Use of Scip after license change

[jjhforrest]

I have been trying to improve Cbc, and searching around, I discovered that the Scip license has changed from the ZIB Academic license to Apache 2. This make it's use much more attractive in Coin. I have added some code to Cbc to use Scip. There is so much documentation, that I found it difficult to find everything I wanted - so I would appreciate help. To use Scip you need to define -DCBC_TRY_SCIP in coinbrew and use -I, -L etc to point to files. Then adding -options usescip will use scip after Cbc does work at root node. You may want to switch off Cgl cuts. Cbc heuristics would be good - but there I need help as to how to pass in a solution to Scip. To encourage someone to play with the code, I put my attempt to use Scip in CbcGeneral.cpp. You will see TODO in several places and that is where I would appreciate help.

I think this could be important.

Since I am here, I will mention two other "improvements". In the standalone solver you can now add -optionfile filename. This can have commands you keep using - to make it easier if you always use several commands. So my file has lines like

• I think this works well
  preprocess equalall
• switches cuts on relaxed version of model with root cuts
  more2 lagrangean2
• for fast solves - this seems to do better than default
  depth 5
  or the file could just have those all on one line as -preprocess equalall -more2 lagrangean2 -depth 5

This is actually a first step to try and make options more readable.

The second "improvement" is to allow more than 2147483647 nodes!

[svigerske]

To pass a solution before SCIP starts solving, better use SCIPcreateOrigSol() or SCIPcreatePartialSol() to create one, use SCIPsetSolVals() to set the values, and add the solution to SCIP via SCIPaddSolFree().
SCIPsetSolVals() should probably get your tempvars and model->bestSolution() arrays as input.
The SCIPcreatePartialSol() is to give a solution that doesn't have all values set. You may need to set some options to have SCIP trying to complete the partial sol to a feasible complete one.

Maybe have a look at the code starting at https://github.com/coin-or/GAMSlinks/blob/master/src/scip/reader_gmo.c#L2127

To run a Cbc heuristic within SCIP, you could add a primal heuristic plugin to SCIP (see https://www.scipopt.org/doc-8.0.3/html/HEUR.php) that communicates with Cbc and passes solution back to SCIP.

[jjhforrest]

Thanks Stefan. I will slowly look at your suggestions. What I would really like is for a student to dig into all this. OsiScpSolverInterface is one idea that comes to mind - to make moving backwards and forwards simpler. Also a way of using Scip's preprocessing instead of CglPreprocess as Scip seems superior. I managed to use Papilo in that way, but that adds extra complications.

Ted - yes probably a good idea to make a separate branch.

[tkralphs]

@jjhforrest Could we maybe do this in a separate branch? I would still like to make a new version out of what is currently in master. If you have the bandwidth, could we maybe try to get what is in master closer to release? I recall that there were some weird regressions with what is in master that we were working through, but it kind of got dropped. I still don't have a lot of bandwidth for this, but have a student working a lot with Cbc, so maybe we could move things forward a bit.