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Z3 negate maximisation objective #786

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Z3 negate maximisation objective #786

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6 changes: 5 additions & 1 deletion cpmpy/solvers/z3.py
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Original file line number Diff line number Diff line change
Expand Up @@ -249,7 +249,9 @@ def solve(self, time_limit=None, assumptions=[], **kwargs):
# translate objective, for optimisation problems only
if self.has_objective():
obj = self.z3_solver.objectives()[0]
self.objective_value_ = sol.evaluate(obj).as_long()
self.objective_value_ = sol.evaluate(obj).as_long()
if not self._minimize:
self.objective_value_ = -1*self.objective_value_ # Z3 negates the objective function to turn a maximisation problem into a minimisation one, undoing negation here

else: # clear values of variables
for cpm_var in self.user_vars:
Expand Down Expand Up @@ -318,8 +320,10 @@ def objective(self, expr, minimize=True):
obj = self._z3_expr(expr)
if minimize:
self.obj_handle = self.z3_solver.minimize(obj)
self._minimize = True # record direction of optimisation
else:
self.obj_handle = self.z3_solver.maximize(obj)
self._minimize = False # record direction of optimisation


def transform(self, cpm_expr):
Expand Down
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