Finding myself up at midnight last night I couldn’t resist trying Day 12’s puzzle, and thankfully I got my answers without staying up until sunrise.

``````(ns aoc.2021.day.12
(:require [hyperfiddle.rcf :as rcf]
[clojure.string :as str]))

(rcf/enable!)
``````

The puzzle is another graph problem. We’re given a graph with a start, an end, and other nodes that are either “big caves” (capital letters) or “small caves” (lower-case letters).

``````;     start
;     /   \
; c--A-----b--d
;     \   /
;      end
``````

We’re asked to find all paths from start to end with the constraint that no small cave is visited more than once in a given path.

The input is given as a list of adjacencies; we’ll parse it into a hash-map for efficiency’s sake.

``````(def input "LP-cb\nPK-yk\nbf-end\nPK-my\nend-cb\nBN-yk\ncd-yk\ncb-lj\nyk-bf\nbf-lj\nBN-bf\nPK-cb\nend-BN\nmy-start\nLP-yk\nPK-bf\nmy-BN\nstart-PK\nyk-EP\nlj-BN\nlj-start\nmy-lj\nbf-LP\n")
(def t-in-sm "start-A\nstart-b\nA-c\nA-b\nb-d\nA-end\nb-end")
(def t-in-med "dc-end\nHN-start\nstart-kj\ndc-start\ndc-HN\nLN-dc\nHN-end\nkj-sa\nkj-HN\nkj-dc")

(->> (str/split-lines input)
(map #(str/split % #"-"))
(mapcat (juxt identity reverse))
(group-by first)
(reduce-kv (fn [m k v] (assoc m k (map second v))) {})))
``````

(It was tempting to use the new `update-vals` as the last step here, but since it takes a map as its first argument I couldn’t use it with the thread-last macro.)

``````(def adjacencies (parse-adjacencies input))
``````

Now for our graph search. BFS or DFS will do, and at midnight one uses whichever comes naturally, so it was DFS for me. The only non-standard thing we need to do is to ensure that “big caves” are never marked as visited, since they may be returned to any number of times. (It’s implied that the puzzle input will not cause infinite loops.)

``````(defn all-caps? [s] (= s (str/upper-case s)))

(letfn [(paths [curr visited]
(if (= curr end)
'((curr))  ; Singleton path [curr] is the only path
(let [to-visit (remove visited (adjacencies curr))
visited (if (all-caps? curr)
visited
(conj visited curr))
visit-neighbor (fn [neighbor]
(map (fn [path] (cons curr path))
(paths neighbor visited)))]
(mapcat visit-neighbor to-visit))))]
(paths start #{})))

(rcf/tests
``````

Part 2 changes the rules – it says that in a given path, a single small cave may be visited twice.

To me there’s no obvious axis on which to generalize this rule – might we want to permit a single small cave to be visited 3 or 4 times, or might we want to permit up to 2 or 3 small caves to be visited twice…? And of course there’s no part 3 to the puzzle anyway; so we’ll solve only the problem we have, using a `did-double` flag to indicate whether we’ve used our twofer-small-cave already, and call it a day.

``````(defn paths-2 [adjacencies start end]
(letfn [(paths [curr visited did-double]
(if (= curr end)
'((curr))
(let [did-double (or did-double (boolean (visited curr)))
(if did-double
visited (if (all-caps? curr)
visited
(conj visited curr))
visit-neighbor (fn [neighbor]
(map (fn [path] (cons curr path))
(paths neighbor
visited
did-double)))]
(mapcat visit-neighbor to-visit))))]
(paths start #{} false)))