diff --git a/2015/README.md b/2015/README.md
index a2b28cf..643eb3d 100644
--- a/2015/README.md
+++ b/2015/README.md
@@ -12,7 +12,7 @@
 | ✓      | 08  | Ruby
 | ✓      | 09  | Rust
 | ✓      | 10  | Ruby
-|        | 11  |
+| ✓      | 11  | Ruby
 | ✓      | 12  | Ruby
 |        | 13  |
 |        | 14  |
diff --git a/2015/day-11/inputs/puzzle.txt b/2015/day-11/inputs/puzzle.txt
new file mode 100644
index 0000000..5915be7
--- /dev/null
+++ b/2015/day-11/inputs/puzzle.txt
@@ -0,0 +1 @@
+cqjxjnds
diff --git a/2015/day-11/inputs/test.txt b/2015/day-11/inputs/test.txt
new file mode 100644
index 0000000..7a31c56
--- /dev/null
+++ b/2015/day-11/inputs/test.txt
@@ -0,0 +1,5 @@
+hijklmmn
+abbceffg
+abbcegjk
+abcdffaa
+ghjaabcc
diff --git a/2015/day-11/main.rb b/2015/day-11/main.rb
new file mode 100755
index 0000000..3221c53
--- /dev/null
+++ b/2015/day-11/main.rb
@@ -0,0 +1,71 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require "pathname"
+require "set"
+
+ValidationFailed = Class.new(StandardError)
+
+class LengthInvalid < ValidationFailed
+  def initialize(pwd) = super("#{pwd} length is not 8")
+end
+
+class NoStraightFound < ValidationFailed
+  def initialize(pwd) = super("#{pwd} doesn't include an increasing straight")
+end
+
+class ForbiddenLetter < ValidationFailed
+  def initialize(pwd) = super("#{pwd} has forbidden letters 'i', 'o', or 'l'")
+end
+
+class NotEnoughPairs < ValidationFailed
+  def initialize(pwd) = super("#{pwd} doesn't include at least two different pairs")
+end
+
+def validate(input)
+  raise LengthInvalid, input unless input.size == 8
+
+  result = input
+    .each_char
+    .chain([nil])
+    .each_cons(3)
+    .each_with_object(straight: false, pairs: Set.new) do |(a, b, c), res|
+      raise ForbiddenLetter, input unless ([a, b, c] & %w[i o l]).empty?
+      res[:straight] ||= (b == a.succ && c == b.succ)
+      res[:pairs].add(a) if (a == b && b != c)
+    end
+
+  raise NoStraightFound, input unless result[:straight]
+  raise NotEnoughPairs, input unless result[:pairs].size >= 2
+
+  input
+end
+
+def next_valid_password(input)
+  validate(input)
+rescue LengthInvalid
+  nil
+rescue ForbiddenLetter => e
+  idx = input.index(/[iol]/)
+  input = input[0..idx].succ + input[(idx+1)..]
+  retry
+rescue ValidationFailed
+  input = input.succ
+  retry
+end
+
+def main(file_paths)
+  file_paths.each do |file_path|
+    puts "File: #{file_path}"
+
+    file_path.each_line do |line|
+      line = line.chomp
+      solution1 = next_valid_password(line)
+      puts "solution 1: #{line} -> #{solution1}"
+      solution2 = next_valid_password(solution1.succ)
+      puts "solution 2: #{solution1} -> #{solution2}"
+    end
+  end
+end
+
+main(ARGV.map { |x| Pathname(x) })