Add Advent 2021 - day 16.

2021/day16/p.rkt

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+#lang racket/base
+(require racket/list)
+(require racket/match)
+
+(define (char16->number c)
+  (string->number (string c) 16))
+
+(define (make-line t)
+  (define numbers (map char16->number (string->list t)))
+  (flatten (for/list ([n numbers])
+             (define l (string->list (number->string n 2)))
+             (if (= (length l) 4)
+                 l
+                 (list (make-list (- 4 (length l)) #\0) l)))))
+
+(define slist (make-line (read-line)))
+
+; part 1
+(define version-sum 0)
+
+; l=list p=initial_position n=number_of_bits
+(define (get-value-bits l p n)
+  (string->number (apply string (for/list ([index (in-range p (+ p n))]) (list-ref l index))) 2))
+
+; l=list p=initial_position
+; returns (values new_position return_value)
+(define (advance-in-list l p)
+  (define (advance-p! n)
+    (set! p (+ p n)))
+
+  ; get version
+  (set! version-sum (+ version-sum (get-value-bits l p 3)))
+  (advance-p! 3)
+
+  ; get type ID
+  (define type-id (get-value-bits l p 3))
+  (advance-p! 3)
+
+  (define v (match type-id
+              ; literal value
+              [4
+               (define sl (reverse (for/list ([i (in-naturals)] #:final (equal? (list-ref l p) #\0))
+                                     (define r (begin0 (get-value-bits l p 5) (advance-p! 5)))
+                                     (if (>= r 16)
+                                         (- r 16)
+                                         r))))
+               (for/sum ([r sl] [i (in-naturals)])
+                 (* r (expt 2 (* i 4))))
+               ]
+              ; one of the operators
+              [operator
+               (define is-bits #t)
+               (define out-len
+                 (let ([c (list-ref l p)])
+                   (advance-p! 1)
+                   (if (equal? c #\0)
+                       ; total length in bits
+                       (let ([len (get-value-bits l p 15)])
+                         (advance-p! 15)
+                         len)
+                       ; number of subpackets
+                       (let ([num (get-value-bits l p 11)])
+                         (advance-p! 11)
+                         (set! is-bits #f)
+                         num))))
+
+               (define r
+                 (if is-bits
+                     (let ([pp p])
+                       (for/list ([i (in-naturals)] #:break (= (- p pp) out-len))
+                         (match-let-values ([(mp mr) (advance-in-list l p)])
+                                           (set! p mp)
+                                           mr)))
+                     (let ([sub-count 0])
+                       (for/list ([i (in-naturals)] #:break (= sub-count out-len))
+                         (match-let-values ([(mp mr) (advance-in-list l p)])
+                                           (set! p mp)
+                                           (set! sub-count (add1 sub-count))
+                                           mr)))
+                     ))
+
+               ; apply operators
+               (match operator
+                 [0 (apply + r)]
+                 [1 (apply * r)]
+                 [2 (apply min r)]
+                 [3 (apply max r)]
+                 [5 (if (apply > r) 1 0)]
+                 [6 (if (apply < r) 1 0)]
+                 [7 (if (apply = r) 1 0)])
+               ]
+              ))
+
+  (values p v))
+
+; returns result for part 2 and goes through input to update version sum
+(advance-in-list slist 0)
+; part 1
+version-sum

2021/day16/test1.txt

@@ -0,0 +1 @@
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