GurobiSolver: Improve get_constraints

miplearn/solvers/gurobi.py

@@ -154,16 +154,55 @@ class GurobiSolver(InternalSolver):
 
     @overrides
     def get_constraints(self) -> Dict[str, Constraint]:
-        assert self.model is not None
+        model = self.model
+        assert model is not None
         self._raise_if_callback()
         if self._dirty:
-            self.model.update()
+            model.update()
             self._dirty = False
+        gp_constrs = model.getConstrs()
+        var_names = model.getAttr("varName", model.getVars())
+        constr_names = model.getAttr("constrName", gp_constrs)
+        rhs = model.getAttr("rhs", gp_constrs)
+        sense = model.getAttr("sense", gp_constrs)
+        dual_value = None
+        sa_rhs_up = None
+        sa_rhs_down = None
+        slack = None
+        basis_status = None
+        if self._has_lp_solution:
+            dual_value = model.getAttr("pi", gp_constrs)
+            sa_rhs_up = model.getAttr("saRhsUp", gp_constrs)
+            sa_rhs_down = model.getAttr("saRhsLow", gp_constrs)
+            basis_status = model.getAttr("cbasis", gp_constrs)
+        if self._has_lp_solution or self._has_mip_solution:
+            slack = model.getAttr("slack", gp_constrs)
         constraints: Dict[str, Constraint] = {}
-        for c in self.model.getConstrs():
-            constr = self._parse_gurobi_constraint(c)
-            assert c.constrName not in constraints
-            constraints[c.constrName] = constr
+        for (i, gp_constr) in enumerate(gp_constrs):
+            expr = model.getRow(gp_constr)
+            lhs = {}
+            for j in range(expr.size()):
+                lhs[var_names[expr.getVar(j).index]] = expr.getCoeff(j)
+            assert (
+                constr_names[i] not in constraints
+            ), f"Duplicated constraint name detected: {constr_names[i]}"
+            constraint = Constraint(lhs=lhs, rhs=rhs[i], sense=sense[i])
+            if dual_value is not None:
+                assert sa_rhs_up is not None
+                assert sa_rhs_down is not None
+                assert basis_status is not None
+                constraint.dual_value = dual_value[i]
+                constraint.sa_rhs_up = sa_rhs_up[i]
+                constraint.sa_rhs_down = sa_rhs_down[i]
+                if gp_constr.cbasis == 0:
+                    constraint.basis_status = "B"
+                elif gp_constr.cbasis == -1:
+                    constraint.basis_status = "N"
+                else:
+                    raise Exception(f"unknown cbasis: {gp_constr.cbasis}")
+            if slack is not None:
+                constraint.slack = slack[i]
+            constraints[constr_names[i]] = constraint
         return constraints
 
     @overrides
@@ -553,31 +592,6 @@ class GurobiSolver(InternalSolver):
         self.model = None
         self.cb_where = None
 
-    def _parse_gurobi_constraint(self, gp_constr: Any) -> Constraint:
-        assert self.model is not None
-        expr = self.model.getRow(gp_constr)
-        lhs: Dict[str, float] = {}
-        for i in range(expr.size()):
-            lhs[expr.getVar(i).varName] = expr.getCoeff(i)
-        constr = Constraint(
-            rhs=gp_constr.rhs,
-            lhs=lhs,
-            sense=gp_constr.sense,
-        )
-        if self._has_lp_solution:
-            constr.dual_value = gp_constr.pi
-            constr.sa_rhs_up = gp_constr.sarhsup
-            constr.sa_rhs_down = gp_constr.sarhslow
-            if gp_constr.cbasis == 0:
-                constr.basis_status = "B"
-            elif gp_constr.cbasis == -1:
-                constr.basis_status = "N"
-            else:
-                raise Exception(f"unknown cbasis: {gp_constr.cbasis}")
-        if self._has_lp_solution or self._has_mip_solution:
-            constr.slack = gp_constr.slack
-        return constr
-
 
 class GurobiTestInstanceInfeasible(Instance):
     @overrides