;; The preparations are finally complete; you and the Elves
;; leave camp on foot and begin to make your way toward the
;; star fruit grove.
;; As you move through the dense undergrowth, one of the
;; Elves gives you a handheld device. He says that it has
;; many fancy features, but the most important one to set up
;; right now is the communication system.
;; However, because he's heard you have significant
;; experience dealing with signal-based systems, he
;; convinced the other Elves that it would be okay to give
;; you their one malfunctioning device - surely you'll have
;; no problem fixing it.
;; As if inspired by comedic timing, the device emits a few
;; colorful sparks.
;; To be able to communicate with the Elves, the device
;; needs to lock on to their signal. The signal is a series
;; of seemingly-random characters that the device receives
;; one at a time.
;; To fix the communication system, you need to add a
;; subroutine to the device that detects a start-of-packet
;; marker in the datastream. In the protocol being used by
;; the Elves, the start of a packet is indicated by a
;; sequence of four characters that are all different.
;; The device will send your subroutine a datastream buffer
;; (your puzzle input); your subroutine needs to identify
;; the first position where the four most recently received
;; characters were all different. Specifically, it needs to
;; report the number of characters from the beginning of the
;; buffer to the end of the first such four-character
;; marker.
;; For example, suppose you receive the following datastream
;; buffer:
;; mjqjpqmgbljsphdztnvjfqwrcgsmlb
;; After the first three characters (mjq) have been
;; received, there haven't been enough characters received
;; yet to find the marker. The first time a marker could
;; occur is after the fourth character is received, making
;; the most recent four characters mjqj. Because j is
;; repeated, this isn't a marker.
;; The first time a marker appears is after the seventh
;; character arrives. Once it does, the last four characters
;; received are jpqm, which are all different. In this case,
;; your subroutine should report the value 7, because the
;; first start-of-packet marker is complete after 7
;; characters have been processed.
;; Here are a few more examples:
;; bvwbjplbgvbhsrlpgdmjqwftvncz: first marker after character 5
;; nppdvjthqldpwncqszvftbrmjlhg: first marker after character 6
;; nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg: first marker after character 10
;; zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw: first marker after character 11
;; How many characters need to be processed before the first
;; start-of-packet marker is detected?
(defun day6/compute-marker-position (line n)
(let ((lst (coerce line 'list)))
(loop for cons on lst
counting cons into idx
when (day6/markerp (subseq cons 0 n))
do (return (+ n (1- idx))))))
;; Your device's communication system is correctly detecting
;; packets, but still isn't working. It looks like it also
;; needs to look for messages.
;; A start-of-message marker is just like a start-of-packet
;; marker, except it consists of 14 distinct characters
;; rather than 4.
;; Here are the first positions of start-of-message markers
;; for all of the above examples:
;; mjqjpqmgbljsphdztnvjfqwrcgsmlb: first marker after character 19
;; bvwbjplbgvbhsrlpgdmjqwftvncz: first marker after character 23
;; nppdvjthqldpwncqszvftbrmjlhg: first marker after character 23
;; nznrnfrfntjfmvfwmzdfjlvtqnbhcprsg: first marker after character 29
;; zcfzfwzzqfrljwzlrfnpqdbhtmscgvjw: first marker after character 26
;; How many characters need to be processed before the first
;; start-of-message marker is detected?
