class Day10
  def initialize(filename)
    @filename = filename
  end

  def solve
    file = File.open(@filename)
    data = file.readline(chomp: true)
    file.close

    presents = data.split(' ').map(&:to_i).sort

    median = presents.size.odd? ? presents[(presents.size / 2)] : presents[(presents.size / 2) - 1, 2].sum / 2.0
    first_median_last = [presents.first, median, presents.last]

    missing = []
    last = nil
    presents.each do |present|
      missing.push(*((last + 1)..(present - 1))) if !last.nil? && present > last + 1
      last = present
    end

    print "#{first_median_last}\n"
    print "#{missing}\n"
  end
end

# Expects the input file to be named 10.txt and in the current working directory
if __FILE__ == $PROGRAM_NAME
  challenge = Day10.new('10.txt')
  challenge.solve
end

Christmas Eve is all about efficiency. In order for Santa to be able to deliver so many presents, Elves put a lot of preliminary work into mapping efficient routes and having the right presents available at the right time. Presents are placed into a specific order that is indicated by a removeable number label. These numbers are placed into the bag in order, from the largest number being deepest in the bag, to smallest number being at the top of the bag. Unfortunately, one of the bags of presents was dropped while being moved to the Present Staging Area.

Problem: In order to get things going again, we need to get all of the presents in presents.txt back into order, as well as make sure that no presents are missing! Luckily, the last several presents were unaffected (as the bag was tipped over, not dumped out). Return 2 lists. The first contains the first number (always 0), the median (middle number), and the last number. The second list contains all of the presents, if any, that are missing (sorted).

Example:

[0,1,3,5,7,9,2,4,8,9]
0 1 2 3 4 5 7 8 9

Should return

[0,4,9]
[6]