diff --git a/2015/README.md b/2015/README.md
index 643eb3d..5cd7c25 100644
--- a/2015/README.md
+++ b/2015/README.md
@@ -14,7 +14,7 @@
 | ✓      | 10  | Ruby
 | ✓      | 11  | Ruby
 | ✓      | 12  | Ruby
-|        | 13  |
+| ✓      | 13  | Ruby
 |        | 14  |
 |        | 15  |
 |        | 16  |
diff --git a/2015/day-13/inputs/puzzle.txt b/2015/day-13/inputs/puzzle.txt
new file mode 100644
index 0000000..a6f9c66
--- /dev/null
+++ b/2015/day-13/inputs/puzzle.txt
@@ -0,0 +1,56 @@
+Alice would gain 2 happiness units by sitting next to Bob.
+Alice would gain 26 happiness units by sitting next to Carol.
+Alice would lose 82 happiness units by sitting next to David.
+Alice would lose 75 happiness units by sitting next to Eric.
+Alice would gain 42 happiness units by sitting next to Frank.
+Alice would gain 38 happiness units by sitting next to George.
+Alice would gain 39 happiness units by sitting next to Mallory.
+Bob would gain 40 happiness units by sitting next to Alice.
+Bob would lose 61 happiness units by sitting next to Carol.
+Bob would lose 15 happiness units by sitting next to David.
+Bob would gain 63 happiness units by sitting next to Eric.
+Bob would gain 41 happiness units by sitting next to Frank.
+Bob would gain 30 happiness units by sitting next to George.
+Bob would gain 87 happiness units by sitting next to Mallory.
+Carol would lose 35 happiness units by sitting next to Alice.
+Carol would lose 99 happiness units by sitting next to Bob.
+Carol would lose 51 happiness units by sitting next to David.
+Carol would gain 95 happiness units by sitting next to Eric.
+Carol would gain 90 happiness units by sitting next to Frank.
+Carol would lose 16 happiness units by sitting next to George.
+Carol would gain 94 happiness units by sitting next to Mallory.
+David would gain 36 happiness units by sitting next to Alice.
+David would lose 18 happiness units by sitting next to Bob.
+David would lose 65 happiness units by sitting next to Carol.
+David would lose 18 happiness units by sitting next to Eric.
+David would lose 22 happiness units by sitting next to Frank.
+David would gain 2 happiness units by sitting next to George.
+David would gain 42 happiness units by sitting next to Mallory.
+Eric would lose 65 happiness units by sitting next to Alice.
+Eric would gain 24 happiness units by sitting next to Bob.
+Eric would gain 100 happiness units by sitting next to Carol.
+Eric would gain 51 happiness units by sitting next to David.
+Eric would gain 21 happiness units by sitting next to Frank.
+Eric would gain 55 happiness units by sitting next to George.
+Eric would lose 44 happiness units by sitting next to Mallory.
+Frank would lose 48 happiness units by sitting next to Alice.
+Frank would gain 91 happiness units by sitting next to Bob.
+Frank would gain 8 happiness units by sitting next to Carol.
+Frank would lose 66 happiness units by sitting next to David.
+Frank would gain 97 happiness units by sitting next to Eric.
+Frank would lose 9 happiness units by sitting next to George.
+Frank would lose 92 happiness units by sitting next to Mallory.
+George would lose 44 happiness units by sitting next to Alice.
+George would lose 25 happiness units by sitting next to Bob.
+George would gain 17 happiness units by sitting next to Carol.
+George would gain 92 happiness units by sitting next to David.
+George would lose 92 happiness units by sitting next to Eric.
+George would gain 18 happiness units by sitting next to Frank.
+George would gain 97 happiness units by sitting next to Mallory.
+Mallory would gain 92 happiness units by sitting next to Alice.
+Mallory would lose 96 happiness units by sitting next to Bob.
+Mallory would lose 51 happiness units by sitting next to Carol.
+Mallory would lose 81 happiness units by sitting next to David.
+Mallory would gain 31 happiness units by sitting next to Eric.
+Mallory would lose 73 happiness units by sitting next to Frank.
+Mallory would lose 89 happiness units by sitting next to George.
diff --git a/2015/day-13/inputs/test.txt b/2015/day-13/inputs/test.txt
new file mode 100644
index 0000000..b67c3c6
--- /dev/null
+++ b/2015/day-13/inputs/test.txt
@@ -0,0 +1,12 @@
+Alice would gain 54 happiness units by sitting next to Bob.
+Alice would lose 79 happiness units by sitting next to Carol.
+Alice would lose 2 happiness units by sitting next to David.
+Bob would gain 83 happiness units by sitting next to Alice.
+Bob would lose 7 happiness units by sitting next to Carol.
+Bob would lose 63 happiness units by sitting next to David.
+Carol would lose 62 happiness units by sitting next to Alice.
+Carol would gain 60 happiness units by sitting next to Bob.
+Carol would gain 55 happiness units by sitting next to David.
+David would gain 46 happiness units by sitting next to Alice.
+David would lose 7 happiness units by sitting next to Bob.
+David would gain 41 happiness units by sitting next to Carol.
diff --git a/2015/day-13/main.rb b/2015/day-13/main.rb
new file mode 100755
index 0000000..e8d62f0
--- /dev/null
+++ b/2015/day-13/main.rb
@@ -0,0 +1,65 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require "pathname"
+require "set"
+
+def parse_file(file)
+  file.each_line.each_with_object({}) do |line, people|
+    name, _, rest = line.chomp.partition(" would ")
+    change, neighbor = rest.split(" happiness units by sitting next to ")
+    change = change.split.then { |v| v.first == "gain" ?  v.last.to_i : -v.last.to_i }
+    people[name] ||= {}
+    people[name][neighbor.delete_suffix(".")] = change
+  end
+end
+
+def optimized_seating(people)
+  people.map do |start|
+    visited = Set.new
+    happiness = {}
+
+    name, neighbors = start
+    start_name = name
+
+    loop do
+      visited.add(name)
+      happiness[name] ||= 0
+
+      nb_name, hn_change = neighbors
+        .reject { |n, _| visited.include?(n) }
+        .max { |(an, ac), (bn, bc)| (ac + people[an][name]) <=> (bc + people[bn][name]) }
+
+      nb_name ||= start_name
+      hn_change ||= neighbors[start_name]
+
+      happiness[nb_name] ||= 0
+      happiness[name] += hn_change
+      happiness[nb_name] += people[nb_name][name]
+
+      name = nb_name
+      neighbors = people[name]
+
+      break if visited.size == people.size
+    end
+
+    happiness.values.sum
+  end
+end
+
+def main(file_paths)
+  file_paths.each do |file_path|
+    puts "File: #{file_path}"
+    people = parse_file(file_path)
+
+    solution1 = optimized_seating(people).max
+    puts "  solution 1: #{solution1}"
+
+    people["Self"] = people.each_with_object({}) { |(n, _), o| o[n] = 0 }
+    people.each { |(_, nb)| nb["Self"] = 0 }
+    solution2 = optimized_seating(people).max
+    puts "  solution 2: #{solution2}"
+  end
+end
+
+main(ARGV.map { |x| Pathname(x) })