diff --git a/2016/ruby/day_04.rb b/2016/ruby/day_04.rb
new file mode 100644
index 0000000..f11a02b
--- /dev/null
+++ b/2016/ruby/day_04.rb
@@ -0,0 +1,48 @@
+class Room
+  attr_reader :encrypted_name, :sector_id
+
+  def initialize(encrypted_name, sector_id)
+    @encrypted_name, @sector_id = encrypted_name, sector_id
+  end
+
+  def checksum
+    encrypted_name
+      .scan(/[a-z]/)
+      .group_by {|x| x }
+      .map {|k,v| [k, v.size] }
+      .sort {|a,b|
+        (a.last == b.last) ? a.first <=> b.first : b.last <=> a.last
+      }
+      .take(5)
+      .map(&:first)
+      .join
+  end
+end
+
+if __FILE__ == $0
+  DATA.read.lines.map {|line|
+    /(?<name>.+)-(?<sector_id>\d+)\[(?<checksum>[a-z]+)\]/ =~ line
+    room = Room.new(name, sector_id.to_i)
+    [room, checksum]
+  }.select {|room, checksum|
+    room.checksum == checksum
+  }.inject(0) {|n, (room, _)|
+    n += room.sector_id
+  }.tap {|count|
+    puts count
+  }
+end
+
+require 'minitest'
+# require 'minitest/autorun'
+
+class TestDay04 < Minitest::Test
+  def test_day_04
+    assert_equal 'abxyz', Room.new('aaaaa-bbb-z-y-x', 123).checksum
+    assert_equal 'abcde', Room.new('a-b-c-d-e-f-g-h', 987).checksum
+    assert_equal 'oarel', Room.new('not-a-real-room', 404).checksum
+    refute_equal 'decoy', Room.new('totally-real-room', 200).checksum
+  end
+end
+
+__END__