diff --git a/2017/ruby/day_13.rb b/2017/ruby/day_13.rb
new file mode 100644
index 0000000..6ec1ff7
--- /dev/null
+++ b/2017/ruby/day_13.rb
@@ -0,0 +1,62 @@
+input = ARGF.read.strip
+# input = <<INPUT
+# 0: 3
+# 1: 2
+# 4: 4
+# 6: 4
+# INPUT
+@input = Hash[input.scan(/(\d+): (\d+)/).map {|x| x.map(&:to_i) }]
+
+class Firewall
+  def initialize(input)
+    @layers = input
+    @scanners = Hash[input.map {|d,_| [d, 0] }]
+    @deltas = Hash[input.map {|d,_| [d, 1] }]
+  end
+
+  def each
+    return enum_for(__method__) unless block_given?
+
+    loop do
+      yield @scanners
+
+      @layers.each do |depth, range|
+        @scanners[depth] += @deltas[depth]
+        @deltas[depth] = case @scanners[depth]
+                         when 0
+                           1
+                         when range - 1
+                           -1
+                         else
+                           @deltas[depth]
+                         end
+      end
+    end
+  end
+
+end
+
+def pass?(delay)
+  f = Firewall.new(@input).each
+  delay.times do
+    f.next
+  end
+
+  severity = 0
+  (0..@input.map(&:first).max).each do |pos|
+    scanners = f.next
+    return false if scanners[pos] == 0
+  end
+  true
+end
+
+def s(t, r)
+  r - (t % (2*r) - r).abs
+end
+
+(0..Float::INFINITY).each do |delay|
+  if @input.all? {|d,r| s(d + delay, r-1) != 0 }
+    p delay
+    exit
+  end
+end