shorten

2017/d19/main.rkt

@@ -6,32 +6,28 @@
   (define lines (port->lines port))
   (strip-context #`(module mod "main.rkt"
                      #,@(for/list ([datum (in-port read (open-input-string (car lines)))])
-                                  datum)
+                          datum)
                      #,@(cdr lines))))
 
 (define-macro (#%mb STARS LINE ...)
   #'(#%module-begin
      (time (STARS (list LINE ...)))))
 
-(define (populate-hash! chars lines)
-  (for* ([(line lidx) (in-indexed lines)]
-         [(c cidx) (in-indexed (string->list line))]
-         #:unless (char=? c #\space))
-        (hash-set! chars (+ lidx (* +i cidx)) c)))
-
 (define (traverse lines #:count-steps [count-steps? #f])
-  (define chars (make-hasheqv))
-  (populate-hash! chars lines)
+  (define chars (for*/hasheqv ([(line lidx) (in-indexed lines)]
+                               [(c cidx) (in-indexed (string->list line))]
+                               #:unless (char=? c #\space))
+                  (values (+ lidx (* +i cidx)) c)))
   (define start (for/first ([k (in-hash-keys chars)]
                             #:when (zero? (real-part k)))
-                           k))
+                  k))
   (define down 1) (define up -1) (define left -i) (define right +i)
   (let loop ([here start][dir down][path null])
     (define here-char (hash-ref chars here #f))
     (cond
       [here-char
        (define next-dir (if (char=? here-char #\+)
-                            (if (hash-has-key? chars (+ here (* dir +i))) (* dir +i) (* dir -i))
+                            (if (hash-has-key? chars (+ here (* dir right))) (* dir right) (* dir left))
                             dir))
        (loop (+ here next-dir) next-dir (cons here-char path))]
       [count-steps? (length path)]