Merge pull request #199 from chelseas/fix_binary

Adding constraints so that binary variables reflect correct activation status

src/optimization/utils/constraints.jl

@@ -89,6 +89,7 @@ function encode_layer!(::AbstractLinearProgram, model::Model, layer::Layer{Id},
     nothing
 end
 
+# All ReLU layers pass through this
 function encode_layer!(LP::AbstractLinearProgram, model::Model, layer::Layer{ReLU}, args...)
     encode_relu.(LP, model, args...)
     nothing
@@ -103,8 +104,15 @@ function encode_layer!(SLP::SlackLP, model::Model, layer::Layer{Id}, ẑᵢ, z
     return nothing
 end
 
+# need to fix δᵢⱼ for BoundedMixedIntegerLP and possibly other types 
+function encode_layer!(::BoundedMixedIntegerLP, model::Model, layer::Layer{Id}, ẑᵢ, zᵢ, δᵢ, args...)
+    @constraint(model, zᵢ .== ẑᵢ)
+    @constraint(model, δᵢ .== 1)
+    return nothing
+end
 
 function encode_ij(LP, model, i, j)
+    # where is this function used? Needs documentation.
     L = model[:network].layers[i]
     params = model_params(LP, model, i)
     if L.activation isa Id
@@ -133,8 +141,10 @@ end
 function encode_relu(::BoundedMixedIntegerLP, model, ẑᵢⱼ, zᵢⱼ, δᵢⱼ, l̂ᵢⱼ, ûᵢⱼ)
     if l̂ᵢⱼ >= 0.0
         @constraint(model, zᵢⱼ == ẑᵢⱼ)
+        @constraint(model, δᵢⱼ == 1)
     elseif ûᵢⱼ <= 0.0
         @constraint(model, zᵢⱼ == 0.0)
+        @constraint(model, δᵢⱼ == 0)
     else
         @constraints(model, begin
                                 zᵢⱼ >= 0.0
@@ -164,11 +174,11 @@ function encode_relu(::TriangularRelaxedLP, model, ẑᵢⱼ, zᵢⱼ, l̂ᵢⱼ
 end
 
 function encode_relu(::LinearRelaxedLP, model, ẑᵢⱼ, zᵢⱼ, δᵢⱼ)
-    @constraint(model, zᵢⱼ == (δᵢⱼ ? ẑᵢⱼ : 0.0))
+    @constraint(model, zᵢⱼ == (δᵢⱼ ? ẑᵢⱼ : 0.0)) # in LinearRelaxedLP δᵢⱼ is a constant not a variable
 end
 
 function encode_relu(::StandardLP, model, ẑᵢⱼ, zᵢⱼ, δᵢⱼ)
-    if δᵢⱼ
+    if δᵢⱼ # in StandardLP δᵢⱼ is a constant, not a variable
         @constraint(model, ẑᵢⱼ >= 0.0)
         @constraint(model, zᵢⱼ == ẑᵢⱼ)
     else