From 2cd1da393224dddbe7a7a7d1f1ef2feabab4f6e3 Mon Sep 17 00:00:00 2001
From: Dustin Swan <dustin@dustinswan.com>
Date: Sat, 28 Dec 2024 23:37:06 -0700
Subject: [PATCH] Day 23. Had to look up some algorithm help. Runs pretty
 slowly, but I\'m happy with it

---
 day23/main.exs | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 day23/main.exs

diff --git a/day23/main.exs b/day23/main.exs
new file mode 100644
index 0000000..fa6a371
--- /dev/null
+++ b/day23/main.exs
@@ -0,0 +1,81 @@
+defmodule Day23 do
+  def run do
+    connections =
+      File.read!("data.txt")
+      |> String.trim()
+      |> String.split("\n")
+      |> Enum.map(fn el -> String.split(el, "-") end)
+
+    all_computers = connections |> Enum.concat() |> Enum.uniq()
+
+    # build a map of computers -> all their connected computers
+    map =
+      Enum.reduce(all_computers, %{}, fn el1, map ->
+        friends =
+          Enum.filter(all_computers, fn el2 ->
+            Enum.member?(connections, [el1, el2]) || Enum.member?(connections, [el2, el1])
+          end)
+
+        Map.put(map, el1, friends)
+      end)
+
+    part1 =
+      expand(map, connections)
+      |> Enum.filter(fn set -> Enum.any?(set, fn el -> String.starts_with?(el, "t") end) end)
+      |> length
+
+    dbg(part1)
+
+    part2 =
+      loop_expand(map, connections)
+      |> hd()
+      |> Enum.join(",")
+
+    dbg(part2)
+  end
+
+  defp loop_expand(map, sets) do
+    case expand(map, sets) do
+      [] -> sets
+      x -> loop_expand(map, x)
+    end
+  end
+
+  defp expand(map, sets) do
+    # look at an element in every set (e.g. the first element)
+    # check all of its connections
+    # if any of those connected computers are connected to all of the set members, you've increased the set size by 1!
+    Enum.reduce(sets, [], fn set, acc ->
+      connections = Map.get(map, hd(set), [])
+
+      res =
+        Enum.reduce(connections, [], fn con, acc3 ->
+          # if it is connected to each member of the set, we've enlarged the set
+          if Enum.all?(set, fn set_el -> is_connected(map, set_el, con) end) do
+            [[con | set] | acc3]
+          else
+            acc3
+          end
+        end)
+
+      if Enum.empty?(res) do
+        acc
+      else
+        if is_list(Enum.at(res, 0)) do
+          Enum.concat(res, acc)
+        else
+          [res | acc]
+        end
+      end
+    end)
+    |> Enum.map(fn l -> Enum.sort(l) end)
+    |> Enum.uniq()
+    |> Enum.sort_by(fn a -> Enum.join(a) end)
+  end
+
+  defp is_connected(map, a, b) do
+    Map.get(map, a, []) |> Enum.member?(b)
+  end
+end
+
+Day23.run()