From 9732c7a8514d4c122fd1c843dabaa0ad7cd2da13 Mon Sep 17 00:00:00 2001
From: WillFlame <flameyanai@gmail.com>
Date: Mon, 25 Dec 2023 13:14:26 -0500
Subject: [PATCH] Add 2023 day 25

---
 2023/day25.clj | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)
 create mode 100644 2023/day25.clj

diff --git a/2023/day25.clj b/2023/day25.clj
new file mode 100644
index 0000000..294dafb
--- /dev/null
+++ b/2023/day25.clj
@@ -0,0 +1,76 @@
+(ns clojure-aoc.core (:gen-class)
+    (:require clojure.string))
+
+(def input (-> (slurp "input.txt") (clojure.string/split #"\n")))
+
+(defn parse-line [adjacency line]
+  (let [[name c] (clojure.string/split line #": ")
+        conns (clojure.string/split c #" ")
+        inserted-name (assoc adjacency name (into (or (get adjacency name) #{}) conns))]
+    (reduce #(assoc %1 %2 (conj (or (get %1 %2) #{}) name)) inserted-name conns)))
+
+(defn shortest-path [start end adjacency]
+  (loop [to-visit (conj clojure.lang.PersistentQueue/EMPTY [start []]) visited #{start}]
+    (if-let [[curr-loc used-conns] (peek to-visit)]
+      (if (= curr-loc end)
+        used-conns
+        (let [neighbours (filter #(not (contains? visited %)) (adjacency curr-loc))
+              new-visits (map #(vector % (conj used-conns [curr-loc %])) neighbours)]
+          (recur (into (pop to-visit) new-visits) (into visited neighbours))))
+      nil)))
+
+;; If the algorithm doesn't find a minimum cut, try increasing the number of nodes y can be.
+;; This increases time usage but is more likely to be correct.
+(defn most-used-connections [adjacency]
+  (let [nodes (map first adjacency)
+        paths (for [x nodes y (take 50 nodes) :while (not= x y)]
+                (shortest-path x y adjacency))]
+    (->> (map seq paths)
+         (apply concat)
+         frequencies)))
+
+(defn visit-all [adjacency node]
+  (loop [to-visit (conj clojure.lang.PersistentQueue/EMPTY node) visited #{node}]
+    (if-let [curr-loc (peek to-visit)]
+      (let [neighbours (filter #(not (contains? visited %)) (adjacency curr-loc))]
+        (recur (into (pop to-visit) neighbours) (into visited neighbours)))
+      visited)))
+
+(defn disconnect [adjacency snips]
+  (let [reducer (fn [acc [n1 n2]]
+                  (let [interm (assoc acc n1 (disj (get acc n1) n2))]
+                    (assoc interm n2 (disj (get acc n2) n1))))]
+  (reduce reducer adjacency snips)))
+
+(defn still-connected? [adjacency snips]
+  (let [new-adjacency (disconnect adjacency snips)
+        connected (visit-all new-adjacency (first (first adjacency)))]
+    [(- (count adjacency) (count connected)) (count connected)]))
+
+(defn all-distinct? [coll]
+  (= (count coll) (count (distinct coll))))
+
+(defn minimum-cut [adjacency top-conns]
+  (let [cuts (for [x top-conns y top-conns z top-conns
+                   :while (not= x y z)
+                   :when (all-distinct? (flatten [x y z]))]
+               [x y z])]
+    (->> cuts
+         (map #(still-connected? adjacency %))
+         (filter #(not= (first %) 0))
+         first)))
+
+;; Takes about 5 minutes to run, but it's good enough.
+;; Finds the shortest path between a bunch of nodes and notes the top 6 connections used.
+;; Tries removing 3 of those connections until the graph is no longer connected.
+(def part1
+  (let [adjacency (reduce parse-line {} input)
+      top-conns (->> (most-used-connections adjacency)
+                 (sort-by second)
+                 (take-last 6)
+                 reverse
+                 (map first))]
+  (minimum-cut adjacency top-conns)))
+
+(defn -main [& args]
+  (println part1))