diff --git a/2020/ruby/day_17.rb b/2020/ruby/day_17.rb
new file mode 100644
index 0000000..a60de7b
--- /dev/null
+++ b/2020/ruby/day_17.rb
@@ -0,0 +1,29 @@
+state = ARGF.read.split("\n").flat_map.with_index {|line,y|
+  line.chars.map.with_index {|c,x| [[0,0,y,x], c == "#"] }
+}.to_h
+
+NEIGHBORS = (0...3**4).map {|n| n.digits(3) }.map {|n| (0..3).map {|x| n.fetch(x, 0) - 1 }} - [[0,0,0,0]]
+
+def tick(state)
+  actives = state.select {|_,c| c }.keys
+  ww, zz, yy, xx = actives.transpose.map(&:minmax).map {|min,max| [min-1, max+1] }
+  Range.new(*ww).flat_map {|w|
+    Range.new(*zz).flat_map {|z|
+      Range.new(*yy).flat_map {|y|
+        Range.new(*xx).map {|x|
+          active_neighbors = NEIGHBORS.map {|dw,dz,dy,dx| state[[w+dw,z+dz,y+dy, x+dx]] }.count(&:itself)
+          wzyx = [w,z,y,x]
+          [ wzyx,
+            state[wzyx] ? (2..3).cover?(active_neighbors) : active_neighbors == 3
+          ]
+        }
+      }
+    }
+  }.to_h
+end
+
+6.times do
+  state = tick(state)
+end
+
+p state.values.count(&:itself)