;; Space needs to be cleared before the last supplies can be
;; unloaded from the ships, and so several Elves have been
;; assigned the job of cleaning up sections of the camp.
;; Every section has a unique ID number, and each Elf is
;; assigned a range of section IDs.
;; However, as some of the Elves compare their section
;; assignments with each other, they've noticed that many of
;; the assignments overlap. To try to quickly find overlaps
;; and reduce duplicated effort, the Elves pair up and make
;; a big list of the section assignments for each pair (your
;; puzzle input).
;; For example, consider the following list of section
;; assignment pairs:
;; Within the first pair of Elves, the first Elf was
;; assigned sections 2-4 (sections 2, 3, and 4), while
;; the second Elf was assigned sections 6-8 (sections 6,
;; 7, 8).
;; The Elves in the second pair were each assigned two
;; sections.
;; The Elves in the third pair were each assigned three
;; sections: one got sections 5, 6, and 7, while the
;; other also got 7, plus 8 and 9.
;; This example list uses single-digit section IDs to make
;; it easier to draw; your actual list might contain larger
;; numbers. Visually, these pairs of section assignments
;; look like this:
;; .234..... 2-4
;; .....678. 6-8
;; .23...... 2-3
;; ...45.... 4-5
;; ....567.. 5-7
;; ......789 7-9
;; .2345678. 2-8
;; ..34567.. 3-7
;; .....6... 6-6
;; ...456... 4-6
;; .23456... 2-6
;; ...45678. 4-8
;; Some of the pairs have noticed that one of their
;; assignments fully contains the other. For example, 2-8
;; fully contains 3-7, and 6-6 is fully contained by 4-6. In
;; pairs where one assignment fully contains the other, one
;; Elf in the pair would be exclusively cleaning sections
;; their partner will already be cleaning, so these seem
;; like the most in need of reconsideration. In this
;; example, there are 2 such pairs.
;; In how many assignment pairs does one range fully contain
;; the other?
(defmethod initialize-instance :after ((r day4/range) &key)
(with-slots (start end) r
(when (> start end)
(error "can't have a range with a start after the end"))))
;; It seems like there is still quite a bit of duplicate
;; work planned. Instead, the Elves would like to know the
;; number of pairs that overlap at all.
;; In the above example, the first two pairs (2-4,6-8 and
;; 2-3,4-5) don't overlap, while the remaining four pairs
;; (5-7,7-9, 2-8,3-7, 6-6,4-6, and 2-6,4-8) do overlap:
;; 5-7,7-9 overlaps in a single section, 7.
;; 2-8,3-7 overlaps all of the sections 3 through 7.
;; 6-6,4-6 overlaps in a single section, 6.
;; 2-6,4-8 overlaps in sections 4, 5, and 6.
;; So, in this example, the number of overlapping assignment
;; pairs is 4.
;; In how many assignment pairs do the ranges overlap?
