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day05.txt
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day05.txt
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--- Day 5: Alchemical Reduction ---
You've managed to sneak in to the prototype suit manufacturing lab. The Elves
are making decent progress, but are still struggling with the suit's size
reduction capabilities.
While the very latest in 1518 alchemical technology might have solved their
problem eventually, you can do better. You scan the chemical composition of the
suit's material and discover that it is formed by extremely long polymers (one
of which is available as your puzzle input).
The polymer is formed by smaller units which, when triggered, react with each
other such that two adjacent units of the same type and opposite polarity are
destroyed. Units' types are represented by letters; units' polarity is
represented by capitalization. For instance, r and R are units with the same
type but opposite polarity, whereas r and s are entirely different types and do
not react.
For example:
- In aA, a and A react, leaving nothing behind.
- In abBA, bB destroys itself, leaving aA. As above, this then destroys
itself, leaving nothing.
- In abAB, no two adjacent units are of the same type, and so nothing
happens.
- In aabAAB, even though aa and AA are of the same type, their polarities
match, and so nothing happens.
Now, consider a larger example, dabAcCaCBAcCcaDA:
dabAcCaCBAcCcaDA The first 'cC' is removed.
dabAaCBAcCcaDA This creates 'Aa', which is removed.
dabCBAcCcaDA Either 'cC' or 'Cc' are removed (the result is the same).
dabCBAcaDA No further actions can be taken.
After all possible reactions, the resulting polymer contains 10 units.
How many units remain after fully reacting the polymer you scanned? (Note: in
this puzzle and others, the input is large; if you copy/paste your input, make
sure you get the whole thing.)
--- Part Two ---
Time to improve the polymer.
One of the unit types is causing problems; it's preventing the polymer from
collapsing as much as it should. Your goal is to figure out which unit type is
causing the most problems, remove all instances of it (regardless of polarity),
fully react the remaining polymer, and measure its length.
For example, again using the polymer dabAcCaCBAcCcaDA from above:
- Removing all A/a units produces dbcCCBcCcD. Fully reacting this polymer
produces dbCBcD, which has length 6.
- Removing all B/b units produces daAcCaCAcCcaDA. Fully reacting this polymer
produces daCAcaDA, which has length 8.
- Removing all C/c units produces dabAaBAaDA. Fully reacting this polymer
produces daDA, which has length 4.
- Removing all D/d units produces abAcCaCBAcCcaA. Fully reacting this polymer
produces abCBAc, which has length 6.
In this example, removing all C/c units was best, producing the answer 4.
What is the length of the shortest polymer you can produce by removing all
units of exactly one type and fully reacting the result?