1 files changed, 78 insertions, 0 deletions
diff --git a/day21/part2.rb b/day21/part2.rb
new file mode 100644
index 0000000..ab88699
--- /dev/null
+++ b/day21/part2.rb
@@ -0,0 +1,78 @@
+def mutate(eqn, tgt)
+  if eqn[6..9] == tgt
+    op = case eqn[11]
+    when ?+; ?-
+    when ?-; ?+
+    when ?*; ?/
+    when ?/; ?*
+    end
+    "#{eqn[6..9]}: #{eqn[0..3]} #{op} #{eqn[13..16]}"
+  else
+    case eqn[11]
+    when ?+
+      "#{eqn[13..16]}: #{eqn[0..3]} - #{eqn[6..9]}"
+    when ?-
+      "#{eqn[13..16]}: #{eqn[6..9]} - #{eqn[0..3]}"
+    when ?*
+      "#{eqn[13..16]}: #{eqn[0..3]} / #{eqn[6..9]}"
+    when ?/
+      "#{eqn[13..16]}: #{eqn[6..9]} / #{eqn[0..3]}"
+    end
+  end
+end
+
+monkeys = $stdin.readlines.reject { |l| l[0...4] == "humn" }
+
+prev = nil
+need = "humn"
+loop do
+  cand = monkeys.
+          select { |m| m.include?(need) }.
+          reject { |m| m[0...4] == prev }[0]
+  cix = monkeys.find_index(cand)
+  break if cand[0...4] == "root"
+  monkeys[cix] = mutate(cand, need)
+  prev = need
+  need = cand[0...4]
+end
+
+root = monkeys.select { |m| m[0...4] == "root" }[0]
+candname = (root[6..9] == need ? root[13..16] : root[6..9])
+monkeys.delete(root)
+
+cand = monkeys.select { |m| m[0...4] == candname }[0]
+cix = monkeys.index(cand)
+monkeys[cix] = "#{need}#{cand[4..16]}"
+
+nums = {}
+
+monkeys.count.times do |mix|
+  if monkeys[mix] =~ /(\d+)/
+    val = $1.to_i
+    nums[monkeys[mix][0...4]] = $1.to_i
+    monkeys[mix] = nil
+  end
+end
+monkeys.compact!
+
+while nums["humn"].nil?
+  monkeys.count.times do |mix|
+    f, a, b = monkeys[mix].scan(/[a-z]{4}/)
+    unless nums[a].nil? || nums[b].nil?
+      case monkeys[mix][11]
+      when ?+
+        nums[f] = nums[a] + nums [b]
+      when ?-
+        nums[f] = nums[a] - nums [b]
+      when ?*
+        nums[f] = nums[a] * nums [b]
+      when ?/
+        nums[f] = nums[a] / nums [b]
+      end
+      monkeys[mix] = nil
+    end
+  end
+  monkeys.compact!
+end
+
+puts nums["humn"]