advent-of-code/advent-of-code.org
2015-12-09 13:35:19 -05:00

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#+TITLE: Advent of Code
#+STARTUP: indent
#+OPTIONS: num:nil ^:nil d:nil
#+PROPERTY: header-args :cache yes
#+DRAWERS: HIDDEN
* Day 1: Not Quite Lisp
Santa was hoping for a white Christmas, but his weather machine's
"snow" function is powered by stars, and he's fresh out! To save
Christmas, he needs you to collect *fifty stars* by December 25th.
Collect stars by helping Santa solve puzzles. Two puzzles will be made
available on each day in the advent calendar; the second puzzle is
unlocked when you complete the first. Each puzzle grants *one star*.
Good luck!
Here's an easy puzzle to warm you up.
Santa is trying to deliver presents in a large apartment building, but
he can't find the right floor - the directions he got are a little
confusing. He starts on the ground floor (floor =0=) and then follows
the instructions one character at a time.
An opening parenthesis, =(=, means he should go up one floor, and a
closing parenthesis, =)=, means he should go down one floor.
The apartment building is very tall, and the basement is very deep; he
will never find the top or bottom floors.
For example:
- =(())= and =()()= both result in floor 0.
- =(((= and =(()(()(= both result in floor 3.
- =))(((((= also results in floor 3.
- =())= and =))(= both result in floor -1 (the first basement level).
- =)))= and =)())())= both result in floor -3.
To what floor do the instructions take Santa?
----------------------------------------------------------------------
:HIDDEN:
#+name: 1-input
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#+END_EXAMPLE
:END:
#+name: 1-solution
#+BEGIN_SRC emacs-lisp :var input=1-input :exports both
(defun day1/parse-directions (directions)
(seq-map
(lambda (char)
(cond ((eq char ?\() 1)
((eq char ?\)) -1)
(t 0)))
directions))
(seq-reduce #'+
(day1/parse-directions input)
0)
#+END_SRC
#+RESULTS[966acea0b884f9a83f09aae9b3c00803a516e3ca]: 1-solution
: 74
** Part 2
Now, given the same instructions, find the position of the first
character that causes him to enter the basement (floor =-1=). The
first character in the instructions has position =1=, the second
character has position =2=, and so on.
For example:
- =)= causes him to enter the basement at character position =1=.
- =()())= causes him to enter the basement at character position =5=.
What is the position of the character that causes Santa to first enter the basement?
----------------------------------------------------------------------
#+name: 1.2-solution
#+BEGIN_SRC emacs-lisp :var input=1-input :exports both
(defun day1/steps-until (predicate reduce-fn sequence initial-value)
(car (seq-reduce
(lambda (acc next)
(let ((counter (car acc))
(value (cdr acc)))
(if (funcall predicate value) acc
(cons (+ 1 counter)
(funcall reduce-fn value next)))))
sequence
(cons 0 initial-value))))
(day1/steps-until
(lambda (floor) (= floor -1))
#'+
(day1/parse-directions input)
0)
#+END_SRC
#+RESULTS[048e5b88e35ca21b8be15e5cffaa117c4e1eca45]: 1.2-solution
: 1795