From 03e14731fb51fafbf7931231674ecf7a621fcbf4 Mon Sep 17 00:00:00 2001
From: Tristan Williams <tgwil@tgwil.net>
Date: Sat, 11 Jan 2025 14:07:50 -0500
Subject: Complete AoC 2015 Day 1

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 create mode 100644 src/2015.01.02-more-lispy-this-time.lisp

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+;;;; 2015.01.02-more-lispy-this-time.lisp
+;;;; Advent of Code 2015 Day 1, first attempt in Lisp
+
+;; ---------------------------------------------------------------------
+;;; 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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