src/HOL/Integ/IntDiv.thy

tuned;

(*  Title:      HOL/IntDiv.thy
    ID:         $Id$
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1999  University of Cambridge

The division operators div, mod and the divides relation "dvd"
*)

IntDiv = IntArith + Recdef + 

constdefs
  quorem :: "(int*int) * (int*int) => bool"
    "quorem == %((a,b), (q,r)).
                      a = b*q + r &
                      (if 0 < b then 0<=r & r<b else b<r & r <= 0)"

  adjust :: "[int, int*int] => int*int"
    "adjust b == %(q,r). if 0 <= r-b then (2*q + 1, r-b)
                         else (2*q, r)"

(** the division algorithm **)

(*for the case a>=0, b>0*)
consts posDivAlg :: "int*int => int*int"
recdef posDivAlg "inv_image less_than (%(a,b). nat(a - b + 1))"
    "posDivAlg (a,b) =
       (if (a<b | b<=0) then (0,a)
        else adjust b (posDivAlg(a, 2*b)))"

(*for the case a<0, b>0*)
consts negDivAlg :: "int*int => int*int"
recdef negDivAlg "inv_image less_than (%(a,b). nat(- a - b))"
    "negDivAlg (a,b) =
       (if (0<=a+b | b<=0) then (-1,a+b)
        else adjust b (negDivAlg(a, 2*b)))"

(*for the general case b~=0*)

constdefs
  negateSnd :: "int*int => int*int"
    "negateSnd == %(q,r). (q,-r)"

  (*The full division algorithm considers all possible signs for a, b
    including the special case a=0, b<0, because negDivAlg requires a<0*)
  divAlg :: "int*int => int*int"
    "divAlg ==
       %(a,b). if 0<=a then
                  if 0<=b then posDivAlg (a,b)
                  else if a=0 then (0,0)
                       else negateSnd (negDivAlg (-a,-b))
               else 
                  if 0<b then negDivAlg (a,b)
                  else         negateSnd (posDivAlg (-a,-b))"

instance
  int :: "Divides.div"        (*avoid clash with 'div' token*)

defs
  div_def   "a div b == fst (divAlg (a,b))"
  mod_def   "a mod b == snd (divAlg (a,b))"


end