;; One Elf has the important job of loading all of the
;; rucksacks with supplies for the jungle journey.
;; Unfortunately, that Elf didn't quite follow the packing
;; instructions, and so a few items now need to be
;; rearranged.
;; Each rucksack has two large compartments. All items of a
;; given type are meant to go into exactly one of the two
;; compartments. The Elf that did the packing failed to
;; follow this rule for exactly one item type per rucksack.
;; The Elves have made a list of all of the items currently
;; in each rucksack (your puzzle input), but they need your
;; help finding the errors. Every item type is identified by
;; a single lowercase or uppercase letter (that is, a and A
;; refer to different types of items).
;; The list of items for each rucksack is given as
;; characters all on a single line. A given rucksack always
;; has the same number of items in each of its two
;; compartments, so the first half of the characters
;; represent items in the first compartment, while the
;; second half of the characters represent items in the
;; second compartment.
;; For example, suppose you have the following list of
;; contents from six rucksacks:
;; The first rucksack contains the items
;; vJrwpWtwJgWrhcsFMMfFFhFp, which means its first
;; compartment contains the items vJrwpWtwJgWr, while
;; the second compartment contains the items
;; hcsFMMfFFhFp. The only item type that appears in both
;; compartments is lowercase p.
;; The second rucksack's compartments contain
;; jqHRNqRjqzjGDLGL and rsFMfFZSrLrFZsSL. The only item
;; type that appears in both compartments is uppercase
;; L.
;; The third rucksack's compartments contain PmmdzqPrV
;; and vPwwTWBwg; the only common item type is uppercase
;; P.
;; The fourth rucksack's compartments only share item
;; type v.
;; The fifth rucksack's compartments only share item
;; type t.
;; The sixth rucksack's compartments only share item
;; type s.
;; To help prioritize item rearrangement, every item type
;; can be converted to a priority:
;; Lowercase item types a through z have priorities 1
;; through 26.
;; Uppercase item types A through Z have priorities 27
;; through 52.
;; In the above example, the priority of the item type that
;; appears in both compartments of each rucksack is 16 (p),
;; 38 (L), 42 (P), 22 (v), 20 (t), and 19 (s); the sum of
;; these is 157.
;; Find the item type that appears in both compartments of
;; each rucksack. What is the sum of the priorities of those
;; item types?
;; As you finish identifying the misplaced items, the Elves
;; come to you with another issue.
;; For safety, the Elves are divided into groups of three.
;; Every Elf carries a badge that identifies their group.
;; For efficiency, within each group of three Elves, the
;; badge is the only item type carried by all three Elves.
;; That is, if a group's badge is item type B, then all
;; three Elves will have item type B somewhere in their
;; rucksack, and at most two of the Elves will be carrying
;; any other item type.
;; The problem is that someone forgot to put this year's
;; updated authenticity sticker on the badges. All of the
;; badges need to be pulled out of the rucksacks so the new
;; authenticity stickers can be attached.
;; Additionally, nobody wrote down which item type
;; corresponds to each group's badges. The only way to tell
;; which item type is the right one is by finding the one
;; item type that is common between all three Elves in each
;; group.
;; Every set of three lines in your list corresponds to a
;; single group, but each group can have a different badge
;; item type. So, in the above example, the first group's
;; rucksacks are the first three lines:
;; In the first group, the only item type that appears in
;; all three rucksacks is lowercase r; this must be their
;; badges. In the second group, their badge item type must
;; be Z.
;; Priorities for these items must still be found to
;; organize the sticker attachment efforts: here, they are
;; 18 (r) for the first group and 52 (Z) for the second
;; group. The sum of these is 70.
;; Find the item type that corresponds to the badges of each
;; three-Elf group. What is the sum of the priorities of
;; those item types?
(defun day3/compute-part2 (lst)
(let ((groups (loop repeat (/ (length lst) 3)
for i = 0 then j
and j = 3 then (+ i 6)
collect (subseq lst i j))))
(apply
#'+
(day3/all-dupe-priorities
(mapcar (lambda (group)
(apply #'day3/intersection group))
groups)))))
