Day 16: Part 1.

input/16.txt

@@ -0,0 +1 @@
+59717513948900379305109702352254961099291386881456676203556183151524797037683068791860532352118123252250974130706958763348105389034831381607519427872819735052750376719383812473081415096360867340158428371353702640632449827967163188043812193288449328058464005995046093112575926165337330100634707115160053682715014464686531460025493602539343245166620098362467196933484413717749680188294435582266877493265037758875197256932099061961217414581388227153472347319505899534413848174322474743198535953826086266146686256066319093589456135923631361106367290236939056758783671975582829257390514211329195992209734175732361974503874578275698611819911236908050184158

src/16.rkt

@@ -0,0 +1,38 @@
+#lang racket
+
+(require "../lib.rkt")
+
+(define (string->numbers str)
+  (map (λ (c) (- (char->integer c) (char->integer #\0)))
+       (string->list str)))
+
+(define input-string (car (problem-input 16)))
+
+(define input
+  (string->numbers input-string))
+
+(define input-long
+  (apply append (make-list 10000 input)))
+
+(define message-offset
+  (string->number (substring input-string 0 7)))
+
+(define (base len n)
+  (let* ([base-n (apply append (map (∂ make-list n) '(0 1 0 -1)))]
+         [repeats (add1 (ceiling (/ len (* n 4))))]
+         [repeated (apply append (make-list repeats base-n))])
+    (take (cdr repeated) len)))
+
+(define (prod-sum l1 l2)
+  (sum (map * l1 l2)))
+
+(define (fft ns)
+  (map (λ (n) (remainder (abs (prod-sum ns (base (length ns) n))) 10))
+       (range 1 (add1 (length ns)))))
+
+(define (part1)
+  (let loop ([signal input]
+             [count 0])
+    (if (= 100 count)
+        (take signal 8)
+        (loop (fft signal) (add1 count)))))