of_int_le_iff [of 1 z] by simp

lemma of_int_le_1_iff [simp]: "of_int z ≤ 1 ⟷ z ≤ 1"
  using of_int_le_iff [of z 1] by simp

lemma of_int_1_less_iff [simp]: "1 < of_int z ⟷ 1 < z"
  using of_int_less_iff [of 1 z] by simp

lemma of_int_less_1_iff [simp]: "of_int z < 1 ⟷ z < 1"
  using of_int_less_iff [of z 1] by simp

lemma of_int_pos: "z > 0 ⟹ of_int z > 0"
  by simp

lemma of_int_nonneg: "z ≥ 0 ⟹ of_int z ≥ 0"
  by simp

lemma of_int_abs [simp]: "of_int ¦x¦ = ¦of_int x¦"
  by (auto simp add: abs_if)

lemma of_int_lessD:
  assumes "¦of_int n¦ < x"
  shows "n = 0 ∨ x > 1"
proof (cases "n = 0")
  case True
  then show ?thesis by simp
next
  case False
  then have "¦n¦ ≠ 0" by simp
  then have "¦n¦ > 0" by simp
  then have "¦n¦ ≥ 1"
    using zless_imp_add1_zle [of 0 "¦n¦"] by simp
  then have "¦of_int n¦ ≥ 1"
    unfolding of_int_1_le_iff [of "¦n¦", symmetric] by simp
  then have "1 < x" using assms by (rule le_less_trans)
  then show ?thesis ..
qed

lemma of_int_leD:
  assumes "¦of_int n¦ ≤ x"
  shows "n = 0 ∨ 1 ≤ x"
proof (cases "n = 0")
  case True
  then show ?thesis by simp
next
  case False
  then have "¦n¦ ≠ 0" by simp
  then have "¦n¦ > 0" by simp
  then have "¦n¦ ≥ 1"
    using zless_imp_add1_zle [of 0 "¦n¦"] by simp
  then have "¦of_int n¦ ≥ 1"
    unfolding of_int_1_le_iff [of "¦n¦", symmetric] by simp
  then have "1 ≤ x" using assms by (rule order_trans)
  then show ?thesis ..
qed

lemma numeral_power_le_of_int_cancel_iff [simp]:
  "numeral x ^ n ≤ of_int a ⟷ numeral x ^ n ≤ a"
  by (metis (mono_tags) local.of_int_eq_numeral_power_cancel_iff of_int_le_iff)

lemma of_int_le_numeral_power_cancel_iff [simp]:
  "of_int a ≤ numeral x ^ n ⟷ a ≤ numeral x ^ n"
  by (metis (mono_tags) local.numeral_power_eq_of_int_cancel_iff of_int_le_iff)

lemma numeral_power_less_of_int_cancel_iff [simp]:
  "numeral x ^ n < of_int a ⟷ numeral x ^ n < a"
  by (metis (mono_tags) local.of_int_eq_numeral_power_cancel_iff of_int_less_iff)

lemma of_int_less_numeral_power_cancel_iff [simp]:
  "of_int a < numeral x ^ n ⟷ a < numeral x ^ n"
  by (metis (mono_tags) local.of_int_eq_numeral_power_cancel_iff of_int_less_iff)

lemma neg_numeral_power_le_of_int_cancel_iff [simp]:
  "(- numeral x) ^ n ≤ of_int a ⟷ (- numeral x) ^ n ≤ a"
  by (metis (mono_tags) of_int_le_iff of_int_neg_numeral of_int_power)

lemma of_int_le_neg_numeral_power_cancel_iff [simp]:
  "of_int a ≤ (- numeral x) ^ n ⟷ a ≤ (- numeral x) ^ n"
  by (metis (mono_tags) of_int_le_iff of_int_neg_numeral of_int_power)

lemma neg_numeral_power_less_of_int_cancel_iff [simp]:
  "(- numeral x) ^ n < of_int a ⟷ (- numeral x) ^ n < a"
  using of_int_less_iff[of "(- numeral x) ^ n" a]
  by simp

lemma of_int_less_neg_numeral_power_cancel_iff [simp]:
  "of_int a < (- numeral x) ^ n ⟷ a < (- numeral x::int) ^ n"
  using of_int_less_iff[of a "(- numeral x) ^ n"]
  by simp

lemma of_int_le_of_int_power_cancel_iff[simp]: "(of_int b) ^ w ≤ of_int x ⟷ b ^ w ≤ x"
  by (metis (mono_tags) of_int_le_iff of_int_power)

lemma of_int_power_le_of_int_cancel_iff[simp]: "of_int x ≤ (of_int b) ^ w⟷ x ≤ b ^ w"
  by (metis (mono_tags) of_int_le_iff of_int_power)

lemma of_int_less_of_int_power_cancel_iff[simp]: "(of_int b) ^ w < of_int x ⟷ b ^ w < x"
  by (metis (mono_tags) of_int_less_iff of_int_power)

lemma of_int_power_less_of_int_cancel_iff[simp]: "of_int x < (of_int b) ^ w⟷ x < b ^ w"
  by (metis (mono_tags) of_int_less_iff of_int_power)

lemma of_int_max: "of_int (max x y) = max (of_int x) (of_int y)"
  by (auto simp: max_def)

lemma of_int_min: "of_int (min x y) = min (of_int x) (of_int y)"
  by (auto simp: min_def)

end

text ‹Comparisons involving @{term of_int}.›

lemma of_int_eq_numeral_iff [iff]: "of_int z = (numeral n :: 'a::ring_char_0) ⟷ z = numeral n"
  using of_int_eq_iff by fastforce

lemma of_int_le_numeral_iff [simp]:
  "of_int z ≤ (numeral n :: 'a::linordered_idom) ⟷ z ≤ numeral n"
  using of_int_le_iff [of z "numeral n"] by simp

lemma of_int_numeral_le_iff [simp]:
  "(numeral n :: 'a::linordered_idom) ≤ of_int z ⟷ numeral n ≤ z"
  using of_int_le_iff [of "numeral n"] by simp

lemma of_int_less_numeral_iff [simp]:
  "of_int z < (numeral n :: 'a::linordered_idom) ⟷ z < numeral n"
  using of_int_less_iff [of z "numeral n"] by simp

lemma of_int_numeral_less_iff [simp]:
  "(numeral n :: 'a::linordered_idom) < of_int z ⟷ numeral n < z"
  using of_int_less_iff [of "numeral n" z] by simp

lemma of_nat_less_of_int_iff: "(of_nat n::'a::linordered_idom) < of_int x ⟷ int n < x"
  by (metis of_int_of_nat_eq of_int_less_iff)

lemma of_int_eq_id [simp]: "of_int = id"
proof
  show "of_int z = id z" for z
    by (cases z rule: int_diff_cases) simp
qed

instance int :: no_top
  apply standard
  apply (rule_tac x="x + 1" in exI)
  apply simp
  done

instance int :: no_bot
  apply standard
  apply (rule_tac x="x - 1" in exI)
  apply simp
  done


subsection ‹Magnitude of an Integer, as a Natural Number: ‹nat››

lift_definition nat :: "int ⇒ nat" is "λ(x, y). x - y"
  by auto

lemma nat_int [simp]: "nat (int n) = n"
  by transfer simp

lemma int_nat_eq [simp]: "int (nat z) = (if 0 ≤ z then z else 0)"
  by transfer clarsimp

lemma nat_0_le: "0 ≤ z ⟹ int (nat z) = z"
  by simp

lemma nat_le_0 [simp]: "z ≤ 0 ⟹ nat z = 0"
  by transfer clarsimp

lemma nat_le_eq_zle: "0 < w ∨ 0 ≤ z ⟹ nat w ≤ nat z ⟷ w ≤ z"
  by transfer (clarsimp, arith)

text ‹An alternative condition is @{term "0 ≤ w"}.›
lemma nat_mono_iff: "0 < z ⟹ nat w < nat z ⟷ w < z"
  by (simp add: nat_le_eq_zle linorder_not_le [symmetric])

lemma nat_less_eq_zless: "0 ≤ w ⟹ nat w < nat z ⟷ w < z"
  by (simp add: nat_le_eq_zle linorder_not_le [symmetric])

lemma zless_nat_conj [simp]: "nat w < nat z ⟷ 0 < z ∧ w < z"
  by transfer (clarsimp, arith)

lemma nonneg_int_cases:
  assumes "0 ≤ k"
  obtains n where "k = int n"
proof -
  from assms have "k = int (nat k)"
    by simp
  then show thesis
    by (rule that)
qed

lemma pos_int_cases:
  assumes "0 < k"
  obtains n where "k = int n" and "n > 0"
proof -
  from assms have "0 ≤ k"
    by simp
  then obtain n where "k = int n"
    by (rule nonneg_int_cases)
  moreover have "n > 0"
    using ‹k = int n› assms by simp
  ultimately show thesis
    by (rule that)
qed

lemma nonpos_int_cases:
  assumes "k ≤ 0"
  obtains n where "k = - int n"
proof -
  from assms have "- k ≥ 0"
    by simp
  then obtain n where "- k = int n"
    by (rule nonneg_int_cases)
  then have "k = - int n"
    by simp
  then show thesis
    by (rule that)
qed

lemma neg_int_cases:
  assumes "k < 0"
  obtains n where "k = - int n" and "n > 0"
proof -
  from assms have "- k > 0"
    by simp
  then obtain n where "- k = int n" and "- k > 0"
    by (blast elim: pos_int_cases)
  then have "k = - int n" and "n > 0"
    by simp_all
  then show thesis
    by (rule that)
qed

lemma nat_eq_iff: "nat w = m ⟷ (if 0 ≤ w then w = int m else m = 0)"
  by transfer (clarsimp simp add: le_imp_diff_is_add)

lemma nat_eq_iff2: "m = nat w ⟷ (if 0 ≤ w then w = int m else m = 0)"
  using nat_eq_iff [of w m] by auto

lemma nat_0 [simp]: "nat 0 = 0"
  by (simp add: nat_eq_iff)

lemma nat_1 [simp]: "nat 1 = Suc 0"
  by (simp add: nat_eq_iff)

lemma nat_numeral [simp]: "nat (numeral k) = numeral k"
  by (simp add: nat_eq_iff)

lemma nat_neg_numeral [simp]: "nat (- numeral k) = 0"
  by simp

lemma nat_2: "nat 2 = Suc (Suc 0)"
  by simp

lemma nat_less_iff: "0 ≤ w ⟹ nat w < m ⟷ w < of_nat m"
  by transfer (clarsimp, arith)

lemma nat_le_iff: "nat x ≤ n ⟷ x ≤ int n"
  by transfer (clarsimp simp add: le_diff_conv)

lemma nat_mono: "x ≤ y ⟹ nat x ≤ nat y"
  by transfer auto

lemma nat_0_iff[simp]: "nat i = 0 ⟷ i ≤ 0"
  for i :: int
  by transfer clarsimp

lemma int_eq_iff: "of_nat m = z ⟷ m = nat z ∧ 0 ≤ z"
  by (auto simp add: nat_eq_iff2)

lemma zero_less_nat_eq [simp]: "0 < nat z ⟷ 0 < z"
  using zless_nat_conj [of 0] by auto

lemma nat_add_distrib: "0 ≤ z ⟹ 0 ≤ z' ⟹ nat (z + z') = nat z + nat z'"
  by transfer clarsimp

lemma nat_diff_distrib': "0 ≤ x ⟹ 0 ≤ y ⟹ nat (x - y) = nat x - nat y"
  by transfer clarsimp

lemma nat_diff_distrib: "0 ≤ z' ⟹ z' ≤ z ⟹ nat (z - z') = nat z - nat z'"
  by (rule nat_diff_distrib') auto

lemma nat_zminus_int [simp]: "nat (- int n) = 0"
  by transfer simp

lemma le_nat_iff: "k ≥ 0 ⟹ n ≤ nat k ⟷ int n ≤ k"
  by transfer auto

lemma zless_nat_eq_int_zless: "m < nat z ⟷ int m < z"
  by transfer (clarsimp simp add: less_diff_conv)

lemma (in ring_1) of_nat_nat [simp]: "0 ≤ z ⟹ of_nat (nat z) = of_int z"
  by transfer (clarsimp simp add: of_nat_diff)

lemma diff_nat_numeral [simp]: "(numeral v :: nat) - numeral v' = nat (numeral v - numeral v')"
  by (simp only: nat_diff_distrib' zero_le_numeral nat_numeral)

lemma nat_abs_triangle_ineq:
  "nat ¦k + l¦ ≤ nat ¦k¦ + nat ¦l¦"
  by (simp add: nat_add_distrib [symmetric] nat_le_eq_zle abs_triangle_ineq)

lemma nat_of_bool [simp]:
  "nat (of_bool P) = of_bool P"
  by auto

lemma split_nat [arith_split]: "P (nat i) ⟷ ((∀n. i = int n ⟶ P n) ∧ (i < 0 ⟶ P 0))"
  (is "?P = (?L ∧ ?R)")
  for i :: int
proof (cases "i < 0")
  case True
  then show ?thesis
    by auto
next
  case False
  have "?P = ?L"
  proof
    assume ?P
    then show ?L using False by auto
  next
    assume ?L
    moreover from False have "int (nat i) = i"
      by (simp add: not_less)
    ultimately show ?P
      by simp
  qed
  with False show ?thesis by simp
qed

lemma all_nat: "(∀x. P x) ⟷ (∀x≥0. P (nat x))"
  by (auto split: split_nat)

lemma ex_nat: "(∃x. P x) ⟷ (∃x. 0 ≤ x ∧ P (nat x))"
proof
  assume "∃x. P x"
  then obtain x where "P x" ..
  then have "int x ≥ 0 ∧ P (nat (int x))" by simp
  then show "∃x≥0. P (nat x)" ..
next
  assume "∃x≥0. P (nat x)"
  then show "∃x. P x" by auto
qed


text ‹For termination proofs:›
lemma measure_function_int[measure_function]: "is_measure (nat ∘ abs)" ..


subsection ‹Lemmas about the Function @{term of_nat} and Orderings›

lemma negative_zless_0: "- (int (Suc n)) < (0 :: int)"
  by (simp add: order_less_le del: of_nat_Suc)

lemma negative_zless [iff]: "- (int (Suc n)) < int m"
  by (rule negative_zless_0 [THEN order_less_le_trans], simp)

lemma negative_zle_0: "- int n ≤ 0"
  by (simp add: minus_le_iff)

lemma negative_zle [iff]: "- int n ≤ int m"
  by (rule order_trans [OF negative_zle_0 of_nat_0_le_iff])

lemma not_zle_0_negative [simp]: "¬ 0 ≤ - int (Suc n)"
  by (subst le_minus_iff) (simp del: of_nat_Suc)

lemma int_zle_neg: "int n ≤ - int m ⟷ n = 0 ∧ m = 0"
  by transfer simp

lemma not_int_zless_negative [simp]: "¬ int n < - int m"
  by (simp add: linorder_not_less)

lemma negative_eq_positive [simp]: "- int n = of_nat m ⟷ n = 0 ∧ m = 0"
  by (force simp add: order_eq_iff [of "- of_nat n"] int_zle_neg)

lemma zle_iff_zadd: "w ≤ z ⟷ (∃n. z = w + int n)"
  (is "?lhs ⟷ ?rhs")
proof
  assume ?rhs
  then show ?lhs by auto
next
  assume ?lhs
  then have "0 ≤ z - w" by simp
  then obtain n where "z - w = int n"
    using zero_le_imp_eq_int [of "z - w"] by blast
  then have "z = w + int n" by simp
  then show ?rhs ..
qed

lemma zadd_int_left: "int m + (int n + z) = int (m + n) + z"
  by simp

text ‹
  This version is proved for all ordered rings, not just integers!
  It is proved here because attribute ‹arith_split› is not available
  in theory ‹Rings›.
  But is it really better than just rewriting with ‹abs_if›?
›
lemma abs_split [arith_split, no_atp]: "P ¦a¦ ⟷ (0 ≤ a ⟶ P a) ∧ (a < 0 ⟶ P (- a))"
  for a :: "'a::linordered_idom"
  by (force dest: order_less_le_trans simp add: abs_if linorder_not_less)

lemma negD: "x < 0 ⟹ ∃n. x = - (int (Suc n))"
  apply transfer
  apply clarsimp
  apply (rule_tac x="b - Suc a" in exI)
  apply arith
  done


subsection ‹Cases and induction›

text ‹
  Now we replace the case analysis rule by a more conventional one:
  whether an integer is negative or not.
›

text ‹This version is symmetric in the two subgoals.›
lemma int_cases2 [case_names nonneg nonpos, cases type: int]:
  "(⋀n. z = int n ⟹ P) ⟹ (⋀n. z = - (int n) ⟹ P) ⟹ P"
  by (cases "z < 0") (auto simp add: linorder_not_less dest!: negD nat_0_le [THEN sym])

text ‹This is the default, with a negative case.›
lemma int_cases [case_names nonneg neg, cases type: int]:
  "(⋀n. z = int n ⟹ P) ⟹ (⋀n. z = - (int (Suc n)) ⟹ P) ⟹ P"
  apply (cases "z < 0")
   apply (blast dest!: negD)
  apply (simp add: linorder_not_less del: of_nat_Suc)
  apply auto
  apply (blast dest: nat_0_le [THEN sym])
  done

lemma int_cases3 [case_names zero pos neg]:
  fixes k :: int
  assumes "k = 0 ⟹ P" and "⋀n. k = int n ⟹ n > 0 ⟹ P"
    and "⋀n. k = - int n ⟹ n > 0 ⟹ P"
  shows "P"
proof (cases k "0::int" rule: linorder_cases)
  case equal
  with assms(1) show P by simp
next
  case greater
  then have *: "nat k > 0" by simp
  moreover from * have "k = int (nat k)" by auto
  ultimately show P using assms(2) by blast
next
  case less
  then have *: "nat (- k) > 0" by simp
  moreover from * have "k = - int (nat (- k))" by auto
  ultimately show P using assms(3) by blast
qed

lemma int_of_nat_induct [case_names nonneg neg, induct type: int]:
  "(⋀n. P (int n)) ⟹ (⋀n. P (- (int (Suc n)))) ⟹ P z"
  by (cases z) auto

lemma sgn_mult_dvd_iff [simp]:
  "sgn r * l dvd k ⟷ l dvd k ∧ (r = 0 ⟶ k = 0)" for k l r :: int
  by (cases r rule: int_cases3) auto

lemma mult_sgn_dvd_iff [simp]:
  "l * sgn r dvd k ⟷ l dvd k ∧ (r = 0 ⟶ k = 0)" for k l r :: int
  using sgn_mult_dvd_iff [of r l k] by (simp add: ac_simps)

lemma dvd_sgn_mult_iff [simp]:
  "l dvd sgn r * k ⟷ l dvd k ∨ r = 0" for k l r :: int
  by (cases r rule: int_cases3) simp_all

lemma dvd_mult_sgn_iff [simp]:
  "l dvd k * sgn r ⟷ l dvd k ∨ r = 0" for k l r :: int
  using dvd_sgn_mult_iff [of l r k] by (simp add: ac_simps)

lemma int_sgnE:
  fixes k :: int
  obtains n and l where "k = sgn l * int n"
proof -
  have "k = sgn k * int (nat ¦k¦)"
    by (simp add: sgn_mult_abs)
  then show ?thesis ..
qed


subsubsection ‹Binary comparisons›

text ‹Preliminaries›

lemma le_imp_0_less:
  fixes z :: int
  assumes le: "0 ≤ z"
  shows "0 < 1 + z"
proof -
  have "0 ≤ z" by fact
  also have "… < z + 1" by (rule less_add_one)
  also have "… = 1 + z" by (simp add: ac_simps)
  finally show "0 < 1 + z" .
qed

lemma odd_less_0_iff: "1 + z + z < 0 ⟷ z < 0"
  for z :: int
proof (cases z)
  case (nonneg n)
  then show ?thesis
    by (simp add: linorder_not_less add.assoc add_increasing le_imp_0_less [THEN order_less_imp_le])
next
  case (neg n)
  then show ?thesis
    by (simp del: of_nat_Suc of_nat_add of_nat_1
        add: algebra_simps of_nat_1 [where 'a=int, symmetric] of_nat_add [symmetric])
qed


subsubsection ‹Comparisons, for Ordered Rings›

lemma odd_nonzero: "1 + z + z ≠ 0"
  for z :: int
proof (cases z)
  case (nonneg n)
  have le: "0 ≤ z + z"
    by (simp add: nonneg add_increasing)
  then show ?thesis
    using le_imp_0_less [OF le] by (auto simp: ac_simps)
next
  case (neg n)
  show ?thesis
  proof
    assume eq: "1 + z + z = 0"
    have "0 < 1 + (int n + int n)"
      by (simp add: le_imp_0_less add_increasing)
    also have "… = - (1 + z + z)"
      by (simp add: neg add.assoc [symmetric])
    also have "… = 0" by (simp add: eq)
    finally have "0<0" ..
    then show False by blast
  qed
qed


subsection ‹The Set of Integers›

context ring_1
begin

definition Ints :: "'a set"  ("ℤ")
  where "ℤ = range of_int"

lemma Ints_of_int [simp]: "of_int z ∈ ℤ"
  by (simp add: Ints_def)

lemma Ints_of_nat [simp]: "of_nat n ∈ ℤ"
  using Ints_of_int [of "of_nat n"] by simp

lemma Ints_0 [simp]: "0 ∈ ℤ"
  using Ints_of_int [of "0"] by simp

lemma Ints_1 [simp]: "1 ∈ ℤ"
  using Ints_of_int [of "1"] by simp

lemma Ints_numeral [simp]: "numeral n ∈ ℤ"
  by (subst of_nat_numeral [symmetric], rule Ints_of_nat)

lemma Ints_add [simp]: "a ∈ ℤ ⟹ b ∈ ℤ ⟹ a + b ∈ ℤ"
  apply (auto simp add: Ints_def)
  apply (rule range_eqI)
  apply (rule of_int_add [symmetric])
  done

lemma Ints_minus [simp]: "a ∈ ℤ ⟹ -a ∈ ℤ"
  apply (auto simp add: Ints_def)
  apply (rule range_eqI)
  apply (rule of_int_minus [symmetric])
  done

lemma Ints_diff [simp]: "a ∈ ℤ ⟹ b ∈ ℤ ⟹ a - b ∈ ℤ"
  apply (auto simp add: Ints_def)
  apply (rule range_eqI)
  apply (rule of_int_diff [symmetric])
  done

lemma Ints_mult [simp]: "a ∈ ℤ ⟹ b ∈ ℤ ⟹ a * b ∈ ℤ"
  apply (auto simp add: Ints_def)
  apply (rule range_eqI)
  apply (rule of_int_mult [symmetric])
  done

lemma Ints_power [simp]: "a ∈ ℤ ⟹ a ^ n ∈ ℤ"
  by (induct n) simp_all

lemma Ints_cases [cases set: Ints]:
  assumes "q ∈ ℤ"
  obtains (of_int) z where "q = of_int z"
  unfolding Ints_def
proof -
  from ‹q ∈ ℤ› have "q ∈ range of_int" unfolding Ints_def .
  then obtain z where "q = of_int z" ..
  then show thesis ..
qed

lemma Ints_induct [case_names of_int, induct set: Ints]:
  "q ∈ ℤ ⟹ (⋀z. P (of_int z)) ⟹ P q"
  by (rule Ints_cases) auto

lemma Nats_subset_Ints: "ℕ ⊆ ℤ"
  unfolding Nats_def Ints_def
  by (rule subsetI, elim imageE, hypsubst, subst of_int_of_nat_eq[symmetric], rule imageI) simp_all

lemma Nats_altdef1: "ℕ = {of_int n |n. n ≥ 0}"
proof (intro subsetI equalityI)
  fix x :: 'a
  assume "x ∈ {of_int n |n. n ≥ 0}"
  then obtain n where "x = of_int n" "n ≥ 0"
    by (auto elim!: Ints_cases)
  then have "x = of_nat (nat n)"
    by (subst of_nat_nat) simp_all
  then show "x ∈ ℕ"
    by simp
next
  fix x :: 'a
  assume "x ∈ ℕ"
  then obtain n where "x = of_nat n"
    by (auto elim!: Nats_cases)
  then have "x = of_int (int n)" by simp
  also have "int n ≥ 0" by simp
  then have "of_int (int n) ∈ {of_int n |n. n ≥ 0}" by blast
  finally show "x ∈ {of_int n |n. n ≥ 0}" .
qed

end

lemma (in linordered_idom) Ints_abs [simp]:
  shows "a ∈ ℤ ⟹ abs a ∈ ℤ"
  by (auto simp: abs_if)

lemma (in linordered_idom) Nats_altdef2: "ℕ = {n ∈ ℤ. n ≥ 0}"
proof (intro subsetI equalityI)
  fix x :: 'a
  assume "x ∈ {n ∈ ℤ. n ≥ 0}"
  then obtain n where "x = of_int n" "n ≥ 0"
    by (auto elim!: Ints_cases)
  then have "x = of_nat (nat n)"
    by (subst of_nat_nat) simp_all
  then show "x ∈ ℕ"
    by simp
qed (auto elim!: Nats_cases)

lemma (in idom_divide) of_int_divide_in_Ints: 
  "of_int a div of_int b ∈ ℤ" if "b dvd a"
proof -
  from that obtain c where "a = b * c" ..
  then show ?thesis
    by (cases "of_int b = 0") simp_all
qed

text ‹The premise involving @{term Ints} prevents @{term "a = 1/2"}.›

lemma Ints_double_eq_0_iff:
  fixes a :: "'a::ring_char_0"
  assumes in_Ints: "a ∈ ℤ"
  shows "a + a = 0 ⟷ a = 0"
    (is "?lhs ⟷ ?rhs")
proof -
  from in_Ints have "a ∈ range of_int"
    unfolding Ints_def [symmetric] .
  then obtain z where a: "a = of_int z" ..
  show ?thesis
  proof
    assume ?rhs
    then show ?lhs by simp
  next
    assume ?lhs
    with a have "of_int (z + z) = (of_int 0 :: 'a)" by simp
    then have "z + z = 0" by (simp only: of_int_eq_iff)
    then have "z = 0" by (simp only: double_zero)
    with a show ?rhs by simp
  qed
qed

lemma Ints_odd_nonzero:
  fixes a :: "'a::ring_char_0"
  assumes in_Ints: "a ∈ ℤ"
  shows "1 + a + a ≠ 0"
proof -
  from in_Ints have "a ∈ range of_int"
    unfolding Ints_def [symmetric] .
  then obtain z where a: "a = of_int z" ..
  show ?thesis
  proof
    assume "1 + a + a = 0"
    with a have "of_int (1 + z + z) = (of_int 0 :: 'a)" by simp
    then have "1 + z + z = 0" by (simp only: of_int_eq_iff)
    with odd_nonzero show False by blast
  qed
qed

lemma Nats_numeral [simp]: "numeral w ∈ ℕ"
  using of_nat_in_Nats [of "numeral w"] by simp

lemma Ints_odd_less_0:
  fixes a :: "'a::linordered_idom"
  assumes in_Ints: "a ∈ ℤ"
  shows "1 + a + a < 0 ⟷ a < 0"
proof -
  from in_Ints have "a ∈ range of_int"
    unfolding Ints_def [symmetric] .
  then obtain z where a: "a = of_int z" ..
  with a have "1 + a + a < 0 ⟷ of_int (1 + z + z) < (of_int 0 :: 'a)"
    by simp
  also have "… ⟷ z < 0"
    by (simp only: of_int_less_iff odd_less_0_iff)
  also have "… ⟷ a < 0"
    by (simp add: a)
  finally show ?thesis .
qed


subsection ‹@{term sum} and @{term prod}›

lemma of_nat_sum [simp]: "of_nat (sum f A) = (∑x∈A. of_nat(f x))"
  by (induct A rule: infinite_finite_induct) auto

lemma of_int_sum [simp]: "of_int (sum f A) = (∑x∈A. of_int(f x))"
  by (induct A rule: infinite_finite_induct) auto

lemma of_nat_prod [simp]: "of_nat (prod f A) = (∏x∈A. of_nat(f x))"
  by (induct A rule: infinite_finite_induct) auto

lemma of_int_prod [simp]: "of_int (prod f A) = (∏x∈A. of_int(f x))"
  by (induct A rule: infinite_finite_induct) auto


subsection ‹Setting up simplification procedures›

lemmas of_int_simps =
  of_int_0 of_int_1 of_int_add of_int_mult

ML_file "Tools/int_arith.ML"
declaration ‹K Int_Arith.setup›

simproc_setup fast_arith
  ("(m::'a::linordered_idom) < n" |
    "(m::'a::linordered_idom) ≤ n" |
    "(m::'a::linordered_idom) = n") =
  ‹K Lin_Arith.simproc›


subsection‹More Inequality Reasoning›

lemma zless_add1_eq: "w < z + 1 ⟷ w < z ∨ w = z"
  for w z :: int
  by arith

lemma add1_zle_eq: "w + 1 ≤ z ⟷ w < z"
  for w z :: int
  by arith

lemma zle_diff1_eq [simp]: "w ≤ z - 1 ⟷ w < z"
  for w z :: int
  by arith

lemma zle_add1_eq_le [simp]: "w < z + 1 ⟷ w ≤ z"
  for w z :: int
  by arith

lemma int_one_le_iff_zero_less: "1 ≤ z ⟷ 0 < z"
  for z :: int
  by arith

lemma Ints_nonzero_abs_ge1:
  fixes x:: "'a :: linordered_idom"
    assumes "x ∈ Ints" "x ≠ 0"
    shows "1 ≤ abs x"
proof (rule Ints_cases [OF ‹x ∈ Ints›])
  fix z::int
  assume "x = of_int z"
    with ‹x ≠ 0› 
  show "1 ≤ ¦x¦"
    apply (auto simp add: abs_if)
    by (metis diff_0 of_int_1 of_int_le_iff of_int_minus zle_diff1_eq)
qed
  
lemma Ints_nonzero_abs_less1:
  fixes x:: "'a :: linordered_idom"
  shows "⟦x ∈ Ints; abs x < 1⟧ ⟹ x = 0"
    using Ints_nonzero_abs_ge1 [of x] by auto
    

subsection ‹The functions @{term nat} and @{term int}›

text ‹Simplify the term @{term "w + - z"}.›

lemma one_less_nat_eq [simp]: "Suc 0 < nat z ⟷ 1 < z"
  using zless_nat_conj [of 1 z] by auto

lemma int_eq_iff_numeral [simp]:
  "int m = numeral v ⟷ m = numeral v"
  by (simp add: int_eq_iff)

lemma nat_abs_int_diff:
  "nat ¦int a - int b¦ = (if a ≤ b then b - a else a - b)"
  by auto

lemma nat_int_add: "nat (int a + int b) = a + b"
  by auto

context ring_1
begin

lemma of_int_of_nat [nitpick_simp]:
  "of_int k = (if k < 0 then - of_nat (nat (- k)) else of_nat (nat k))"
proof (cases "k < 0")
  case True
  then have "0 ≤ - k" by simp
  then have "of_nat (nat (- k)) = of_int (- k)" by (rule of_nat_nat)
  with True show ?thesis by simp
next
  case False
  then show ?thesis by (simp add: not_less)
qed

end

lemma transfer_rule_of_int:
  fixes R :: "'a::ring_1 ⇒ 'b::ring_1 ⇒ bool"
  assumes [transfer_rule]: "R 0 0" "R 1 1"
    "rel_fun R (rel_fun R R) plus plus"
    "rel_fun R R uminus uminus"
  shows "rel_fun HOL.eq R of_int of_int"
proof -
  note transfer_rule_of_nat [transfer_rule]
  have [transfer_rule]: "rel_fun HOL.eq R of_nat of_nat"
    by transfer_prover
  show ?thesis
    by (unfold of_int_of_nat [abs_def]) transfer_prover
qed

lemma nat_mult_distrib:
  fixes z z' :: int
  assumes "0 ≤ z"
  shows "nat (z * z') = nat z * nat z'"
proof (cases "0 ≤ z'")
  case False
  with assms have "z * z' ≤ 0"
    by (simp add: not_le mult_le_0_iff)
  then have "nat (z * z') = 0" by simp
  moreover from False have "nat z' = 0" by simp
  ultimately show ?thesis by simp
next
  case True
  with assms have ge_0: "z * z' ≥ 0" by (simp add: zero_le_mult_iff)
  show ?thesis
    by (rule injD [of "of_nat :: nat ⇒ int", OF inj_of_nat])
      (simp only: of_nat_mult of_nat_nat [OF True]
         of_nat_nat [OF assms] of_nat_nat [OF ge_0], simp)
qed

lemma nat_mult_distrib_neg: "z ≤ 0 ⟹ nat (z * z') = nat (- z) * nat (- z')"
  for z z' :: int
  apply (rule trans)
   apply (rule_tac [2] nat_mult_distrib)
   apply auto
  done

lemma nat_abs_mult_distrib: "nat ¦w * z¦ = nat ¦w¦ * nat ¦z¦"
  by (cases "z = 0 ∨ w = 0")
    (auto simp add: abs_if nat_mult_distrib [symmetric]
      nat_mult_distrib_neg [symmetric] mult_less_0_iff)

lemma int_in_range_abs [simp]: "int n ∈ range abs"
proof (rule range_eqI)
  show "int n = ¦int n¦" by simp
qed

lemma range_abs_Nats [simp]: "range abs = (ℕ :: int set)"
proof -
  have "¦k¦ ∈ ℕ" for k :: int
    by (cases k) simp_all
  moreover have "k ∈ range abs" if "k ∈ ℕ" for k :: int
    using that by induct simp
  ultimately show ?thesis by blast
qed

lemma Suc_nat_eq_nat_zadd1: "0 ≤ z ⟹ Suc (nat z) = nat (1 + z)"
  for z :: int
  by (rule sym) (simp add: nat_eq_iff)

lemma diff_nat_eq_if:
  "nat z - nat z' =
    (if z' < 0 then nat z
     else
      let d = z - z'
      in if d < 0 then 0 else nat d)"
  by (simp add: Let_def nat_diff_distrib [symmetric])

lemma nat_numeral_diff_1 [simp]: "numeral v - (1::nat) = nat (numeral v - 1)"
  using diff_nat_numeral [of v Num.One] by simp


subsection ‹Induction principles for int›

text ‹Well-founded segments of the integers.›

definition int_ge_less_than :: "int ⇒ (int × int) set"
  where "int_ge_less_than d = {(z', z). d ≤ z' ∧ z' < z}"

lemma wf_int_ge_less_than: "wf (int_ge_less_than d)"
proof -
  have "int_ge_less_than d ⊆ measure (λz. nat (z - d))"
    by (auto simp add: int_ge_less_than_def)
  then show ?thesis
    by (rule wf_subset [OF wf_measure])
qed

text ‹
  This variant looks odd, but is typical of the relations suggested
  by RankFinder.›

definition int_ge_less_than2 :: "int ⇒ (int × int) set"
  where "int_ge_less_than2 d = {(z',z). d ≤ z ∧ z' < z}"

lemma wf_int_ge_less_than2: "wf (int_ge_less_than2 d)"
proof -
  have "int_ge_less_than2 d ⊆ measure (λz. nat (1 + z - d))"
    by (auto simp add: int_ge_less_than2_def)
  then show ?thesis
    by (rule wf_subset [OF wf_measure])
qed

(* `set:int': dummy construction *)
theorem int_ge_induct [case_names base step, induct set: int]:
  fixes i :: int
  assumes ge: "k ≤ i"
    and base: "P k"
    and step: "⋀i. k ≤ i ⟹ P i ⟹ P (i + 1)"
  shows "P i"
proof -
  have "⋀i::int. n = nat (i - k) ⟹ k ≤ i ⟹ P i" for n
  proof (induct n)
    case 0
    then have "i = k" by arith
    with base show "P i" by simp
  next
    case (Suc n)
    then have "n = nat ((i - 1) - k)" by arith
    moreover have k: "k ≤ i - 1" using Suc.prems by arith
    ultimately have "P (i - 1)" by (rule Suc.hyps)
    from step [OF k this] show ?case by simp
  qed
  with ge show ?thesis by fast
qed

(* `set:int': dummy construction *)
theorem int_gr_induct [case_names base step, induct set: int]:
  fixes i k :: int
  assumes gr: "k < i"
    and base: "P (k + 1)"
    and step: "⋀i. k < i ⟹ P i ⟹ P (i + 1)"
  shows "P i"
  apply (rule int_ge_induct[of "k + 1"])
  using gr apply arith
   apply (rule base)
  apply (rule step)
   apply simp_all
  done

theorem int_le_induct [consumes 1, case_names base step]:
  fixes i k :: int
  assumes le: "i ≤ k"
    and base: "P k"
    and step: "⋀i. i ≤ k ⟹ P i ⟹ P (i - 1)"
  shows "P i"
proof -
  have "⋀i::int. n = nat(k-i) ⟹ i ≤ k ⟹ P i" for n
  proof (induct n)
    case 0
    then have "i = k" by arith
    with base show "P i" by simp
  next
    case (Suc n)
    then have "n = nat (k - (i + 1))" by arith
    moreover have k: "i + 1 ≤ k" using Suc.prems by arith
    ultimately have "P (i + 1)" by (rule Suc.hyps)
    from step[OF k this] show ?case by simp
  qed
  with le show ?thesis by fast
qed

theorem int_less_induct [consumes 1, case_names base step]:
  fixes i k :: int
  assumes less: "i < k"
    and base: "P (k - 1)"
    and step: "⋀i. i < k ⟹ P i ⟹ P (i - 1)"
  shows "P i"
  apply (rule int_le_induct[of _ "k - 1"])
  using less apply arith
   apply (rule base)
  apply (rule step)
   apply simp_all
  done

theorem int_induct [case_names base step1 step2]:
  fixes k :: int
  assumes base: "P k"
    and step1: "⋀i. k ≤ i ⟹ P i ⟹ P (i + 1)"
    and step2: "⋀i. k ≥ i ⟹ P i ⟹ P (i - 1)"
  shows "P i"
proof -
  have "i ≤ k ∨ i ≥ k" by arith
  then show ?thesis
  proof
    assume "i ≥ k"
    then show ?thesis
      using base by (rule int_ge_induct) (fact step1)
  next
    assume "i ≤ k"
    then show ?thesis
      using base by (rule int_le_induct) (fact step2)
  qed
qed


subsection ‹Intermediate value theorems›

lemma nat_intermed_int_val:
  "∃i. m ≤ i ∧ i ≤ n ∧ f i = k"
  if "∀i. m ≤ i ∧ i < n ⟶ ¦f (Suc i) - f i¦ ≤ 1"
    "m ≤ n" "f m ≤ k" "k ≤ f n"
  for m n :: nat and k :: int
proof -
  have "(∀i<n. ¦f (Suc i) - f i¦ ≤ 1) ⟹ f 0 ≤ k ⟹ k ≤ f n
    ⟹ (∃i ≤ n. f i = k)"
  for n :: nat and f
    apply (induct n)
     apply auto
    apply (erule_tac x = n in allE)
    apply (case_tac "k = f (Suc n)")
     apply (auto simp add: abs_if split: if_split_asm intro: le_SucI)
    done
  from this [of "n - m" "f ∘ plus m"] that show ?thesis
    apply auto
    apply (rule_tac x = "m + i" in exI)
    apply auto
    done
qed

lemma nat0_intermed_int_val:
  "∃i≤n. f i = k"
  if "∀i<n. ¦f (i + 1) - f i¦ ≤ 1" "f 0 ≤ k" "k ≤ f n"
  for n :: nat and k :: int
  using nat_intermed_int_val [of 0 n f k] that by auto


subsection ‹Products and 1, by T. M. Rasmussen›

lemma abs_zmult_eq_1:
  fixes m n :: int
  assumes mn: "¦m * n¦ = 1"
  shows "¦m¦ = 1"
proof -
  from mn have 0: "m ≠ 0" "n ≠ 0" by auto
  have "¬ 2 ≤ ¦m¦"
  proof
    assume "2 ≤ ¦m¦"
    then have "2 * ¦n¦ ≤ ¦m¦ * ¦n¦" by (simp add: mult_mono 0)
    also have "… = ¦m * n¦" by (simp add: abs_mult)
    also from mn have "… = 1" by simp
    finally have "2 * ¦n¦ ≤ 1" .
    with 0 show "False" by arith
  qed
  with 0 show ?thesis by auto
qed

lemma pos_zmult_eq_1_iff_lemma: "m * n = 1 ⟹ m = 1 ∨ m = - 1"
  for m n :: int
  using abs_zmult_eq_1 [of m n] by arith

lemma pos_zmult_eq_1_iff:
  fixes m n :: int
  assumes "0 < m"
  shows "m * n = 1 ⟷ m = 1 ∧ n = 1"
proof -
  from assms have "m * n = 1 ⟹ m = 1"
    by (auto dest: pos_zmult_eq_1_iff_lemma)
  then show ?thesis
    by (auto dest: pos_zmult_eq_1_iff_lemma)
qed

lemma zmult_eq_1_iff: "m * n = 1 ⟷ (m = 1 ∧ n = 1) ∨ (m = - 1 ∧ n = - 1)"
  for m n :: int
  apply (rule iffI)
   apply (frule pos_zmult_eq_1_iff_lemma)
   apply (simp add: mult.commute [of m])
   apply (frule pos_zmult_eq_1_iff_lemma)
   apply auto
  done

lemma infinite_UNIV_int: "¬ finite (UNIV::int set)"
proof
  assume "finite (UNIV::int set)"
  moreover have "inj (λi::int. 2 * i)"
    by (rule injI) simp
  ultimately have "surj (λi::int. 2 * i)"
    by (rule finite_UNIV_inj_surj)
  then obtain i :: int where "1 = 2 * i" by (rule surjE)
  then show False by (simp add: pos_zmult_eq_1_iff)
qed


subsection ‹The divides relation›

lemma zdvd_antisym_nonneg: "0 ≤ m ⟹ 0 ≤ n ⟹ m dvd n ⟹ n dvd m ⟹ m = n"
  for m n :: int
  by (auto simp add: dvd_def mult.assoc zero_le_mult_iff zmult_eq_1_iff)

lemma zdvd_antisym_abs:
  fixes a b :: int
  assumes "a dvd b" and "b dvd a"
  shows "¦a¦ = ¦b¦"
proof (cases "a = 0")
  case True
  with assms show ?thesis by simp
next
  case False
  from ‹a dvd b› obtain k where k: "b = a * k"
    unfolding dvd_def by blast
  from ‹b dvd a› obtain k' where k': "a = b * k'"
    unfolding dvd_def by blast
  from k k' have "a = a * k * k'" by simp
  with mult_cancel_left1[where c="a" and b="k*k'"] have kk': "k * k' = 1"
    using ‹a ≠ 0› by (simp add: mult.assoc)
  then have "k = 1 ∧ k' = 1 ∨ k = -1 ∧ k' = -1"
    by (simp add: zmult_eq_1_iff)
  with k k' show ?thesis by auto
qed

lemma zdvd_zdiffD: "k dvd m - n ⟹ k dvd n ⟹ k dvd m"
  for k m n :: int
  using dvd_add_right_iff [of k "- n" m] by simp

lemma zdvd_reduce: "k dvd n + k * m ⟷ k dvd n"
  for k m n :: int
  using dvd_add_times_triv_right_iff [of k n m] by (simp add: ac_simps)

lemma dvd_imp_le_int:
  fixes d i :: int
  assumes "i ≠ 0" and "d dvd i"
  shows "¦d¦ ≤ ¦i¦"
proof -
  from ‹d dvd i› obtain k where "i = d * k" ..
  with ‹i ≠ 0› have "k ≠ 0" by auto
  then have "1 ≤ ¦k¦" and "0 ≤ ¦d¦" by auto
  then have "¦d¦ * 1 ≤ ¦d¦ * ¦k¦" by (rule mult_left_mono)
  with ‹i = d * k› show ?thesis by (simp add: abs_mult)
qed

lemma zdvd_not_zless:
  fixes m n :: int
  assumes "0 < m" and "m < n"
  shows "¬ n dvd m"
proof
  from assms have "0 < n" by auto
  assume "n dvd m" then obtain k where k: "m = n * k" ..
  with ‹0 < m› have "0 < n * k" by auto
  with ‹0 < n› have "0 < k" by (simp add: zero_less_mult_iff)
  with k ‹0 < n› ‹m < n› have "n * k < n * 1" by simp
  with ‹0 < n› ‹0 < k› show False unfolding mult_less_cancel_left by auto
qed

lemma zdvd_mult_cancel:
  fixes k m n :: int
  assumes d: "k * m dvd k * n"
    and "k ≠ 0"
  shows "m dvd n"
proof -
  from d obtain h where h: "k * n = k * m * h"
    unfolding dvd_def by blast
  have "n = m * h"
  proof (rule ccontr)
    assume "¬ ?thesis"
    with ‹k ≠ 0› have "k * n ≠ k * (m * h)" by simp
    with h show False
      by (simp add: mult.assoc)
  qed
  then show ?thesis by simp
qed

lemma int_dvd_int_iff [simp]:
  "int m dvd int n ⟷ m dvd n"
proof -
  have "m dvd n" if "int n = int m * k" for k
  proof (cases k)
    case (nonneg q)
    with that have "n = m * q"
      by (simp del: of_nat_mult add: of_nat_mult [symmetric])
    then show ?thesis ..
  next
    case (neg q)
    with that have "int n = int m * (- int (Suc q))"
      by simp
    also have "… = - (int m * int (Suc q))"
      by (simp only: mult_minus_right)
    also have "… = - int (m * Suc q)"
      by (simp only: of_nat_mult [symmetric])
    finally have "- int (m * Suc q) = int n" ..
    then show ?thesis
      by (simp only: negative_eq_positive) auto
  qed
  then show ?thesis by (auto simp add: dvd_def)
qed

lemma dvd_nat_abs_iff [simp]:
  "n dvd nat ¦k¦ ⟷ int n dvd k"
proof -
  have "n dvd nat ¦k¦ ⟷ int n dvd int (nat ¦k¦)"
    by (simp only: int_dvd_int_iff)
  then show ?thesis
    by simp
qed

lemma nat_abs_dvd_iff [simp]:
  "nat ¦k¦ dvd n ⟷ k dvd int n"
proof -
  have "nat ¦k¦ dvd n ⟷ int (nat ¦k¦) dvd int n"
    by (simp only: int_dvd_int_iff)
  then show ?thesis
    by simp
qed

lemma zdvd1_eq [simp]: "x dvd 1 ⟷ ¦x¦ = 1" (is "?lhs ⟷ ?rhs")
  for x :: int
proof
  assume ?lhs
  then have "nat ¦x¦ dvd nat ¦1¦"
    by (simp only: nat_abs_dvd_iff) simp
  then have "nat ¦x¦ = 1"
    by simp
  then show ?rhs
    by (cases "x < 0") simp_all
next
  assume ?rhs
  then have "x = 1 ∨ x = - 1"
    by auto
  then show ?lhs
    by (auto intro: dvdI)
qed

lemma zdvd_mult_cancel1:
  fixes m :: int
  assumes mp: "m ≠ 0"
  shows "m * n dvd m ⟷ ¦n¦ = 1"
    (is "?lhs ⟷ ?rhs")
proof
  assume ?rhs
  then show ?lhs
    by (cases "n > 0") (auto simp add: minus_equation_iff)
next
  assume ?lhs
  then have "m * n dvd m * 1" by simp
  from zdvd_mult_cancel[OF this mp] show ?rhs
    by (simp only: zdvd1_eq)
qed

lemma nat_dvd_iff: "nat z dvd m ⟷ (if 0 ≤ z then z dvd int m else m = 0)"
  using nat_abs_dvd_iff [of z m] by (cases "z ≥ 0") auto

lemma eq_nat_nat_iff: "0 ≤ z ⟹ 0 ≤ z' ⟹ nat z = nat z' ⟷ z = z'"
  by (auto elim: nonneg_int_cases)

lemma nat_power_eq: "0 ≤ z ⟹ nat (z ^ n) = nat z ^ n"
  by (induct n) (simp_all add: nat_mult_distrib)

lemma numeral_power_eq_nat_cancel_iff [simp]:
  "numeral x ^ n = nat y ⟷ numeral x ^ n = y"
  using nat_eq_iff2 by auto

lemma nat_eq_numeral_power_cancel_iff [simp]:
  "nat y = numeral x ^ n ⟷ y = numeral x ^ n"
  using numeral_power_eq_nat_cancel_iff[of x n y]
  by (metis (mono_tags))

lemma numeral_power_le_nat_cancel_iff [simp]:
  "numeral x ^ n ≤ nat a ⟷ numeral x ^ n ≤ a"
  using nat_le_eq_zle[of "numeral x ^ n" a]
  by (auto simp: nat_power_eq)

lemma nat_le_numeral_power_cancel_iff [simp]:
  "nat a ≤ numeral x ^ n ⟷ a ≤ numeral x ^ n"
  by (simp add: nat_le_iff)

lemma numeral_power_less_nat_cancel_iff [simp]:
  "numeral x ^ n < nat a ⟷ numeral x ^ n < a"
  using nat_less_eq_zless[of "numeral x ^ n" a]
  by (auto simp: nat_power_eq)

lemma nat_less_numeral_power_cancel_iff [simp]:
  "nat a < numeral x ^ n ⟷ a < numeral x ^ n"
  using nat_less_eq_zless[of a "numeral x ^ n"]
  by (cases "a < 0") (auto simp: nat_power_eq less_le_trans[where y=0])

lemma zdvd_imp_le: "z dvd n ⟹ 0 < n ⟹ z ≤ n"
  for n z :: int
  apply (cases n)
  apply auto
  apply (cases z)
   apply (auto simp add: dvd_imp_le)
  done

lemma zdvd_period:
  fixes a d :: int
  assumes "a dvd d"
  shows "a dvd (x + t) ⟷ a dvd ((x + c * d) + t)"
    (is "?lhs ⟷ ?rhs")
proof -
  from assms have "a dvd (x + t) ⟷ a dvd ((x + t) + c * d)"
    by (simp add: dvd_add_left_iff)
  then show ?thesis
    by (simp add: ac_simps)
qed


subsection ‹Finiteness of intervals›

lemma finite_interval_int1 [iff]: "finite {i :: int. a ≤ i ∧ i ≤ b}"
proof (cases "a ≤ b")
  case True
  then show ?thesis
  proof (induct b rule: int_ge_induct)
    case base
    have "{i. a ≤ i ∧ i ≤ a} = {a}" by auto
    then show ?case by simp
  next
    case (step b)
    then have "{i. a ≤ i ∧ i ≤ b + 1} = {i. a ≤ i ∧ i ≤ b} ∪ {b + 1}" by auto
    with step show ?case by simp
  qed
next
  case False
  then show ?thesis
    by (metis (lifting, no_types) Collect_empty_eq finite.emptyI order_trans)
qed

lemma finite_interval_int2 [iff]: "finite {i :: int. a ≤ i ∧ i < b}"
  by (rule rev_finite_subset[OF finite_interval_int1[of "a" "b"]]) auto

lemma finite_interval_int3 [iff]: "finite {i :: int. a < i ∧ i ≤ b}"
  by (rule rev_finite_subset[OF finite_interval_int1[of "a" "b"]]) auto

lemma finite_interval_int4 [iff]: "finite {i :: int. a < i ∧ i < b}"
  by (rule rev_finite_subset[OF finite_interval_int1[of "a" "b"]]) auto


subsection ‹Configuration of the code generator›

text ‹Constructors›

definition Pos :: "num ⇒ int"
  where [simp, code_abbrev]: "Pos = numeral"

definition Neg :: "num ⇒ int"
  where [simp, code_abbrev]: "Neg n = - (Pos n)"

code_datatype "0::int" Pos Neg


text ‹Auxiliary operations.›

definition dup :: "int ⇒ int"
  where [simp]: "dup k = k + k"

lemma dup_code [code]:
  "dup 0 = 0"
  "dup (Pos n) = Pos (Num.Bit0 n)"
  "dup (Neg n) = Neg (Num.Bit0 n)"
  by (simp_all add: numeral_Bit0)

definition sub :: "num ⇒ num ⇒ int"
  where [simp]: "sub m n = numeral m - numeral n"

lemma sub_code [code]:
  "sub Num.One Num.One = 0"
  "sub (Num.Bit0 m) Num.One = Pos (Num.BitM m)"
  "sub (Num.Bit1 m) Num.One = Pos (Num.Bit0 m)"
  "sub Num.One (Num.Bit0 n) = Neg (Num.BitM n)"
  "sub Num.One (Num.Bit1 n) = Neg (Num.Bit0 n)"
  "sub (Num.Bit0 m) (Num.Bit0 n) = dup (sub m n)"
  "sub (Num.Bit1 m) (Num.Bit1 n) = dup (sub m n)"
  "sub (Num.Bit1 m) (Num.Bit0 n) = dup (sub m n) + 1"
  "sub (Num.Bit0 m) (Num.Bit1 n) = dup (sub m n) - 1"
  by (simp_all only: sub_def dup_def numeral.simps Pos_def Neg_def numeral_BitM)

text ‹Implementations.›

lemma one_int_code [code]: "1 = Pos Num.One"
  by simp

lemma plus_int_code [code]:
  "k + 0 = k"
  "0 + l = l"
  "Pos m + Pos n = Pos (m + n)"
  "Pos m + Neg n = sub m n"
  "Neg m + Pos n = sub n m"
  "Neg m + Neg n = Neg (m + n)"
  for k l :: int
  by simp_all

lemma uminus_int_code [code]:
  "uminus 0 = (0::int)"
  "uminus (Pos m) = Neg m"
  "uminus (Neg m) = Pos m"
  by simp_all

lemma minus_int_code [code]:
  "k - 0 = k"
  "0 - l = uminus l"
  "Pos m - Pos n = sub m n"
  "Pos m - Neg n = Pos (m + n)"
  "Neg m - Pos n = Neg (m + n)"
  "Neg m - Neg n = sub n m"
  for k l :: int
  by simp_all

lemma times_int_code [code]:
  "k * 0 = 0"
  "0 * l = 0"
  "Pos m * Pos n = Pos (m * n)"
  "Pos m * Neg n = Neg (m * n)"
  "Neg m * Pos n = Neg (m * n)"
  "Neg m * Neg n = Pos (m * n)"
  for k l :: int
  by simp_all

instantiation int :: equal
begin

definition "HOL.equal k l ⟷ k = (l::int)"

instance
  by standard (rule equal_int_def)

end

lemma equal_int_code [code]:
  "HOL.equal 0 (0::int) ⟷ True"
  "HOL.equal 0 (Pos l) ⟷ False"
  "HOL.equal 0 (Neg l) ⟷ False"
  "HOL.equal (Pos k) 0 ⟷ False"
  "HOL.equal (Pos k) (Pos l) ⟷ HOL.equal k l"
  "HOL.equal (Pos k) (Neg l) ⟷ False"
  "HOL.equal (Neg k) 0 ⟷ False"
  "HOL.equal (Neg k) (Pos l) ⟷ False"
  "HOL.equal (Neg k) (Neg l) ⟷ HOL.equal k l"
  by (auto simp add: equal)

lemma equal_int_refl [code nbe]: "HOL.equal k k ⟷ True"
  for k :: int
  by (fact equal_refl)

lemma less_eq_int_code [code]:
  "0 ≤ (0::int) ⟷ True"
  "0 ≤ Pos l ⟷ True"
  "0 ≤ Neg l ⟷ False"
  "Pos k ≤ 0 ⟷ False"
  "Pos k ≤ Pos l ⟷ k ≤ l"
  "Pos k ≤ Neg l ⟷ False"
  "Neg k ≤ 0 ⟷ True"
  "Neg k ≤ Pos l ⟷ True"
  "Neg k ≤ Neg l ⟷ l ≤ k"
  by simp_all

lemma less_int_code [code]:
  "0 < (0::int) ⟷ False"
  "0 < Pos l ⟷ True"
  "0 < Neg l ⟷ False"
  "Pos k < 0 ⟷ False"
  "Pos k < Pos l ⟷ k < l"
  "Pos k < Neg l ⟷ False"
  "Neg k < 0 ⟷ True"
  "Neg k < Pos l ⟷ True"
  "Neg k < Neg l ⟷ l < k"
  by simp_all

lemma nat_code [code]:
  "nat (Int.Neg k) = 0"
  "nat 0 = 0"
  "nat (Int.Pos k) = nat_of_num k"
  by (simp_all add: nat_of_num_numeral)

lemma (in ring_1) of_int_code [code]:
  "of_int (Int.Neg k) = - numeral k"
  "of_int 0 = 0"
  "of_int (Int.Pos k) = numeral k"
  by simp_all


text ‹Serializer setup.›

code_identifier
  code_module Int  (SML) Arith and (OCaml) Arith and (Haskell) Arith

quickcheck_params [default_type = int]

hide_const (open) Pos Neg sub dup


text ‹De-register ‹int› as a quotient type:›

lifting_update int.lifting
lifting_forget int.lifting


subsection ‹Duplicates›

lemmas int_sum = of_nat_sum [where 'a=int]
lemmas int_prod = of_nat_prod [where 'a=int]
lemmas zle_int = of_nat_le_iff [where 'a=int]
lemmas int_int_eq = of_nat_eq_iff [where 'a=int]
lemmas nonneg_eq_int = nonneg_int_cases
lemmas double_eq_0_iff = double_zero

lemmas int_distrib =
  distrib_right [of z1 z2 w]
  distrib_left [of w z1 z2]
  left_diff_distrib [of z1 z2 w]
  right_diff_distrib [of w z1 z2]
  for z1 z2 w :: int

end