1 files changed, 83 insertions, 0 deletions
diff --git a/advent-of-code/2023/10/1.rkt b/advent-of-code/2023/10/1.rkt
new file mode 100644
index 0000000..e4911aa
--- /dev/null
+++ b/advent-of-code/2023/10/1.rkt
@@ -0,0 +1,83 @@
+#lang racket
+
+(require "../../lib/utils.rkt")
+
+(define lines
+  (call-with-input-file "input"
+    (λ (fp)
+      (get-lines fp))))
+
+(define diagram (list->vector lines))
+(define height (vector-length diagram))
+(define width (string-length (vector-ref diagram 0)))
+(define (char-at x y)
+  (string-ref (vector-ref diagram y) x))
+
+(define (find-start)
+  (let loop ((x 0) (y 0))
+    (if (>= y height)
+      #f
+      (if (>= x width)
+        (loop 0 (+ y 1))
+        (if (char=? #\S (char-at x y))
+          (cons x y)
+          (loop (+ x 1) y))))))
+
+(define start (find-start))
+
+(define (valid-pos? pos)
+  (define x (car pos))
+  (define y (cdr pos))
+  (and (and (>= x 0) (< x width))
+       (and (>= y 0) (< y height))))
+
+(define first-pos
+  (let ()
+    (define up (cons (car start) (- (cdr start) 1)))
+    (define down (cons (car start) (+ (cdr start) 1)))
+    (define left (cons (- (car start) 1) (cdr start)))
+    (define right (cons (+ (car start) 1) (cdr start)))
+      (cond ((and (valid-pos? up)
+                  (member (char-at (car up) (cdr up)) (list #\| #\F #\7)))
+            up)
+            ((and (valid-pos? down)
+                  (member (char-at (car down) (cdr down)) (list #\| #\L #\J )))
+            down)
+            ((and (valid-pos? left)
+                  (member (char-at (car left) (cdr left)) (list #\- #\F #\L )))
+            left)
+            ((and (valid-pos? right)
+                  (member (char-at (car right) (cdr right)) (list #\- #\7 #\J)))
+            right))))
+
+(define (next cur recent)
+  (define up (cons (car cur) (- (cdr cur) 1)))
+  (define down (cons (car cur) (+ (cdr cur) 1)))
+  (define left (cons (- (car cur) 1) (cdr cur)))
+  (define right (cons (+ (car cur) 1) (cdr cur)))
+  (define c (char-at (car cur) (cdr cur)))
+  (define possible-pos
+    (cond ((char=? c #\|) (list up down))
+          ((char=? c #\-) (list left right))
+          ((char=? c #\F) (list right down))
+          ((char=? c #\L) (list up right))
+          ((char=? c #\7) (list left down))
+          ((char=? c #\J) (list up left))))
+  (let loop ((cur possible-pos))
+    (if (not (equal? (car cur) recent))
+      (car cur)
+      (loop (cdr cur)))))
+
+(define (simulate)
+  (let loop ((i 0) (cur first-pos) (recent start))
+    (if (equal? cur start)
+      i
+      (let ()
+        (define next-loc (next cur recent))
+        (loop (+ i 1) next-loc cur)))))
+
+(define loop-length (simulate))
+
+(if (= 0 (modulo loop-length 2))
+  (/ loop-length 2)
+  (+ 1 (quotient loop-length 2)))