Base.length dispatch for ArrayPartition (StaticArrays)

[dehann]

This only works for partition elements that themselves have a length-on-type function, e.g. SVector or SMatrix.

I'm trying to use this line of code from NearestNeighbors.jl (issue on the call length(::Type{ArrayPartition...}}):
https://github.com/KristofferC/NearestNeighbors.jl/blob/fb84d15b96a2d7f94f691db4ff6f42e994194c93/src/tree_data.jl#L14

MWE I'm working to fix:

using Manifolds, NearestNeighbors, Distances, StaticArrays
import Manifolds: ArrayPartition

Base.@kwdef struct DistSE2 <: Distances.Metric
  M = SpecialEuclidean(2)
end 
(d::DistSE2)(p,q) = Manifolds.ManifoldsBase.distance(d.M,p,q)

pts = [ArrayPartition(SA[randn(2)...], SMatrix{2,2}([1 0; 0 1.])) for _ in 1:10]
dSE2 = DistSE2()

bt = NearestNeighbors.BallTree(pts, dSE2)

# adding this PR corrects the error
# Base.length(::Type{<:ArrayPartition{F,T}}) where {F,T <: Tuple} = T.parameters .|> length |> sum

---

julia> bt = NearestNeighbors.BallTree(pts, dSE2)
ERROR: MethodError: no method matching length(::Type{ArrayPartition{Float64, Tuple{SVector{2, Float64}, SMatrix{2, 2, Float64, 4}}}})
Closest candidates are:
  length(::Union{Base.KeySet, Base.ValueIterator}) at /usr/local/share/julia-1.7.3/share/julia/base/abstractdict.jl:58
  length(::Union{DataStructures.OrderedRobinDict, DataStructures.RobinDict}) at ~/.julia/packages/DataStructures/59MD0/src/ordered_robin_dict.jl:86
  length(::Union{DataStructures.SortedDict, DataStructures.SortedMultiDict, DataStructures.SortedSet}) at ~/.julia/packages/DataStructures/59MD0/src/container_loops.jl:322
  ...
Stacktrace:
 [1] NearestNeighbors.TreeData(data::Vector{ArrayPartition{Float64, Tuple{SVector{2, Float64}, SMatrix{2, 2, Float64, 4}}}}, leafsize::Int64)
   @ NearestNeighbors ~/.julia/packages/NearestNeighbors/VZzTb/src/tree_data.jl:14
 [2] BallTree(data::Vector{ArrayPartition{Float64, Tuple{SVector{2, Float64}, SMatrix{2, 2, Float64, 4}}}}, metric::DistSE2; leafsize::Int64, reorder::Bool, storedata::Bool, reorderbuffer::Vector{ArrayPartition{Float64, Tuple{SVector{2, Float64}, SMatrix{2, 2, Float64, 4}}}})
   @ NearestNeighbors ~/.julia/packages/NearestNeighbors/VZzTb/src/ball_tree.jl:39
 [3] BallTree(data::Vector{ArrayPartition{Float64, Tuple{SVector{2, Float64}, SMatrix{2, 2, Float64, 4}}}}, metric::DistSE2)
   @ NearestNeighbors ~/.julia/packages/NearestNeighbors/VZzTb/src/ball_tree.jl:37
 [4] top-level scope
   @ REPL[11]:1

Currently there are further errors that I'm working separately.

---

xref: Using Manifolds.jl objects represented as ArrayPartition,

• Ball/Metric Tree using NearestNeighbors.jl / Manifolds.jl / Distances.jl / CoordinateTransformations.jl JuliaRobotics/ApproxManifoldProducts.jl#41 (comment)

[dehann]

cc @mateuszbaran , @Affie

[mateuszbaran]

Yes, that seems necessary for NearestNeighbors.jl. Here is a test that can be added for that method:

p3 = ArrayPartition(SA[1.0, 2.0], MMatrix{2,2}([3.0 4.0; 3.0 5.0]))
@test length(typeof(p3)) == 6

[dehann]

Thanks, I added the test too!

[dehann]

I included the suggestions also thanks

[mateuszbaran]

You're welcome 🙂 .