Add Advent 2021 - day 12.

2021/day12/p.rkt

@@ -0,0 +1,43 @@
+#lang racket/base
+(require racket/string)
+(require racket/set)
+
+(define (cdrm l)
+  (car (cdr l)))
+
+(define (make-line t)
+  (sort (string-split t "-") string<?))
+
+(define sset (for/set ([t (in-lines)]) (make-line t)))
+
+(define caves (for/mutable-set ([s sset]) (car s)))
+(set-union! caves (for/set ([s sset]) (cdrm s)))
+(set-subtract! caves (set "start" "end"))
+
+(define (is-small s)
+  (char-lower-case? (string-ref s 0)))
+
+(define (valid-path p1 p2)
+  (set-member? sset (sort (list p1 p2) string<?)))
+
+(define (cannot-use-cave c used)
+  ;(hash-has-key? used c)) ; part 1
+  (and (hash-has-key? used c) (hash-has-key? used 0))) ; part 2
+
+(define (hash-handle! used c)
+  (hash-update! used c add1 0)
+  (when (= (hash-ref used c) 2)
+    (hash-set! used 0 #t)))
+
+(for/sum ([cave caves])
+  (define (recurse-path path used c)
+    (when (is-small c) (hash-handle! used c))
+    (set! path (append path (list c)))
+    (+ (if (valid-path "end" c) 1 0)
+       (for/sum ([nc caves]
+                 #:unless (or (equal? nc c) (cannot-use-cave nc used) (not (valid-path c nc))))
+         (recurse-path path (hash-copy used) nc))))
+
+  (if (valid-path "start" cave)
+      (recurse-path null (make-hash) cave)
+      0))

2021/day12/test1.txt

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+start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end

2021/day12/test2.txt

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+fs-end
+he-DX
+fs-he
+start-DX
+pj-DX
+end-zg
+zg-sl
+zg-pj
+pj-he
+RW-he
+fs-DX
+pj-RW
+zg-RW
+start-pj
+he-WI
+zg-he
+pj-fs
+start-RW

2021/day12/test3.txt

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+end-MY
+MY-xc
+ho-NF
+start-ho
+NF-xc
+NF-yf
+end-yf
+xc-TP
+MY-qo
+yf-TP
+dc-NF
+dc-xc
+start-dc
+yf-MY
+MY-ho
+EM-uh
+xc-yf
+ho-dc
+uh-NF
+yf-ho
+end-uh
+start-NF