UUID

[Wout4]

Added an ID fields to variables, using uuid to get unique id's.
Naming stays the same, for display/print reasons.
When posting to the solver I decided to use name + id because

• it will still be unique
• minizinc names have to start with a letter, this ensures that and makes it consistent for all solvers
• might make debugging easier as the var name is still part of the solver var name

Let me know if you think this is silly.

[363734]

I think this will also be useful for the CSE, as you can then create easily a unique hash for each expression based on the type of operation and the argument of it

[Wout4]

defining a unique hash for every expression is a great idea, should probably go in another pr though

[JoD]

typo?

[JoD]

This is hard to review because of the autoformatting changes. Would it be possible to add these in a separate MR? Also, provide an explanation for all collaborators on how to use the same autoformatter, so that the autoformatting does not get broken in future MRs.

[JoD]

Replace str(var) + str(var.id) and var.name + str(var.id) with helper function var.solver_name().

[JoD]

I'm lukewarm on using a uuid. The alternative is using a global variable counter. The advantage of a global counter is that it will also be faster: long string names of variables do have a measurable impact (both for memory and efficiency) in scenario's with lots of variables passed to a solver, so keeping them short (via a counter starting from 0) is better.

However, this is just an optimization issue, so if you think the current approach is fine, that's fine by me.

[363734]

global variable counter does not work with pickled models, as the counter is not pickled. For example, you define a first model with variable id 1 to 10, counter is at 10, you pickle the model, fine. Then you unpickle it in another run, counter is resetted, you decide to add new var to model, var id 1 reused, conflict

[JoD]

Ah, did not think of that. The counter is indeed part of the state and should be pickled too, but I can see that this may tip the scales in the balance between simplicity and efficiency.

[Wout4]

I will try to make a case again for why we need unique identifiers :) (to implement CSE)

After looking into python dicts/sets it turns out that in the event of a hash collision __eq__ gets called on the keys, to decide if they are the same (so not the pointer, nor is)
Since we overwrite eq for cpmpy expressions to return a new comparison object, eq always returns a 'truthy' value.
So 2 expressions with the same hash can never co-exist in a dictionary where we use the expressions themselves as keys.
This means we will need unique hashes if we want to use expressions as keys. (-> uuid needed)

On the other hand, we could use something else as the key, to avoid the problem with overwriting eq. But then we would still need a unique property of expressions to use as the key, since the string representation could not be unique (when the user gives particular names or descriptions to the variables/expressions) (-> uuid needed)