diff --git a/ruby/day_18.rb b/ruby/day_18.rb
new file mode 100644
index 0000000..f5e58ef
--- /dev/null
+++ b/ruby/day_18.rb
@@ -0,0 +1,129 @@
+class Grid < Struct.new(:state)
+  def next
+    state_ = Array.new(100) {|x| Array.new(100) {|y|
+      light = self.state[x][y]
+      neighbors = self.neighbors(x,y).count(?#)
+      (light == ?#) ? (([2,3].include?(neighbors)) ? ?# : ?.) : ((neighbors == 3) ? ?# : ?. )
+    }}
+    Grid.new(state_)
+  end
+
+  def neighbors(x,y)
+    (-1..1).flat_map {|i| (-1..1).map {|j| [x+i, y+j] }}
+           .reject {|ij| ij == [x,y] }
+           .select {|i,j| (0..99).cover?(i) && (0..99).cover?(j) }
+           .map {|i,j| self.state[i][j] }
+  end
+
+  def to_s
+    self.state.map {|i| i.join }.join("\n")
+  end
+end
+
+grid = Grid.new(DATA.read.split("\n").map {|i| i.split(//) })
+100.times do
+  grid = grid.next
+end
+puts grid.state.map {|x| x.count(?#) }.inject(:+)
+
+__END__
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