#!/usr/bin/env -S sbcl --script

;; Copyright (C) 2022
;; This program is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with this program.  If not, see <https://www.gnu.org/licenses/>.

(require 'asdf)
(require 'uiop)
(require 'iterate)
(require 'str)

(defpackage advent-of-code-day-9
  (:use :cl :iterate)
  (:import-from :uiop :if-let))

(in-package :advent-of-code-day-9)

(defclass point ()
  ((x
    :initarg :x
    :reader x)
   (y
    :initarg :y
    :reader y)))

(defmacro new-point (x y)
  `(make-instance 'point :x ,x :y ,y))

(defmethod print-object ((obj point) stream)
  (print-unreadable-object (obj stream :type t)
    (format stream "x: ~a, y: ~a" (x obj) (y obj))))

(defun point+ (a b)
  (new-point
   (+ (x a) (x b))
   (+ (y a) (y b))))

(defun point- (a b)
  (new-point
   (- (x a) (x b))
   (- (y a) (y b))))

(defun point= (a b)
  (= (x a) (x b) (y a) (y b)))

(defclass rope ()
  ((head
    :initarg :head
    :accessor head)
   (tail
    :initarg :tail
    :accessor tail)))

(defmacro new-rope (head tail)
  `(make-instance 'rope :head ,head :tail ,tail))

(defmethod print-object ((obj rope) stream)
  (print-unreadable-object (obj stream :type t)
    (format stream "head: ~a, tail: ~a" (head obj) (tail obj))))

(defun touching? (rope)
  (let ((delta (point- (head rope) (tail rope))))
    (and (<= (abs (x delta)) 1)
         (<= (abs (y delta)) 1))))

(defun aligned-along-y? (rope)
  (= (x (head rope)) (x (tail rope))))

(defun aligned-along-x? (rope)
  (= (y (head rope)) (y (tail rope))))

(defun aligned? (rope)
  (or (aligned-along-x? rope)
      (aligned-along-y? rope)))


;; Not sure what to name this function, but this is used to move the tail
;; towards the head when they are not touching but aligned by row or column,
;; which means we just step once in one direction instead of a diagonal hop.

(defun catch-up-directional (rope)
  (let ((delta (point- (head rope) (tail rope))))
    (setf (tail rope) (point+ (tail rope) (if (aligned-along-x? rope)
                                              (if (plusp (x delta))
                                                  (new-point 1 0)
                                                  (new-point -1 0))
                                              (if (plusp (y delta))
                                                  (new-point 0 1)
                                                  (new-point 0 -1)))))))

(defun catch-up-diagonal (rope)
  (let ((delta (point- (head rope) (tail rope))))
    (setf (tail rope) (point+ (tail rope) (cond ((and (plusp (x delta)) (plusp (y delta)))
                                                 (new-point 1 1))
                                                ((and (minusp (x delta)) (plusp (y delta)))
                                                 (new-point -1 1))
                                                ((and (plusp (x delta)) (minusp (y delta)))
                                                 (new-point 1 -1))
                                                ((and (minusp (x delta)) (minusp (y delta)))
                                                 (new-point -1 -1)))))))

(defun catch-up (rope)
  (with-accessors ((head head)
                   (tail tail)
                   (history history))
      rope
    (unless (touching? rope)
      (if (aligned? rope)
          (catch-up-directional rope)
          (catch-up-diagonal rope)))))

(defun process-move (rope direction)
  (setf (head rope) (point+ (head rope) (case direction
                                          (:right (new-point 1 0))
                                          (:left (new-point -1 0))
                                          (:up (new-point 0 1))
                                          (:down (new-point 0 -1)))))
  (catch-up rope))

(defun parse-input (input)
  (iter (for line in input)
        (for parts = (str:split " " line :omit-nulls t))        
        (for direction = (car parts))
        (for steps = (parse-integer (cadr parts)))
        (collect (cons (cond ((string= direction "R")
                              :right)
                             ((string= direction "L")
                              :left)
                             ((string= direction "U")
                              :up)
                             ((string= direction "D")
                              :down))
                       steps))))

(defun solution-part-1 (input)
  (let ((rope (new-rope (new-point 0 0) (new-point 0 0)))
        (moves (parse-input input))
        (history (list (new-point 0 0))))
    (iter (for (direction . count) in moves)                   
          (iter (for x from 0 to count)
                (process-move rope direction)
                (debug-print rope t)
                (unless (find (tail rope) history :test 'point=)
                  (push (tail rope) history)))
          (finally (return history)))))

(defun debug-print (rope stream)
  (format stream "~A~%" (tail rope))
  (iter (for y from 0 to 5)        
        (iter (for x from 0 to 6)              
              (cond ((point= (head rope) (new-point x y))
                     (format stream "H "))
                    ((point= (tail rope) (new-point x y))
                     (format stream "T "))
                    (t
                     (format stream "* ")))
              (finally (format stream "~%")))
        (finally (format stream "~%"))))

(assert (touching? (new-rope (new-point 5 5) (new-point 4 4))))
(assert (not (touching? (new-rope (new-point 5 5) (new-point 4 3)))))
(assert (aligned? (new-rope (new-point 1 0) (new-point 2 0))))

(solution-part-1 (uiop:read-file-lines "example.txt"))

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