diff --git a/2021/ruby/day_20.rb b/2021/ruby/day_20.rb
new file mode 100644
index 0000000..0a1ea1c
--- /dev/null
+++ b/2021/ruby/day_20.rb
@@ -0,0 +1,50 @@
+algorithm, image = ARGF.read.strip.split("\n\n")
+
+algorithm = algorithm.delete("\n") # for the example
+algorithm = algorithm.chars.map.with_index { [_2, _1 == ?#] }.to_h
+
+image = image.split("\n").flat_map.with_index {|row, y|
+  row.chars.map.with_index {|pixel, x| [[y, x], pixel == ?#] }
+}.to_h
+image.default = false
+
+def each(image)
+  return enum_for(__method__, image) unless block_given?
+
+  ys = image.keys.map(&:first).minmax
+  xs = image.keys.map(&:last).minmax
+
+  (ys[0]-1..ys[1]+1).each do |y|
+    (xs[0]-1..xs[1]+1).each do |x|
+      yield [y, x]
+    end
+  end
+end
+
+def debug(image)
+  ys = image.keys.map(&:first).minmax
+  xs = image.keys.map(&:last).minmax
+
+  puts (ys[0]-1..ys[1]+1).map {|y|
+    (xs[0]-1..xs[1]+1).map {|x|
+      image[[y,x]] ? ?# : ?.
+    }.join
+  }.join("\n")
+end
+
+50.times {
+  ys = image.keys.map(&:first).minmax
+  xs = image.keys.map(&:last).minmax
+
+  prev_default = image.default
+  image = each(image).to_h {|y,x|
+    area = (-1..1).flat_map {|dy| (-1..1).map {|dx| image[[y+dy, x+dx]] }}
+    index = area.map { _1 ? 1 : 0 }.join.to_i(2)
+    pixel = algorithm.fetch(index)
+    [[y,x], pixel]
+  }
+
+  image.default = prev_default ? algorithm.fetch(511) : algorithm.fetch(0)
+}
+
+p image.count { _2 }