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Diffstat (limited to 'aoc2023-racket/day-23/day-23.rkt')
| -rw-r--r-- | aoc2023-racket/day-23/day-23.rkt | 89 |
1 files changed, 0 insertions, 89 deletions
diff --git a/aoc2023-racket/day-23/day-23.rkt b/aoc2023-racket/day-23/day-23.rkt deleted file mode 100644 index c048013..0000000 --- a/aoc2023-racket/day-23/day-23.rkt +++ /dev/null @@ -1,89 +0,0 @@ -#lang racket - -(require advent-of-code - threading - graph) - -(define input (fetch-aoc-input (find-session) 2023 23 #:cache #true)) -(define trails - (for*/hash ([(row r) (in-indexed (string-split input "\n"))] - [(col c) (in-indexed row)] - #:when (not (equal? col #\#))) - ; for now, we don't actually need to detect paths and slopes, just not-rocks - ; in part 1, all forks in the road go right or down, so we can just infer the path - ; direction from the shape of the junction and form a network of junctions from there - ; in part 2, the slopes are removed anyway - (values (cons (add1 r) (add1 c)) col))) - -(define max-row (~> input (string-split "\n") length)) - -(define start (findf (λ (p) (= (car p) 1)) (hash-keys trails))) -(define end (findf (λ (p) (= (car p) max-row)) (hash-keys trails))) - -(define (get-neighbors posn type) - (match-define (cons r c) posn) - (match type - ['junction - (~> (set (cons (add1 r) c) (cons r (add1 c))))] - [_ - (~> (list (cons (add1 r) c) (cons (sub1 r) c) (cons r (add1 c)) (cons r (sub1 c))) - (filter (curry hash-has-key? trails) _) - list->set)])) - -(define junction-points - (for/set ([(k v) (in-hash trails)] - #:when (not (= (set-count (get-neighbors k v)) 2))) - k)) - -(define trails-with-junctions - (for/hash ([k (in-hash-keys trails)]) - (cond - [(set-member? junction-points k) (values k 'junction)] - [else (values k 'trail)]))) - -(define (walk-to-next-junction start current [length 1] [seen (set start)]) - (define next (~> current - (get-neighbors _ 'trail) - (set-subtract seen) set-first)) - (cond - [(equal? (hash-ref trails-with-junctions next) 'junction) - (list (- (add1 length)) start next)] ; weird format is due to graph library - [else - (walk-to-next-junction start next (add1 length) (set-add seen current))])) - -(define routes-to-junctions - (for*/list ([j (in-set junction-points)] - [neighbor (in-set (get-neighbors j 'junction))] - #:when (hash-has-key? trails neighbor)) - (walk-to-next-junction j neighbor))) - -;; part 1 -- using graph library for Bellman-Ford on negative weighted graph -;; Bellman-Ford finds the shortest path, but negating all the path weights -;; will give us the longest path instead -;; -;; unlike Dijkstra which can't handle negative path lengths, Bellman-Ford -;; works as long as the graph is currently directed and acyclic -(define slippery-trail (weighted-graph/directed routes-to-junctions)) -(match-define-values (distances _) (bellman-ford slippery-trail start)) -(- (hash-ref distances end)) - -;; part 2 -- rolling my own DFS that can reject seen junctions and dead ends -(define routes-to-junctions-with-traction - (for/fold ([trails (hash)]) ([route (in-list routes-to-junctions)]) - (match-define (list (app - weight) from to) route) - (~> trails - (hash-update _ from (curry cons (cons to weight)) '()) - (hash-update _ to (curry cons (cons from weight)) '())))) - -(define (dfs g from to [acc 0] [seen (set from)]) - (cond - [(equal? from to) acc] - [else - (define choices (filter (λ (path) (not (set-member? seen (car path)))) (hash-ref g from))) - (if (empty? choices) 0 ; long dead-ends don't count - (for/fold ([best acc]) - ([path (in-list choices)] - #:do [(match-define (cons next dist) path)]) - (max best (dfs g next to (+ acc dist) (set-add seen next)))))])) - -(dfs routes-to-junctions-with-traction start end)
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