From 1ae744093d86d8d557d9781e0519a0c6363ef2c6 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Wed, 9 Dec 2020 23:11:47 -0800
Subject: [PATCH] day 10

---
 2020/10.rkt  |  31 +++++++++++++++
 2020/10.rktd | 108 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 139 insertions(+)
 create mode 100644 2020/10.rkt
 create mode 100644 2020/10.rktd

diff --git a/2020/10.rkt b/2020/10.rkt
new file mode 100644
index 0000000..c4d04c7
--- /dev/null
+++ b/2020/10.rkt
@@ -0,0 +1,31 @@
+#lang br
+(require racket/file rackunit)
+
+(define adapters (map string->number (file->lines "10.rktd")))
+(define sources (sort `(0 ,@adapters ,(+ (apply max adapters) 3)) <))
+
+(define diffs (for/list ([left sources]
+                         [right (cdr sources)])
+                (- right left)))
+
+(check-equal? (* (count (curry = 1) diffs) (count (curry = 3) diffs)) 2590)
+
+(define (sequential-subsequences sources [seqs null])
+  (match sources
+    [(? null?) seqs]
+    [(list head tail ...)
+     (let loop ([head (list head)][tail tail])
+       (match tail
+         [(list first rest ...) #:when (eq? (car head) (sub1 first))
+          (loop (cons first head) rest)]
+         [_ (sequential-subsequences tail (cons head seqs))]))]))
+
+(define (subseq->multiplier subseq)
+  (match (length subseq)
+    [3 2] ; abc | ac
+    [4 4] ; abcd | acd | abd | ad
+    [5 7] ; abcde | acde | abde | abce | abe | ace | ade
+    [_ 1]))
+
+(check-equal? (for/product ([subseq (sequential-subsequences sources)])
+                (subseq->multiplier subseq)) 226775649501184)
diff --git a/2020/10.rktd b/2020/10.rktd
new file mode 100644
index 0000000..895ecdd
--- /dev/null
+++ b/2020/10.rktd
@@ -0,0 +1,108 @@
+105
+78
+37
+153
+10
+175
+62
+163
+87
+22
+24
+92
+46
+5
+115
+61
+124
+128
+8
+60
+17
+93
+166
+29
+90
+148
+113
+55
+141
+134
+79
+101
+49
+133
+38
+53
+33
+30
+66
+159
+23
+132
+145
+147
+121
+94
+146
+21
+135
+56
+176
+118
+44
+138
+85
+169
+111
+9
+1
+83
+36
+59
+140
+149
+160
+43
+131
+69
+2
+25
+84
+39
+28
+171
+172
+100
+18
+15
+114
+70
+86
+97
+155
+152
+40
+122
+77
+16
+11
+170
+52
+45
+139
+76
+102
+63
+54
+142
+14
+158
+80
+154
+112
+91
+108
+73
+127
+123
\ No newline at end of file