Exact solution hints (#396)

* implement solution hinting in exact

* update error

* make tests for solution hinting

* update test

cpmpy/solvers/exact.py

@@ -22,6 +22,9 @@
 import sys  # for stdout checking
 import time
 
+import pkg_resources
+from pkg_resources import VersionConflict
+
 from .solver_interface import SolverInterface, SolverStatus, ExitStatus
 from ..expressions.core import *
 from ..expressions.variables import _BoolVarImpl, NegBoolView, _IntVarImpl, _NumVarImpl, intvar
@@ -65,6 +68,9 @@ def supported():
             return True
         except ImportError as e:
             return False
+        except VersionConflict:
+            warnings.warn(f"CPMpy requires Exact version >=1.1.5 is required but you have version {pkg_resources.get_distribution('exact').version}")
+            return False
 
 
     def __init__(self, cpm_model=None, subsolver=None):
@@ -574,3 +580,16 @@ def get_core(self):
         # return cpm_variables corresponding to Exact core
         return [self.assumption_dict[i][1] for i in self.xct_solver.getLastCore()]
 
+
+    def solution_hint(self, cpm_vars, vals):
+        """
+        Exact supports warmstarting the solver with a partial feasible assignment.
+            Requires version >= 1.2.1
+        :param cpm_vars: list of CPMpy variables
+        :param vals: list of (corresponding) values for the variables
+        """
+        try:
+            pkg_resources.require("exact>=1.1.5")
+            self.xct_solver.setSolutionHints(self.solver_vars(cpm_vars), vals)
+        except VersionConflict:
+            raise NotSupportedError("Upgrade Exact version to >=1.2.1 to support solution hinting")

cpmpy/solvers/solver_interface.py

@@ -267,7 +267,7 @@ def solution_hint(self, cpm_vars, vals):
         :param cpm_vars: list of CPMpy variables
         :param vals: list of (corresponding) values for the variables
         """
-        raise NotImplementedError("Solver does not support solution hinting")
+        raise NotSupportedError("Solver does not support solution hinting")
 
     def get_core(self):
         """
@@ -279,7 +279,7 @@ def get_core(self):
         (a literal is either a `_BoolVarImpl` or a `NegBoolView` in case of its negation, e.g. x or ~x)
         Setting these literals to True makes the model UNSAT, setting any to False makes it SAT
         """
-        raise NotImplementedError("Solver does not support unsat core extraction")
+        raise NotSupportedError("Solver does not support unsat core extraction")
 
 
     # shared helper functions

tests/test_solvers_solhint.py

@@ -0,0 +1,38 @@
+import unittest
+
+import cpmpy as cp
+from cpmpy.exceptions import NotSupportedError
+
+
+class TestSolutionHinting(unittest.TestCase):
+
+
+    def test_hints(self):
+
+        a,b = cp.boolvar(shape=2)
+        model = cp.Model(a ^ b)
+
+        for n, solver_class in cp.SolverLookup.base_solvers():
+            if not solver_class.supported():
+                continue
+            slv = solver_class(model)
+            try:
+                args = {"cp_model_presolve": False} if n == "ortools" else {}  # hints are not taken into account in presolve
+
+                slv.solution_hint([a,b], [True, False])
+                self.assertTrue(slv.solve(**args))
+                self.assertEqual(a.value(), True)
+                self.assertEqual(b.value(), False)
+
+                slv.solution_hint([a,b], [False, True]) # check hints are correctly overwritten
+                self.assertTrue(slv.solve(**args))
+                self.assertEqual(a.value(), False)
+                self.assertEqual(b.value(), True)
+
+                slv.solution_hint([a,b], [True,True])
+                self.assertTrue(slv.solve(**args)) # should also work with an UNSAT hint
+
+            except NotSupportedError:
+                continue
+
+