Initial commit

2019/day16/day16.rkt

@@ -0,0 +1,73 @@
+#lang racket
+
+(require racket/sequence)
+
+(module+ test
+  (require rackunit))
+
+(define (pattern n i)
+  (define index (remainder (add1 i) (* 4 n)))
+  (cond
+    [(< index n) 0]
+    [(< index (* 2 n)) 1]
+    [(< index (* 3 n)) 0]
+    [(< index (* 4 n)) -1]))
+
+(define (last-digit n)
+  (abs (remainder n 10)))
+
+(define (phase signal)
+  (for/list ([i (in-range (length signal))])
+    (last-digit (for/sum ([d (in-list signal)]
+                          [j (in-naturals)]
+                          #:when (>= j i))
+                  (* d (pattern (add1 i) j))))))
+
+(define (number->digits n)
+  (define (aux n)
+    (define-values (q r) (quotient/remainder n 10))
+    (if (> q 0)
+        (cons r (aux q))
+        (list r)))
+  (reverse (aux n)))
+
+(define (digits->number lst)
+  (for/sum ([d (in-list (reverse lst))]
+            [i (in-naturals)])
+    (* d (expt 10 i))))
+
+(define (apply-n-times f n x)
+  (for/fold ([val x]) ([i (in-range n)])
+    (f val)))
+
+(define (part1 n)
+  (digits->number (take (apply-n-times phase 100 n) 8)))
+
+(module+ test
+  (check-eq? (part1 (number->digits 80871224585914546619083218645595)) 24176176)
+  (check-eq? (part1 (number->digits 19617804207202209144916044189917)) 73745418)
+  (check-eq? (part1 (number->digits 69317163492948606335995924319873)) 52432133))
+
+(module+ main
+  (define input (number->digits
+                 (string->number
+                  (string-trim
+                   (port->string (open-input-file "input") #:close? #t)))))
+  (displayln (part1 input)))
+
+(define (phase2 signal)
+  (define-values (s res)
+    (for/fold ([s (apply + signal)]
+               [res '()])
+              ([d (in-list signal)])
+      (values (- s d) (cons (last-digit s) res))))
+  (reverse res))
+
+(define (part2 input)
+  (define offset (digits->number (take input 7)))
+  (define n (drop input offset))
+  (digits->number (take (apply-n-times phase2 100 n) 8)))
+
+(module+ main
+  (define input2 (flatten (make-list 10000 input)))
+  (displayln (part2 input2)))

2019/day16/input

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