Add MOI.LagrangeMultipliers attribute

src/Test/test_nonlinear.jl

@@ -2382,3 +2382,133 @@ function setup_test(
 end
 
 version_added(::typeof(test_vector_nonlinear_oracle_no_hessian)) = v"1.46.0"
+
+function test_VectorNonlinearOracle_LagrangeMultipliers_MAX_SENSE(
+    model::MOI.ModelLike,
+    config::MOI.Test.Config{T},
+) where {T}
+    @requires _supports(config, MOI.optimize!)
+    @requires _supports(config, MOI.ConstraintDual)
+    @requires _supports(config, MOI.LagrangeMultipliers)
+    @requires MOI.supports_constraint(
+        model,
+        MOI.VectorOfVariables,
+        MOI.VectorNonlinearOracle{T},
+    )
+    set = MOI.VectorNonlinearOracle(;
+        dimension = 2,
+        l = T[typemin(T)],
+        u = T[1],
+        eval_f = (ret, x) -> (ret[1] = x[1]^2 + x[2]^2),
+        jacobian_structure = [(1, 1), (1, 2)],
+        eval_jacobian = (ret, x) -> ret .= T(2) .* x,
+        hessian_lagrangian_structure = [(1, 1), (2, 2)],
+        eval_hessian_lagrangian = (ret, x, u) -> ret .= T(2) .* u[1],
+    )
+    x = MOI.add_variables(model, 2)
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MAX_SENSE)
+    f = one(T) * x[1] + one(T) * x[2]
+    MOI.set(model, MOI.ObjectiveFunction{typeof(f)}(), f)
+    c = MOI.add_constraint(model, MOI.VectorOfVariables(x), set)
+    MOI.optimize!(model)
+    y = T(1) / sqrt(T(2))
+    @test isapprox(MOI.get(model, MOI.VariablePrimal(), x), [y, y], config)
+    @test isapprox(MOI.get(model, MOI.ConstraintDual(), c), T[-1, -1], config)
+    @test isapprox(MOI.get(model, MOI.LagrangeMultipliers(), c), T[-y])
+    return
+end
+
+function setup_test(
+    ::typeof(test_VectorNonlinearOracle_LagrangeMultipliers_MAX_SENSE),
+    model::MOIU.MockOptimizer,
+    config::Config{T},
+) where {T}
+    F, S = MOI.VectorOfVariables, MOI.VectorNonlinearOracle{T}
+    y = T(1) / sqrt(T(2))
+    MOI.Utilities.set_mock_optimize!(
+        model,
+        mock -> begin
+            MOI.Utilities.mock_optimize!(
+                mock,
+                config.optimal_status,
+                T[y, y],
+                (F, S) => [T[-1, -1]],
+            )
+            ci = only(MOI.get(mock, MOI.ListOfConstraintIndices{F,S}()))
+            MOI.set(mock, MOI.LagrangeMultipliers(), ci, T[-y])
+        end,
+    )
+    model.eval_variable_constraint_dual = false
+    return () -> model.eval_variable_constraint_dual = true
+end
+
+function version_added(
+    ::typeof(test_VectorNonlinearOracle_LagrangeMultipliers_MAX_SENSE),
+)
+    return v"1.47.0"
+end
+
+function test_VectorNonlinearOracle_LagrangeMultipliers_MIN_SENSE(
+    model::MOI.ModelLike,
+    config::MOI.Test.Config{T},
+) where {T}
+    @requires _supports(config, MOI.optimize!)
+    @requires _supports(config, MOI.ConstraintDual)
+    @requires _supports(config, MOI.LagrangeMultipliers)
+    @requires MOI.supports_constraint(
+        model,
+        MOI.VectorOfVariables,
+        MOI.VectorNonlinearOracle{T},
+    )
+    set = MOI.VectorNonlinearOracle(;
+        dimension = 2,
+        l = T[-1],
+        u = T[typemax(T)],
+        eval_f = (ret, x) -> (ret[1] = -x[1]^2 - x[2]^2),
+        jacobian_structure = [(1, 1), (1, 2)],
+        eval_jacobian = (ret, x) -> ret .= -T(2) .* x,
+        hessian_lagrangian_structure = [(1, 1), (2, 2)],
+        eval_hessian_lagrangian = (ret, x, u) -> ret .= -T(2) .* u[1],
+    )
+    x = MOI.add_variables(model, 2)
+    MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
+    f = one(T) * x[1] + one(T) * x[2]
+    MOI.set(model, MOI.ObjectiveFunction{typeof(f)}(), f)
+    c = MOI.add_constraint(model, MOI.VectorOfVariables(x), set)
+    MOI.optimize!(model)
+    y = T(1) / sqrt(T(2))
+    @test isapprox(MOI.get(model, MOI.VariablePrimal(), x), [-y, -y], config)
+    @test isapprox(MOI.get(model, MOI.ConstraintDual(), c), T[1, 1], config)
+    @test isapprox(MOI.get(model, MOI.LagrangeMultipliers(), c), T[y])
+    return
+end
+
+function setup_test(
+    ::typeof(test_VectorNonlinearOracle_LagrangeMultipliers_MIN_SENSE),
+    model::MOIU.MockOptimizer,
+    config::Config{T},
+) where {T}
+    F, S = MOI.VectorOfVariables, MOI.VectorNonlinearOracle{T}
+    y = T(1) / sqrt(T(2))
+    MOI.Utilities.set_mock_optimize!(
+        model,
+        mock -> begin
+            MOI.Utilities.mock_optimize!(
+                mock,
+                config.optimal_status,
+                T[-y, -y],
+                (F, S) => [T[1, 1]],
+            )
+            ci = only(MOI.get(mock, MOI.ListOfConstraintIndices{F,S}()))
+            MOI.set(mock, MOI.LagrangeMultipliers(), ci, T[y])
+        end,
+    )
+    model.eval_variable_constraint_dual = false
+    return () -> model.eval_variable_constraint_dual = true
+end
+
+function version_added(
+    ::typeof(test_VectorNonlinearOracle_LagrangeMultipliers_MIN_SENSE),
+)
+    return v"1.47.0"
+end

src/Utilities/mockoptimizer.jl

@@ -76,6 +76,10 @@ mutable struct MockOptimizer{MT<:MOI.ModelLike,T} <: MOI.AbstractOptimizer
         Dict{Int,MOI.BasisStatusCode},
     }
     variable_basis_status::Dict{MOI.VariableIndex,Dict{Int,MOI.BasisStatusCode}}
+    constraint_attributes::Dict{
+        MOI.AbstractConstraintAttribute,
+        Dict{MOI.ConstraintIndex,Any},
+    }
 end
 
 function MockOptimizer(
@@ -133,6 +137,7 @@ function MockOptimizer(
         # Basis status
         Dict{MOI.ConstraintIndex,Dict{Int,MOI.BasisStatusCode}}(),
         Dict{MOI.VariableIndex,Dict{Int,MOI.BasisStatusCode}}(),
+        Dict{MOI.AbstractConstraintAttribute,Dict{MOI.ConstraintIndex,Any}}(),
     )
 end
 
@@ -421,7 +426,14 @@ function MOI.set(
     idx::MOI.ConstraintIndex,
     value,
 )
-    MOI.set(mock.inner_model, attr, xor_index(idx), value)
+    if MOI.is_set_by_optimize(attr)
+        ret = get!(mock.constraint_attributes, attr) do
+            return Dict{MOI.ConstraintIndex,Any}()
+        end
+        ret[idx] = value
+    else
+        MOI.set(mock.inner_model, attr, xor_index(idx), value)
+    end
     return
 end
 
@@ -660,6 +672,9 @@ function MOI.get(
 )
     # If it is thrown by `mock.inner_model`, the index will be xor'ed.
     MOI.throw_if_not_valid(mock, idx)
+    if MOI.is_set_by_optimize(attr)
+        return mock.constraint_attributes[attr][idx]
+    end
     return MOI.get(mock.inner_model, attr, xor_index(idx))
 end
 

src/attributes.jl

@@ -3272,6 +3272,70 @@ function get_fallback(
     return supports_constraint(model, F, S) ? 0.0 : Inf
 end
 
+"""
+    LagrangeMultipliers(result_index::Int = 1)
+
+An [`AbstractConstraintAttribute`](@ref) for the Lagrange multipliers associated
+with a constraint.
+
+## Relationship to `ConstraintDual`
+
+In most cases, the value of this attribute is equivalent to
+[`ConstraintDual`](@ref), and querying the value of [`LagrangeMultipliers`](@ref)
+will fallback to querying the value of [`ConstraintDual`](@ref).
+
+The attribute values differ in one important case.
+
+When there is a [`VectorNonlinearOracle`](@ref) constraint of the form:
+```math
+x \\in VectorNonlinearOracle
+```
+the associated [`ConstraintDual`](@ref) is ``\\mu^\\top \\nabla f(x)``, and the
+value of [`LagrangeMultipliers`](@ref) is the vector ``\\mu`` directly.
+
+Both values are useful in different circumstances.
+
+## DualStatus
+
+Before quering this attribute you should first check [`DualStatus`](@ref) to
+confirm that a dual solution is avaiable.
+
+If the [`DualStatus`](@ref) is [`NO_SOLUTION`](@ref) the result of querying
+this attribute is undefined.
+
+## `result_index`
+
+The optimizer may return multiple dual solutions. See [`ResultCount`](@ref)
+for information on how the results are ordered.
+
+If the solver does not have a dual value for the constraint because the
+`result_index` is beyond the available solutions (whose number is indicated by
+the [`ResultCount`](@ref) attribute), getting this attribute must throw a
+[`ResultIndexBoundsError`](@ref).
+
+## Implementation
+
+Optimizers should implement the following methods:
+```
+MOI.get(::Optimizer, ::MOI.LagrangeMultipliers, ::MOI.ConstraintIndex)
+```
+They should not implement [`set`](@ref) or [`supports`](@ref).
+
+"""
+struct LagrangeMultipliers <: AbstractConstraintAttribute
+    result_index::Int
+
+    LagrangeMultipliers(result_index::Int = 1) = new(result_index)
+end
+
+function get_fallback(
+    model::ModelLike,
+    attr::LagrangeMultipliers,
+    ci::ConstraintIndex,
+)
+    return get(model, ConstraintDual(attr.result_index), ci)
+end
+
 """
     is_set_by_optimize(::AnyAttribute)
 
@@ -3330,6 +3394,7 @@ function is_set_by_optimize(
         ConstraintDual,
         ConstraintBasisStatus,
         VariableBasisStatus,
+        LagrangeMultipliers,
     },
 )
     return true