1 files changed, 59 insertions, 0 deletions
diff --git a/racket/aoc2021/day-20/day-20.rkt b/racket/aoc2021/day-20/day-20.rkt
new file mode 100644
index 0000000..b7ed092
--- /dev/null
+++ b/racket/aoc2021/day-20/day-20.rkt
@@ -0,0 +1,59 @@
+#lang racket
+(require "../../jj-aoc.rkt"
+         threading)
+
+(struct pixel (i j) #:transparent)
+
+(define-values (raw-enhancement raw-image)
+  (~> (open-day 20 2021) port->string (string-split "\n\n") (apply values _)))
+
+(define (char->pixel c)
+  (if (char=? #\# c) 'light 'dark))
+(define (pixel->char c)
+  (if (equal? 'light c) #\# #\.))
+
+(define enhancement-algorithm
+  (for/hash ([(c i) (in-indexed (~> raw-enhancement (string-replace "\n" "")))])
+    (values i (char->pixel c))))
+
+(define image-hash
+  (for*/hash ([(row i) (in-indexed (string-split raw-image "\n"))] [(c j) (in-indexed row)])
+    (values (pixel i j) (char->pixel c))))
+
+(define (window->new-pixel p ps bg)
+  (match-define (pixel i j) p)
+  (~> (for*/list ([di '(-1 0 1)] [dj '(-1 0 1)])
+        (if (equal? 'dark (hash-ref ps (pixel (+ i di) (+ j dj)) bg)) #\0 #\1))
+      (apply string _)
+      (string->number _ 2)
+      (hash-ref enhancement-algorithm _)))
+
+(define (pad-hash ps bg)
+  (define coords (hash-keys ps))
+  (define i-min (~>> coords (map pixel-i) (apply min)))
+  (define i-max (~>> coords (map pixel-i) (apply max)))
+  (define j-min (~>> coords (map pixel-j) (apply min)))
+  (define j-max (~>> coords (map pixel-j) (apply max)))
+  (for*/hash ([i (in-inclusive-range (- i-min 2) (+ i-max 2))]
+              [j (in-inclusive-range (- j-min 2) (+ j-max 2))])
+    (values (pixel i j) (hash-ref ps (pixel i j) bg))))
+
+(define (enhance-image ps bg)
+  (for/hash ([(p _) (in-hash (pad-hash ps bg))])
+    (values p (window->new-pixel p ps bg))))
+
+;; part 1
+;; looking at the enhancement algorithm, since a window of 0 -> light and 512 -> dark,
+;; the infinite background flips colors every other enhancement step
+;; instead of trying to account for this algorithmically I just hardcoded it in
+(~> image-hash
+    (enhance-image 'dark)
+    (enhance-image 'light)
+    hash-values
+    (count (curry equal? 'light) _))
+
+;; part 2
+(~> (for/fold ([img image-hash]) ([_ (in-range 25)])
+      (~> img (enhance-image 'dark) (enhance-image 'light)))
+    hash-values
+    (count (curry equal? 'light) _))