1 files changed, 45 insertions, 0 deletions
diff --git a/rb/day13.rb b/rb/day13.rb
new file mode 100755
index 0000000..3967d22
--- /dev/null
+++ b/rb/day13.rb
@@ -0,0 +1,45 @@
+#!/usr/bin/env ruby
+
+deltas = {}
+whos = []
+
+File.readlines("day13.input").map(&:strip).each do |line|
+  who, dir, delta, neighbour = line.scan(/(\w+) would (gain|lose) (\d+) happiness units by sitting next to (\w+)\./)[0]
+  delta = delta.to_i
+  delta = -delta if dir == "lose"
+  deltas[who] = {} if deltas[who].nil?
+  deltas[who][neighbour] = delta
+  whos << who
+end
+
+whos.uniq!
+num = whos.length
+
+change = whos.permutation.map { |perm|
+  num.times.map { |i|
+    left = (i - 1) % num
+    right = (i + 1) % num
+    [deltas[perm[i]][perm[left]], deltas[perm[i]][perm[right]]]
+  }.flatten.sum
+}.max
+
+puts "Part 1: #{change}"
+
+deltas["me"] = {}
+whos.each do |who|
+  deltas["me"][who] = 0
+  deltas[who]["me"] = 0
+end
+
+whos << "me"
+num += 1
+
+change = whos.permutation.map { |perm|
+  num.times.map { |i|
+    left = (i - 1) % num
+    right = (i + 1) % num
+    [deltas[perm[i]][perm[left]], deltas[perm[i]][perm[right]]]
+  }.flatten.sum
+}.max
+
+puts "Part 2: #{change}"