Initial implementation of the 3.0 solver
This is a simple backtracking brute-force solver with heurisitcs,
this initial version has the following known gaps:
- Errors are not kept from branches, the error reporting after
backtracking isn't particularly useful.
- We cannot show automatically removed packages
- We cannot replace packages with others
- We do not have conflict-driven clause learning yet
Untested:
- Multi-arch
This solver is fundamentally different in key aspects:
- It solves smaller dependency groups before larger ones, leading
us to avoid installing A in A|B if B is installed more often and
more consistently.
- It only keeps the automatic packages reachable via the strongest
path. Currently it only implements autoremoval, but not display
of autoremoval as we simply enqueue all automatically installed
packages at the end when not doing automatic removal.
This will need some translation where we Solve() first, and then
Solve() again with the automatically installed packages added such
that we can mark them as Garbage for display purposes.
- It does not remove manually installed packages.
Hook the solver in via the EDSP framework, this allows us to achieve
easy initial integration without lots of issues.
A lot of this work was planned and executed in my free time and then
some leaked into work time I suppose.
Implementation notes:
- Restore the full backlog of items
The annoying thing is that we record only when an item was enqueued
and not the level at which it was installed, so when going back a
decision level we might have to reinstall packages that were queued
at an earlier decision level because they were only installed at a
later decision level.
- When picking one version, reject the others
- Propagate conflicts up to reverse dependencies
This will recursively mark every reverse dependency that can
no longer be satisfied as MUSTNOT.
Also make sure to recursively call Reject(Ver) from Reject(Pkg)
to make sure we trigger the Rejections there.
This means we now end up having Recursion in the algorithm. An
alternative approach would be to push *reject* items to the heap
and then do them, but this is not entirely straight forward and
it may simply not be necessary.
- Sort upgrades before other optional installs containing subsets
If I want to upgrade a package A, I schedule A3|A2|A1; if another
thing depends specifically on A1; we'd not be installed. Hence we
need to sort upgrades first.
This only is needed for optional packages; manual packages will
figure this out naturally.
- Rescoring is lazily implemented. Instead of calling make_heap()
after rescoring items, we just mark the items as dirty and reinsert
them. We also only rescore from the main solve loop, Reject() marks
the heap as needing a rescore due to a Conflict (as some versions will
no longer be installable), and RescoreWorkIfNeeded() then will do the
rescoring.
- Recursive unit propagation: Install() and Reject() recursively call
each other to promote decisions across single-version dependencies
(or across not-anymore satisfiable reverse-depends).
- Make Reason constructors explicit, this enhances readability
This makes calls like the one in here be
Reject(object, Reason(otherObject))
Ensuring that it's clear that the 2nd argument is a reason at the
caller side.
- Split Decision into Decision and Hint vs. first draft
When branching/deciding, we do not want to override SHOULD and MAY.
We do not actually use them yet, and we do actually clean them when
backtracking, but let's at least keep the data structure correct.
Convert the enum to a 16-bit integer so we can still fit in the same
space as before.
