129 lines
4.7 KiB
Plaintext
129 lines
4.7 KiB
Plaintext
Advent of Code
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--- Day 7: Recursive Circus ---
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Wandering further through the circuits of the computer, you come upon a tower
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of programs that have gotten themselves into a bit of trouble. A recursive
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algorithm has gotten out of hand, and now they're balanced precariously in a
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large tower.
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One program at the bottom supports the entire tower. It's holding a large
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disc, and on the disc are balanced several more sub-towers. At the bottom of
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these sub-towers, standing on the bottom disc, are other programs, each
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holding their own disc, and so on. At the very tops of these
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sub-sub-sub-...-towers, many programs stand simply keeping the disc below
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them balanced but with no disc of their own.
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You offer to help, but first you need to understand the structure of these
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towers. You ask each program to yell out their name, their weight, and (if
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they're holding a disc) the names of the programs immediately above them
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balancing on that disc. You write this information down (your puzzle input).
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Unfortunately, in their panic, they don't do this in an orderly fashion; by
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the time you're done, you're not sure which program gave which information.
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For example, if your list is the following:
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pbga (66)
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xhth (57)
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ebii (61)
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havc (66)
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ktlj (57)
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fwft (72) -> ktlj, cntj, xhth
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qoyq (66)
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padx (45) -> pbga, havc, qoyq
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tknk (41) -> ugml, padx, fwft
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jptl (61)
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ugml (68) -> gyxo, ebii, jptl
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gyxo (61)
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cntj (57)
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...then you would be able to recreate the structure of the towers that looks
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like this:
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gyxo
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/
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ugml - ebii
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/ \
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| jptl
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| pbga
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/ /
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tknk --- padx - havc
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\ \
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| qoyq
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| ktlj
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\ /
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fwft - cntj
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\
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xhth
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In this example, tknk is at the bottom of the tower (the bottom program), and
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is holding up ugml, padx, and fwft. Those programs are, in turn, holding up
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other programs; in this example, none of those programs are holding up any
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other programs, and are all the tops of their own towers. (The actual tower
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balancing in front of you is much larger.)
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Before you're ready to help them, you need to make sure your information is
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correct. What is the name of the bottom program?
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Your puzzle answer was ___________.
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--- Part Two ---
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The programs explain the situation: they can't get down. Rather, they could
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get down, if they weren't expending all of their energy trying to keep the
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tower balanced. Apparently, one program has the wrong weight, and until it's
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fixed, they're stuck here.
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For any program holding a disc, each program standing on that disc forms a
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sub-tower. Each of those sub-towers are supposed to be the same weight, or
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the disc itself isn't balanced. The weight of a tower is the sum of the
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weights of the programs in that tower.
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In the example above, this means that for ugml's disc to be balanced, gyxo,
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ebii, and jptl must all have the same weight, and they do: 61.
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However, for tknk to be balanced, each of the programs standing on its disc
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and all programs above it must each match. This means that the following sums
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must all be the same:
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* ugml + (gyxo + ebii + jptl) = 68 + (61 + 61 + 61) = 251
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* padx + (pbga + havc + qoyq) = 45 + (66 + 66 + 66) = 243
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* fwft + (ktlj + cntj + xhth) = 72 + (57 + 57 + 57) = 243
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As you can see, tknk's disc is unbalanced: ugml's stack is heavier than the
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other two. Even though the nodes above ugml are balanced, ugml itself is too
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heavy: it needs to be 8 units lighter for its stack to weigh 243 and keep the
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towers balanced. If this change were made, its weight would be 60.
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Given that exactly one program is the wrong weight, what would its weight
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need to be to balance the entire tower?
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Your puzzle answer was ______.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, you should return to your advent calendar and try another
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puzzle.
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If you still want to see it, you can get your puzzle input.
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/2017
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. http://adventofcode.com/2017/day/7/input
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