2 files changed, 112 insertions(+), 0 deletions(-)

A day16.lisp
A input/day16.txt

@@ 0,0 1,111 @@
+(defpackage #:advent2021.day16
+  (:use #:cl #:alexandria #:cl-ppcre #:queues #:advent2021.util)
+  (:export #:solve-part-1 #:solve-part-2))
+
+(in-package #:advent2021.day16)
+
+
+
+(defun hex2val (c)
+  (if (<= (char-code #\0) (char-code c) (char-code #\9))
+      (- (char-code c) (char-code #\0))
+      (- (char-code (char-upcase c)) (char-code #\A) -10)))
+
+(defparameter +input+
+  (loop :with line := (with-puzzle-file (stream)
+                        (read-line stream))
+        :with array := (make-array (* 4 (length line))
+                                   :element-type 'bit)
+        :for c :across line
+        :for nybble := (hex2val c)
+        :for i :upfrom 0 :by 4
+        :do (setf (aref array i) (logand 1 (ash nybble -3)))
+        :do (setf (aref array (+ 1 i)) (logand 1 (ash nybble -2)))
+        :do (setf (aref array (+ 2 i)) (logand 1 (ash nybble -1)))
+        :do (setf (aref array (+ 3 i)) (logand 1 (ash nybble -0)))
+        :finally (return array)))
+
+(defun reader ()
+  (cons 0 nil))
+
+(defun pos (reader)
+  (car reader))
+
+(defun (setf pos) (new-value reader)
+  (setf (car reader) new-value))
+
+(defun read-bits (reader size)
+  (loop :with start := (pos reader)
+        :with result := 0
+        :for i :from start :below (+ start size)
+        :do (setf result (+ (ash result 1)
+                            (aref +input+ i)))
+        :finally (incf (pos reader) size)
+                 (return result)))
+
+(defun read-literal (reader)
+  (loop :with result := 0
+        :for group := (read-bits reader 5)
+        :do (setf result (logior (ash result 4)
+                                 (logand group #b1111)))
+        :while (= (logand #b10000 group) #b10000)
+        :finally (return result)))
+
+(defun read-subpackets-to-length (reader length)
+  (loop :with start := (pos reader)
+        :collect (read-packet reader)
+        :while (< (- (pos reader) start) length)))
+
+(defun read-packet (reader)
+  (let ((version (read-bits reader 3))
+        (type (read-bits reader 3)))
+    (cond
+      ((= type 4)
+       (list :version version :type type
+             :value (read-literal reader)))
+      ((= 0 (read-bits reader 1))
+       (list :version version :type type
+             :subpackets (read-subpackets-to-length
+                           reader (read-bits reader 15))))
+      (t
+       (list :version version :type type
+             :subpackets (loop :repeat (read-bits reader 11)
+                               :collect (read-packet reader)))))))
+
+(defun sum-versions (packet)
+  (if (= 4 (getf packet :type))
+      (getf packet :version)
+      (+ (getf packet :version)
+         (loop :for subpacket :in (getf packet :subpackets)
+               :sum (sum-versions subpacket)))))
+
+(defun solve-part-1 ()
+  (sum-versions (read-packet (reader))))
+
+(defun packet-type (packet)
+  (getf packet :type))
+
+(defun subpackets (packet)
+  (getf packet :subpackets))
+
+(defun eval-packet (packet)
+  (eswitch (packet :key #'packet-type)
+    (0 (reduce #'+ (mapcar #'eval-packet (subpackets packet))))
+    (1 (reduce #'* (mapcar #'eval-packet (subpackets packet))))
+    (2 (reduce #'min (mapcar #'eval-packet (subpackets packet))))
+    (3 (reduce #'max (mapcar #'eval-packet (subpackets packet))))
+    (4 (getf packet :value))
+    (5 (if (> (eval-packet (first (subpackets packet)))
+              (eval-packet (second (subpackets packet))))
+           1 0))
+    (6 (if (< (eval-packet (first (subpackets packet)))
+              (eval-packet (second (subpackets packet))))
+           1 0))
+    (7 (if (= (eval-packet (first (subpackets packet)))
+              (eval-packet (second (subpackets packet))))
+           1 0))))
+
+(defun solve-part-2 ()
+  (eval-packet (read-packet (reader))))
+
+

@@ 0,0 1,1 @@
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