part 1! Easy!

But oooooh, slow for part 2.

2021/ruby/day06.rb

@@ -0,0 +1,70 @@
+class Day06
+  def self.go
+    day = new
+    puts "Part 1: #{day.part1}"
+    puts "Part 2: #{day.part2}"
+  end
+
+  attr_reader :input
+
+  def initialize(input=nil)
+    @input = input || real_input
+  end
+
+  # @example 18 days
+  #   s = LanternFishSchool.new(EXAMPLE_INPUT)
+  #   18.times { s.a_day_passes }
+  #   s.school.count #=> 26
+  #
+  # @example 80 days
+  #   d = Day06.new(EXAMPLE_INPUT)
+  #   d.part1 #=> 5934
+  def part1
+    s = LanternFishSchool.new(input)
+    80.times { s.a_day_passes }
+    s.school.count
+  end
+
+  def part2
+  end
+
+  def real_input
+    File.read('../inputs/day06-input.txt')
+  end
+
+  EXAMPLE_INPUT = <<~INPUT
+    3,4,3,1,2
+  INPUT
+end
+
+class LanternFishSchool
+  attr_reader :size, :school
+
+  # @example
+  #   LanternFishSchool
+  #     .new('1,2,3')
+  #     .school #=> [1,2,3]
+  def initialize(list='')
+    @school = list.chomp.split(",").map(&:to_i)
+  end
+
+  # @example 1 day
+  #   LanternFishSchool
+  #     .new('3,4,3,1,2')
+  #     .a_day_passes
+  #     .school #=> [2,3,2,0,1]
+  # @example 2 days
+  #   LanternFishSchool
+  #     .new('3,4,3,1,2')
+  #     .a_day_passes
+  #     .a_day_passes
+  #     .school #=> [1,2,1,6,0,8]
+  #
+  def a_day_passes
+    new_fish = @school.count(0)
+    @school = @school.map { |fish_days|
+      fish_days == 0 ? 6 : fish_days - 1
+    }.append([8] * new_fish).flatten
+    self
+  end
+end