feat: retain original equations of the system in TearingState

Author: AayushSabharwal

src/systems/systems.jl

@@ -80,7 +80,6 @@ function __mtkcompile(sys::AbstractSystem; simplify = false,
 
     @unpack structure, fullvars = state
     @unpack graph, var_to_diff, var_types = structure
-    eqs = equations(state)
     brown_vars = Int[]
     new_idxs = zeros(Int, length(var_types))
     idx = 0
@@ -98,7 +97,8 @@ function __mtkcompile(sys::AbstractSystem; simplify = false,
         Is = Int[]
         Js = Int[]
         vals = Num[]
-        new_eqs = copy(eqs)
+        make_eqs_zero_equals!(state)
+        new_eqs = copy(equations(state))
         dvar2eq = Dict{Any, Int}()
         for (v, dv) in enumerate(var_to_diff)
             dv === nothing && continue

src/systems/systemstructure.jl

@@ -203,6 +203,7 @@ end
 mutable struct TearingState{T <: AbstractSystem} <: AbstractTearingState{T}
     """The system of equations."""
     sys::T
+    original_eqs::Vector{Equation}
     """The set of variables of the system."""
     fullvars::Vector{BasicSymbolic}
     structure::SystemStructure
@@ -219,6 +220,7 @@ end
 TransformationState(sys::AbstractSystem) = TearingState(sys)
 function system_subset(ts::TearingState, ieqs::Vector{Int})
     eqs = equations(ts)
+    @set! ts.original_eqs = ts.original_eqs[ieqs]
     @set! ts.sys.eqs = eqs[ieqs]
     @set! ts.original_eqs = ts.original_eqs[ieqs]
     @set! ts.structure = system_subset(ts.structure, ieqs)
@@ -524,7 +526,7 @@ function TearingState(sys; quick_cancel = false, check = true, sort_eqs = true)
 
     eq_to_diff = DiffGraph(nsrcs(graph))
 
-    ts = TearingState(sys, fullvars,
+    ts = TearingState(sys, original_eqs, fullvars,
         SystemStructure(complete(var_to_diff), complete(eq_to_diff),
             complete(graph), nothing, var_types, false),
         Any[], param_derivative_map, original_eqs, Equation[])
@@ -810,6 +812,22 @@ function Base.show(io::IO, mime::MIME"text/plain", ms::MatchedSystemStructure)
     printstyled(io, " SelectedState")
 end
 
+function make_eqs_zero_equals!(ts::TearingState)
+    neweqs = map(enumerate(get_eqs(ts.sys))) do kvp
+        i, eq = kvp
+        isalgeq = true
+        for j in 𝑠neighbors(ts.structure.graph, i)
+            isalgeq &= invview(ts.structure.var_to_diff)[j] === nothing
+        end
+        if isalgeq
+            return 0 ~ eq.rhs - eq.lhs
+        else
+            return eq
+        end
+    end
+    copyto!(get_eqs(ts.sys), neweqs)
+end
+
 function mtkcompile!(state::TearingState; simplify = false,
         check_consistency = true, fully_determined = true, warn_initialize_determined = true,
         inputs = Any[], outputs = Any[],
@@ -836,6 +854,7 @@ function mtkcompile!(state::TearingState; simplify = false,
         """))
     end
     if length(tss) > 1
+        make_eqs_zero_equals!(tss[continuous_id])
         # simplify as normal
         sys = _mtkcompile!(tss[continuous_id]; simplify,
             inputs = [inputs; clocked_inputs[continuous_id]], outputs, disturbance_inputs,