day 8 meow,,

8.rkt

@@ -4,50 +4,30 @@
 
 ;; solution for day 8
 
-(define (follow mapl cur inst)
-  (define node
-    (for/first ([entry (in-list mapl)]
-                #:when (equal? (first entry) cur))
-      entry))
-  (match inst
-    [#\L (second node)]
-    [#\R (third node)]))
-
-(define (part1 input)
-  (for/fold ([cur "AAA"] [cnt 0] #:result cnt)
+(define (part1 input [cur "AAA"] [break? #{equal? % "ZZZ"}])
+  (for/fold ([cur cur] [cnt 0] #:result cnt)
             ([inst (in-cycle (in-list (first input)))])
-            #:break (equal? cur "ZZZ")
-    (define next (follow (second input) cur inst))
-    (values next (add1 cnt))))
+            #:break (break? cur)
+    (values (inst (hash-ref (second input) cur))
+            (add1 cnt))))
 
 (define (part2 input)
-  (define all-as
-    (for/list ([x (in-list (second input))] #:when (regexp-match? #px"^..A$" (first x)))
-      (first x)))
-  (define (end-z? c)
-    (regexp-match? #px"^..Z$" c))
-
-  (define (run-item item)
-    (for/fold ([cur item] [cnt 0] #:result cnt)
-              ([inst (in-cycle (in-list (first input)))])
-              #:break (end-z? cur)
-      (define next (follow (second input) cur inst))
-      (values next (add1 cnt))))
-
-  (apply lcm (map run-item all-as)))
-
+  (~> (filter #{string-suffix? % "A"}
+               (hash-keys (second input)))
+      (map #{part1 input % #{string-suffix? % "Z"}} _)
+      (apply lcm _)))
 
 (define (parse fname)
-  (match-define (list instl mapl) (string-split (file->string fname) "\n\n"))
-  (list (string->list instl)
-        (for/list ([line (in-list (string-split mapl "\n"))])
-          (match-define (pregexp #px"([A-Z0-9]{3}) = .([A-Z0-9]{3}), ([A-Z0-9]{3})."
-                                 (list _ node left right))
-            line)
-          (list node left right))))
-
-; (part2 (parse "inputs/8-test1"))
-; (error)
+  (match-define (list* instl _ mapl) (file->lines fname))
+  (list
+    (for/list ([c (in-string instl)])
+      (match c
+        [#\L first]
+        [#\R second]))
+    (for/hash ([line (in-list mapl)])
+      (match-define (list node left right)
+        (regexp-match* #px"[A-Z0-9]{3}" line))
+      (values node (list left right)))))
 
 (module+ main
   (define input (parse "inputs/8"))