add an alias_elimination function

Author: shashi

src/ModelingToolkit.jl

@@ -113,6 +113,8 @@ include("systems/pde/pdesystem.jl")
 include("systems/reaction/reactionsystem.jl")
 include("systems/dependency_graphs.jl")
 
+include("systems/reduction.jl")
+
 include("latexify_recipes.jl")
 include("build_function.jl")
 

src/systems/reduction.jl

@@ -0,0 +1,30 @@
+export alias_elimination
+
+function substitute_aliases(diffeqs, outputs)
+    lhss(diffeqs) .~ substitute.(rhss(diffeqs), (Dict(lhss(outputs) .=> rhss(outputs)),))
+end
+
+function alias_elimination(sys::ODESystem)
+    eqs = equations(sys)
+
+    new_stateops = map(eqs) do eq
+        if eq.lhs isa Operation && eq.lhs.op isa Differential
+            get_variables(eq.lhs)
+        else
+            []
+        end
+    end |> Iterators.flatten |> collect |> unique
+    newstates = convert.(Variable, new_stateops)
+
+    alg_idxs = findall(x->x.lhs isa Constant && iszero(x.lhs), eqs)
+
+    eliminate = setdiff(states(sys), newstates)
+    outputs = solve_for(eqs[alg_idxs], map(x->x(sys.iv()), eliminate))
+
+    diffeqs = eqs[setdiff(1:length(eqs), alg_idxs)]
+
+    diffeqs′ = substitute_aliases(diffeqs, outputs)
+
+    ODESystem(diffeqs′, sys.iv(), new_stateops, parameters(sys), observed=outputs)
+end
+