From 49f2419361381880a14e53c6d6a11770b9e763ab Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Wed, 23 Dec 2020 22:44:43 -0800
Subject: [PATCH] day 24

---
 2020/24.rkt  |  61 +++++++
 2020/24.rktd | 492 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 553 insertions(+)
 create mode 100644 2020/24.rkt
 create mode 100644 2020/24.rktd

diff --git a/2020/24.rkt b/2020/24.rkt
new file mode 100644
index 0000000..bb9d271
--- /dev/null
+++ b/2020/24.rkt
@@ -0,0 +1,61 @@
+#lang br
+(require racket/file rackunit)
+
+(define (lex-1 port)
+  (match (regexp-match #rx"^se|nw|sw|ne|w|e" port)
+    [#false eof]
+    [(list str) str]))
+
+(define tokss (for/list ([ln (in-list (file->lines "24.rktd"))])
+                (for/list ([tok (in-port lex-1 (open-input-string ln))])
+                  tok)))
+
+(define (tok->delta tok)
+  (match tok
+    [#"w" -1]
+    [#"e" 1]
+    [#"nw" -i]
+    [#"ne" 1-i]
+    [#"sw" -1+i]
+    [#"se" +i]))
+
+(define white -1)
+(define black 1)
+(define black? positive?)
+
+(define (make-initial-floor tokss)
+  (define floor (make-hasheqv))
+  (for ([toks (in-list tokss)])
+    (hash-update! floor (apply + (map tok->delta toks)) - white))
+  floor)
+
+(define deltas '(1 -1 -i 1-i -1+i +i))
+
+(define (grow! floor)
+  (for* ([k (in-list (hash-keys floor))]
+         [delta (in-list deltas)])
+    (hash-ref! floor (+ k delta) white)))
+
+(define (adjacent-black-tiles floor k)
+  (for/sum ([delta (in-list deltas)]
+            #:when (black? (hash-ref floor (+ k delta) white)))
+    1))
+
+(define (iterate floor num)
+  (for ([i (in-range num)])
+    (grow! floor)
+    (define fliplist
+      (for/list ([(k v) (in-hash floor)]
+                 #:when (let ([abt (adjacent-black-tiles floor k)])
+                          (cond
+                            [(and (black? v) (or (zero? abt) (< 2 abt)))]
+                            [(and (not (black? v)) (= 2 abt))]
+                            [else #false])))
+        k))
+    (for ([k (in-list fliplist)])
+      (hash-update! floor k -)))
+  floor)
+    
+(define floor (make-initial-floor tokss))
+(check-equal? (count black? (hash-values floor)) 436)
+(check-equal? (count black? (hash-values (iterate floor 100))) 4133)
\ No newline at end of file
diff --git a/2020/24.rktd b/2020/24.rktd
new file mode 100644
index 0000000..45c3ff3
--- /dev/null
+++ b/2020/24.rktd
@@ -0,0 +1,492 @@
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