From b7faf7c96193044b80604db5a7cd6a6a296fed5c Mon Sep 17 00:00:00 2001
From: Jonathan Chan <jonathanhwchan@gmail.com>
Date: Thu, 10 Dec 2020 00:03:11 -0800
Subject: [PATCH] Day 10.

---
 input/10.txt | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/10.rkt   | 24 ++++++++++++++
 2 files changed, 116 insertions(+)
 create mode 100644 input/10.txt
 create mode 100644 src/10.rkt

diff --git a/input/10.txt b/input/10.txt
new file mode 100644
index 0000000..3286c1b
--- /dev/null
+++ b/input/10.txt
@@ -0,0 +1,92 @@
+47
+61
+131
+15
+98
+123
+32
+6
+137
+111
+25
+28
+107
+20
+99
+36
+2
+97
+88
+124
+138
+75
+112
+52
+122
+78
+46
+110
+41
+64
+63
+16
+93
+104
+105
+91
+27
+45
+119
+14
+1
+65
+62
+118
+37
+79
+77
+19
+71
+35
+130
+69
+5
+44
+9
+48
+125
+136
+103
+140
+53
+126
+106
+55
+129
+139
+87
+68
+21
+85
+76
+31
+113
+12
+100
+24
+96
+82
+13
+70
+72
+86
+26
+117
+58
+132
+114
+40
+54
+133
+51
+92
diff --git a/src/10.rkt b/src/10.rkt
new file mode 100644
index 0000000..8f09885
--- /dev/null
+++ b/src/10.rkt
@@ -0,0 +1,24 @@
+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(define input (sort (map string->number (problem-input 10)) <=))
+
+(define part1
+  (let* ([input (cons 0 input)]
+         [diffs (for/list ([i (rest input)] [j input]) (- i j))]
+         [ones (count #{= % 1} diffs)]
+         [threes (add1 (count #{= % 3} diffs))])
+    (* ones threes)))
+
+(define arrangements
+  (for*/fold ([arrs (hash 0 1)])
+             ([joltage input]
+              [i (range 1 4)])
+    (let ([prev (hash-ref arrs (- joltage i) 0)])
+      (hash-update arrs joltage #{+ % prev} 0))))
+
+(define part2
+  (hash-ref arrangements (last input)))
+
+(show-solution part1 part2)