From 41f9a5899396236262b99d4efde1d64f82f48e6a Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jcxz@cs.ubc.ca>
Date: Sat, 12 Dec 2020 22:41:31 -0800
Subject: [PATCH] Day 13.

---
 input/13.txt |  2 ++
 src/13.rkt   | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 input/13.txt
 create mode 100644 src/13.rkt

diff --git a/input/13.txt b/input/13.txt
new file mode 100644
index 0000000..91abf0b
--- /dev/null
+++ b/input/13.txt
@@ -0,0 +1,2 @@
+1000104
+41,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,659,x,x,x,x,x,x,x,23,x,x,x,x,13,x,x,x,x,x,19,x,x,x,x,x,x,x,x,x,29,x,937,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17
diff --git a/src/13.rkt b/src/13.rkt
new file mode 100644
index 0000000..decd2b7
--- /dev/null
+++ b/src/13.rkt
@@ -0,0 +1,21 @@
+#lang curly-fn racket
+
+(require math/number-theory
+         "../lib.rkt")
+
+(define input (problem-input 13))
+(define timestamp (string->number (first input)))
+(define busses (map string->number (string-split (second input) ",")))
+(define ids (filter identity busses))
+
+(define part1
+  (let* ([waits (map #{- (* % (add1 (quotient timestamp %))) timestamp} ids)]
+         [wait (apply min waits)]
+         [index (index-of waits wait)])
+    (* wait (list-ref ids index))))
+
+(define part2
+  (let ([offsets (filter-map #{and %1 (negate %2)} busses (range (length busses)))])
+    (solve-chinese offsets ids)))
+
+(show-solution part1 part2)