Complete AoC 2015 Day 1

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+;;;; 2015.01.01-first-attempt.lisp
+;;;; Advent of Code 2015 Day 1, first attempt in Lisp
+
+;; ---------------------------------------------------------------------
+;;; Package Definition
+;; ---------------------------------------------------------------------
+
+(defpackage :2015-01-first-attempt
+  (:use #:common-lisp))
+(in-package :2015-01-first-attempt)
+
+
+
+
+;; ---------------------------------------------------------------------
+;;; Definitions
+;; ---------------------------------------------------------------------
+
+(defparameter *PUZZLE-INPUT-FILE* "~/proj/Learn/aoc/src/2015.01._PUZZLE-INPUT.txt"
+  "File to be used as input to the program.")
+
+(defparameter *PUZZLE-INPUT-STRING* (uiop:read-file-string *PUZZLE-INPUT-FILE*)
+  "String version of the text in `*PUZZLE-INPUT-FILE*'.")
+
+(defparameter *PUZZLE-INPUT* (loop for char across *PUZZLE-INPUT-STRING*
+                                   collect char)
+  "Puzzle input in lisp list form.")
+
+
+
+
+;; ---------------------------------------------------------------------
+;;; 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 parse-input (INPUT)
+  "Parse `INPUT' to determine where the instructions take Santa."
+  (let ((FLOOR-NUMBER 0))
+    (+ FLOOR-NUMBER (cond ((eql (first INPUT) nil) 0)
+                          ((eql (first INPUT) #\() (+ 1 (parse-input (rest INPUT))))
+                          ((eql (first INPUT) #\)) (- (parse-input (rest INPUT)) 1))))))
+
+;; Test for correct answer
+(assert (= 138 (parse-input *PUZZLE-INPUT*)))
+
+
+;; I have elected to just throw this away and not do part 2 in this line of thinking.
+;; Keeping it for posterity.