feat: add ProblemState

Author: AayushSabharwal

docs/src/api.md

@@ -88,4 +88,5 @@ symbolic_evaluate
 
 ```@docs
 SymbolCache
+ProblemState
 ```

docs/src/usage.md

@@ -22,10 +22,10 @@ Consider the following example:
 
 ```@example Usage
 using ModelingToolkit, OrdinaryDiffEq, SymbolicIndexingInterface, Plots
+using ModelingToolkit: t_nounits as t, D_nounits as D
 
 @parameters σ ρ β
-@variables t x(t) y(t) z(t) w(t)
-D = Differential(t)
+@variables x(t) y(t) z(t) w(t)
 
 eqs = [D(D(x)) ~ σ * (y - x),
     D(y) ~ x * (ρ - z) - y,
@@ -121,6 +121,30 @@ output, the following shorthand is used:
 sol[allvariables] # equivalent to sol[all_variable_symbols(sol)]
 ```
 
+### Evaluating expressions
+
+`getu` also generates functions for expressions if the object passed to it supports
+[`observed`](@ref). For example:
+
+```@example Usage
+getu(prob, x + y + z)(prob)
+```
+
+To evaluate this function using values other than the ones contained in `prob`, we need
+an object that supports [`state_values`](@ref), [`parameter_values`](@ref),
+[`current_time`](@ref). SymbolicIndexingInterface provides the [`ProblemState`](@ref) type,
+which has trivial implementations of the above functions. We can thus do:
+
+```@example Usage
+temp_state = ProblemState(; u = [0.1, 0.2, 0.3, 0.4], p = parameter_values(prob))
+getu(prob, x + y + z)(temp_state)
+```
+
+Note that providing all of the state vector, parameter object and time may not be
+necessary if the function generated by `observed` does not access them. ModelingToolkit.jl
+generates functions that access the parameters regardless of whether they are used in the
+expression, and thus it needs to be provided to the `ProblemState`.
+
 ## Parameter Indexing: Getting and Setting Parameter Values
 
 Parameters cannot be obtained using this syntax, and instead require using [`getp`](@ref) and [`setp`](@ref).

src/SymbolicIndexingInterface.jl

@@ -31,6 +31,9 @@ include("parameter_indexing.jl")
 export state_values, set_state!, current_time, getu, setu
 include("state_indexing.jl")
 
+export ProblemState
+include("problem_state.jl")
+
 export ParameterIndexingProxy
 include("parameter_indexing_proxy.jl")
 

src/problem_state.jl

@@ -0,0 +1,23 @@
+"""
+    struct ProblemState
+    function ProblemState(; u = nothing, p = nothing, t = nothing)
+
+A struct which can be used as an argument to the function returned by [`getu`](@ref) or
+[`setu`](@ref). It stores the state vector, parameter object and current time, and
+forwards calls to [`state_values`](@ref), [`parameter_values`](@ref),
+[`current_time`](@ref), [`set_state!`](@ref), [`set_parameter!`](@ref) to the contained
+objects.
+"""
+struct ProblemState{U, P, T}
+    u::U
+    p::P
+    t::T
+end
+
+ProblemState(; u = nothing, p = nothing, t = nothing) = ProblemState(u, p, t)
+
+state_values(ps::ProblemState) = ps.u
+parameter_values(ps::ProblemState) = ps.p
+current_time(ps::ProblemState) = ps.t
+set_state!(ps::ProblemState, val, idx) = set_state!(ps.u, val, idx)
+set_parameter!(ps::ProblemState, val, idx) = set_parameter!(ps.p, val, idx)

test/problem_state_test.jl

@@ -0,0 +1,15 @@
+using SymbolicIndexingInterface
+using Test
+
+sys = SymbolCache([:x, :y, :z], [:a, :b, :c], :t)
+prob = ProblemState(; u = [1.0, 2.0, 3.0], p = [0.1, 0.2, 0.3], t = 0.5)
+
+for (i, sym) in enumerate(variable_symbols(sys))
+    @test getu(sys, sym)(prob) == prob.u[i]
+end
+for (i, sym) in enumerate(parameter_symbols(sys))
+    @test getp(sys, sym)(prob) == prob.p[i]
+end
+@test getu(sys, :t)(prob) == prob.t
+
+@test getu(sys, :(x + a + t))(prob) == 1.6

test/runtests.jl

@@ -26,3 +26,6 @@ end
 @safetestset "Remake test" begin
     @time include("remake_test.jl")
 end
+@safetestset "ProblemState test" begin
+    @time include("problem_state_test.jl")
+end