Re: Comparing version numbers

[Shark8 <onewingedshark@gmail.com>]

On Sunday, November 12, 2017 at 7:39:15 PM UTC-7, Victor Porton wrote:
> Not sure if it is quite on-topic, because it is more about general 
> programming ideas rather than about Ada. (However, a solution in Python or 
> Ruby or JavaScript would more likely used regexps than Ada.)

Please don't use regular expressions -- speaking as a software engineer who's done maintenance for most of my career Regular Expressions are horridly brittle and often times the problem isn't Regular (and thus impossible for Regular Expressions to handle) *or* the data you're operating on changes so you'd have to rewrite and/or include another Regular Expression to handle it.

> 
> Suppose I have two version numbers of a software. I need to check which of 
> the two is greater.
> 
> Lexical order string comparison does not work:
> 
> "2.3" vs "11.4".
> 
> I could split it by "." and compare the numbers. But a component of a 
> version numbers may be nonnumeric like:
> 
> "1.2beta".
> 
> What to do?

Hm, I would make a type for handling version-numbers explicitly:

Package Versioning is
   Type Version_Number is private;
   
   -- Whatever you actually have in a version.
   Function Create( Major, Minor, Natural; Build : Positive; Beta : Boolean := False ) return Version_Number;

   -- Plus accessors:
   Function Major( Version : Version_Number ) return Natural;
   --...

   -- Plus operations.
   Procedure Increment_Build (Version in out : Version_Number);
   Procedure Reset_Build     (Version in out : Version_Number);
   Procedure Increment       (Version in out : Version_Number; Major : Boolean := False );
   --...
   Function Parse( Input : String ) return Version_Number;
   Function Image( Input : Version_Number ) return String;
Private
   Type Version_Number is record
     Major_Number,
     Minor_Number : Natural;
     Build        : Positive;
     Beta         : Boolean := False
   end record;
End Versioning;

The advantage of doing something like this is that it forces you to think about the structure you're using, not blindly flailing about with strings that "look like" versions -- remember, in Ada model the problem-space first, not the solution-space; take advantage of the Fundamental Theory of Ada: Types (see http://blog.kickin-the-darkness.com/2007/08/fundamental-theory-of-ada.html ).

> 
> It should work for example for all Debian packages containing interpreters 
> (for example, "2.7" for "python" package).

I'm unfamiliar w/ Debian's scheme, but would be unsurprised if the version field is unstructured text (Unix, and Linux too, seems to be rather enamored with unstructured text).