Limit concurrency of async functions

concurrent.coffee

# Returns a function that invokes `fn`, with maximum concurrency of `max`
# Generalized version of https://gist.github.com/4071443
concurrent = (free, fn) ->
  queue = []
  r = (a..., cb) ->
    # Queue the call if we have too many running
    return queue.push [a..., cb] unless free
    --free
    fn a..., (resp...) ->
      ++free
      # Run the next queued request, if we have one
      r queue.shift()... if queue.length
      cb resp...

# Usage:

# To modify an existing function
limitedFoo = concurrent 15, foo
# And just use `limitedFoo()` like you would normally use `foo()`

# Or simply as a decorator
foo = concurrent 15, (cb) -> setTimeout cb, 1000