60 lines
2.1 KiB
Markdown
60 lines
2.1 KiB
Markdown
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## \-\-- Day 16: Permutation Promenade \-\--
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You come upon a very unusual sight; a group of programs here appear to
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be [dancing](https://www.youtube.com/watch?v=lyZQPjUT5B4&t=53).
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There are sixteen programs in total, named `a` through `p`. They start
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by standing in a line: `a` stands
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in position `0`, `b` stands in position `1`, and so on until `p`, which
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stands in position `15`.
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The programs\' *dance* consists of a sequence of *dance moves*:
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- *Spin*, written `sX`, makes `X` programs move from the end to the
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front, but maintain their order otherwise. (For example, `s3` on
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`abcde` produces `cdeab`).
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- *Exchange*, written `xA/B`, makes the programs at positions `A` and
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`B` swap places.
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- *Partner*, written `pA/B`, makes the programs named `A` and `B` swap
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places.
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For example, with only five programs standing in a line (`abcde`), they
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could do the following dance:
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- `s1`, a spin of size `1`: `eabcd`.
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- `x3/4`, swapping the last two programs: `eabdc`.
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- `pe/b`, swapping programs `e` and `b`: `baedc`.
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After finishing their dance, the programs end up in order `baedc`.
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You watch the dance for a while and record their dance moves (your
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puzzle input). *In what order are the programs standing* after their
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dance?
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Your puzzle answer was `ehdpincaogkblmfj`.
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The first half of this puzzle is complete! It provides one gold star: \*
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## \-\-- Part Two \-\-- {#part2}
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Now that you\'re starting to get a feel for the dance moves, you turn
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your attention to *the dance as a whole*.
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Keeping the positions they ended up in from their previous dance, the
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programs perform it again and again: including the first dance, a total
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of *one billion* (`1000000000`) times.
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In the example above, their second dance would *begin* with the order
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`baedc`, and use the same dance moves:
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- `s1`, a spin of size `1`: `cbaed`.
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- `x3/4`, swapping the last two programs: `cbade`.
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- `pe/b`, swapping programs `e` and `b`: `ceadb`.
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*In what order are the programs standing* after their billion dances?
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Answer:
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Although it hasn\'t changed, you can still [get your puzzle
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input](16/input).
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