day 5: idk maybe slightly better

5.rkt

@@ -4,54 +4,31 @@
 
 ;; solution for day 5
 
-(define (part1 input)
-  (match-define (list stacks moves) input)
-  (define end-stacks
-    (for/fold ([stacks stacks]) ([move (in-list moves)])
+(define (solution input reverse?)
+  (for/fold ([stacks (first input)] #:result (list->string (map first stacks)))
+            ([move (in-list (second input))])
     (match-define (list ct (app sub1 from) (app sub1 to)) move)
-      (define movement (take (list-ref stacks from) ct))
-      (for/list ([stack (in-list stacks)] [i (in-naturals)])
-        (match i
-          [(== from) (drop stack ct)]
-          [(== to) (append (reverse movement) stack)]
-          [else stack]))))
-  (apply string-append (map first end-stacks)))
+    (define movement ((if reverse? reverse identity)
+                      (take (list-ref stacks from) ct)))
+    (list-update (list-update stacks from #{drop % ct})
+                 to #{append movement %})))
 
-(define (part2 input)
-  (match-define (list stacks moves) input)
-  (define end-stacks
-    (for/fold ([stacks stacks]) ([move (in-list moves)])
-      (match-define (list ct (app sub1 from) (app sub1 to)) move)
-      (define movement (take (list-ref stacks from) ct))
-      (for/list ([stack (in-list stacks)] [i (in-naturals)])
-        (match i
-          [(== from) (drop stack ct)]
-          [(== to) (append movement stack)]
-          [else stack]))))
-  (apply string-append (map first end-stacks)))
+(define (part1 input) (solution input #t))
+(define (part2 input) (solution input #f))
 
 (define (parse fname)
   (match-define (list stacks moves) (string-split (file->string fname) "\n\n"))
-  (define stack-lines (string-split stacks "\n"))
-  (define columns
-    (for/list ([x (in-list (string->list (last stack-lines)))]
-               [i (in-naturals)]
-               #:unless (equal? x #\space))
-      i))
-  (define stacks-data
+  (match-define (list stack-lines ... (app string->list cols-line)) (string-split stacks "\n"))
+  (define columns (indexes-where cols-line #{not (equal? % #\space)}))
+  (list
     (for/list ([col (in-list columns)])
-      (reverse
-        (for*/list ([line (in-list (rest (reverse stack-lines)))]
-                    [letter (in-value (substring line col (add1 col)))]
-                    #:unless (equal? " " letter))
-          letter))))
-  (define moves-data
+      (for*/list ([line (in-list stack-lines)]
+                  [letter (in-value (string-ref line col))]
+                  #:unless (equal? #\space letter))
+        letter))
     (for/list ([line (in-list (string-split moves "\n"))])
-      (define parts (string-split line " "))
-      (list (string->number (second parts))
-            (string->number (fourth parts))
-            (string->number (sixth parts)))))
-  (list stacks-data moves-data))
+      (match-define (pregexp #px"move (\\d+) from (\\d+) to (\\d+)" (app rest move-data)) line)
+      (map string->number move-data))))
 
 (module+ main
   (define input (parse "inputs/5"))