index_holded_list.rb

class IndexHoldedList < ::Hash
  def [](n)
    raise ArgumentError unless Integer === n
    self.fetch(n)
  end

  def []=(n, value)
    raise ArgumentError unless Integer === n
    self.store(n, value)
  end

  def each(&block)
    self.values.each(&block)
  end

  def remove(n)
    self[n] = nil
  end

  def prepend(val)
    if self.any? {|i| i.nil? }
      idx = self.find_index {|i| i.nil? }
      self[idx] = val
    else
      self[(self.keys.sort.last || -1) + 1] = val
    end
  end
end