99 lines
3.3 KiB
Markdown
99 lines
3.3 KiB
Markdown
## \-\-- Day 11: Plutonian Pebbles \-\--
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The ancient civilization on [Pluto](/2019/day/20) was known for its
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ability to manipulate spacetime, and while The Historians explore their
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infinite corridors, you\'ve noticed a strange set of physics-defying
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stones.
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At first glance, they seem like normal stones: they\'re arranged in a
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perfectly *straight line*, and each stone has a *number* engraved on it.
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The strange part is that every time you
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blink, the stones
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*change*.
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Sometimes, the number engraved on a stone changes. Other times, a stone
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might *split in two*, causing all the other stones to shift over a bit
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to make room in their perfectly straight line.
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As you observe them for a while, you find that the stones have a
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consistent behavior. Every time you blink, the stones each
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*simultaneously* change according to the *first applicable rule* in this
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list:
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- If the stone is engraved with the number `0`, it is replaced by a
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stone engraved with the number `1`.
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- If the stone is engraved with a number that has an *even* number of
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digits, it is replaced by *two stones*. The left half of the digits
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are engraved on the new left stone, and the right half of the digits
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are engraved on the new right stone. (The new numbers don\'t keep
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extra leading zeroes: `1000` would become stones `10` and `0`.)
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- If none of the other rules apply, the stone is replaced by a new
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stone; the old stone\'s number *multiplied by 2024* is engraved on
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the new stone.
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No matter how the stones change, their *order is preserved*, and they
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stay on their perfectly straight line.
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How will the stones evolve if you keep blinking at them? You take a note
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of the number engraved on each stone in the line (your puzzle input).
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If you have an arrangement of five stones engraved with the numbers
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`0 1 10 99 999` and you blink once, the stones transform as follows:
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- The first stone, `0`, becomes a stone marked `1`.
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- The second stone, `1`, is multiplied by 2024 to become `2024`.
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- The third stone, `10`, is split into a stone marked `1` followed by
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a stone marked `0`.
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- The fourth stone, `99`, is split into two stones marked `9`.
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- The fifth stone, `999`, is replaced by a stone marked `2021976`.
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So, after blinking once, your five stones would become an arrangement of
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seven stones engraved with the numbers `1 2024 1 0 9 9 2021976`.
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Here is a longer example:
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Initial arrangement:
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125 17
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After 1 blink:
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253000 1 7
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After 2 blinks:
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253 0 2024 14168
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After 3 blinks:
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512072 1 20 24 28676032
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After 4 blinks:
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512 72 2024 2 0 2 4 2867 6032
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After 5 blinks:
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1036288 7 2 20 24 4048 1 4048 8096 28 67 60 32
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After 6 blinks:
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2097446912 14168 4048 2 0 2 4 40 48 2024 40 48 80 96 2 8 6 7 6 0 3 2
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In this example, after blinking six times, you would have `22` stones.
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After blinking 25 times, you would have `55312` stones!
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Consider the arrangement of stones in front of you. *How many stones
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will you have after blinking 25 times?*
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Your puzzle answer was `211306`.
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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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The Historians sure are taking a long time. To be fair, the infinite
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corridors *are* very large.
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*How many stones would you have after blinking a total of 75 times?*
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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](11/input).
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