optimize get_ith (#286)

Co-authored-by: Pietro Vertechi <[email protected]>

Author: aplavin

src/structarray.jl

@@ -338,12 +338,7 @@ to
 map(c -> c[I...], Tuple(cols))
 ```
 """
-@inline get_ith(cols::NamedTuple, I...) = get_ith(Tuple(cols), I...)
-@inline function get_ith(cols::Tuple, I...)
-    @inbounds r = first(cols)[I...]
-    return (r, get_ith(Base.tail(cols), I...)...)
-end
-@inline get_ith(::Tuple{}, I...) = ()
+@inline @generated get_ith(cols::Tup, I...) = :(Base.Cartesian.@ntuple $(fieldcount(cols)) i -> @inbounds cols[i][I...])
 
 Base.@propagate_inbounds Base.getindex(x::StructArray, I...) = _getindex(x, to_indices(x, I)...)