From 2c519f882bdbebc0fc6b800522849491e2a84257 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Wed, 16 Dec 2020 22:14:06 -0800
Subject: [PATCH] day 17

---
 2020/17.rkt  | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2020/17.rktd |  8 ++++++++
 2 files changed, 66 insertions(+)
 create mode 100644 2020/17.rkt
 create mode 100644 2020/17.rktd

diff --git a/2020/17.rkt b/2020/17.rkt
new file mode 100644
index 0000000..7435842
--- /dev/null
+++ b/2020/17.rkt
@@ -0,0 +1,58 @@
+#lang br
+(require racket/file rackunit)
+
+(define grid
+  (for*/hash ([(row ridx) (in-indexed (file->lines "17.rktd"))]
+              [(col cidx) (in-indexed row)])
+    (values (list 0 (make-rectangular cidx ridx)) col)))
+
+(define (seat-count grid)
+  (count (λ (val) (char=? val #\#)) (hash-values grid)))
+
+(define offsets-3d (for*/list ([z '(1 0 -1)]
+                               [xy '(-1-1i -1 -1+i +i 1+i 1 1-i -i 0)]
+                               #:unless (and (zero? z) (zero? xy)))
+                     (list z xy)))
+
+(define offsets-4d
+  (for*/list ([offset (cons '(0 0) offsets-3d)]
+              [zadj '(+i 0 -i)]
+              [nextval (in-value
+                        (list (+ zadj (first offset)) (second offset)))]
+              #:unless (equal? nextval '(0 0)))
+    nextval))
+
+(define (adjacent-occupied-seats grid offsets k)
+  (for/sum ([offset (in-list offsets)])
+    (let loop ([k (map + k offset)] [sum 0])
+      (match (hash-ref grid k #false)
+        [#\# 1]
+        [_ 0]))))
+
+(define (grow grid offsets)
+  (define newgrid (make-hash))
+  (for* ([k (in-hash-keys grid)]
+         [offset (cons '(0 0) offsets)])
+    (define newk (map + k offset))
+    (hash-ref! newgrid newk (hash-ref grid newk #\.)))
+  newgrid)
+
+(define (iterate grid offsets)
+  (for/hash ([(k v) (in-hash (grow grid offsets))])
+    (match v
+      [#\# (values k (if (<= 2 (adjacent-occupied-seats grid offsets k) 3) #\# #\.))]
+      [_ (values k (if (= (adjacent-occupied-seats grid offsets k) 3) #\# #\.))])))
+
+(define (count-active grid)
+  (count (λ (val) (char=? val #\#)) (hash-values grid)))
+
+(define (solve offsets)
+  (for/fold ([grid grid]
+             #:result (count-active grid))
+            ([i 6])
+    (iterate grid offsets)))
+
+(check-equal? (solve offsets-3d) 395)
+
+;; warning: slow
+#;(check-equal? (solve offsets-4d) 2296)
\ No newline at end of file
diff --git a/2020/17.rktd b/2020/17.rktd
new file mode 100644
index 0000000..85254df
--- /dev/null
+++ b/2020/17.rktd
@@ -0,0 +1,8 @@
+.#######
+#######.
+###.###.
+#....###
+.#..##..
+#.#.###.
+###..###
+.#.#.##.
\ No newline at end of file