http://codereview.stackexchange.com/questions/19608/functional-sundarams-sieve

sieve_sundaram

sieve_sundaram = (limit) ->

  start = 4
  half = limit/2

  numbers = (n for n in [3...limit+1] by 2)
  steps = numbers[..] # copy of numbers

  callback = (start_and_nums,step) ->

    start = start_and_nums[0]
    nums = start_and_nums[1..]

    if start > half
      result = (n for n in nums when n > 0)
      result.unshift start 
      return result        

      # We can't stop iterating inside the reduce() although we'd like to right now.
      # Also, we slice the first element every time our function gets called,
      # so we'll put it back here for now. We'll deal with it after reduce() finishes.
      # Also, we mustn't forget about the 2!

    idxs = (idx-1 for idx in [start...half] by step)
    new_nums = ((if i in idxs then 0 else num) for num,i in nums)
    new_start = start + 2 * (step + 1)

    new_start_and_nums = new_nums[..]
    new_start_and_nums.unshift new_start
    return new_start_and_nums
  
  start_and_nums = numbers[..]
  start_and_nums.unshift start
  result = (steps.reduce(callback,start_and_nums))
  result.shift()
  result.unshift 2
  return result