#+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