Advent of Code 2017 day 6 solution

adventday6.el

; Returns the index of the largest value
(defun maxv (xs)
  (let* ((idx 1)
	 (mxidx 0)
	 (mxval (aref xs 0)))
    (while (< idx (length xs))
      (if (> (aref xs idx) mxval)
	  (progn
	    (setq mxval (aref xs idx))
	    (setq mxidx idx)))
      (setq idx (+ idx 1)))
    mxidx))

(defun distribute (xs)
  (let* ((mx-idx (maxv xs))
	 (mx-val (aref xs mx-idx))
	 (idx (+ mx-idx 1)))
    (aset xs mx-idx 0) ;set largest bank to 0
    (while (> mx-val 0)
      (if (>= idx (length xs))
	  (setq idx 0)) ;check for rollover
      (aset xs idx (+ (aref xs idx) 1))
      (setq mx-val (- mx-val 1))
      (setq idx (+ idx 1)))
    )
  )

(defun solve (raw)
  (let* ((xs (copy-sequence raw))
	 (st (make-hash-table :test 'equal))
	 (cycle nil)
	 (num-steps 0)
	 (num-iters 0))
    (while (not cycle)
      (puthash (md5 (mapconcat 'number-to-string xs ",")) num-steps st)
      (distribute xs)
      (setq num-steps (+ num-steps 1))
      (if (gethash (md5 (mapconcat 'number-to-string xs ",")) st)
	  (setq cycle t))
      )
    (let ((iter-count (gethash (md5 (mapconcat 'number-to-string xs ",")) st)))
      `((steps . ,num-steps)
	(iters-to-cycle . ,(- num-steps iter-count)))
      )
    )
  )