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|
(*
//
// HX-2014-03-28:
// A program to solve the 8-queens problem
// based on lazy evaluation
//
*)
(* ****** ****** *)
//
#include
"share/atspre_define.hats"
#include
"share/atspre_staload.hats"
//
#include
"share/HATS/atspre_staload_libats_ML.hats"
//
(* ****** ****** *)
//
staload UN = $UNSAFE
//
(* ****** ****** *)
fun path_test
(
xs: list0 (int), x0: int
) : bool = let
//
fun aux
(
xs: list0 (int), df: int
) : bool =
(
case+ xs of
| nil0 () => true
| cons0 (x, xs) =>
if x0 != x && abs (x0 - x) != df then aux (xs, df+1) else false
// end of [cons0]
)
//
in
aux (xs, 1(*df*))
end // end of [path_test]
(* ****** ****** *)
#define N 8
(* ****** ****** *)
fun path_next
(
xs: list0(int)
) : stream(list0(int)) = $delay
(
let
val n = length (xs)
in
//
if n < N
then path_next2 (xs, 0)
else let
val-cons0 (x, xs) = xs
in
path_next2 (xs, x + 1)
end // end of [else]
//
end
) // end of [path_next]
and path_next2
(
xs: list0(int), x0: int
) : stream_con(list0(int)) =
(
if x0 < N
then let
val pass = path_test (xs, x0)
in
if pass
then let
val xs = cons0 (x0, xs)
in
stream_cons (xs, path_next (xs))
end // end of [then]
else path_next2 (xs, x0 + 1)
// end of [if]
end // end of [then]
else (
case+ xs of
| cons0 (x, xs) => path_next2 (xs, x+1) | nil0 () => stream_nil(*void*)
) (* end of [else] *)
) (* end of [path_next2] *)
(* ****** ****** *)
val theSolutions =
stream_filter_cloref (path_next (nil0(*void*)), lam (xs) => length (xs) >= N)
// end of [theSolutions]
(* ****** ****** *)
fun fprint_row
(
out: FILEref, i: int
) : void = let
//
fun aux (i: int): void =
if i > 0 then (fprint (out, ". "); aux (i-1)) else ()
//
val () = aux (i)
val () = fprint (out, "Q ")
val () = aux (N-1-i)
//
in
// nothing
end // end of [fprint_row]
(* ****** ****** *)
fun fprint_board
(
out: FILEref, xs: list0(int)
) : void = let
//
implement
fprint_val<int> (out, x) = fprint_row (out, x)
//
val () =
fprint_list0_sep (out, xs, "\n")
val () = fprint_newline (out)
//
in
// nothing
end // end of [fprint_board]
(* ****** ****** *)
val () = let
//
fun printall
(
xss: stream(list0(int)), i: int
) : void =
(
case+ !xss of
| stream_cons
(xs, xss) =>
{
val () =
println! ("Solution #", i, ":")
val () = fprint_board (stdout_ref, list0_reverse(xs))
val () = print_newline ()
val () = printall (xss, i+1)
}
| stream_nil () => ()
)
//
in
printall (theSolutions, 1)
end // end of [val]
(* ****** ****** *)
implement main0 () = ()
(* ****** ****** *)
(* end of [queens_lazy.dats] *)
|
