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#!/usr/bin/env -S sbcl --script
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;; Copyright (C) 2022
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;; This program is free software: you can redistribute it and/or modify
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;; it under the terms of the GNU General Public License as published by
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;; the Free Software Foundation, either version 3 of the License, or
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;; (at your option) any later version.
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;; This program is distributed in the hope that it will be useful,
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;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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;; GNU General Public License for more details.
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;; You should have received a copy of the GNU General Public License
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;; along with this program. If not, see <https://www.gnu.org/licenses/>.
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(require 'asdf)
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(require 'uiop)
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(require 'iterate)
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(require 'str)
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(defpackage advent-of-code-day-9
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(:use :cl :iterate)
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(:import-from :uiop :if-let))
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(in-package :advent-of-code-day-9)
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(defclass point ()
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((x
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:initarg :x
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:reader x)
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(y
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:initarg :y
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:reader y)))
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(defun new-point (x y)
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(make-instance 'point :x x :y y))
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(defmethod print-object ((obj point) stream)
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(print-unreadable-object (obj stream :type t)
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(format stream "x: ~a, y: ~a" (x obj) (y obj))))
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(defun point+ (a b)
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(new-point
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(+ (x a) (x b))
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(+ (y a) (y b))))
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(defun point- (a b)
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(new-point
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(- (x a) (x b))
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(- (y a) (y b))))
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(defun point= (a b)
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(and (= (x a) (x b))
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(= (y a) (y b))))
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(defclass rope ()
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((knots
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:initarg :knots
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:accessor knots)))
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(defun new-rope (len)
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(make-instance 'rope :knots (iter (for x from 1 to len)
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(collect (new-point 0 0)))))
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(defun touching? (head tail)
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(let ((delta (point- head tail)))
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(and (<= (abs (x delta)) 1)
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(<= (abs (y delta)) 1))))
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(defun aligned-along-y? (head tail)
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(= (x head) (x tail)))
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(defun aligned-along-x? (head tail)
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(= (y head) (y tail)))
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(defun aligned? (head tail)
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(or (aligned-along-x? head tail)
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(aligned-along-y? head tail)))
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;; Not sure what to name this function, but this is used to move the tail
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;; towards the head when they are not touching but aligned by row or column,
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;; which means we just step once in one direction instead of a diagonal hop.
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(defun catch-up-directional (head tail)
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(let ((delta (point- head tail)))
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(point+ tail (if (aligned-along-x? head tail)
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(if (plusp (x delta))
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(new-point 1 0)
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(new-point -1 0))
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(if (plusp (y delta))
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(new-point 0 1)
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(new-point 0 -1))))))
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(defun catch-up-diagonal (head tail)
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(let ((delta (point- head tail)))
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(point+ tail (cond ((and (plusp (x delta)) (plusp (y delta)))
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(new-point 1 1))
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((and (minusp (x delta)) (plusp (y delta)))
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(new-point -1 1))
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((and (plusp (x delta)) (minusp (y delta)))
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(new-point 1 -1))
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((and (minusp (x delta)) (minusp (y delta)))
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(new-point -1 -1))))))
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(defun catch-up (rope)
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(iter (for (head . rest) on (knots rope))
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(while rest)
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(for tail = (car rest))
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(unless (touching? head tail)
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(setf (car rest) (if (aligned? head tail)
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(catch-up-directional head tail)
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(catch-up-diagonal head tail))))))
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(defun move-rope (rope direction)
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(with-accessors ((knots knots))
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rope
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(setf (car knots) (point+ (car knots) (case direction
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(:right (new-point 1 0))
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(:left (new-point -1 0))
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(:up (new-point 0 1))
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(:down (new-point 0 -1))))))
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(catch-up rope))
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(defun parse-input (input)
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(iter (for line in input)
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(for parts = (str:split " " line :omit-nulls t))
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(for direction = (car parts))
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(for steps = (parse-integer (cadr parts)))
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(collect (cons (cond ((string= direction "R")
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:right)
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((string= direction "L")
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:left)
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((string= direction "U")
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:up)
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((string= direction "D")
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:down))
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steps))))
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(defun solution (input rope-length)
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(iter (with rope = (new-rope rope-length))
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(with moves = (parse-input input))
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(with history = nil)
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(for (direction . count) in moves)
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(iter (repeat count)
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(move-rope rope direction)
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(for tail = (car (last (knots rope))))
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(unless (find tail history :test 'point=)
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(push tail history)))
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(finally (return (length history)))))
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(defun debug-print (&key stream width height rope)
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(iter (for y from height downto 0)
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(iter (for x from 0 to width)
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(for current-point = (new-point x y))
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(for position = (position current-point (knots rope) :test 'point=))
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(if position
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(format stream "~a " position)
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(format stream "* ")))
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(format stream "~%")
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(finally (format stream "~%"))))
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(assert (= 13 (solution (uiop:read-file-lines "example.txt") 2)))
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(assert (= 6044 (solution (uiop:read-file-lines "input.txt") 2)))
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(assert (= 1 (solution (uiop:read-file-lines "example.txt") 10)))
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(assert (= 36 (solution (uiop:read-file-lines "example-large.txt") 10)))
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(assert (= 2384 (solution (uiop:read-file-lines "input.txt") 10)))
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;; Local Variables:
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;; mode: lisp
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;; End:
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