From aea1a3ac1e23e1abb094940bf3c6b6cb099a01fb Mon Sep 17 00:00:00 2001
From: Joaquim Garcia <joaquimgarcia@local.psrservices.net>
Date: Wed, 24 May 2023 02:14:45 -0300
Subject: [PATCH] add ReducedCost

---
 src/MOI/MOI_wrapper.jl               | 31 ++++++++++++++++++++++++++++
 test/MathOptInterface/MOI_wrapper.jl | 29 ++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/src/MOI/MOI_wrapper.jl b/src/MOI/MOI_wrapper.jl
index 37886714..e2556b0e 100644
--- a/src/MOI/MOI_wrapper.jl
+++ b/src/MOI/MOI_wrapper.jl
@@ -3092,6 +3092,10 @@ function _farkas_variable_dual(model::Optimizer, col::Int64)
     return sum(v * model.cached_solution.linear_dual[i + 1] for (i, v) in zip(mrwind, dmatval))
 end
 
+#=
+    Duals
+=#
+
 function MOI.get(
     model::Optimizer, attr::MOI.ConstraintDual,
     c::MOI.ConstraintIndex{MOI.VariableIndex, MOI.LessThan{Float64}}
@@ -4396,4 +4400,31 @@ function _get_constraint_names(model)
     end
     con_names = split(all_names, '\0')[1:num_constraints]
     return strip.(con_names)
+end
+
+#=
+    Reduced costs
+=#
+
+struct ReducedCost <: MOI.AbstractVariableAttribute
+    result_index::Int
+    ReducedCost(result_index::Int = 1) = new(result_index)
+end
+
+function MOI.supports(
+    ::Optimizer, ::ReducedCost, ::Type{MOI.VariableIndex})
+    return true
+end
+
+MOI.is_set_by_optimize(::ReducedCost) = true
+
+MOI.attribute_value_type(::ReducedCost) = Float64
+
+function MOI.get(
+    model::Optimizer, attr::ReducedCost, vi::MOI.VariableIndex
+)
+    _throw_if_optimize_in_progress(model, attr)
+    MOI.check_result_index_bounds(model, attr)
+    column = _info(model, vi).column
+    return model.cached_solution.variable_dual[column]
 end
\ No newline at end of file
diff --git a/test/MathOptInterface/MOI_wrapper.jl b/test/MathOptInterface/MOI_wrapper.jl
index 49a6f728..3c0fd204 100644
--- a/test/MathOptInterface/MOI_wrapper.jl
+++ b/test/MathOptInterface/MOI_wrapper.jl
@@ -976,6 +976,35 @@ function test_dummy_nlp()
     return nothing
 end
 
+function test_multiple_modifications()
+    model = Xpress.Optimizer(OUTPUTLOG = 0)
+
+    x = MOI.add_variable(model)
+
+    c = MOI.add_constraint(model, x, MOI.GreaterThan(1.0))
+
+    MOI.set(
+        model,
+        MOI.ObjectiveFunction{MOI.ScalarAffineFunction{Float64}}(),
+        2.0 * x,
+    )
+
+    MOI.set(
+        model,
+        MOI.ObjectiveSense(),
+        MOI.MIN_SENSE,
+    )
+
+    MOI.optimize!(model)
+
+    @test MOI.get(model, MOI.VariablePrimal(), x) == 1.0
+
+    @test MOI.get(model, MOI.ConstraintDual(), c) == 2.0
+    @test MOI.get(model, Xpress.ReducedCost(), x) == 2.0
+
+    return
+end
+
 end
 
 TestMOIWrapper.runtests()