Add more flexible add_accumulations

Author: YingboMa

src/systems/diffeqs/odesystem.jl

@@ -455,14 +455,30 @@ $(SIGNATURES)
 Add accumulation variables for `vars`.
 """
 function add_accumulations(sys::ODESystem, vars = states(sys))
+    avars = [rename(v, Symbol(:accumulation_, getname(v))) for v in vars]
+    add_accumulations(sys, avars .=> vars)
+end
+
+"""
+$(SIGNATURES)
+
+Add accumulation variables for `vars`. `vars` is a vector of pairs in the form
+of
+
+```julia
+[cumulative_var1 => x + y, cumulative_var2 => x^2]
+```
+Then, cumulative variables `cumulative_var1` and `cumulative_var2` that computes
+the comulative `x + y` and `x^2` would be added to `sys`.
+"""
+function add_accumulations(sys::ODESystem, vars::Vector{<:Pair})
     eqs = get_eqs(sys)
-    accs = filter(x -> startswith(string(x), "accumulation_"), states(sys))
-    if !isempty(accs)
-        error("$accs variable names start with \"accumulation_\"")
+    avars = map(first, vars)
+    if (ints = intersect(avars, states(sys)); !isempty(ints))
+        error("$ints already exist in the system!")
     end
-    avars = [rename(v, Symbol(:accumulation_, getname(v))) for v in vars]
     D = Differential(get_iv(sys))
-    @set! sys.eqs = [eqs; Equation[D(a) ~ v for (a, v) in zip(avars, vars)]]
+    @set! sys.eqs = [eqs; Equation[D(a) ~ v[2] for (a, v) in zip(avars, vars)]]
     @set! sys.states = [get_states(sys); avars]
     @set! sys.defaults = merge(get_defaults(sys), Dict(a => 0.0 for a in avars))
 end

test/odesystem.jl

@@ -348,6 +348,14 @@ eqs = [0 ~ x + z
        D(accumulation_z) ~ z
        D(x) ~ y]
 @test sort(equations(asys), by = string) == eqs
+@variables ac(t)
+asys = add_accumulations(sys, [ac => (x + y)^2])
+eqs = [0 ~ x + z
+       0 ~ x - y
+       D(ac) ~ (x + y)^2
+       D(x) ~ y]
+@test sort(equations(asys), by = string) == eqs
+
 sys2 = ode_order_lowering(sys)
 M = ModelingToolkit.calculate_massmatrix(sys2)
 @test M == Diagonal([1, 0, 0])