(* Title: HOL/Proofs/Extraction/QuotRem.thy
Author: Stefan Berghofer, TU Muenchen
*)
section \<open>Quotient and remainder\<close>
theory QuotRem
imports Util
begin
text \<open>Derivation of quotient and remainder using program extraction.\<close>
theorem division: "\<exists>r q. a = Suc b * q + r \<and> r \<le> b"
proof (induct a)
case 0
have "0 = Suc b * 0 + 0 \<and> 0 \<le> b" by simp
then show ?case by iprover
next
case (Suc a)
then obtain r q where I: "a = Suc b * q + r" and "r \<le> b" by iprover
from nat_eq_dec show ?case
proof
assume "r = b"
with I have "Suc a = Suc b * (Suc q) + 0 \<and> 0 \<le> b" by simp
then show ?case by iprover
next
assume "r \<noteq> b"
with \<open>r \<le> b\<close> have "r < b" by (simp add: order_less_le)
with I have "Suc a = Suc b * q + (Suc r) \<and> (Suc r) \<le> b" by simp
then show ?case by iprover
qed
qed
extract division
text \<open>
The program extracted from the above proof looks as follows
@{thm [display] division_def [no_vars]}
The corresponding correctness theorem is
@{thm [display] division_correctness [no_vars]}
\<close>
lemma "division 9 2 = (0, 3)" by eval
end