Increasing number of batches causes an error in map_pairwise!

[efaulhaber]

Say we have a cell list.

x = zeros(3, 1)
search_radius = 0.1
box = Box(limits(x), search_radius)
cell_list = CellList(x, box)

Then we get some more particles and update our cell list accordingly:

x2 = rand(3, 100)
box = Box(limits(x2), search_radius)
UpdateCellList!(x2, box, cell_list)

The cell list is now using only a single thread if we used too few particles at the beginning:

julia> cell_list.nbatches
  Number of batches for cell list construction: 1
  Number of batches for function mapping: 1

With #87 fixed, we can set the number of batches as follows.

CellListMap.set_number_of_batches!(cell_list)

But now, we get an error.

julia> map_pairwise!(0, box, cell_list, parallel=true) do x, y, i, j, d2, output
           return output
       end
ERROR: TaskFailedException

    nested task error: BoundsError: attempt to access 0-element Vector{CellListMap.ProjectedParticle{3, Float64}} at index [1]
    Stacktrace:
     [1] throw_boundserror(A::Vector{CellListMap.ProjectedParticle{3, Float64}}, I::Tuple{Int64})
       @ Base ./essentials.jl:14
     [2] setindex!
       @ ./array.jl:975 [inlined]
     [3] project_particles!(projected_particles::Vector{…}, cellⱼ::CellListMap.Cell{…}, cellᵢ::CellListMap.Cell{…}, Δc::SVector{…}, Δc_norm::Float64, box::Box{…})
       @ CellListMap ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:357
     [4] _vicinal_cell_interactions!(f::var"#249#250", box::Box{…}, cellᵢ::CellListMap.Cell{…}, cellⱼ::CellListMap.Cell{…}, cl::CellList{…}, output::Int64, ibatch::Int64)
       @ CellListMap ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:272
     [5] inner_loop!(f::var"#249#250", _neighbor_cells::typeof(CellListMap.neighbor_cells_forward), box::Box{…}, cellᵢ::CellListMap.Cell{…}, cl::CellList{…}, output::Int64, ibatch::Int64)
       @ CellListMap ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:220
     [6] inner_loop!
       @ ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:200 [inlined]
     [7] batch(f::var"#249#250", ibatch::Int64, nbatches::Int64, n_cells_with_real_particles::Int64, output_threaded::Vector{…}, box::Box{…}, cl::CellList{…}, p::Nothing)
       @ CellListMap ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:124
     [8] (::CellListMap.var"#111#113"{var"#249#250", Int64, Int64, Int64, Vector{…}, Box{…}, CellList{…}, Nothing})()
       @ CellListMap ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:147
    Some type information was truncated. Use `show(err)` to see complete types.

...and 4 more exceptions.

Stacktrace:
 [1] sync_end(c::Channel{Any})
   @ Base ./task.jl:466
 [2] macro expansion
   @ ./task.jl:499 [inlined]
 [3] map_pairwise_parallel!(f::var"#249#250", output::Int64, box::Box{…}, cl::CellList{…}; output_threaded::Nothing, reduce::typeof(CellListMap.reduce), show_progress::Bool)
   @ CellListMap ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:146
 [4] map_pairwise_parallel!
   @ ~/.julia/packages/CellListMap/0BWXe/src/CoreComputing.jl:133 [inlined]
 [5] #map_pairwise!#124
   @ ~/.julia/packages/CellListMap/0BWXe/src/CellListMap.jl:112 [inlined]
 [6] top-level scope
   @ REPL[96]:1
Some type information was truncated. Use `show(err)` to see complete types.

[efaulhaber]

Okay, I see that the update uses the number of batches. So one cannot just change the number of batches without updating afterward.

[lmiq]

Thanks for reporting. I have seen your PR, thanks. I'm thinking about the best interface to this, maybe I'll just update the number of batches by default except if they are set manually.

Anyway, since this can introduce correctness issues, I'll only release after very careful testing.