diff --git a/2020/ruby/day_11.rb b/2020/ruby/day_11.rb
new file mode 100644
index 0000000..1328360
--- /dev/null
+++ b/2020/ruby/day_11.rb
@@ -0,0 +1,72 @@
+layout = ARGF.read.split("\n").map(&:chars)
+Y_MAX = layout.size
+X_MAX = layout.first.size
+
+# NEIGHBORS = (-1..1).flat_map {|y| (-1..1).map {|x| [y, x] }} - [[0, 0]]
+
+# def tick(layout)
+#   (0...Y_MAX).map {|y|
+#     (0...X_MAX).map {|x|
+#       occupied = NEIGHBORS
+#         .map {|dy,dx| [y+dy, x+dx] }
+#         .select {|y,x| (0...Y_MAX).cover?(y) && (0...X_MAX).cover?(x) }
+#         .select {|y,x| layout[y][x] == ?# }
+#         .count
+#       case layout[y][x]
+#       when ?.
+#         ?.
+#       when ?L
+#         occupied == 0 ? ?# : ?L
+#       when ?#
+#         occupied >= 4 ? ?L : ?#
+#       else
+#         fail
+#       end
+#     }
+#   }
+# end
+
+DIRS = (-1..1).flat_map {|y| (-1..1).map {|x| [y, x] }} - [[0, 0]]
+
+def tick(layout)
+  (0...Y_MAX).map {|y|
+    (0...X_MAX).map {|x|
+      next ?. if layout[y][x] == ?.
+      occupied = DIRS
+        .map {|dy,dx|
+          (1..)
+            .lazy
+            .map {|i| [dy*i+y, dx*i+x] }
+            .map {|y,x|
+              if (0...Y_MAX).cover?(y) && (0...X_MAX).cover?(x)
+                layout[y][x]
+              else
+                nil
+              end
+            }
+            .find {|s| s != ?. }
+        }
+        .select {|s| s == ?# }
+        .count
+      case occupied
+      when 0
+        ?#
+      when (5..)
+        ?L
+      else
+        layout[y][x]
+      end
+    }
+  }
+end
+
+loop do
+  next_layout = tick(layout)
+  break if next_layout == layout
+  layout = next_layout
+
+  puts layout.map(&:join).join("\n")
+  puts
+end
+
+p layout.map {|row| row.count(?#) }.sum