1 files changed, 64 insertions, 0 deletions
diff --git a/2015/day13.exs b/2015/day13.exs
new file mode 100644
index 0000000..e23dc34
--- /dev/null
+++ b/2015/day13.exs
@@ -0,0 +1,64 @@
+pairings =
+  File.stream!("inputs/day13.txt")
+  |> Enum.reduce(%{}, fn line, acc ->
+    [_, from, dir, amount, to] =
+      Regex.run(
+        ~r/^(\w+) would (gain|lose) (\d+) happiness units by sitting next to (\w+)./,
+        line
+      )
+
+    {amount, ""} = Integer.parse(amount)
+
+    amount =
+      if dir == "lose" do
+        -amount
+      else
+        +amount
+      end
+
+    {_, newAcc} =
+      Map.get_and_update(acc, from, fn
+        nil -> {nil, %{to => amount}}
+        existing -> {existing, Map.put(existing, to, amount)}
+      end)
+
+    newAcc
+  end)
+
+defmodule A do
+  def permutations([lastElement]) do
+    [[lastElement]]
+  end
+
+  def permutations(list) do
+    Enum.flat_map(list, fn start ->
+      rest = List.delete(list, start)
+
+      Enum.map(permutations(rest), fn perm ->
+        [start | perm]
+      end)
+    end)
+  end
+
+  def maxHappiness(pairings, people) do
+    A.permutations(people)
+    |> Enum.map(fn tableSorting ->
+      offsetSorting = Stream.cycle(tableSorting) |> Stream.drop(1)
+
+      Enum.zip(tableSorting, offsetSorting)
+      |> Enum.map(fn {p1, p2} ->
+        oneToTwo = Map.get(pairings, p1, %{}) |> Map.get(p2, 0)
+        twoToOne = Map.get(pairings, p2, %{}) |> Map.get(p1, 0)
+        oneToTwo + twoToOne
+      end)
+      |> Enum.sum()
+    end)
+    |> Enum.max()
+  end
+end
+
+maxHappinessWithoutMe = A.maxHappiness(pairings, Map.keys(pairings))
+IO.puts("Maximum happiness without me: #{maxHappinessWithoutMe}")
+
+maxHappinessWithMe = A.maxHappiness(pairings, ["Me" | Map.keys(pairings)])
+IO.puts("Maximum happiness with me: #{maxHappinessWithMe}")