`dd_reorder(f,π)`: (Explicit) Variable (Re)Ordering

[ssoelvsten]

Currently, Adiar only uses the identity variable ordering, i.e. where x_i has level i and so on. We would like to support different variable orderings.

• Rename label to level

  To this end, we want the BDD representation to only thinks about levels; these levels are then mapped back to the user's variables elsewhere. Hence, we should replace the variable label with the name level where appropriate:

  • ptr / uid
  • node
  • request
  • ...?

• Add variable ordering file

  Then, the BDD's level file or even better a separate shared variable ordering file will translate these back to actual variables.

• Resolve mismatching orders in bdd_apply and other multi-BDD operations

  Having each BDD own its own ordering opens up some interesting opportunities. Yet, it also can create a headache. For example, what should happen if bdd_and(f,g) is called with an f and g with mismatching orderings? Here are the following strategies, which should be available for the end user via the exec_policy.

  • Throw an exception. [default]
  • Reorder one into the other (based on some heuristic?) [optional]
  • Reorder both into some compromise (based on some heuristic?) [optional]

• Add dd_reorder(f,π)

  At this point, we can finally add dd_reorder which is a shallow wrapper around dd_replace. The main difference from dd_replace is that it not only affects the levels but also changes the associated variable ordering.

[ssoelvsten]

Currently, I have given this issue the deprecation label, but question is whether some parts of the public API will end up with a breaking change? It might be a good idea to already do this as part of #397 for v2.0 rather than v2.1 .