fix: FMI varname parse bug #3934

ext/MTKFMIExt.jl

@@ -362,11 +362,24 @@ function parseFMIVariableName(name::AbstractString)
     name = replace(name, "." => "__")
     der = 0
     if startswith(name, "der(")
-        idx = findfirst(',', name)
+
+        # account for multi-dimensional array variable derivatives, e.g. der(x[1,2], 2)
+        array_variable_pattern = r"\[\d+?,\d+?\]"
+        patternmatches = match(array_variable_pattern, name)
+        if (patternmatches !== nothing)
+            safe_array_index_str = replace(String(patternmatches.match), "," => "_")
+            safe_name = replace(name, array_variable_pattern => safe_array_index_str)
+        else
+            safe_name = name
+        end
+
+
+        idx = findfirst(',', safe_name)
         if idx === nothing
             name = @view name[5:(end - 1)]
             der = 1
         else
+
             der = parse(Int, @view name[(idx + 1):(end - 1)])
             name = @view name[5:(idx - 1)]
         end

test/fmi/fmi.jl

@@ -274,6 +274,12 @@ end
             sol.t; idxs = [sys.adder1.c, sys.adder2.c]).u rtol=1e-3
     end
 
+    @testset "multiDimArray Support" begin
+        path_to_FMU = joinpath(FMU_DIR, "SimpleArrayModel.fmu")
+        fmu = loadFMU(path_to_FMU)
+        @named model = MTK.FMIComponent(Val(2); fmu, type = :ME)
+        @test model !== nothing
+    end
     function build_looped_sspace(sspace1, sspace2)
         @variables x(t) = 1
         @mtkcompile sys = System([D(x) ~ x, sspace1.u ~ sspace2.x, sspace2.u ~ sspace1.y],

test/fmi/fmus/SimpleArrayModel/README.MD

@@ -0,0 +1,5 @@
+SimpleArrayModel.fmu is a v2 ME FMU that contains multidimensional array variables with derivatives. It is included to test SciML issue number 3934. It is generated from the Modelica model in test_mtk_2darray_bug.mo using  Dymola 2022x. The translation command is
+
+```julia
+translateModelFMU("SimpleArrayModel", false, "", "2", "me", true, 0, fill("",0));
+```

test/fmi/fmus/SimpleArrayModel/test_mtk_2darray_bug.mo

@@ -0,0 +1,25 @@
+model SimpleArrayModel
+    "Simple model with a 2D array variable and its derivative"
+
+    parameter Integer n = 3 "First dimension size";
+    parameter Integer m = 2 "Second dimension size";
+
+    Real x[n,m] "Two-dimensional array variable";
+    Real dx[n,m] "Derivative of the array variable";
+
+equation
+    // Define the derivative relationship
+    der(x) = dx;
+
+    // Simple example dynamics for the derivative
+    for i in 1:n loop
+        for j in 1:m loop
+            dx[i,j] = -x[i,j] + sin(time);
+        end for;
+    end for;
+
+initial equation
+    // Initial conditions
+    x = zeros(n,m);
+
+end SimpleArrayModel;