Re: Fixed vs float and precision and conversions

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 2020-07-08 0:10, Björn Lundin wrote:
> 
> Hi !
> 
> I've for years run an interface towards extenal part on a raspberry pi
> that communicates with JSON over http (JSONRPC2)
> 
> the versions are
> 
> bnl@pibetbot:~ $ gnatls -v
> 
> GNATLS 6.3.0
> Copyright (C) 1997-2016, Free Software Foundation, Inc.
> ...
> 
> bnl@pibetbot:~ $ uname -a
> Linux pibetbot 4.14.79-v7+ #1159 SMP Sun Nov 4 17:50:20 GMT 2018 armv7l 
> GNU/Linux
> bnl@pibetbot:~ $ cat /etc/deb
> debconf.conf    debian_version
> bnl@pibetbot:~ $ cat /etc/deb
> debconf.conf    debian_version
> bnl@pibetbot:~ $ cat /etc/debian_version
> 9.4
> 
> 
> I have found this reliable but suddenly I have got some rounding 
> troubles. Or I perhaps just discovered it now.
> 
> 
> I have a fixed type
>    type Fixed_Type is delta 0.001 digits 18;
> 
> but JSON does not support that. So I get floats instead.
> 
> 
> I use gnatcoll.json as parser by the way
> 
> 
>    procedure Get_Value(Container: in    Json_Value;
>                        Field    : in     String;
>                        Target   : in out Fixed_Type;
>                        Found    :    out Boolean ) is
>      Tmp : Float := 0.0;
>    begin
>      if Container.Has_Field(Field) then
>        Tmp := Container.Get(Field);
>        Found := True;
>        Target := Fixed_Type(Tmp);
>      else
>        Found := False;
>      end if;
>    end Get_Value;
> 
> 
> The message (JSON) contains a value 5.10 (in a float), but that is 
> converted (sometimes I think) to the fixed_type variable with value 
> 5.099. This gets me into trouble further down in the code.
> 
> So - What should I do instead?

According to RM 4.6(31), conversion to a decimal fixed-point type does 
not round, but truncates toward zero if the operand is not a multiple of 
the "small" of the target type, which is usually the case here if Floats 
are base-two.

You should perhaps change the conversion (Target := Fixed_Type(Tmp)) to 
round, by doing Target := Fixed_Point'Round (Tmp).

Note, I haven't tried it.

-- 
Niklas Holsti
niklas holsti tidorum fi
       .      @       .