Solutions for Advent of Code
Anders Ytterström
bfe94864e9
Part 2 requires more clever code. In the first part, I kept track on
every single fish on order, much like the output of the example in the
puzzle.
However, by increasing the day count to 256, my code became obese and
would not run to the end: the OS killed it before it was done. It halted
around 150 days.
Browsing the sub reddit, I quickly realised that the current state of
each fish individually did not matter. For example:
3,4,3,1,2
This says 2 fishes has a timer set to 3, and 1 fish each has a timer of
1, 2 and 4.
Add one day, and we have
2,3,2,0,1
This says 2 fishes has a timer set to 2, and 1 fish each has a timer of
0, 1 and 3.
Notice how **the relative count does not change** (2, 1, 1, 1).
By using the excellent collections.counter, you get
>>> from collections import Counter
>>> Counter([3,4,3,1,2])
Counter({3: 2, 4: 1, 1: 1, 2: 1})
>>> Counter([2,3,2,0,1])
Counter({2: 2, 3: 1, 0: 1, 1: 1})
>>>
If visualized in a tabulat view, you get this:
0 1 2 3 4 5 6
----------------------------------
Initial 1 1 2 1
Day 1 1 1 2 1
The sequence moves **one step to the left each day**. Add another day,
and something extra happens.
0 1 2 3 4 5 6 7 8
----------------------------------------
Initial 1 1 2 1
Day 1 1 1 2 1
Day 2 1 2 1 1 1
2 things:
* A value higher than zero pops out on the left. By rules, 1 fish
has created 1 new fish, and resets its timer to 6.
* The new fish created has a timer of 8, since it will need 2 days
before it is able to start the create timer.
There are now 6 fishes.
Now, what happens on day 3?
0 1 2 3 4 5 6 7 8
----------------------------------------
Initial 1 1 2 1
Day 1 1 1 2 1
Day 2 1 2 1 1 1
Day 3 2 1 1 1 1 1
Now this happened:
* The fish on 0 creates a new fish, resets to 6.
* The created fish, once again, has a timer of 8.
* The fish created on day 2 decreases timer to 7.
There are now 7 fishes.
And so it continues. In Python, the most memory efficient and performant
method of leftpadding a list of values is to use collections.deque, with
the functions popleft and append.
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| 2015-elixir | ||
| 2016-python | ||
| 2017-python | ||
| 2018-elixir | ||
| 2018-python | ||
| 2019-elixir | ||
| 2020-elixir | ||
| 2020-python | ||
| 2021-elixir | ||
| 2021-python | ||
| README.md | ||
Advent of Code 2015–
Here all solved puzzles for Advent of Code are kept for reference. Most solutions are written in Python, but there are also solutions written in Elixir.
- 2016–2020 are squashed in version control, so the original thoughts/rants about the solution are forgotten for future generations.
- 2021 onwards will have each solution as a separate commit.