Another update from ingydotnet^djgoku

Task/N-queens-problem/ATS/n-queens-problem.ats

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+(* ****** ****** *)
+//
+// Solving N-queen puzzle
+//
+(* ****** ****** *)
+//
+// How to test:
+// ./queens
+// How to compile:
+// patscc -DATS_MEMALLOC_LIBC -o queens queens.dats
+//
+(* ****** ****** *)
+//
+#include
+"share/atspre_staload.hats"
+//
+#include
+"share/HATS/atspre_staload_libats_ML.hats"
+//
+(* ****** ****** *)
+
+fun
+solutions(N:int) = let
+//
+fun
+show
+(
+  board: list0(int)
+) : void =
+(
+  list0_foreach<int>
+  ( list0_reverse(board)
+  , lam(n) => ((N).foreach()(lam(i) => print_string(if i = n then " Q" else " _")); print_newline())
+  ) ;
+  print_newline()
+)
+//
+fun
+safe
+(
+  i: int, j: int, k: int, xs: list0(int)
+) : bool =
+(
+  case+ xs of
+  | nil0() => true
+  | cons0(x, xs) => x != i && x != j && x != k && safe(i, j+1, k-1, xs)
+)
+//
+fun
+loop
+(
+  col: int, xs: list0(int)
+) : void =
+(N).foreach()
+(
+lam(i) =>
+if
+safe(i, i+1, i-1, xs)
+then let
+  val xs = cons0(i, xs)
+in
+  if col = N then show(xs) else loop(col+1, xs)
+end // end of [then]
+)
+//
+in
+  loop(1, nil0())
+end // end of [solutions]
+
+(* ****** ****** *)
+
+val () = solutions(8)
+
+(* ****** ****** *)
+
+implement main0() = ()
+
+(* ****** ****** *)
+
+(* end of [queens.dats] *)