Day6a.rb

def advance(fish)
  new_fish = Hash.new(0)
  fish.each do |timer, count|
    if timer > 0
      new_fish[timer - 1] += count
    elsif timer == 0
      new_fish[6] += count
      new_fish[8] += count
    else
      raise "wtf?"
    end
  end
  new_fish
end

count_by_timer = Hash.new(0)

gets.chomp.split(',').map(&:to_i).each do |n|
  count_by_timer[n] += 1
end

80.times do
  count_by_timer = advance count_by_timer
end

print count_by_timer.sum {|_, count| count}