Day 8

Added solution for day 8.

advent-of-code-2018.asd

@@ -15,4 +15,5 @@
                (:file "day4")
                (:file "day5")
                (:file "day6")
-               (:file "day7")))
+               (:file "day7")
+               (:file "day8")))

day8.lisp

@@ -0,0 +1,32 @@
+;;;; day8.lisp
+
+(in-package :advent-of-code-2018)
+
+(defun day8-build-tree ()
+  (with-open-file (in (puzzlepath "input8.txt"))
+    (labels ((build-node ()
+               (let ((num-children (read in))
+                     (num-metadata (read in)))
+                 (list (loop :repeat num-children
+                             :collect (build-node))
+                       (loop :repeat num-metadata
+                             :collect (read in))))))
+      (build-node))))
+
+(defun day8-sum-metadata (node)
+  (+ (reduce #'+ (mapcar #'day8-sum-metadata (first node)))
+     (reduce #'+ (second node))))
+
+(defun day8-node-value (node)
+  (flet ((nth-child-value (n)
+           (if (<= 1 n (length (first node)))
+               (day8-node-value (nth (1- n) (first node)))
+               0)))
+    (if (null (first node))
+        (reduce #'+ (second node))
+        (reduce #'+ (mapcar #'nth-child-value (second node))))))
+
+(defun day8 ()
+  (let ((root (day8-build-tree)))
+    (format t "The sum of all metadata is ~a and the score for the root node is ~a.~%"
+            (day8-sum-metadata root) (day8-node-value root))))