first working prototype

src/dense/generic_dense.jl

@@ -315,6 +315,22 @@ function evaluate_interpolant(f, Θ, dt, timeseries, i₋, i₊, cache, idxs,
     end
 end
 
+function get_differential_vars(f, len)
+    differential_vars = trues(len)
+    if hasproperty(f, :mass_matrix)
+        mm = f.mass_matrix
+        if mm isa UniformScaling
+            # already correct
+        elseif isdiag(mm)
+            differential_vars = Diagonal(mm).diag .!= 0
+        else
+            @show typeof(mm)
+            error("QR factorizations is annoying")
+        end
+    end
+    @show differential_vars
+end
+
 """
 ode_interpolation(tvals,ts,timeseries,ks)
 
@@ -326,6 +342,7 @@ function ode_interpolation(tvals, id::I, idxs, deriv::D, p,
     @unpack ts, timeseries, ks, f, cache = id
     @inbounds tdir = sign(ts[end] - ts[1])
     idx = sortperm(tvals, rev = tdir < 0)
+    differential_vars = get_differential_vars(f, length(timeseries[begin]))
     # start the search thinking it's ts[1]-ts[2]
     i₋₊ref = Ref((1, 2))
     vals = map(idx) do j
@@ -363,6 +380,7 @@ function ode_interpolation!(vals, tvals, id::I, idxs, deriv::D, p,
     @unpack ts, timeseries, ks, f, cache = id
     @inbounds tdir = sign(ts[end] - ts[1])
     idx = sortperm(tvals, rev = tdir < 0)
+    differential_vars = get_differential_vars(f, length(timeseries[begin]))
 
     # start the search thinking it's in ts[1]-ts[2]
     i₋ = 1
@@ -448,20 +466,7 @@ function ode_interpolation(tval::Number, id::I, idxs, deriv::D, p,
     @unpack ts, timeseries, ks, f, cache = id
     @inbounds tdir = sign(ts[end] - ts[1])
 
-    differential_vars = trues(length(timeseries[begin]))
-    if hasproperty(f, :mass_matrix)
-        mm = f.mass_matrix
-        if mm isa UniformScaling
-            # already correct
-        elseif isdiag(mm)
-            differential_vars = Diagonal(mm).diag .!= 0
-        else
-            @show typeof(mm)
-            error("QR factorizations is annoying")
-        end
-    end
-    @show differential_vars
-
+    differential_vars = get_differential_vars(f, length(timeseries[begin]))
 
     if continuity === :left
         # we have i₋ = i₊ = 1 if tval = ts[1], i₊ = i₋ + 1 = lastindex(ts) if tval > ts[end],
@@ -511,20 +516,7 @@ function ode_interpolation!(out, tval::Number, id::I, idxs, deriv::D, p,
     @unpack ts, timeseries, ks, f, cache = id
     @inbounds tdir = sign(ts[end] - ts[1])
 
-    differential_vars = trues(length(timeseries[begin]))
-    if hasproperty(f, :mass_matrix)
-        mm = f.mass_matrix
-        if mm isa UniformScaling
-            # already correct
-        elseif isdiag(mm)
-            differential_vars = Diagonal(mm).diag .!= 0
-        else
-            # @show typeof(mm)
-            error("QR factorizations is annoying")
-        end
-    end
-    @show "!" differential_vars
-
+    differential_vars = get_differential_vars(f, length(timeseries[begin]))
 
     if continuity === :left
         # we have i₋ = i₊ = 1 if tval = ts[1], i₊ = i₋ + 1 = lastindex(ts) if tval > ts[end],
@@ -634,24 +626,42 @@ 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
+    lin = linear_interpolant(Θ, dt, y₀, y₁, linidxs, T)
+    res = similar(y₀, eltype(y₀), size(sel))
+    res[sel] = diff
+    res[.!sel] = lin
     res
 end
 
+function _ode_interpolant(Θ, dt, y₀, y₁, k, cache, idxs::Nothing, T::Type{Val{TI}}, differential_vars) where {TI}
+    _ode_interpolant(Θ, dt, y₀, y₁, k, cache, eachindex(y₀), T, differential_vars)
+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::AbstractArray, Θ, dt, y₀, y₁, k, cache, idxs::AbstractArray, T::Type{Val{TI}}, differential_vars) where {TI}
+    sel = differential_vars[idxs]
+    diffidxs = idxs[sel]
+    linidxs = idxs[.!sel]
+    hermite_interpolant!(@view(out[sel]), Θ, dt, y₀, y₁, k, diffidxs, T)
+    linear_interpolant!(@view(out[.!sel]), Θ, dt, y₀, y₁, linidxs, T)
+    out
+end
+
+function _ode_interpolant!(out, Θ, dt, y₀, y₁, k, cache, idxs::Nothing, T::Type{Val{TI}}, differential_vars) where {TI}
+    _ode_interpolant!(out, Θ, dt, y₀, y₁, k, cache, eachindex(y₀), T, differential_vars)
+end
+
 function _ode_interpolant!(out, Θ, dt, y₀, y₁, k, cache, idxs, T::Type{Val{TI}}, differential_vars) where {TI}
+    @assert all(differential_vars)
+    @show differential_vars
     hermite_interpolant!(out, Θ, dt, y₀, y₁, k, idxs, T)
 end