feat: add kwarg to allow disabling sorting of equations in structural_simplify

Author: AayushSabharwal

src/systems/systems.jl

@@ -24,6 +24,7 @@ topological sort of the observed equations in `sys`.
 + When `simplify=true`, the `simplify` function will be applied during the tearing process.
 + `allow_symbolic=false`, `allow_parameter=true`, and `conservative=false` limit the coefficient types during tearing. In particular, `conservative=true` limits tearing to only solve for trivial linear systems where the coefficient has the absolute value of ``1``.
 + `fully_determined=true` controls whether or not an error will be thrown if the number of equations don't match the number of inputs, outputs, and equations.
++ `sort_eqs=true` controls whether equations are sorted lexicographically before simplification or not.
 """
 function structural_simplify(
         sys::AbstractSystem, io = nothing; additional_passes = [], simplify = false, split = true,
@@ -69,10 +70,11 @@ function __structural_simplify(sys::SDESystem, args...; kwargs...)
     return __structural_simplify(ODESystem(sys), args...; kwargs...)
 end
 
-function __structural_simplify(sys::AbstractSystem, io = nothing; simplify = false,
+function __structural_simplify(
+        sys::AbstractSystem, io = nothing; simplify = false, sort_eqs = true,
         kwargs...)
     sys = expand_connections(sys)
-    state = TearingState(sys)
+    state = TearingState(sys; sort_eqs)
 
     @unpack structure, fullvars = state
     @unpack graph, var_to_diff, var_types = structure

src/systems/systemstructure.jl

@@ -260,7 +260,7 @@ function is_time_dependent_parameter(p, iv)
             (args = arguments(p); length(args)) == 1 && isequal(only(args), iv))
 end
 
-function TearingState(sys; quick_cancel = false, check = true)
+function TearingState(sys; quick_cancel = false, check = true, sort_eqs = true)
     sys = flatten(sys)
     ivs = independent_variables(sys)
     iv = length(ivs) == 1 ? ivs[1] : nothing
@@ -381,11 +381,13 @@ function TearingState(sys; quick_cancel = false, check = true)
     neqs = length(eqs)
     symbolic_incidence = symbolic_incidence[eqs_to_retain]
 
-    # sort equations lexicographically to reduce simplification issues
-    # depending on order due to NP-completeness of tearing.
-    sortidxs = Base.sortperm(eqs, by = string)
-    eqs = eqs[sortidxs]
-    symbolic_incidence = symbolic_incidence[sortidxs]
+    if sort_eqs
+        # sort equations lexicographically to reduce simplification issues
+        # depending on order due to NP-completeness of tearing.
+        sortidxs = Base.sortperm(eqs, by = string)
+        eqs = eqs[sortidxs]
+        symbolic_incidence = symbolic_incidence[sortidxs]
+    end
 
     ### Handle discrete variables
     lowest_shift = Dict()