d24 p1

2019/24.rkt

@@ -0,0 +1,47 @@
+#lang br
+(require racket/file racket/dict rackunit)
+
+(define (bug? x) (char=? x #\#))
+
+(define (parse-grid lns)
+  (for*/list ([(ln row) (in-indexed lns)]
+              [(c col) (in-indexed ln)])
+    (cons (make-rectangular col row) c)))
+
+(define (adjacent-bugs g tile)
+  (for/sum ([delta '(+i -i 1 -1)]
+            #:when (bug? (dict-ref g (+ tile delta) #\.)))
+    1))
+
+(define (step g)
+  (for*/list ([row (in-range 5)]
+              [col (in-range 5)])
+    (define tile (make-rectangular col row))
+    (cons tile
+          (match (cons (dict-ref g tile) (adjacent-bugs g tile))
+            [(cons #\# 1) #\#]
+            [(cons #\# _) #\.]
+            [(cons #\. (or 1 2)) #\#]
+            [(cons other _) other]))))
+
+(define (biodiversity-rating g)
+  (for/sum ([(rec idx) (in-indexed g)]
+            #:when (bug? (cdr rec)))
+    (expt 2 idx)))
+
+(define (solve lns)
+  (let loop ([gs (list (parse-grid lns))])
+    (define next-g (step (car gs)))
+    (cond
+      [(member next-g gs) (biodiversity-rating next-g)]
+      [else (loop (cons next-g gs))])))
+
+(check-eq?
+ (solve (string-split "....#
+#..#.
+#..##
+..#..
+#....")) 2129920)
+
+;; 1
+(check-eq? (solve (file->lines "24.rktd")) 18407158)

2019/24.rktd

@@ -0,0 +1,5 @@
+..#.#
+#####
+.#...
+...#.
+##...