Re: Stream_Element_Array 2 String

[Simon Wright <simon@pushface.org>]

Marius Amado Alves <maa@liacc.up.pt> writes:

> > Im using Ada.Streams.Stream_IO for read block of files and i can convert
> > a Stream_Element_Array to String type.
> > how can i do?
> 
> Ada.Unchecked_Conversion is your friend.
> 
> But make sure you need that, instead of basic stream I/O e.g.
> 
> with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO;
> 
> procedure Strstr is
>    F : File_Type;
>    S3 : String (1 .. 3);
> begin
>    Create (F);
>    String'Write (Stream (F), "abc");
>    Reset (F);
>    S3 := "***";
>    Set_Mode (F, In_File);
>    String'Read (Stream (F), S3);
>    pragma Assert (S3 = "abc");
> end;

Also, it depends whether performance or clarity is what you need!

There is a third way, using address overlays, which is possibly rather
easier to get wrong .. I always feel uncomfortable with it, but here
goes:

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Unchecked_Conversion;
with Ada.Streams;
with BC.Support.High_Resolution_Time;

procedure String_To_Stream_Element_Array is

   function To_Stream_Element_Array_Using_Unchecked_Conversion
     (S : String) return Ada.Streams.Stream_Element_Array;

   function To_Stream_Element_Array_Using_Overlay
     (S : String) return Ada.Streams.Stream_Element_Array;

   function To_Stream_Element_Array_Using_Unchecked_Conversion
     (S : String) return Ada.Streams.Stream_Element_Array is
      subtype Source is String (S'Range);
      subtype Result is Ada.Streams.Stream_Element_Array
        (Ada.Streams.Stream_Element_Offset (S'First)
           .. Ada.Streams.Stream_Element_Offset (S'Last));
      function To_Array is new Ada.Unchecked_Conversion (Source, Result);
   begin
      return To_Array (S);
   end To_Stream_Element_Array_Using_Unchecked_Conversion;

   function To_Stream_Element_Array_Using_Overlay
     (S : String) return Ada.Streams.Stream_Element_Array is
      Array_View : Ada.Streams.Stream_Element_Array
        (Ada.Streams.Stream_Element_Offset (S'First)
           .. Ada.Streams.Stream_Element_Offset (S'Last));
      for Array_View'Address use S'Address;
   begin
      return Array_View;
   end To_Stream_Element_Array_Using_Overlay;
   
   Start, Finished : BC.Support.High_Resolution_Time.Time;
   Took : Duration;
   
   use type BC.Support.High_Resolution_Time.Time;

begin
   
   Start := BC.Support.High_Resolution_Time.Clock;
   declare
      A : constant Ada.Streams.Stream_Element_Array :=
	To_Stream_Element_Array_Using_Unchecked_Conversion
	("now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country");
   begin
      Finished := BC.Support.High_Resolution_Time.Clock;
      Took := Finished - Start;
   end;
   Put_Line ("unchecked conversion took" & Took'Img);
   
   Start := BC.Support.High_Resolution_Time.Clock;
   declare
      A : constant Ada.Streams.Stream_Element_Array :=
	To_Stream_Element_Array_Using_Overlay
	("now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country"
	   & "now is the time for all good men"
	   & " to come to the aid of the country");
   begin
      Finished := BC.Support.High_Resolution_Time.Clock;
      Took := Finished - Start;
   end;
   Put_Line ("overlay took" & Took'Img);
   
end String_To_Stream_Element_Array;

Runing on this box (which is a bit antiquated now), I get (GNAT 3.15p)

smaug.pushface.org[64]$ ./string_to_stream_element_array 
Clock rate is 0.400915179 GHz
unchecked conversion took 0.000008465
overlay took 0.000002641


You can find the Booch Components at http://pushface.org/components/bc/.

The high resolution time stuff is for GNAT and Intel (well, "must
support the rdtsc instruction") only.


Shouldn't that be ".. to the aid of the party"? oh well ..


-- 
Simon Wright                               100% Ada, no bugs.