[FileFormats] fix default lower bound in LP reader (#1810)

Author: odow

src/FileFormats/LP/LP.jl

@@ -377,13 +377,15 @@ mutable struct _ReadCache
     constraint_name::String
     num_constraints::Int
     name_to_variable::Dict{String,MOI.VariableIndex}
+    has_default_bound::Set{MOI.VariableIndex}
     function _ReadCache()
         return new(
             MOI.ScalarAffineFunction(MOI.ScalarAffineTerm{Float64}[], 0.0),
             MOI.ScalarAffineFunction(MOI.ScalarAffineTerm{Float64}[], 0.0),
             "",
             0,
             Dict{String,MOI.VariableIndex}(),
+            Set{MOI.VariableIndex}(),
         )
     end
 end
@@ -403,6 +405,10 @@ function _get_variable_from_name(model::Model, cache::_ReadCache, name::String)
     end
     x = MOI.add_variable(model)
     MOI.set(model, MOI.VariableName(), x, name)
+    # By default, all variables have a lower bound of 0 unless otherwise
+    # specified.
+    MOI.add_constraint(model, x, MOI.GreaterThan(0.0))
+    push!(cache.has_default_bound, x)
     cache.name_to_variable[name] = x
     return x
 end
@@ -596,7 +602,9 @@ function _parse_section(
 )
     tokens = _tokenize(line)
     if length(tokens) == 2 && tokens[2] == "free"
-        return  # Do nothing. Variable is free
+        x = _get_variable_from_name(model, cache, tokens[1])
+        _delete_default_lower_bound_if_present(model, cache, x)
+        return
     end
     lb, ub, name = -Inf, Inf, ""
     if length(tokens) == 5
@@ -629,17 +637,35 @@ function _parse_section(
     end
     x = _get_variable_from_name(model, cache, name)
     if lb == ub
+        _delete_default_lower_bound_if_present(model, cache, x)
         MOI.add_constraint(model, x, MOI.EqualTo(lb))
     elseif -Inf < lb < ub < Inf
+        _delete_default_lower_bound_if_present(model, cache, x)
         MOI.add_constraint(model, x, MOI.Interval(lb, ub))
     elseif -Inf < lb
+        _delete_default_lower_bound_if_present(model, cache, x)
         MOI.add_constraint(model, x, MOI.GreaterThan(lb))
     else
+        if ub < 0
+            # We only need to delete the default lower bound if the upper bound
+            # is less than 0.
+            _delete_default_lower_bound_if_present(model, cache, x)
+        end
         MOI.add_constraint(model, x, MOI.LessThan(ub))
     end
     return
 end
 
+function _delete_default_lower_bound_if_present(model, cache, x)
+    if !(x in cache.has_default_bound)
+        return
+    end
+    c = MOI.ConstraintIndex{MOI.VariableIndex,MOI.GreaterThan{Float64}}(x.value)
+    MOI.delete(model, c)
+    delete!(cache.has_default_bound, x)
+    return
+end
+
 # _KW_INTEGER
 
 function _parse_section(::typeof(_KW_INTEGER), model, cache, line)

test/FileFormats/LP/LP.jl

@@ -408,6 +408,42 @@ function test_read_maximum_length_error()
     return
 end
 
+function test_default_bound()
+    io = IOBuffer()
+    write(io, "minimize\nobj: x + y")
+    seekstart(io)
+    model = LP.Model()
+    MOI.read!(io, model)
+    x = MOI.get(model, MOI.ListOfVariableIndices())
+    F, S = MOI.VariableIndex, MOI.GreaterThan{Float64}
+    c = [MOI.ConstraintIndex{F,S}(xi.value) for xi in x]
+    sets = MOI.get.(model, MOI.ConstraintSet(), c)
+    @test all(s -> s == MOI.GreaterThan(0.0), sets)
+    @test length(sets) == 2
+    return
+end
+
+function test_default_bound_double_bound()
+    io = IOBuffer()
+    write(io, "minimize\nobj: x\nsubject to\nbounds\n x <= -1\n x >= -2")
+    seekstart(io)
+    model = LP.Model()
+    MOI.read!(io, model)
+    x = first(MOI.get(model, MOI.ListOfVariableIndices()))
+    F = MOI.VariableIndex
+    @test MOI.get(
+        model,
+        MOI.ConstraintSet(),
+        MOI.ConstraintIndex{F,MOI.GreaterThan{Float64}}(x.value),
+    ) == MOI.GreaterThan(-2.0)
+    @test MOI.get(
+        model,
+        MOI.ConstraintSet(),
+        MOI.ConstraintIndex{F,MOI.LessThan{Float64}}(x.value),
+    ) == MOI.LessThan(-1.0)
+    return
+end
+
 function runtests()
     for name in names(@__MODULE__, all = true)
         if startswith("$(name)", "test_")