Test case : 8 Queens Problem using Ada

-- queens
with Ada.Text_Io;
use Ada.Text_Io;
with Interfaces;
use Interfaces;

procedure Queens is 
   subtype Index is Natural range 1..8;
   Queens : array (Index) of Unsigned_8;  

   function Row (
         Vertex  : Unsigned_8; 
         Diag    : Unsigned_8; 
         The_Row : Index       ) 
     return Boolean is 
      M0 : Unsigned_8;  
      M1 : Unsigned_8;
      C1 : constant Unsigned_8 := 16#10#;
      C3 : constant Unsigned_8 := 7;
      Urow : Unsigned_8 := Unsigned_8(The_Row); 
   begin
      -- set diagonal initial bits
      M1 := (Shift_Left(C1, The_Row) or (Shift_Left(1, Natural((C3 xor Urow)))));

      M0 := 1;
      while M0 <= 255 loop
         -- see if lines not already attacked
         if 0 = ((Vertex and M0) or (Diag and M1)) then
            -- recurse on next row after attacking these lines
            if (The_Row = Index'Last) or else
                  (The_Row < Index'Last and then
                     Row((Vertex or M0), (Diag or M1), The_Row + 1)) then
               Queens(The_Row) := M0;
               return True;
            end if;
         end if;
         M0 := M0 + M0;
         M1 := M1 + M1;
      end loop;
      return False;
   end Row;

   procedure Display_Solution is 
   begin
      for I in Index'range loop
         for J in 0..7 loop
            if Queens(I) = Shift_Left(1, J) then
               Put("Q");
            else
               Put("_");
            end if;
         end loop;
         New_Line;
      end loop;
   end Display_Solution;

begin
   if Row(0,0,1) then
      Display_Solution;
   else
      Put_Line("No Solution");
   end if;
end Queens;