Don't reduce inputs

Author: YingboMa

src/structural_transformation/StructuralTransformations.jl

@@ -16,7 +16,8 @@ using ModelingToolkit
 using ModelingToolkit: ODESystem, AbstractSystem,var_from_nested_derivative, Differential,
                        states, equations, vars, Symbolic, diff2term, value,
                        operation, arguments, Sym, Term, simplify, solve_for,
-                       isdiffeq, isdifferential, get_structure, get_iv, independent_variables,
+                       isdiffeq, isdifferential, isinput,
+                       get_structure, get_iv, independent_variables,
                        get_structure, defaults, InvalidSystemException,
                        ExtraEquationsSystemException,
                        ExtraVariablesSystemException,

src/structural_transformation/utils.jl

@@ -276,7 +276,9 @@ function find_solvables!(sys)
         term = value(eq.rhs - eq.lhs)
         for j in 𝑠neighbors(graph, i)
             isalgvar(s, j) || continue
-            a, b, islinear = linear_expansion(term, fullvars[j])
+            var = fullvars[j]
+            isinput(var) && continue
+            a, b, islinear = linear_expansion(term, var)
             a = unwrap(a)
             if islinear && (!(a isa Symbolic) && a isa Number && a != 0)
                 add_edge!(solvable_graph, i, j)

src/systems/systemstructure.jl

@@ -5,7 +5,8 @@ using Symbolics: linear_expansion, unwrap
 using SymbolicUtils: istree, operation, arguments, Symbolic
 using ..ModelingToolkit
 import ..ModelingToolkit: isdiffeq, var_from_nested_derivative, vars!, flatten,
-    value, InvalidSystemException, isdifferential, _iszero, isparameter, independent_variables
+    value, InvalidSystemException, isdifferential, _iszero, isparameter,
+    independent_variables, isinput
 using ..BipartiteGraphs
 using LightGraphs
 using UnPack
@@ -230,7 +231,7 @@ function find_linear_equations(sys)
             var = fullvars[j]
             a, b, islinear = linear_expansion(term, var)
             a = unwrap(a)
-            if islinear && !(a isa Symbolic) && a isa Number
+            if islinear && !(a isa Symbolic) && a isa Number && !isinput(var)
                 if a == 1 || a == -1
                     a = convert(Integer, a)
                     linear_term += a * var

src/variables.jl

@@ -11,6 +11,15 @@ Symbolics.option_to_metadata_type(::Val{:description}) = VariableDescriptionType
 Symbolics.option_to_metadata_type(::Val{:input}) = VariableInput
 Symbolics.option_to_metadata_type(::Val{:output}) = VariableOutput
 
+function isvarkind(m, x)
+    p = Symbolics.getparent(x, nothing)
+    p === nothing || (x = p)
+    getmetadata(x, m, false)
+end
+
+isinput(x) = isvarkind(VariableInput, x)
+isoutput(x) = isvarkind(VariableOutput, x)
+
 """
 $(SIGNATURES)