WIP

Author: YingboMa

src/systems/diffeqs/index_reduction.jl

@@ -0,0 +1,14 @@
+struct BiGraph{T}
+    data::Vector{Vector{T}}
+end
+
+function sys2bigraph(sys)
+    ss = states(sys)
+    data = Operation[]
+    for eq in sys.eqs
+        es = []
+        lhs = eq.lhs
+        lhs.op isa Differential && push!(eq, lhs)
+        push!(data, es)
+    end
+end

test/index_reduction.jl

@@ -0,0 +1,25 @@
+using ModelingToolkit
+using ModelingToolkit: sys2bigraph
+using DiffEqBase
+using Test
+
+# Define some variables
+@parameters t L g
+@variables x(t) y(t) w(t) z(t) T(t)
+@derivatives D'~t
+
+eqs2 = [D(D(x)) ~ T*x,
+        D(D(y)) ~ T*y - g,
+        0 ~ x^2 + y^2 - L^2]
+pendulum2 = ODESystem(eqs, t, [x, y, T], [L, g], name=:pendulum)
+ModelingToolkit.ode_order_lowering(pendulum2)
+
+# Simple pendulum in cartesian coordinates
+eqs = [D(x) ~ w,
+       D(y) ~ z,
+       D(w) ~ T*x,
+       D(z) ~ T*y - g,
+       0 ~ x^2 + y^2 - L^2]
+pendulum = ODESystem(eqs, t, [x, y, w, z, T], [L, g], name=:pendulum)
+
+sys2bigraph(pendulum)