feat: add get_save_idxs_and_saved_subsystem

src/solutions/save_idxs.jl

@@ -347,3 +347,40 @@ function SymbolicIndexingInterface.with_updated_parameter_timeseries_values(
 
     return ps
 end
+
+"""
+    $(TYPEDSIGNATURES)
+
+Given a SciMLProblem `prob` and (possibly symbolic) `save_idxs`, return the `save_idxs`
+corresponding to the state variables and a `SavedSubsystem` to pass to `build_solution`.
+
+The second return value (corresponding to the `SavedSubsystem`) may be `nothing` in case
+one is not required. `save_idxs` may be a scalar or `nothing`.
+"""
+function get_save_idxs_and_saved_subsystem(prob, save_idxs)
+    if save_idxs === nothing
+        saved_subsystem = nothing
+    else
+        if !(save_idxs isa AbstractArray) || symbolic_type(save_idxs) != NotSymbolic()
+            _save_idxs = [save_idxs]
+        else
+            _save_idxs = save_idxs
+        end
+        saved_subsystem = SavedSubsystem(prob, parameter_values(prob), _save_idxs)
+        if saved_subsystem !== nothing
+            _save_idxs = get_saved_state_idxs(saved_subsystem)
+            if isempty(_save_idxs)
+                # no states to save
+                save_idxs = Int[]
+            elseif !(save_idxs isa AbstractArray) || symbolic_type(save_idxs) != NotSymbolic()
+                # only a single state to save, and save it as a scalar timeseries instead of
+                # single-element array
+                save_idxs = only(_save_idxs)
+            else
+                save_idxs = _save_idxs
+            end
+        end
+    end
+
+    return save_idxs, saved_subsystem
+end

test/downstream/solution_interface.jl

@@ -327,4 +327,35 @@ end
             parameter_values(ptc), rpidx.timeseries_idx => vals)
         @test newp[ridx] == 2prob.ps[r]
     end
+
+    @testset "get_save_idxs_and_saved_subsystem" begin
+        @variables x(t) y(t)
+        @parameters p q(t) r(t) s(t) u(t)
+        evs = [0.1 => [q ~ q + 1, s ~ s - 1], 0.3 => [r ~ 2r, u ~ u / 2]]
+        @mtkbuild sys = ODESystem([D(x) ~ x + p * y, D(y) ~ 2p + x^2], t, [x, y],
+            [p, q, r, s, u], discrete_events = evs)
+        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])
+
+        _idxs, _ss = SciMLBase.get_save_idxs_and_saved_subsystem(prob, nothing)
+        @test _idxs === _ss === nothing
+        _idxs, _ss = SciMLBase.get_save_idxs_and_saved_subsystem(prob, 1)
+        @test _idxs == 1
+        @test _ss isa SciMLBase.SavedSubsystem
+        _idxs, _ss = SciMLBase.get_save_idxs_and_saved_subsystem(prob, [1])
+        @test _idxs == [1]
+        @test _ss isa SciMLBase.SavedSubsystem
+        _idxs, _ss = SciMLBase.get_save_idxs_and_saved_subsystem(prob, x)
+        @test _idxs == 1
+        @test _ss isa SciMLBase.SavedSubsystem
+        _idxs, _ss = SciMLBase.get_save_idxs_and_saved_subsystem(prob, [x])
+        @test _idxs == [1]
+        @test _ss isa SciMLBase.SavedSubsystem
+        _idxs, _ss = SciMLBase.get_save_idxs_and_saved_subsystem(prob, [x, q])
+        @test _idxs == [1]
+        @test _ss isa SciMLBase.SavedSubsystem
+        _idxs, _ss = SciMLBase.get_save_idxs_and_saved_subsystem(prob, [q])
+        @test _idxs == Int[]
+        @test _ss isa SciMLBase.SavedSubsystem
+    end
 end