group.rb

class Array
  def group p
    arr = self
    res = []
    start = 0
    last = 0
    while start + p <= arr.length
      last = start + p -1 
      res.push(arr[start..last])
      start = last + 1
    end
    res
  end
end

print [1,2,3,4,5,6].group(2)