Limited Queue based on suggestions from http://stereopsis.com/async/

LimitedQueue.coffee

# On user event -> push event into a n-element queue.
# If the number of outstanding requests is <n, fire request immediately.
# When an outstanding request works or fails, fire a pending queued request if one is waiting.

EventEmitter = process.EventEmitter or require?('events').EventEmitter or {}

LimitedQueue = (@n = 2, @log = false) ->
    @log = console.log if @log is true and console? and console.log
    @q = []
    @req = []
    this

LimitedQueue.prototype = new EventEmitter()

# On user event -> push event into a n-element queue.
LimitedQueue::add = (fn) ->
    # If the number of outstanding requests is <n, fire request immediately.
    if  @req.length < @n
        @req.push fn
        done = @done.bind(this)
        # call done when fn finishes
        fn(done,done)
        @emit?("start", arguments)
    else 
        @q.push fn
    this

# When an outstanding request works or fails, fire a pending queued request if one is waiting.
LimitedQueue::done = (err, success) ->
    @req.pop()
    @emit?("done", @len())
    @fire()

LimitedQueue::fire = ->
    next = @q.shift()
    @add next if next
    if  @empty()
        @log "Queue empty" if @log
        @emit?("alldone")

LimitedQueue::empty = ->
    return 0 is @len()

# Some IE browsers do not support Function::bind
LimitedQueue::done.bind ?= (obj, args...) -> 
    return -> LimitedQueue::done.call(obj, args); 

LimitedQueue::len = -> 
    @q.length + @req.length

module.exports = LimitedQueue if module?