Day 2, Part 2

advent-of-code.org

@@ -1190,3 +1190,60 @@ feet of wrapping paper* should they order?
 #+RESULTS[f343fb2890e11ccbcc408d25b441e9a808073e1f]: 2-solution
 : 1606483
 
+** Part 2
+The elves are also running low on ribbon. Ribbon is all the same
+width, so they only have to worry about the length they need to order,
+which they would again like to be exact.
+
+The ribbon required to wrap a present is the shortest distance around
+its sides, or the smallest perimeter of any one face. Each present
+also requires a bow made out of ribbon as well; the feet of ribbon
+required for the perfect bow is equal to the cubic feet of volume of
+the present. Don't ask how they tie the bow, though; they'll never
+tell.
+
+For example:
+
+- A present with dimensions =2x3x4= requires =2+2+3+3 = 10= feet of
+  ribbon to wrap the present plus =2*3*4 = 24= feet of ribbon for the
+  bow, for a total of =34= feet.
+- A present with dimensions =1x1x10= requires =1+1+1+1 = 4= feet of
+  ribbon to wrap the present plus =1*1*10 = 10= feet of ribbon for the
+  bow, for a total of =14= feet.
+
+How many total feet of ribbon should they order?
+
+----------------------------------------------------------------------
+
+#+BEGIN_SRC emacs-lisp :var input=2-input :exports both
+  (defun day2/smallest-perimeter (w h l)
+    (min (* 2 (+ l w))
+         (* 2 (+ w h))
+         (* 2 (+ h l))))
+
+  (defun day2/volume (w h l)
+    (* w h l))
+
+  (defun day2/ribbon (w h l)
+    (+ (day2/smallest-perimeter w h l)
+       (day2/volume w h l)))
+
+  (ert-deftest day2/ribbon ()
+    (should (eq 34 (day2/ribbon 2 3 4)))
+    (should (eq 14 (day2/ribbon 1 1 10))))
+
+  (defun day2/sum-ribbon (input)
+    (seq-reduce
+     #'+
+     (seq-map (lambda (dimension-string)
+                (let ((dimensions (day2/parse-dimensions dimension-string)))
+                  (eval (cons 'day2/ribbon dimensions))))
+              (split-string input))
+     0))
+
+  (day2/sum-ribbon input)
+#+END_SRC
+
+#+RESULTS[c13958a4756029347386f7b48c44959c7b8e3dea]:
+: 3842356
+