day 5: cleaned up version

5.rkt

@@ -7,29 +7,29 @@
 ;; helper functions here
 
 (define (part1 input)
-  (apply max
-    (for/list ([line (in-list input)])
-      (match-define (list _ rowstr colstr) (regexp-match #px"^([FB]{7})([LR]{3})$" line))
-      (define row (string->number (string-replace (string-replace rowstr "F" "0") "B" "1") 2))
-      (define col (string->number (string-replace (string-replace colstr "L" "0") "R" "1") 2))
-      (+ (* row 8) col))))
+  (apply max input))
 
 (define (part2 input)
-  (define seats
-    (for/list ([line (in-list input)])
-      (match-define (list _ rowstr colstr) (regexp-match #px"^([FB]{7})([LR]{3})$" line))
-      (define row (string->number (string-replace (string-replace rowstr "F" "0") "B" "1") 2))
-      (define col (string->number (string-replace (string-replace colstr "L" "0") "R" "1") 2))
-      (+ (* row 8) col)))
-
-  (for/first ([id (in-range (apply min seats) (apply max seats))]
-              #:when (and (not (member id seats))
-                          (member (add1 id) seats)
-                          (member (sub1 id) seats)))
+  (for/first ([id (in-range (apply min input) (apply max input))]
+              #:when (and (not (member id input))
+                          (member (add1 id) input)
+                          (member (sub1 id) input)))
     id))
 
 (module+ main
-  (define input (file->lines "inputs/5"))
+  (define input (for/list ([line (in-list (file->lines "inputs/5"))])
+                  ;; ok what is this fuckery???
+                  ;; the trick is the whole number can just be treated as binary
+                  ;; first, notice that the entry is a concatenation of row and column, and both
+                  ;; are basically binary numbers, B and R are a 1 digit and F and L are a 0 digit
+                  ;; finally... the ID is <first part> * 8 + <second part>, and conveniently the
+                  ;; second part is 3 digits
+                  ;; so we parse the whole thing as a single binary number. rather than
+                  ;; string-replacing the chars and then number->string with radix 2, i used a
+                  ;; manual fold to build the number
+                  (for/fold ([val 0]) ([char (in-string line)])
+                    (+ (* 2 val) (if ((or/c #\B #\R) char) 1 0)))))
+
   ;; part 1
   (answer 5 1 (part1 input))