From 0f5e14363bb223fd34699685e74953c2afa8b3ff Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jonathanhwchan@gmail.com>
Date: Mon, 14 Dec 2020 21:46:39 -0800
Subject: [PATCH] Day 15.

---
 .gitignore |  1 +
 src/15.rkt | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)
 create mode 100644 src/15.rkt

diff --git a/.gitignore b/.gitignore
index 072f362..f8acb17 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 **/*.bak
 **/*.rkt~
 **/*.txt~
+**/*.exe
 **/compiled/
diff --git a/src/15.rkt b/src/15.rkt
new file mode 100644
index 0000000..d283602
--- /dev/null
+++ b/src/15.rkt
@@ -0,0 +1,21 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input '(5 1 9 18 13 8 0))
+
+(define (play end)
+  (define turns
+    (make-hash (map cons input (range 1 (add1 (length input))))))
+  (let loop ([turn (length input)] [curr (last input)])
+    (cond
+      [(>= turn end) curr]
+      [(hash-has-key? turns curr)
+       (let ([next (- turn (hash-ref turns curr))])
+         (hash-set! turns curr turn)
+         (loop (add1 turn) next))]
+      [else
+       (hash-set! turns curr turn)
+       (loop (add1 turn) 0)])))
+
+(show-solution (play 2020) (play 30000000))