Re: Translating an embedded C algorithm

[Niklas Holsti <niklas.holsti@nospam.please>]

Jeffrey Carter wrote:
> Niklas Holsti wrote:
> 
>>    package ADC is
>>
>>       type Count_T is range 0 .. 1023;
> 
> 
> Why not mod 2 ** 10?

Yes, that would be possible. But the code did not need logical bit-wise 
operations nor shifting (because, following the OP, I assumed that the 
compiler is smart enough to use shifting for divisions and remainders 
where the right operand is a power of 2). Modular arithmetic for ADC 
readings also seems undesirable; if it overflows I don't want 
wrap-around, I want Constraint_Error (or saturation, but Ada has no 
predefined forms of saturating integer types).

> Is
> 
> for Count_T'Size use 10;
> 
> appropriate here?

I don't see any reason for it in this example. If we had some packed 
data structures with Count_T components, sure.

> Seems to me these are only used to produce temperatures, and so should 
> be hidden from the rest of the application.

As I replied to Ludovic, I assumed that the same ADC is used to measure 
other sensors, too. The question of the public visibility of ADC.Count_T 
is a question of "programming in the large", for example which part of 
the SW is actually reading the ADC, and whether we can encapsulate all 
ADC interactions under the hierarchy of package ADC, in which case the 
type Count_T could be hidden in this hierarchy, as you suggest.

>>       Saturation : Boolean := True;
> 
> 
> Control coupling through global variables. Terrible SW engineering. What 
> happens when one task changes the value when another is part way through 
> the function?

I considered the alternative of making Saturation a parameter with a 
default value:

    function Temperature (
       Count      : Count;
       Saturation : Boolean := True)
    return Reading_T;

but I wanted to demonstrate mutable package state in the simplest 
possible way. I agree that "in the large" such freely mutable variables 
are usually a bad idea, and especially in multi-threaded programs, as 
you say.

If we want to follow the original C code exactly then the saturation 
option should of course be removed (assumed always true).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .