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;;; https://gitlab.com/Syndamia/senzill
(require :senzill)
(use-package :senzill.math)
(use-package :senzill.collections)
(use-package :senzill.pair)
(use-package :senzill.io)
(defparameter +rope-length+ 10)
(ask-for-stream (prog-input)
(let ((rope '()) (visited '((0 . 0))))
(flet ((norm>sqrt2p (p) ; If c's eucledian length is less than sqrt(2). In other words, if c is outside the 3x3 square.
"Optimized version of (> (norm p) (sqrt 2))"
(or (> (abs (car p)) 1) (> (abs (cdr p)) 1)))
(dir-to-pair (direction)
(cond ((char= direction #\R) '( 1 . 0))
((char= direction #\L) '(-1 . 0))
((char= direction #\U) '( 0 . 1))
((char= direction #\D) '( 0 . -1)))))
(dotimes (i +rope-length+)
(push '(0 . 0) rope))
(doread-lines (inpt :read-line-options (prog-input NIL))
(loop for i from 1 to (parse-integer inpt :start 2)
for dirvec = (dir-to-pair (char inpt 0))
do
(pair+= (first rope) dirvec) ; First knot is the head
;;; We update the knot positions from left to right (from head to tail)
;;; Since each knot depends on the previous, we iterate from head to tail-1
;;; but always update "the second element" (from head+1 to tail)
(loop for knot on rope
while (cdr knot)
for movec = (pair- (first knot) (second knot)) ; head - tail
if (norm>sqrt2p movec)
do
;; dist-resize so we don't put the knot on top of the other, but behind it
(pair+= (second knot) (pair-round (dist-resize-by movec -1)))
finally
(if (not (member-if (lambda (x) (pair= x (car knot))) visited))
(push (car knot) visited)))))
(print (length visited)))))
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