attempts to slice and splice hermite

src/dense/generic_dense.jl

@@ -624,11 +624,34 @@ end
 ##################### Hermite Interpolants
 
 # If no dispatch found, assume Hermite
-function _ode_interpolant(Θ, dt, y₀, y₁, k, cache, idxs, T::Type{Val{TI}}, differential_vars=trues(size(y₀))) where {TI}
+function _ode_interpolant(Θ, dt, y₀, y₁, k, cache, idxs, T::Type{Val{TI}}) where {TI}
     hermite_interpolant(Θ, dt, y₀, y₁, k, Val{cache isa OrdinaryDiffEqMutableCache}, idxs, T)
 end
 
-function _ode_interpolant!(out, Θ, dt, y₀, y₁, k, cache, idxs, T::Type{Val{TI}}, differential_vars=trues(size(y₀))) where {TI}
+function _ode_interpolant!(out, Θ, dt, y₀, y₁, k, cache, idxs, T::Type{Val{TI}}) where {TI}
+    hermite_interpolant!(out, Θ, dt, y₀, y₁, k, idxs, T)
+end
+
+function _ode_interpolant(Θ, dt, y₀::AbstractArray, y₁::AbstractArray, k, cache, idxs::AbstractArray, T::Type{Val{TI}}, differential_vars) where {TI}
+    sel = differential_vars[idxs]
+    @show y₀ idxs differential_vars sel
+    diffidxs = idxs[sel]
+    linidxs = idxs[.!sel]
+    diff = hermite_interpolant(Θ, dt, y₀, y₁, k, Val{cache isa OrdinaryDiffEqMutableCache}, diffidxs, T)
+    @show lin = linear_interpolant(Θ, dt, y₀, y₁, linidxs, T)
+    @show res = similar(y₀, eltype(y₀), size(sel))
+    res[diffidxs] = diff
+    res[linidxs] = lin
+    res
+end
+
+function _ode_interpolant(Θ, dt, y₀, y₁, k, cache, idxs, T::Type{Val{TI}}, differential_vars) where {TI}
+    @assert all(differential_vars)
+    @show differential_vars
+    hermite_interpolant(Θ, dt, y₀, y₁, k, Val{cache isa OrdinaryDiffEqMutableCache}, idxs, T)
+end
+
+function _ode_interpolant!(out, Θ, dt, y₀, y₁, k, cache, idxs, T::Type{Val{TI}}, differential_vars) where {TI}
     hermite_interpolant!(out, Θ, dt, y₀, y₁, k, idxs, T)
 end