Add fast path for forwarddiff_color_jacobian! with sparse J

Partial fix for #138

Bodges in a fast path for the case where J and sparsity are are both SparseMatrixCSC
and have the same number of columns and stored values.

Author: Stuart Daines

src/differentiation/compute_jacobian_ad.jl

@@ -264,8 +264,20 @@ function forwarddiff_color_jacobian!(J::AbstractMatrix{<:Number},
     chunksize = jac_cache.chunksize
     color_i = 1
     maxcolor = maximum(colorvec)
+    
     fill!(J, zero(eltype(J)))
 
+    # fast path if J and sparsity are both SparseMatrixCSC and have the same number of columns and stored values
+    sparseCSC_common = (J isa SparseMatrixCSC && 
+                        sparsity isa SparseMatrixCSC && 
+                        length(J.colptr) == length(sparsity.colptr) &&
+                        length(J.nzval) == length(sparsity.nzval))
+
+    if sparseCSC_common
+        J.colptr .= sparsity.colptr
+        J.rowval .= sparsity.rowval
+    end
+
     if FiniteDiff._use_findstructralnz(sparsity)
         rows_index, cols_index = ArrayInterface.findstructralnz(sparsity)
     else
@@ -287,9 +299,14 @@ function forwarddiff_color_jacobian!(J::AbstractMatrix{<:Number},
         if !(sparsity isa Nothing)
             for j in 1:chunksize
                 dx .= partials.(fx, j)
+
                 if ArrayInterface.fast_scalar_indexing(dx)
                     #dx is implicitly used in vecdx
-                    FiniteDiff._colorediteration!(J,sparsity,rows_index,cols_index,vecdx,colorvec,color_i,ncols)
+                    if sparseCSC_common
+                        _colorediteration!(J,vecdx,colorvec,color_i,ncols)
+                    else
+                        FiniteDiff._colorediteration!(J,sparsity,rows_index,cols_index,vecdx,colorvec,color_i,ncols)
+                    end
                 else
                     #=
                     J.nzval[rows_index] .+= (colorvec[cols_index] .== color_i) .* dx[rows_index]
@@ -316,3 +333,17 @@ function forwarddiff_color_jacobian!(J::AbstractMatrix{<:Number},
     end
     return J
 end
+
+# fast version of FiniteDiff._colorediteration! for the case where J and sparsity have the same sparsity pattern
+@inline function _colorediteration!(Jsparsity::SparseMatrixCSC,vfx,colorvec,color_i,ncols)
+    @inbounds for col_index in 1:ncols
+        if colorvec[col_index] == color_i
+            @inbounds for spidx in nzrange(Jsparsity, col_index)
+                row_index = Jsparsity.rowval[spidx]
+                Jsparsity.nzval[spidx]=vfx[row_index]
+            end
+        end
+    end
+end
+
+