[FileFormats.LP] fix reading off-diagonals of quadratic functions (#2182)

Author: odow

src/FileFormats/LP/LP.jl

@@ -596,11 +596,12 @@ function _parse_function(
             push!(f.terms, term::MOI.ScalarAffineTerm{Float64})
         elseif term isa MOI.ScalarQuadraticTerm{Float64}
             push!(cache.quad_terms, term::MOI.ScalarQuadraticTerm{Float64})
-            if tokens[offset-1] == "]"
+            if tokens[offset-1] in ("]", "]/2")
+                scale = tokens[offset-1] == "]/2" ? 0.5 : 1
                 for (i, term) in enumerate(cache.quad_terms)
                     x, y = term.variable_1, term.variable_2
-                    scale = (x == y ? 2 : 1) * term.coefficient
-                    cache.quad_terms[i] = MOI.ScalarQuadraticTerm(scale, x, y)
+                    coef = scale * (x == y ? 2 : 1) * term.coefficient
+                    cache.quad_terms[i] = MOI.ScalarQuadraticTerm(coef, x, y)
                 end
             end
         else

test/FileFormats/LP/LP.jl

@@ -459,7 +459,7 @@ function test_read_model1_tricky()
     seekstart(io)
     file = read(io, String)
     @test occursin("maximize", file)
-    @test occursin("obj: -1 Var4 + 1 V5 + [ 1 Var4 ^ 2 - 2.4 V5 * V1 ]/2", file)
+    @test occursin("obj: -1 Var4 + 1 V5 + [ 1 Var4 ^ 2 - 1.2 V5 * V1 ]/2", file)
     @test occursin("CON3: 1 V3 <= 2.5", file)
     @test occursin("CON4: 1 V5 + 1 V6 + 1 V7 <= 1", file)
     @test occursin("CON1: 1 V1 >= 0", file)
@@ -632,8 +632,8 @@ function test_read_tricky_quadratic()
     @test occursin("minimize", file)
     @test occursin("obj: [ 2 x ^ 2 + 2 x * y ]/2", file)
     @test occursin("c1: [ 1 x ^ 2 - 1 x * y ] <= 0", file)
-    @test occursin("c2: [ 0.5 x ^ 2 - 1 x * y ] <= 0", file)
-    @test occursin("c3: [ 0.5 x ^ 2 - 1 x * y ] <= 0", file)
+    @test occursin("c2: [ 0.5 x ^ 2 - 0.5 x * y ] <= 0", file)
+    @test occursin("c3: [ 0.5 x ^ 2 - 0.5 x * y ] <= 0", file)
     @test occursin("x free", file)
     @test occursin("y free", file)
     return
@@ -832,6 +832,61 @@ function test_reading_bounds()
     return
 end
 
+function _test_read_quadratic(input::String, output::String = input)
+    io = IOBuffer()
+    input_text = """
+    minimize
+    obj: $input
+    subject to
+    Bounds
+    x >= 0
+    y >= 0
+    End
+    """
+    write(io, input_text)
+    model = MOI.FileFormats.LP.Model()
+    seekstart(io)
+    read!(io, model)
+    out = IOBuffer()
+    write(out, model)
+    seekstart(out)
+    output_text = """
+    minimize
+    obj: $output
+    subject to
+    Bounds
+    x >= 0
+    y >= 0
+    End
+    """
+    @test read(out, String) == output_text
+    return
+end
+
+function test_read_quadratic()
+    _test_read_quadratic("1 x + 1 y + [ 1 x * y + 1 y ^ 2 ]/2")
+    _test_read_quadratic("1 x + 1 y + [ 2 x * y + 1 y ^ 2 ]/2")
+    _test_read_quadratic("1 x + 1 y + [ 1 x * y + 2 y ^ 2 ]/2")
+    _test_read_quadratic("1 x + 1 y + [ 2 x * y + 2 y ^ 2 ]/2")
+    _test_read_quadratic(
+        "1 x + 1 y + [ 1 x * y + 1 y ^ 2 ]",
+        "1 x + 1 y + [ 2 x * y + 2 y ^ 2 ]/2",
+    )
+    _test_read_quadratic(
+        "1 x + 1 y + [ 2 x * y + 1 y ^ 2 ]",
+        "1 x + 1 y + [ 4 x * y + 2 y ^ 2 ]/2",
+    )
+    _test_read_quadratic(
+        "1 x + 1 y + [ 1 x * y + 2 y ^ 2 ]",
+        "1 x + 1 y + [ 2 x * y + 4 y ^ 2 ]/2",
+    )
+    _test_read_quadratic(
+        "1 x + 1 y + [ 2 x * y + 2 y ^ 2 ]",
+        "1 x + 1 y + [ 4 x * y + 4 y ^ 2 ]/2",
+    )
+    return
+end
+
 function runtests()
     for name in names(@__MODULE__, all = true)
         if startswith("$(name)", "test_")