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;;;; 2015.01.02-more-lispy-this-time.lisp
;;;; Advent of Code 2015
;;;; Day 1: Not Quite Lisp
;;;; Attempt 2: More Lispy This Time
;; ---------------------------------------------------------------------
;;; Package Definition
;; ---------------------------------------------------------------------
(defpackage :2015-01-02-more-lispy-this-time
(:use #:common-lisp))
(in-package :2015-01-02-more-lispy-this-time)
;; ---------------------------------------------------------------------
;;; Definitions
;; ---------------------------------------------------------------------
(defconstant +PUZZLE-INPUT-FILE+ "~/proj/Learn/aoc/src/2015.01._PUZZLE-INPUT.txt"
"File to be used as input to the program.")
(defconstant +PUZZLE-INPUT-STRING+ (uiop:read-file-string +PUZZLE-INPUT-FILE+)
"String version of the text in `+PUZZLE-INPUT-FILE+'.")
(defconstant +PUZZLE-INPUT-LIST+ (loop for char across +PUZZLE-INPUT-STRING+
collect char)
"Puzzle input in lisp list form.")
(defconstant +PUZZLE-INPUT-VALUES-LIST+ (mapcar (lambda (INPUT-LIST)
(cond ((eql INPUT-LIST #\() 1)
((eql INPUT-LIST #\)) -1)))
+PUZZLE-INPUT-LIST+)
"Puzzle input converted into floor direction values.")
;; ---------------------------------------------------------------------
;;; Part 1
;; -------
;; 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?
;; ---------------------------------------------------------------------
(defun final-floor (INPUT)
"Calculate the final floor Santa ends up on given the provided `INPUT'."
(apply #'+ INPUT))
(assert (= 138 (final-floor +PUZZLE-INPUT-VALUES-LIST+)))
;; ---------------------------------------------------------------------
;;; 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?
;; ---------------------------------------------------------------------
(defun next-floor ()
"Determine the result of the next button press, go in that direction, and destroy the information so the Nazis don't get their dirty hands on it."
(let ((FLOOR-NUMBER 0)
(INPUT-POSITION 0)
(INPUT +PUZZLE-INPUT-VALUES-LIST+))
(lambda ()
(list (incf FLOOR-NUMBER (pop INPUT))
(incf INPUT-POSITION)))))
(defvar next-floor (next-floor))
(defun first-basement-input ()
"Determine the position of the input in `+PUZZLE-INPUT-VALUES-LIST' which results in the first entry into the basement (floor -1)."
(let ((RESULT (funcall next-floor)))
(loop until (= -1 (car RESULT))
do (setf RESULT (funcall next-floor)))
(cdr RESULT)))
;; Test for correct result
(assert (= 1771 (first-basement-input)))
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