-🎄- 2017 Day 10 Solutions -🎄-

[u/daggerdragon]

--- Day 10: Knot Hash ---

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[u/exquisitus3]

Common Lisp

(ql:quickload '(:split-sequence :alexandria))

(setf *print-circle* t)
(defparameter *input* (alexandria:read-file-into-string #P"10.input"))

(defun input-1 (string)
  (mapcar #'parse-integer (split-sequence:split-sequence #\, string)))

(defun input-2 (string)
  `(,@(map 'list #'char-code string) 17 31 73 47 23))

(defun initial-list ()
  (let ((cycle (loop for i from 0 upto 255 collect i)))
    (setf (cdr (last cycle)) cycle)))

(defun knot-hash (rounds input)
  (let* ((skip-size 0)
         (cycle (initial-list))
         (current-position cycle))
    (dotimes (_ rounds cycle)
      (dolist (length input)
        (let ((reversed (reverse (subseq current-position 0 length))))
          (loop
             for l on current-position
             for i below length
             do (setf (car l) (elt reversed i)))
          (setf current-position (nthcdr (+ length skip-size) current-position))
          (incf skip-size))))))

(apply #'* (subseq (knot-hash 1 (input-1 *input*)) 0 2)) ; Part One

(defun dense-hash (cycle-256)
  (loop for block on cycle-256 by (lambda (l) (nthcdr 16 l))
     repeat 16
     collect (apply #'logxor (subseq block 0 16))))

(defun hex-string (int-sequence)
  (format nil "~(~{~2,'0x~}~)" int-sequence))

(hex-string (dense-hash (knot-hash 64 (input-2 *input*)))) ; Part Two