feat: add get_saved_state_idxs, handle problems with no system in SavedSubsystem constructor

src/solutions/save_idxs.jl

@@ -44,6 +44,13 @@ function as_diffeq_array(vt::Vector{VectorTemplate}, t)
     return DiffEqArray(typeof(TupleOfArraysWrapper(vt))[], t, (1, 1))
 end
 
+function is_empty_indp(indp)
+    isempty(variable_symbols(indp)) && isempty(parameter_symbols(indp)) &&
+        isempty(independent_variable_symbols(indp))
+end
+
+# Everything from this point on is public API
+
 """
     $(TYPEDSIGNATURES)
 
@@ -104,6 +111,12 @@ function SavedSubsystem(indp, pobj, saved_idxs)
         return nothing
     end
 
+    # this is required because problems with no system have an empty `SymbolCache`
+    # as their symbolic container.
+    if is_empty_indp(indp)
+        return nothing
+    end
+
     # array state symbolics must be scalarized
     saved_idxs = collect(Iterators.flatten(map(saved_idxs) do sym
         if symbolic_type(sym) == NotSymbolic()
@@ -226,6 +239,20 @@ function SavedSubsystem(indp, pobj, saved_idxs)
         timeseries_partition_templates, indexes_in_partition, ts_idx_to_count)
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Given a `SavedSubsystem`, return the subset of state indexes of the original system that are
+saved, in the order they are saved.
+"""
+function get_saved_state_idxs(ss::SavedSubsystem)
+    idxs = Vector{valtype(ss.state_map)}(undef, length(ss.state_map))
+    for (k, v) in ss.state_map
+        idxs[v] = k
+    end
+    return idxs
+end
+
 """
     $(TYPEDEF)
 

test/downstream/solution_interface.jl

@@ -193,6 +193,8 @@ end
 
         ode_sol = solve(prob, Tsit5(); save_idxs = xidx)
         subsys = SciMLBase.SavedSubsystem(sys, prob.p, [xidx])
+        @test SciMLBase.get_saved_state_idxs(subsys) == [xidx]
+
         # FIXME: hack for save_idxs
         SciMLBase.@reset ode_sol.saved_subsystem = subsys
 
@@ -257,6 +259,7 @@ end
         sol = solve(prob; save_idxs = xidx)
         xvals = sol[x]
         subsys = SciMLBase.SavedSubsystem(sys, prob.p, [x, q, r])
+        @test SciMLBase.get_saved_state_idxs(subsys) == [xidx]
         qvals = sol.ps[q]
         rvals = sol.ps[r]
         # FIXME: hack for save_idxs
@@ -290,6 +293,7 @@ end
         prob = ODEProblem(sys, [x => 1.0, y => 1.0], (0.0, 5.0),
             [p => 0.5, q => 0.0, r => 1.0, s => 10.0, u => 4096.0])
         ss = SciMLBase.SavedSubsystem(sys, prob.p, [x, q, s, r])
+        @test SciMLBase.get_saved_state_idxs(ss) == [xidx]
         sswf = SciMLBase.SavedSubsystemWithFallback(ss, sys)
         xidx = variable_index(sys, x)
         qidx = parameter_index(sys, q)