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(load "lib")
(defparameter input (uiop:read-file-string "9-input"))
(defparameter lines (get-lines input))
(defun move (P X Y)
(cons (+ (car P) X) (+ (cdr P) Y)))
(defun -1unit (x)
(if (< x 0)
(+ x 1)
(- x 1)))
(defun update-tail (TAIL HEAD)
(let ((xd (- (car HEAD) (car TAIL)))
(yd (- (cdr HEAD) (cdr TAIL))))
(if (and (<= (abs xd) 1) (<= (abs yd) 1))
;; Leave as is
TAIL
(cond
;; Diagonal movements
((and (= 1 (abs xd)) (= 2 (abs yd)))
(move TAIL xd (-1unit yd)))
((and (= 1 (abs yd)) (= 2 (abs xd)))
(move TAIL (-1unit xd) yd))
;; Linear movements on an axis
((= (abs xd) 2)
(move TAIL (-1unit xd) 0))
((= (abs yd) 2)
(move TAIL 0 (-1unit yd)))))))
(defun move-direction (direction P)
(case direction
(R (move P 1 0))
(L (move P -1 0))
(U (move P 0 1))
(D (move P 0 -1))))
(defun print-grid (head tail)
(destructuring-bind (hx . hy) head
(destructuring-bind (tx . ty) tail
(loop for x from 0 to 6
do
(format t "-"))
(format t "~%")
(loop for y from 6 downto 0
do
(progn
(format t "~d: " y)
(loop for x from 0 to 6
do
(format t "~s"
(cond
((and (= hx x) (= hy y)) 'H)
((and (= tx x) (= ty y)) 'T)
(t '-))))
(format t "~%"))))))
(defun parse-line (line)
(cons (intern (subseq line 0 1))
(parse-integer (subseq line 2))))
(defun coords-eq? (x y)
(and (= (car x) (car y)) (= (cdr x) (cdr y))))
(defun single-knot-execute-line (line head tail points)
(destructuring-bind (direction . magnitude) (parse-line line)
(dotimes (x magnitude)
(setq head (move-direction direction head))
(setq tail (update-tail tail head))
(setq points (adjoin tail points :test #'coords-eq?))))
(values head tail points))
(defun single-knot-execute-lines (lines &optional head tail points)
(if (null lines)
(values
head tail points)
(let ((line (car lines))
(head (or head `(0 . 0)))
(tail (or tail `(0 . 0)))
(points (or points nil)))
(multiple-value-bind (head tail points) (single-knot-execute-line line head tail points)
(single-knot-execute-lines (cdr lines) head tail points)))))
(format t "round 1: ~s~%"
(length (car (last (multiple-value-list (single-knot-execute-lines lines))))))
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