GNAT Community 2020 (20200818-93): Big_Integer

[Frank Jørgen Jørgensen]

Hi

I'm running the below program with GNAT Community 2020 (20200818-93)
on Windows 11 Home.
I have some problems trying to save big numbers to a file - so I noticed this behaviour:
If I open Test.dat in Visual Studio Hex editor, it seems like this program saves this big number with a different bit pattern each time.
Is that as expected?
I do have some problems reading back the big numbers in my real code.
When I compile I get the warning: "Ada.Numerics.Big_Numbers.Big_Integers" is an Ada 202x unit.

Frank

--
with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO;
with Ada.Numerics.Big_Numbers.Big_Integers;

procedure Test is

B1 : Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer;
F1 : File_Type;
S1 : Stream_Access;
begin
B1 := 1;

Ada.Streams.Stream_IO.Create (F1, Out_File, "Test.dat");
S1 := Ada.Streams.Stream_IO.Stream (F1);
Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer'Write(S1, B1);
Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer'Output(S1, B1);
Ada.Streams.Stream_IO.Close (F1);
end Test;

[Dmitry A. Kazakov]

On 2023-06-30 21:28, Frank Jørgen Jørgensen wrote:

> I'm running the below program with GNAT Community 2020 (20200818-93)
> on Windows 11 Home.
> I have some problems trying to save big numbers to a file - so I noticed this behaviour:
> If I open Test.dat in Visual Studio Hex editor, it seems like this program saves this big number with a different bit pattern each time.
> Is that as expected?
> I do have some problems reading back the big numbers in my real code.
> When I compile I get the warning: "Ada.Numerics.Big_Numbers.Big_Integers" is an Ada 202x unit.
>

> --
> with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO;
> with Ada.Numerics.Big_Numbers.Big_Integers;
>
> procedure Test is
>
> B1 : Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer;
> F1 : File_Type;
> S1 : Stream_Access;
> begin
> B1 := 1;
>
> Ada.Streams.Stream_IO.Create (F1, Out_File, "Test.dat");
> S1 := Ada.Streams.Stream_IO.Stream (F1);
> Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer'Write(S1, B1);
> Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer'Output(S1, B1);
> Ada.Streams.Stream_IO.Close (F1);
> end Test;

As a general rule, you should never use predefined implementations of
stream attributes except for Stream_Element or Character. Anything else
you must always override or else not use.

If you want to serialize signed integers use some portable format for
it. E.g. a chained encoding.

Here is a test program for a straightforward implementation of chained
store/restore:
-------------------------

with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO;
with Ada.Numerics.Big_Numbers.Big_Integers;

with Ada.Exceptions;
with Ada.IO_Exceptions;

procedure Test is

use Ada.Streams;
use Ada.Numerics.Big_Numbers.Big_Integers;
use Ada.Exceptions;
use Ada.Streams.Stream_IO;

Two : constant Big_Integer := To_Big_Integer (2);

package Conversions is new Unsigned_Conversions (Stream_Element);
use Conversions;

function Get
( Stream : in out Root_Stream_Type'Class
) return Big_Integer is
Result : Big_Integer;
Power : Natural := 6;
Negative : Boolean;
Buffer : Stream_Element_Array (1..1);
Last : Stream_Element_Offset;
This : Stream_Element renames Buffer (1);
begin
Stream.Read (Buffer, Last);
if Last /= 1 then
raise End_Error;
end if;
Result := To_Big_Integer ((This and 2#0111_1110#) / 2);
Negative := 0 /= (This and 1);
if 0 = (This and 16#80#) then
if Negative then
return -Result - 1;
else
return Result;
end if;
end if;
loop
Stream.Read (Buffer, Last);
if Last /= 1 then
raise End_Error;
end if;
Result := Result +
Two**Power * To_Big_Integer (This and 16#7F#);
if 0 = (This and 16#80#) then
if Negative then
return -Result - 1;
else
return Result;
end if;
end if;
Power := Power + 7;
end loop;
end Get;

procedure Put
( Stream : in out Root_Stream_Type'Class;
Value : Big_Integer
) is
Item : Big_Integer := Value;
Buffer : Stream_Element_Array (1..1);
This : Stream_Element renames Buffer (1);
begin
if Item >= 0 then
Item := Value;
This := From_Big_Integer (Item mod (16#40#)) * 2;
else
Item := -(Value + 1);
This := From_Big_Integer (Item mod (16#40#)) * 2 + 1;
end if;
Item := Item / 16#40#;
if Item = 0 then
Stream.Write (Buffer);
return;
end if;
This := This or 16#80#;
Stream.Write (Buffer);
loop
This := From_Big_Integer (Item mod 16#80#) or 16#80#;
Item := Item / 16#80#;
if Item = 0 then
This := This and 16#7F#;
Stream.Write (Buffer);
return;
end if;
Stream.Write (Buffer);
end loop;
end Put;

F : File_Type;
begin
Create (F, Out_File, "Test.dat");
for I in -1_000_000..1_000_000 loop
Put (Stream (F).all, To_Big_Integer (I));
end loop;
Close (F);
Open (F, In_File, "Test.dat");
for I in -1_000_000..1_000_000 loop
declare
Value : constant Big_Integer := Get (Stream (F).all);
begin
if Value /= To_Big_Integer (I) then
raise Data_Error;
end if;
end;
end loop;
Close (F);
end Test;
-------------------------
The above could be optimized to work with buffers rather than
reading/writing stream octets one by one. It is a long story, but
normally you would implement some data blocks with the length count on
top of the stream in order to avoid inefficient octet by octet reading
and add an error correction layer.

--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

[Simon Wright]

Frank Jørgen Jørgensen <f...@itema.no> writes:

> I'm running the below program with GNAT Community 2020 (20200818-93)
> on Windows 11 Home.
> I have some problems trying to save big numbers to a file - so I
> noticed this behaviour:
> If I open Test.dat in Visual Studio Hex editor, it seems like this
> program saves this big number with a different bit pattern each time.
> Is that as expected?
> I do have some problems reading back the big numbers in my real code.

Looking at the source for Big_Integer[1] I see no trace of streaming
support; what you got streamed was the address of some allocated data
used under the hood to contain the big integer.

I don't know whether the ARM requires/expects Big_Integer to be
streamable, but I think you had every reason to expect it!

[1] https://github.com/gcc-mirror/gcc/blob/7e904d6c7f252ee947c237ed32dd43b2c248384d/gcc/ada/libgnat/a-nbnbin.ads#L171

[Randy Brukardt]

"Simon Wright" <si...@pushface.org> wrote in message
news:lyr0prq...@pushface.org...
...

> I don't know whether the ARM requires/expects Big_Integer to be
> streamable, but I think you had every reason to expect it!

See 13.13.1(54/1):

For every subtype S of a language-defined nonlimited specific type T, the
output generated by S'Output or S'Write shall be readable by S'Input or
S'Read, respectively. This rule applies across partitions if the
implementation conforms to the Distributed Systems Annex.

Ada.Numerics.Big_Numbers.Big_Integers.Big_integer surely is a
language-defined nonlimited specific type, so the above applies. Streaming
should work, if it doesn't, that's a bug.

Randy.