Theory Greatest_Common_Divisor

(*  Title:      HOL/Proofs/Extraction/Greatest_Common_Divisor.thy
    Author:     Stefan Berghofer, TU Muenchen
    Author:     Helmut Schwichtenberg, LMU Muenchen
*)

section ‹Greatest common divisor›

theory Greatest_Common_Divisor
imports QuotRem
begin

theorem greatest_common_divisor:
  "⋀n::nat. Suc m < n ⟹
    ∃k n1 m1. k * n1 = n ∧ k * m1 = Suc m ∧
    (∀l l1 l2. l * l1 = n ⟶ l * l2 = Suc m ⟶ l ≤ k)"
proof (induct m rule: nat_wf_ind)
  case (1 m n)
  from division obtain r q where h1: "n = Suc m * q + r" and h2: "r ≤ m"
    by iprover
  show ?case
  proof (cases r)
    case 0
    with h1 have "Suc m * q = n" by simp
    moreover have "Suc m * 1 = Suc m" by simp
    moreover have "l * l1 = n ⟹ l * l2 = Suc m ⟹ l ≤ Suc m" for l l1 l2
      by (cases l2) simp_all
    ultimately show ?thesis by iprover
  next
    case (Suc nat)
    with h2 have h: "nat < m" by simp
    moreover from h have "Suc nat < Suc m" by simp
    ultimately have "∃k m1 r1. k * m1 = Suc m ∧ k * r1 = Suc nat ∧
        (∀l l1 l2. l * l1 = Suc m ⟶ l * l2 = Suc nat ⟶ l ≤ k)"
      by (rule 1)
    then obtain k m1 r1 where h1': "k * m1 = Suc m"
      and h2': "k * r1 = Suc nat"
      and h3': "⋀l l1 l2. l * l1 = Suc m ⟹ l * l2 = Suc nat ⟹ l ≤ k"
      by iprover
    have mn: "Suc m < n" by (rule 1)
    from h1 h1' h2' Suc have "k * (m1 * q + r1) = n"
      by (simp add: add_mult_distrib2 mult.assoc [symmetric])
    moreover have "l ≤ k" if ll1n: "l * l1 = n" and ll2m: "l * l2 = Suc m" for l l1 l2
    proof -
      have "l * (l1 - l2 * q) = Suc nat"
        by (simp add: diff_mult_distrib2 h1 Suc [symmetric] mn ll1n ll2m [symmetric])
      with ll2m show "l ≤ k" by (rule h3')
    qed
    ultimately show ?thesis using h1' by iprover
  qed
qed

extract greatest_common_divisor

text ‹
  The extracted program for computing the greatest common divisor is
  @{thm [display] greatest_common_divisor_def}
›

instantiation nat :: default
begin

definition "default = (0::nat)"

instance ..

end

instantiation prod :: (default, default) default
begin

definition "default = (default, default)"

instance ..

end

instantiation "fun" :: (type, default) default
begin

definition "default = (λx. default)"

instance ..

end

lemma "greatest_common_divisor 7 12 = (4, 3, 2)" by eval

end