some work on iis

Author: Joao-Dionisio

src/pyscipopt/iisfinder.pxi

@@ -3,7 +3,8 @@
 cdef class IISfinder:
     cdef public Model model
     cdef public str name
-    cdef SCIP_IIS* _iisfinder
+    cdef SCIP_IISFINDER* _iisfinder
+    cdef SCIP_IIS* _iis
 
     def iisfinderfree(self):
         '''calls destructor and frees memory of iis finder'''

src/pyscipopt/scip.pxd

@@ -1223,6 +1223,10 @@ cdef extern from "scip/scip.h":
     SCIP_RETCODE SCIPiisGreedyMakeIrreducible(SCIP_IIS* iis)
     SCIP_Bool SCIPiisIsSubscipInfeasible(SCIP_IIS* iis)
     SCIP_Bool SCIPiisIsSubscipIrreducible(SCIP_IIS* iis)
+    SCIP_IIS* SCIPgetIIS(SCIP* scip)
+    SCIP_Real SCIPiisGetTime(SCIP_IIS* scip)
+    SCIP_Bool SCIPiisIsSubscipIrreducible(SCIP_IIS* scip)
+    SCIP_Longint SCIPiisGetNNodes(SCIP_IIS* scip)
     SCIP* SCIPiisGetSubscip(SCIP_IIS* iis)
 
     #Relaxation plugin
@@ -2212,6 +2216,8 @@ cdef class Model:
     cdef int _generated_event_handlers_count
     # store references to Benders subproblem Models for proper cleanup
     cdef _benders_subproblems
+    # store iis, if found
+    cdef SCIP_IIS* _iis
 
     @staticmethod
     cdef create(SCIP* scip)

src/pyscipopt/scip.pxi

@@ -2663,6 +2663,23 @@ cdef class _VarArray:
         if self.ptr != NULL:
             free(self.ptr)
 
+cdef class IIS:
+    cdef SCIP_IIS* _iis
+    cdef SCIP* subscip
+    cdef public object time
+    cdef public object irreducible
+    cdef public object nodes
+    cdef public object model
+
+    def __init__(self, Model model):
+        self._iis = SCIPgetIIS(model._scip)
+        self.time = SCIPiisGetTime(self._iis)
+        self.irreducible = SCIPiisIsSubscipIrreducible(self._iis)
+        self.nodes = SCIPiisGetNNodes(self._iis)
+        subscip = SCIPiisGetSubscip(self._iis)
+        self.model = Model.create(subscip)
+        model._iis = self._iis
+
 # - remove create(), includeDefaultPlugins(), createProbBasic() methods
 # - replace free() by "destructor"
 # - interface SCIPfreeProb()
@@ -2706,6 +2723,7 @@ cdef class Model:
         self._modelvars = {}
         self._generated_event_handlers_count = 0
         self._benders_subproblems = []  # Keep references to Benders subproblem Models
+        self._iis = NULL
 
         if not createscip:
             # if no SCIP instance should be created, then an empty Model object is created.
@@ -9283,16 +9301,40 @@ cdef class Model:
         """
         Generates an Irreducible Infeasible Subsystem (IIS) from the current
         problem.
+
+        Returns
+        -------
+        IIS
+
+        """
+        PY_SCIP_CALL( SCIPgenerateIIS(self._scip) )
+        iis = IIS(self)
+        return iis
+    
+    def getIIS(self):
+        """
+        Get the IIS object.
+        Note: Needs to be called after generateIIS, or after a single execution of the iisfinderExec.
+        
+        Returns
+        -------
+        IIS 
         """
+        assert self._iis != NULL, "No IIS exists. You need to first call generateIIS() or run the iisfinderexec method of your custom IISfinder class."
 
-        PY_SCIP_CALL(SCIPgenerateIIS(self._scip))
+        return IIS(self)
 
-    def iisGreedyMakeIrreducible(self, IISfinder iisfinder):
+    def iisGreedyMakeIrreducible(self, IIS iis):
        """
        Perform the greedy deletion algorithm with singleton batches to obtain an irreducible infeasible subsystem (IIS)
+
+       Parameters
+       ----------
+       iis : IIS
+            The IIS to apply the greedy deletion algorithm to.
        """
 
-       PY_SCIP_CALL(SCIPiisGreedyMakeIrreducible(iisfinder._iisfinder))
+       PY_SCIP_CALL(SCIPiisGreedyMakeIrreducible(iis._iis))
 
     def includeRelax(self, Relax relax, name, desc, priority=10000, freq=1):
         """

tests/test_iis.py

@@ -12,30 +12,37 @@ def infeasible_model():
     m.addCons(x1 + x2 == 1, name="c1")
     m.addCons(x2 + x3 == 1, name="c2")
     m.addCons(x1 + x3 == 1, name="c3")
+    m.addCons(x1 + x2 + x3 <= 0, name="c4")
 
     return m
 
 def test_generate_iis():
     m = infeasible_model()
 
     # make sure IIS generation doesn't raise any exceptions
-    m.generateIIS()
-
+    iis = m.generateIIS()
+    assert iis.irreducible
+    assert iis.model.getNConss() == 2
+    assert iis.nodes == 0
+    iis.time
 
 class myIIS(IISfinder):
-    def __init__(self, model):
+    def __init__(self, model, skip=False):
         super().__init__()
         self.model = model
         self.size = 0
         self.iis = None
+        self.skip = skip
 
     def iisfinderexec(self):
-        n_infeasibilities, aux_vars = get_infeasible_constraints(self.model.__repr__.__self__)
+        if self.skip:
+            return {"result": SCIP_RESULT.SUCCESS} # success to attempt to skip further processing
+
+        n_infeasibilities, _ = get_infeasible_constraints(self.model.__repr__.__self__)
         if n_infeasibilities == 0:
             return {"result": SCIP_RESULT.DIDNOTFIND}
 
         self.size = n_infeasibilities
-        self.iis = aux_vars
         return {"result": SCIP_RESULT.SUCCESS}
 
 def test_custom_iis_finder():        
@@ -46,4 +53,16 @@ def test_custom_iis_finder():
     m.includeIISfinder(my_iis, "", "")
 
     m.generateIIS()
-    assert my_iis.size == 1
+    iis = m.getIIS()
+    assert iis.model.getNConss() == my_iis.size
+
+def test_iisGreddyMakeIrreducible():
+    m = infeasible_model()
+    my_iis = myIIS(m, skip=True)
+    m.includeIISfinder(my_iis, "", "")
+    iis = m.generateIIS()
+    with pytest.raises(AssertionError):
+        assert not iis.irreducible # currently breaking. do SCIP IIS methods enter after custom iisfinder?
+
+    m.iisGreedyMakeIrreducible(iis)
+    assert iis.irreducible