Add ToZerosBridge

src/Bridges/Bridges.jl

@@ -19,6 +19,7 @@ include("SplitIntervalBridge.jl")
 include("FlipSignBridge.jl")
 include("NonlinearBridge.jl")
 include("ToSOS1Bridge.jl")
+include("ToZerosBridge.jl")
 
 """
     add_all_bridges(model::MOI.ModelLike, ::Type{T} = Float64)
@@ -40,6 +41,7 @@ function add_all_bridges(model::MOI.ModelLike, ::Type{T} = Float64) where {T}
     MOI.Bridges.add_bridge(model, SplitIntervalBridge{T})
     MOI.Bridges.add_bridge(model, FlipSignBridge{T})
     MOI.Bridges.add_bridge(model, ToSOS1Bridge{T})
+    MOI.Bridges.add_bridge(model, ToZerosBridge{T})
     MOI.Bridges.add_bridge(model, VerticalBridge{T})
     MOI.Bridges.add_bridge(model, NonlinearBridge{T})
     return

src/Bridges/ToZerosBridge.jl

@@ -0,0 +1,105 @@
+# Copyright (c) 2025 François Pacaud, Benoît Legat, and contributors
+#
+# 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.
+
+"""
+    ToZerosBridge{T} <: MOI.Bridges.Constraint.AbstractBridge
+
+`ToZerosBridge` implements the following reformulation:
+
+  * `[x₁, x₂]` in `ComplementsWithSetType{Zeros}` into `[x₁]` in
+    [`MOI.Zeros`](@ref)
+
+`ComplementsWithSetType{Zeros}` means that the activity `x₁` is complementary
+to a variable `x₂` whose reference set is a single point (it is either free or
+fixed). In both cases the complementarity condition forces the activity to be
+zero, so the constraint reduces to `x₁ = 0`.
+
+## Source node
+
+`ToZerosBridge` supports:
+
+  * [`MOI.VectorOfVariables`](@ref) in
+    [`ComplementsWithSetType{MOI.Zeros}`](@ref)
+
+## Target nodes
+
+`ToZerosBridge` creates:
+
+  * [`MOI.VectorOfVariables`](@ref) in [`MOI.Zeros`](@ref)
+
+"""
+struct ToZerosBridge{T} <: MOI.Bridges.Constraint.AbstractBridge
+    zeros::MOI.ConstraintIndex{MOI.VectorOfVariables,MOI.Zeros}
+    func::MOI.VectorOfVariables
+end
+
+function MOI.Bridges.Constraint.bridge_constraint(
+    ::Type{ToZerosBridge{T}},
+    model::MOI.ModelLike,
+    func::MOI.VectorOfVariables,
+    set::ComplementsWithSetType{MOI.Zeros},
+) where {T}
+    @assert set.dimension == 2
+    x1 = func.variables[1]
+    ci = MOI.add_constraint(model, MOI.VectorOfVariables([x1]), MOI.Zeros(1))
+    return ToZerosBridge{T}(ci, func)
+end
+
+function MOI.supports_constraint(
+    ::Type{<:ToZerosBridge},
+    ::Type{MOI.VectorOfVariables},
+    ::Type{ComplementsWithSetType{MOI.Zeros}},
+)
+    return true
+end
+
+function MOI.Bridges.Constraint.concrete_bridge_type(
+    ::Type{<:ToZerosBridge{T}},
+    ::Type{MOI.VectorOfVariables},
+    ::Type{ComplementsWithSetType{MOI.Zeros}},
+) where {T}
+    return ToZerosBridge{T}
+end
+
+# Bridge metadata
+
+function MOI.Bridges.added_constrained_variable_types(::Type{<:ToZerosBridge})
+    return Tuple{Type}[]
+end
+
+function MOI.Bridges.added_constraint_types(::Type{ToZerosBridge{T}}) where {T}
+    return Tuple{Type,Type}[(MOI.VectorOfVariables, MOI.Zeros)]
+end
+
+function MOI.get(
+    ::ToZerosBridge,
+    ::MOI.NumberOfConstraints{MOI.VectorOfVariables,MOI.Zeros},
+)::Int64
+    return 1
+end
+
+function MOI.get(
+    bridge::ToZerosBridge,
+    ::MOI.ListOfConstraintIndices{MOI.VectorOfVariables,MOI.Zeros},
+)
+    return [bridge.zeros]
+end
+
+function MOI.get(
+    ::MOI.ModelLike,
+    ::MOI.ConstraintFunction,
+    bridge::ToZerosBridge,
+)
+    return bridge.func
+end
+
+function MOI.get(::MOI.ModelLike, ::MOI.ConstraintSet, ::ToZerosBridge)
+    return ComplementsWithSetType{MOI.Zeros}(2)
+end
+
+function MOI.delete(model::MOI.ModelLike, bridge::ToZerosBridge)
+    MOI.delete(model, bridge.zeros)
+    return
+end

src/MOI_wrapper.jl

@@ -205,13 +205,13 @@ function _sos1_bridge_type(
     return Bridges.ToSOS1Bridge{T}
 end
 
-# VOV-in-Zeros → ToSOS1Bridge (trivial complementarity)
+# VOV-in-Zeros → ToZerosBridge (trivial complementarity: activity is zero)
 function _sos1_bridge_type(
     ::Type{T},
     ::Type{MOI.VectorOfVariables},
     ::Type{MOI.Zeros},
 ) where {T}
-    return Bridges.ToSOS1Bridge{T}
+    return Bridges.ToZerosBridge{T}
 end
 
 # Any non-VOV function → VerticalBridge (create slacks, then re-enter as VOV)

test/Bridges/test_ToZerosBridge.jl

@@ -0,0 +1,41 @@
+# Copyright (c) 2025 François Pacaud, Benoît Legat, and contributors
+#
+# 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.
+
+module TestToZerosBridge
+
+using Test
+
+import MathOptComplements
+import MathOptInterface as MOI
+
+const M = "TestToZerosBridge.MathOptComplements"
+
+function runtests()
+    is_test(name) = startswith("$name", "test_")
+    @testset "$name" for name in filter(is_test, names(@__MODULE__; all = true))
+        getfield(@__MODULE__, name)()
+    end
+    return
+end
+
+function test_ToZerosBridge()
+    MOI.Bridges.runtests(
+        MathOptComplements.Bridges.ToZerosBridge,
+        """
+        variables: x, y
+        [x, y] in $M.ComplementsWithSetType{MOI.Zeros}(2)
+        """,
+        """
+        variables: x, y
+        [x] in Zeros(1)
+        """;
+        cannot_unbridge = true,
+    )
+    return
+end
+
+end  # TestToZerosBridge
+
+TestToZerosBridge.runtests()

test/test_optimizer.jl

@@ -197,13 +197,15 @@ function test_Optimizer_bridge_dispatch()
         MOI.VectorAffineFunction{Float64},
         S,
     ) == MathOptComplements.Bridges.NonlinearBridge{Float64,MOI.Nonnegatives}
-    # SOS1 path: VectorOfVariables in ComplementsWithSetType{Zeros} → ToSOS1Bridge
+    # SOS1 path: VectorOfVariables in ComplementsWithSetType{Zeros} →
+    # ToZerosBridge (the activity is complementary to a fixed/free variable, so
+    # it is forced to zero).
     opt_sos1 = MathOptComplements.Optimizer(
         MOI.Bridges.full_bridge_optimizer(HiGHS.Optimizer(), Float64),
     )
     S_zeros = MathOptComplements.ComplementsWithSetType{MOI.Zeros}
     @test MOI.Bridges.bridge_type(opt_sos1, MOI.VectorOfVariables, S_zeros) ==
-          MathOptComplements.Bridges.ToSOS1Bridge{Float64}
+          MathOptComplements.Bridges.ToZerosBridge{Float64}
     return
 end