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87 lines
2.9 KiB
Markdown
87 lines
2.9 KiB
Markdown
2 years ago
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https://adventofcode.com/2021/day/14
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## \--- Day 14: Extended Polymerization ---
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The incredible pressures at this depth are starting to put a strain on your
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submarine. The submarine has
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[polymerization](https://en.wikipedia.org/wiki/Polymerization) equipment that
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would produce suitable materials to reinforce the submarine, and the nearby
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volcanically-active caves should even have the necessary input elements in
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sufficient quantities.
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The submarine manual contains instructions for finding the optimal polymer
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formula; specifically, it offers a _polymer template_ and a list of _pair
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insertion_ rules (your puzzle input). You just need to work out what polymer
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would result after repeating the pair insertion process a few times.
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For example:
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[code]
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NNCB
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CH -> B
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HH -> N
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CB -> H
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NH -> C
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HB -> C
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HC -> B
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HN -> C
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NN -> C
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BH -> H
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NC -> B
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NB -> B
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BN -> B
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BB -> N
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BC -> B
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CC -> N
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CN -> C
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[/code]
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The first line is the _polymer template_ \- this is the starting point of the
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process.
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The following section defines the _pair insertion_ rules. A rule like `AB ->
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C` means that when elements `A` and `B` are immediately adjacent, element `C`
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should be inserted between them. These insertions all happen simultaneously.
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So, starting with the polymer template `NNCB`, the first step simultaneously
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considers all three pairs:
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* The first pair (`NN`) matches the rule `NN -> C`, so element `_C_` is inserted between the first `N` and the second `N`.
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* The second pair (`NC`) matches the rule `NC -> B`, so element `_B_` is inserted between the `N` and the `C`.
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* The third pair (`CB`) matches the rule `CB -> H`, so element `_H_` is inserted between the `C` and the `B`.
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Note that these pairs overlap: the second element of one pair is the first
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element of the next pair. Also, because all pairs are considered
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simultaneously, inserted elements are not considered to be part of a pair
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until the next step.
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After the first step of this process, the polymer becomes `N _C_ N _B_ C _H_
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B`.
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Here are the results of a few steps using the above rules:
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[code]
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Template: NNCB
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After step 1: NCNBCHB
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After step 2: NBCCNBBBCBHCB
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After step 3: NBBBCNCCNBBNBNBBCHBHHBCHB
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After step 4: NBBNBNBBCCNBCNCCNBBNBBNBBBNBBNBBCBHCBHHNHCBBCBHCB
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[/code]
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This polymer grows quickly. After step 5, it has length 97; After step 10, it
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has length 3073. After step 10, `B` occurs 1749 times, `C` occurs 298 times,
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`H` occurs 161 times, and `N` occurs 865 times; taking the quantity of the
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most common element (`B`, 1749) and subtracting the quantity of the least
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common element (`H`, 161) produces `1749 - 161 = _1588_`.
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Apply 10 steps of pair insertion to the polymer template and find the most and
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least common elements in the result. _What do you get if you take the quantity
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of the most common element and subtract the quantity of the least common
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element?_
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