Advent of Code
--- Day 11: Plutonian Pebbles ---
The ancient civilization on [16]Pluto was known for its ability to
manipulate spacetime, and while The Historians explore their infinite
corridors, you've noticed a strange set of physics-defying stones.
At first glance, they seem like normal stones: they're arranged in a
perfectly straight line, and each stone has a number engraved on it.
The strange part is that every time you blink, the stones change.
Sometimes, the number engraved on a stone changes. Other times, a stone
might split in two, causing all the other stones to shift over a bit to
make room in their perfectly straight line.
As you observe them for a while, you find that the stones have a
consistent behavior. Every time you blink, the stones each simultaneously
change according to the first applicable rule in this list:
• If the stone is engraved with the number 0, it is replaced by a stone
engraved with the number 1.
• If the stone is engraved with a number that has an even number of
digits, it is replaced by two stones. The left half of the digits are
engraved on the new left stone, and the right half of the digits are
engraved on the new right stone. (The new numbers don't keep extra
leading zeroes: 1000 would become stones 10 and 0.)
• If none of the other rules apply, the stone is replaced by a new
stone; the old stone's number multiplied by 2024 is engraved on the
new stone.
No matter how the stones change, their order is preserved, and they stay
on their perfectly straight line.
How will the stones evolve if you keep blinking at them? You take a note
of the number engraved on each stone in the line (your puzzle input).
If you have an arrangement of five stones engraved with the numbers 0 1 10
99 999 and you blink once, the stones transform as follows:
• The first stone, 0, becomes a stone marked 1.
• The second stone, 1, is multiplied by 2024 to become 2024.
• The third stone, 10, is split into a stone marked 1 followed by a
stone marked 0.
• The fourth stone, 99, is split into two stones marked 9.
• The fifth stone, 999, is replaced by a stone marked 2021976.
So, after blinking once, your five stones would become an arrangement of
seven stones engraved with the numbers 1 2024 1 0 9 9 2021976.
Here is a longer example:
Initial arrangement:
125 17
After 1 blink:
253000 1 7
After 2 blinks:
253 0 2024 14168
After 3 blinks:
512072 1 20 24 28676032
After 4 blinks:
512 72 2024 2 0 2 4 2867 6032
After 5 blinks:
1036288 7 2 20 24 4048 1 4048 8096 28 67 60 32
After 6 blinks:
2097446912 14168 4048 2 0 2 4 40 48 2024 40 48 80 96 2 8 6 7 6 0 3 2
In this example, after blinking six times, you would have 22 stones. After
blinking 25 times, you would have 55312 stones!
Consider the arrangement of stones in front of you. How many stones will
you have after blinking 25 times?
Your puzzle answer was 231278.
--- Part Two ---
The Historians sure are taking a long time. To be fair, the infinite
corridors are very large.
How many stones would you have after blinking a total of 75 times?
Your puzzle answer was 274229228071551.
Both parts of this puzzle are complete! They provide two gold stars: **
At this point, you should [17]return to your Advent calendar and try
another puzzle.
If you still want to see it, you can [18]get your puzzle input.
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