qpromise.rb

require 'thread'

def QPromise(*args, &block)
  QPromise.new(*args, &block)
end

class QPromise < BasicObject

  # Allows you to change the defaults, e.g.:
  #   QPromise.config[:thread_limit] = 100
  # Changes to these settings will have an instant effect to all existing QPromises.
  @@config = {
    thread_limit: 10,
    wait_time: 0.1
  }
  cattr_accessor :config

  def initialize(*args, &block)
    @item = (::Thread.current[:qpromise_queue] ||= ::QPromise::Queue.new).push(args, block)
  end

  def method_missing(*args, &block)
    @item.value.send(*args, &block)
  end

  def respond_to?(method)
    @item.value.respond_to?(method)
  end

  def ==(obj)
    @item.value == obj
  end

  class Item
    def initialize(args, block, queue)
      @args = args
      @block = block
      @thread = nil
      @queue = queue
    end

    def run!
      @thread = ::Thread.new(*@args, &@block)
    end

    def run
      @queue.run(self)
    end

    def value
      run unless @thread
      @thread.value
    end

    def running?
      !@thread.nil?
    end

    def completed?
      @thread && @thread.status == false
    end
  end

  class Queue
    def initialize
      @work = []
      @current = 0
    end

    def push(*args, &block)
      item = ::QPromise::Item.new(args, block, self)
      @work << item
      check_queue!
      item
    end

    def check_queue!(run_new_items = true)
      sweep = []
      @work.each do |item|
        if item.completed?
          @current -= 1
          sweep << item
        end
        if run_new_items && !item.running? && @current < ::QPromise.config[:thread_limit]
          item.run!
          @current += 1
        end
      end
      sweep.each do |item|
        @work.delete(item)
      end
    end

    def run(item)
      loop do
        check_queue!(false)
        break if @current < ::QPromise.config[:thread_limit]
        sleep(::QPromise.config[:wait_time])
      end
      item.run!
      @current += 1
      if @current < @limit
        check_queue!
      end
    end
  end

end