Hi all, I'm new here.

I'm experienced embedded C developer and I've started 2 years ago an implementation of one of the specifications kind of popular in the
payment industry. I also hope to make it open source one day.

I've implemented most of it in C, but now I have a problem, because I
have to guarantee consistency of a huge configurable data structure,
that can be changed by an entity which in theory may not be in my
control. And adding all checks quickly became nightmarish and then I've
found an Ada language.

I already did some experiments, but it doesn't look right, because my experience in Ada is around 1 week. And I want to quickly assess if it
is worth doing at all.

So my requirements:

1. Important: Syntax to accessing, setting, comparing, allocating
values should be as simple as possible, because the body of code is
generated from diagrams and it should be understandable by a non
programmer, ie payments expert. In C I've wrote some macros for it.
2. Memory management should be unobtrusive to the visual look of
diagrams. In C I've used a memory pool, so I just have to do a
single de-allocation after each transaction.
3. It should support optional, mandatory and conditional fields. In C
modeled as pointers.
4. It should support bit fields that may have different meaning if some
flag is set (in C a union of bitfields)
5. Consistency between elements, if a particular flag is set to True
then some other fields must have a certain value as well. Probably
doable by Dynamic_Predicate.
6. Data elements should be accessible by name `db.processingStatus' or
by it's tag value like `db (16#CA#)' and also it should support any
additional tag that can be supplied by the card, even if its
symbolic name isn't known (in C it's void*).
7. I have to guarantee consistency of a different views of the same
value eg. amount represented as BCD array – usual representation
or 32 bit unsigned integer when requested by tag 16#81#.
8. It should be possible to modify data elements both symbolically or
using binary arrays with encoded data. Either because it is
specified this way or I have to set it to the unspecified value. In
C I've achieved this by having a union of char[] and a bitfield.
9. And as a bonus it should be serializable to EMV's variant of TLV
which isn't the same as you have in ASN.1, because EMV specification
was standardized before ASN.1 BER and those committees had little to
no cooperation.

I'm asking Ada wizards if it is at all doable in Ada? Or maybe some
links to opensource project where something similar was already achieved
so I can look it up?

--
Best Regards,
Alex

P.S. My code is available on github if someone is interested.

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Aleksy Grabowski <hurufu@gmail.com> writes:

I'm experienced embedded C developer and I've started 2 years ago an implementation of one of the specifications kind of popular in the
payment industry. I also hope to make it open source one day.

Can you say what specification it is?

have to guarantee consistency of a huge configurable data structure,
that can be changed by an entity which in theory may not be in my
control. And adding all checks quickly became nightmarish and then I've
found an Ada language.

It sounds like you waht to define a datatype for this structure, with
access and update procedures (OOP is not necessary but it's the same
idea) that make sure all the rules are followed. Is there more to it
than that?

Ada sounds like a reasonable choice, C sounds terrible, other
possibilities depend on the hardware and software environment. Would
this have to run on a smart card cpu or anything like that?

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On 2022-01-09 10:49, Aleksy Grabowski wrote:

I also have some concrete question, how to properly implement optional element? Right now I have something like this:

    generic
        type T is private;
    package TypeUtils is
        type Optional(exists : Boolean) is
        record
            case exists is
                when True => value : T;
                when False => null;
            end case;
        end record;
    end TypeUtils;

But it looks like it doesn't work, because it depends on the
discriminant `exists' but it is set once in .ads file and I can't modify
it in runtime.

Provide a default value for the discriminant:

type Optional (Defined : Boolean := False) is record
case Present is
when True =>
Value : T;
when False =>
null;
end case;
end record;

Now it is a definite type and you can place it into another record type:

type Container is record
...
Optional_Field : Optional;
...
end record;

and you can always update it:

X : Container;

X.Optional_Field := (True, Value_1);
...
X.Optional_Field := (Defined => False);
...
X.Optional_Field := (True, Value_2);
...

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

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First of all thanks for an answer.

On 1/9/22 03:36, Paul Rubin wrote:

Can you say what specification it is?

Yeah sure, it's called nexo. It is an open specification and it is
available for free after a registration on nexo-standards.org. It is
basically a high level description of an abstract payment terminal. They specify all the data structures and full flow of control. Basically they
try to combine all EMV books and all regional payment schemes into a one
huge beast, to make a universal terminal.

It sounds like you waht to define a datatype for this structure, with
access and update procedures (OOP is not necessary but it's the same
idea) that make sure all the rules are followed. Is there more to it
than that?

Something more. The first thing is update procedures, Right now I have
two modules trusted and non-trusted. Trusted part just does whatever it
wants to and non-trusted uses geters and setters. But the other thing is
that I want to model data as close to the spec as possible, so if it is
set to update some enum to a value 16#3F00# I do this. The another part
is because they tried to unify the whole payment zoo into single spec
there a lot of bizarre things there.

I also have some concrete question, how to properly implement optional
element? Right now I have something like this:

generic
type T is private;
package TypeUtils is
type Optional(exists : Boolean) is
record
case exists is
when True => value : T;
when False => null;
end case;
end record;
end TypeUtils;

But it looks like it doesn't work, because it depends on the
discriminant `exists' but it is set once in .ads file and I can't modify
it in runtime.

Ada sounds like a reasonable choice, C sounds terrible, other
possibilities depend on the hardware and software environment. Would
this have to run on a smart card cpu or anything like that?

Not really, arm CPU is enough. The one I have is MAX32590. I'm also not
really worried about memory consumption. Those embedded platforms have a
plenty of RAM nowadays.

How large this project can be? I'm already two years into it :)

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