[Nonlinear] add support for is_empty and empty\!

src/Nonlinear/model.jl

@@ -4,6 +4,27 @@
 # Use of this source code is governed by an MIT-style license that can be found
 # in the LICENSE.md file or at https://opensource.org/licenses/MIT.
 
+function MOI.empty!(model::Model)
+    model.objective = nothing
+    empty!(model.expressions)
+    empty!(model.constraints)
+    empty!(model.parameters)
+    model.operators = OperatorRegistry()
+    model.last_constraint_index = 0
+    return
+end
+
+function MOI.is_empty(model::Model)
+    return model.objective === nothing &&
+        isempty(model.expressions) &&
+        isempty(model.constraints) &&
+        isempty(model.parameters) &&
+        isempty(model.operators.registered_univariate_operators) &&
+        isempty(model.operators.registered_multivariate_operators) &&
+        model.last_constraint_index === 0
+    return
+end
+
 function Base.copy(::Model)
     return error("Copying nonlinear problems not yet implemented")
 end

test/Nonlinear/Nonlinear.jl

@@ -1103,6 +1103,34 @@ function test_parse_unsupported_operator()
     return
 end
 
+function test_is_empty()
+    model = MOI.Nonlinear.Model()
+    @test MOI.is_empty(model)
+    x = MOI.VariableIndex(1)
+    Nonlinear.set_objective(model, :(log($x)))
+    @test !MOI.is_empty(model)
+    MOI.empty!(model)
+    @test MOI.is_empty(model)
+    Nonlinear.add_constraint(model, :(log($x)), MOI.GreaterThan(1.0))
+    @test !MOI.is_empty(model)
+    MOI.empty!(model)
+    @test MOI.is_empty(model)
+    Nonlinear.add_expression(model, :(sin($x)^2))
+    @test !MOI.is_empty(model)
+    MOI.empty!(model)
+    @test MOI.is_empty(model)
+    Nonlinear.add_parameter(model, 1.2)
+    @test !MOI.is_empty(model)
+    MOI.empty!(model)
+    @test MOI.is_empty(model)
+    f(x) = log(x + 1)
+    Nonlinear.register_operator(model, :f, 1, f)
+    @test !MOI.is_empty(model)
+    MOI.empty!(model)
+    @test MOI.is_empty(model)
+    return
+end
+
 end
 
 TestNonlinear.runtests()