day 3: genious solution

3.rkt

@@ -4,24 +4,17 @@
 
 ;; solution for day 3
 
-(define priorities
-  (string->list "_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"))
+(define (char->prio chr)
+  (- (char->integer chr) (if (char-upper-case? chr) 38 96)))
 
 (define (part1 input)
   (for/sum ([line (in-list input)])
     (define-values [left right] (split-at line (/ (length line) 2)))
-    (~>> (set-intersect left right)
-         (set-first)
-         (index-of priorities))))
+    (~> (set-intersect left right) (set-first) (char->prio))))
 
 (define (part2 input)
-  (match input
-    ['() 0]
-    [(list* a b c rst)
-     (+ (~>> (set-intersect a b c)
-             (set-first)
-             (index-of priorities))
-        (part2 rst))]))
+  (for/sum ([group (in-slice 3 input)])
+    (~> (apply set-intersect group) (set-first) (char->prio))))
 
 (define (parse fname)
   (map string->list (file->lines fname)))