diff --git a/2021/ruby/day_10.rb b/2021/ruby/day_10.rb
new file mode 100644
index 0000000..bd1e075
--- /dev/null
+++ b/2021/ruby/day_10.rb
@@ -0,0 +1,74 @@
+require "strscan"
+
+input = ARGF.read.split("\n")
+
+class IllegalCharacter < StandardError
+  attr_reader :char
+  def initialize(char)
+    @char = char
+  end
+end
+
+def parse(line)
+  ss = StringScanner.new(line)
+  stack = []
+
+  until ss.eos?
+    case
+    when ss.scan(/\(/)
+      stack << ?(
+    when ss.scan(/\[/)
+      stack << ?[
+    when ss.scan(/\{/)
+      stack << ?{
+    when ss.scan(/</)
+      stack << ?<
+    when ss.scan(/\)/)
+      raise IllegalCharacter.new(?)) unless stack.pop == ?(
+    when ss.scan(/\]/)
+      raise IllegalCharacter.new(?]) unless stack.pop == ?[
+    when ss.scan(/\}/)
+      raise IllegalCharacter.new(?}) unless stack.pop == ?{
+    when ss.scan(/>/)
+      raise IllegalCharacter.new(?>) unless stack.pop == ?<
+    else
+      fail
+    end
+  end
+
+  stack
+end
+
+# points = {
+#   ?) => 3,
+#   ?] => 57,
+#   ?} => 1197,
+#   ?> => 25137,
+# }
+
+# p input.sum {|line|
+#   begin
+#     parse(line)
+#     0
+#   rescue IllegalCharacter => e
+#     points.fetch(e.char)
+#   end
+# }
+
+POINTS = ")]}>".chars.map.with_index.to_h.transform_values { _1 + 1 }
+def score(closing)
+  total = 0
+  closing.each do |char|
+    total *= 5
+    total += POINTS.fetch(char)
+  end
+  total
+end
+
+complements = "() [] {} <>".split(" ").map(&:chars).to_h
+scores = input
+  .map { parse(_1) rescue [] }
+  .reject(&:empty?)
+  .map {|remaining| remaining.reverse.map { complements.fetch(_1) }}
+  .map { score(_1) }
+p scores.sort.fetch(scores.length / 2)