Day 03: Use for/fold instead of loop.

lib.rkt

@@ -73,6 +73,11 @@
 (define (string->binary str)
   (string->number (string-append "#b" str)))
 
+;; chars->binary : (listof char?) -> number?
+;; Given a list of characters representing the bits of a binary number,
+;; convert it into the number it represents
+(define chars->binary (∘ string->binary list->string))
+
 ;; string->vector : string? -> (vectorof char?)
 (define (string->vector str)
   (list->vector (string->list str)))

src/03.rkt

@@ -6,26 +6,25 @@
   (for/list ([b (problem-input 3)])
     (string->list b)))
 
-(define chars->binary
-  (∘ string->binary list->string))
+(define bits (length (first input)))
 
 (define (nth-bit nums p n)
   (let ([bs (map #{list-ref % n} nums)])
-    (if (p (count (∂ char=? #\1) bs) (/ (length nums) 2))
+    (if (p (count #{char=? % #\1} bs) (/ (length nums) 2))
         #\1 #\0)))
 
 (define (squimsh p)
-  (~> (range (length (first input)))
+  (~> (range bits)
       (map #{nth-bit input p %} _)
       chars->binary))
 
 (define (search p)
-  (let loop ([nums input]
-             [n 0])
-    (if (= (length nums) 1)
-        (chars->binary (first nums))
-        (let ([bit (nth-bit nums p n)])
-          (loop (filter #{char=? bit (list-ref % n)} nums) (add1 n))))))
+  (for/fold ([nums input]
+             #:result (chars->binary (first nums)))
+            ([n (range bits)]
+             #:break (singleton? nums))
+    (define bit (nth-bit nums p n))
+    (filter #{char=? bit (list-ref % n)} nums)))
 
 (define part1
   (* (squimsh >=) (squimsh <)))