[feature] finish_in_order - process finish hook in the original order about parallel

[grosser]

the results that Parallel.map returns are sorted, so you could do:

out = Parallel.each_with_index(ITEMS, in_threads: 6) { |item, index| "my output" }
puts out

capturing stdout would not work since that relies on replacing $stdout and that would not be thread local
if you want to get super hacky you could replace $stdout with a thread-aware StringIO that stores each threads output separately and then returns prints it in order when all processes have finished

[shaicoleman]

That doesn't do the same thing as the example I've provided.
I'm interested in the ability to have a callback to print things (or save to disk, etc.) in order as they finish executing, not at the end

[grosser]

if you want to print as things are finishing you can use a finish hook docs

in that case it would just print the result

[shaicoleman]

The finish hook with the example from the docs will be out of order.
Parallel.map('A'..'Z', in_threads: 6, finish: -> (item, i, result) { puts item }) { sleep rand }

I want the finish hook to be triggered in order. Try to run the example I gave if I'm unclear

[grosser]

require 'parallel'

want = 0
stack = []
finish = -> (item, i, result) do
  if i == want
    puts item # print current
    want += 1
    
    # print things already ready
    stack[(i + 1)...].to_a.each do |old|
      break unless old
      puts old
      want += 1
    end
  else
    stack[i] = item # store for later
  end
end

Parallel.map('A'..'Z', in_threads: 6, finish: finish) { sleep rand }

[shaicoleman]

Yes, that's a more elegant and efficient solution.

I'm still hoping this would be built as a reusable solution into the parallel gem

[grosser]

PR welcome :)

finish_in_order: true or so 🤷