solve day 13 part 1

13/1.rkt

@@ -0,0 +1,40 @@
+#lang racket
+
+(require "../lib/utils.rkt")
+
+(define lines
+  (call-with-input-file "input"
+    (λ (fp) (get-lines fp))))
+
+(define patterns (split-list-by "" lines))
+
+(define (find-reflection lines)
+  (define lines-vec (list->vector lines))
+  (define len (vector-length lines-vec))
+  (define (find-first)
+    (let loop ((i 0) (ret '()))
+      (if (>= i (- len 1))
+          (reverse ret)
+          (if (string=? (vector-ref lines-vec i)
+                        (vector-ref lines-vec (+ i 1)))
+              (loop (+ 1 i) (cons i ret))
+              (loop (+ 1 i) ret)))))
+  (define (count start)
+    (let loop ((cnt 1) (i start))
+      (if (or (< i 0)
+              (>= (+ start cnt) len))
+          (+ start 1)
+          (if (string=? (vector-ref lines-vec i)
+                        (vector-ref lines-vec (+ start cnt)))
+              (loop (+ 1 cnt) (- i 1))
+              0))))
+  (define start (find-first))
+  (if (null? start)
+      0
+      (apply max (map count start))))
+
+(define (score pattern)
+  (+ (* 100 (find-reflection pattern))
+     (find-reflection (transpose-list pattern))))
+
+(apply + (map score patterns))

lib/utils.rkt

@@ -3,7 +3,8 @@
 (provide get-lines
          enumerate
          repeat
-         split-list-by)
+         split-list-by
+         transpose-list)
 
 (define (repeat n e)
   (let loop ((i 0) (ret '()))
@@ -41,3 +42,14 @@
         (if (cmp e (car lst))
             (loop (cons (reverse cur) ret) '() (cdr lst))
             (loop ret (cons (car lst) cur) (cdr lst))))))
+
+(define (transpose-list lines)
+  (define width (string-length (car lines)))
+  (define new-lines (make-vector width))
+  (let loop ((i 0))
+    (if (>= i width)
+      (vector->list new-lines)
+      (let ()
+        (define char-list (map (λ (l) (string-ref l i)) lines))
+        (vector-set! new-lines i (list->string char-list))
+        (loop (+ i 1))))))