Add more complete test for ODESystem with difference operator

Author: sharanry

src/systems/diffeqs/abstractodesystem.jl

@@ -117,7 +117,7 @@ function generate_difference_cb(sys::ODESystem, dvs = states(sys), ps = paramete
     rhss = [ 
         begin
             ind = findfirst(eq -> isdifference(eq.lhs) && isequal(arguments(eq.lhs)[1], s), eqs)
-            ind === nothing ? Val{0} : eqs[ind].rhs
+            ind === nothing ? 0 : eqs[ind].rhs
         end
         for s in dvs ]
 

test/odesystem.jl

@@ -398,20 +398,37 @@ eqs = [
     δ(y) ~ -c*y + d*x*y,
     D(x) ~ y
 ]
-
 de = ODESystem(eqs,t,[x,y],[a,b,c,d])
-
 @test generate_difference_cb(de) isa ModelingToolkit.DiffEqCallbacks.DiscreteCallback
 
-prob = ODEProblem(ODEFunction{false}(de),[1.0,1.0],(0.0,1.0),[1.5,1.0,3.0,1.0])
+# doesn't work with ODEFunction
+# prob = ODEProblem(ODEFunction{false}(de),[1.0,1.0],(0.0,1.0),[1.5,1.0,3.0,1.0])
 
 prob = ODEProblem(de,[1.0,1.0],(0.0,1.0),[1.5,1.0,3.0,1.0], check_length=false)
-
 @test prob.kwargs[:difference_cb] isa ModelingToolkit.DiffEqCallbacks.DiscreteCallback
 
-sol = solve(prob, Tsit5(); cb=prob.kwargs[:difference_cb], tstops=prob.tspan[1]:0.1:prob.tspan[2])
+sol = solve(prob, Tsit5(); callback=prob.kwargs[:difference_cb], tstops=prob.tspan[1]:0.1:prob.tspan[2])
+# using Plots
+# plot(sol)
+
+# Direct implementation
+
+function lotka(du,u,p,t)
+    x = u[1]
+    y = u[2]
+    du[1] = p[1]*x - p[2]*x*y
+    du[2] = -p[3]*y + p[4]*x*y
+end
+
+prob2 = ODEProblem(lotka,[1.0,1.0],(0.0,1.0),[1.5,1.0,3.0,1.0])
+function periodic_difference_affect!(int)
+    int.u += [int.u[2], 0]
+end
+
+difference_cb = ModelingToolkit.PeriodicCallback(periodic_difference_affect!, 0.1) 
+
+sol2 = solve(prob2, Tsit5(); callback=difference_cb, tstops=collect(prob.tspan[1]:0.1:prob.tspan[2])[2:end]
+)
+# plot(sol2)
 
-#=
-using Plots
-plot(sol)
-=#
+@test sol(0:0.01:1) ≈ sol2(0:0.01:1)