Presolvers are used to reduce the size of the model by removing irrelevant information like redundant constraints, to strengthen the LP relaxation by exploiting integrality information, and to extract useful information in the presolving step. Constraint based presolving is done in the CONSPRESOL callback methods of the constraint handlers, see CONSPRESOL. Some propagation steps can already be applied in presolving via the PROPRESOL callback methods of propagators, see PROPPRESOL. The presolver plugins complement these by additional, usually optimality based, presolving reductions.
A complete list of all presolvers contained in this release can be found here.
We now explain how users can add their own presolvers. Take the trivial presolver (src/scip/presol_trivial.c) as an example. As all other default plugins, it is written in C. C++ users can easily adapt the code by using the scip::ObjPresol wrapper base class and implement the scip_...() virtual methods instead of the SCIP_DECL_PRESOL... callback methods.
Additional documentation for the callback methods of a presolver, in particular for their input parameters, can be found in the file type_presol.h.
Here is what you have to do to implement a presolver:
At the top of the new file "presol_mypresolver.c", you can find the presolver properties. These are given as compiler defines. In the C++ wrapper class, you have to provide the presolver properties by calling the constructor of the abstract base class scip::ObjPresol from within your constructor. The properties you have to set have the following meaning:
Below the header "Data structures" you can find a struct which is called "struct SCIP_PresolData". In this data structure, you can store the data of your presolver. For example, you should store the adjustable parameters of the presolver in this data structure. If you are using C++, you can add presolver data as usual as object variables to your class.
Defining presolver data is optional. You can leave this struct empty.
At the bottom of "presol_mypresolver.c", you can find the interface method SCIPincludePresolMypresolver(), which also appears in "presol_mypresolver.h" SCIPincludePresolMypresolver() is called by the user, if (s)he wants to include the presolver, i.e., if (s)he wants to use the presolver in his/her application.
This method only has to be adjusted slightly. It is responsible for notifying SCIP of the presence of the presolver. For this, you can either call SCIPincludePresol(), or SCIPincludePresolBasic() since SCIP version 3.0. In the latter variant, additional callbacks must be added via setter functions as, e.g., SCIPsetPresolCopy(). We recommend this latter variant because it is more stable towards future SCIP versions which might have more callbacks, whereas source code using the first variant must be manually adjusted with every SCIP release containing new callbacks for presolvers in order to compile.
If you are using presolver data, you have to allocate the memory for the data at this point. You can do this by calling:
You also have to initialize the fields in struct SCIP_PresolData afterwards. For freeing the presolver data, see PRESOLFREE.
You may also add user parameters for your presolver, see How to add additional user parameters for how to add user parameters and the method SCIPincludePresolTrivial() in src/scip/presol_trivial.c for an example.
The fundamental callback methods of the plugins are the ones that have to be implemented in order to obtain an operational algorithm. They are passed together with the presolver itself to SCIP using SCIPincludePresol() or SCIPincludePresolBasic(), see Interface Methods.
Presolver plugins have only one fundamental callback method, namely the PRESOLEXEC method. This method has to be implemented for every presolver; the other callback methods are optional. In the C++ wrapper class scip::ObjPresol, the scip_exec() method (which corresponds to the PRESOLEXEC callback) is a virtual abstract member function. You have to implement it in order to be able to construct an object of your presolver class.
Additional documentation for the callback methods, in particular to their input parameters, can be found in type_presol.h.
The PRESOLEXEC callback is called inside the presolving loop and should perform the actual presolving reductions. It should inspect the problem instance at hand and simplify it by tightening bounds of variables, aggregating or fixing variables, changing the type of variables, modifying the graph that represents the instance of your application, and the like.
The additional callback methods do not need to be implemented in every case. However, some of them have to be implemented for most applications, they can be used, for example, to initialize and free private data. Additional callbacks can either be passed directly with SCIPincludePresol() to SCIP or via specific setter functions after a call of SCIPincludePresolBasic(), see also Interface Methods.
If you have allocated memory for fields in your presolver data, remember to free this memory before freeing the presolver data itself. If you are using the C++ wrapper class, this method is not available. Instead, just use the destructor of your class to free the member variables of your class.
The PRESOLINIT callback is executed after the problem is transformed. The presolver may, e.g., use this call to initialize its presolver data. The difference between the original and the transformed problem is explained in "What is this thing with the original and the transformed problem about?" on Frequently Asked Questions (FAQ).
The PRESOLCOPY callback is executed when a SCIP instance is copied, e.g. to solve a sub-SCIP. By defining this callback as
NULL the user disables the execution of the specified presolver for all copied SCIP instances. This may deteriorate the performance of primal heuristics using sub-SCIPs.
The PRESOLEXIT callback is executed before the transformed problem is freed. In this method, the presolver should free all resources that have been allocated for the solving process in PRESOLINIT.
The PRESOLINITPRE callback is executed when the presolving is about to begin. The presolver may use this call to initialize its presolving data which only need to exist during the presolving stage.
The PRESOLEXITPRE callback is executed after presolving finishes and before the branch-and-bound process begins. The presolver should use this call to clean up its presolving data, which was allocated in PRESOLINITPRE.