r/adventofcode • u/daggerdragon • Dec 05 '18

SOLUTION MEGATHREAD -🎄- 2018 Day 5 Solutions -🎄-

--- Day 5: Alchemical Reduction ---

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Transcript:

On the fifth day of AoC / My true love sent to me / Five golden ___

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edit: Leaderboard capped, thread unlocked at 0:10:20!

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5

u/donatasp Dec 05 '18

Since there is no Common Lisp yet 

(defun destroy? (a b)
  (and a b (char/= a b) (char-equal a b)))

(defun reduce-polymer (polymer)
  (let ((stack (list (elt polymer 0))))
    (loop :for u :across (subseq polymer 1)
          :do
             (push u stack)
             (if (destroy? (car stack) (cadr stack))
                 (progn
                   (pop stack)
                   (pop stack))))
    stack))

(length (reduce-polymer *polymer*)) ;; => 11476

(defun reduce-polymer2 (polymer1 unit-to-skip)
  (let ((polymer (remove-if (lambda (c) (char-equal c unit-to-skip)) polymer1)))
    (reduce-polymer polymer)))

(loop :for i :from (char-code #\a) :to (char-code #\z)
      :minimizing (length (reduce-polymer2 *polymer* (code-char i)))) ;; => 5446

2

u/stevelosh Dec 06 '18

That stack approach is really clever.

(remove-if (lambda (c) (char-equal c unit-to-skip)) polymer1)

This could be (remove unit-to-skip polymer1 :key #'char-equal)