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(* ****** ****** *)
//
// HX-2013-04-03:
// this example illustrates an idea for
// addressing integer arithmetic overflow
//
(* ****** ****** *)
stacst INTMIN : int and INTMAX : int
stadef isintb (i:int): bool = (INTMIN <= i && i <= INTMAX)
(* ****** ****** *)
abst@ype intb (i: int) = int // bounded integers
(* ****** ****** *)
extern
praxi lemma_INTMINMAX (): [INTMIN < ~0X7FFF ; INTMAX >= 0x7FFF] void
(* ****** ****** *)
extern
castfn intb2int {i:int} (i: intb i): int (i)
extern
castfn int2intb {i:int | isintb(i)} (i: int i): intb (i)
(* ****** ****** *)
extern
praxi lemma_intb_param {i: int} (i: intb i): [isintb(i)] void
extern
fun add_intb_intb
{i,j:int | isintb(i+j)} (i: intb (i), j: intb (j)):<> intb (i+j)
overload + with add_intb_intb
extern
fun sub_intb_intb
{i,j:int | isintb(i-j)} (i: intb (i), j: intb (j)):<> intb (i-j)
overload - with sub_intb_intb
extern
fun half_intb {i:nat} (i: intb (i)):<> intb (ndiv(i,2))
overload half with half_intb
extern
fun lt_intb_intb {i,j:int} (i: intb i, j: intb j):<> bool (i < j)
overload < with lt_intb_intb
(* ****** ****** *)
extern
fun{a:t@ype}
bsearch{n:nat}
(
A: &(@[a][n]), n: intb n, x0: &a, cmp: (&a, &a) -> int
) : bool // end of [bsearch]
implement
{a}
bsearch{n}
(A, n, x0, cmp) = let
//
#define i2b int2intb
//
prval () = lemma_INTMINMAX ()
prval () = lemma_intb_param (n)
//
fun loop
{l,r:nat | l <= r; r <= n} .<r-l>.
(
A: &(@[a][n]), x0: &a, l: intb l, r: intb r
) :<cloref1> bool = let
in
//
if l < r then let
val m = l + half (r - l)
(*
//
// HX: typechecking fails
// if the next line replaces the above one
// as arith overflow may potentially occur
//
val m = (l + r) / 2
*)
val m2 = intb2int (m)
val sgn = cmp (x0, A.[m2])
in
if sgn < 0 then loop (A, x0, l, m)
else if sgn > 0 then loop (A, x0, m+i2b(1), r)
else true (*found*)
end else false (*~found*)
//
end // end of [loop]
//
in
loop (A, x0, i2b(0), n)
end // end of [bsearch]
(* ****** ****** *)
implement main () = 0
(* ****** ****** *)
(* end of [arith_overflow.dats] *)
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