Re: Reading the while standard input into a String

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Natasha Kerensikova wrote on comp.lang.ada:
> On 2011-06-06, Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
>> I'd rather process the input one fixed-length chunk at a time and
>> discard each chunk after processing (possibly by recycling the in-
>> memory buffer for the next chunk).
>
> I would do so if online processing was an option. However in the program
> containing code similar to what I posted, I cannot do any processing
> before reaching the end of input, because the processed format allows
> forward references, so references have to be gathered in a first pass on
> the whole input, before doing a second pass where dereference can take
> place.

OK, fair enough.

>> Note that reading one character at a time from a stream is quite fast
>> since streams normally use the operating system's buffered IO. Adding
>> a second layer of buffering inside your program is probably counter-
>> productive.
>
> That second layer of buffer was actually meant to reduce the number of
> Ada.Strings.Unbounded.Append calls (and the expensive reallocations that
> go with them).

Actually, Ada.Strings.Unbounded.Append itself has mechanisms to reduce
memory reallocations.  But I see your point.

> However I thought that something similar to C's fread(), which can use
> efficient bulk memory copy, would be better than a loop of single
> character read (even when buffered in memory).

How about:

procedure Read_In_Chunks (Stream : in out
Ada.Streams.Root_Stream_Type'Class;
                          Result : out
Ada.Strings.Unbounded.Unbounded_String) is
   Chunk : Ada.Streams.Stream_Element_Array (1 .. 1024);
   Chunk_As_String : String (1 .. 1024);
   for Chunk_As_String'Address use Chunk'Address;
   Last : Natural;
begin
   loop
      Ada.Streams.Read (Stream => Standard_Input,
                        Item   => Chunk,
                        Last   => Last);
      exit when Last = Chunk'First - 1; -- end of stream reached
      Ada.Strings.Unbounded.Append (Result, Chunk_As_String (1 ..
Last));
      exit when Last < Chunk'Last; -- end of stream reached
   end loop;
end Read_In_Chunks;

--
Ludovic Brenta.