60 lines
2 KiB
Text
60 lines
2 KiB
Text
Day 5 Notes
|
|
|
|
+--------+
|
|
| Part 1 |
|
|
+--------+
|
|
|
|
$ elixir day5part1.exs
|
|
906
|
|
|
|
Thoughts:
|
|
Could do something clever with binaries or the Bitwise module?
|
|
The number of rows is even (128), but we're zero-indexed (0-127), so how to deal with calculating half?
|
|
- in binary, can just shift one bit. But again, we need to halve a range (e.g. 32 to 63) so not sure
|
|
how to do that.
|
|
|
|
Instead of storing the range, I decided to store the current "front" seat, and how many rows back to count.
|
|
Then we can work with 128 which divides by two without worrying about rounding problems.
|
|
|
|
When there are only two seats left, just return the correct one based on the flag.
|
|
|
|
+--------+
|
|
| Part 2 |
|
|
+--------+
|
|
|
|
$ elixir day5part2.exs
|
|
519
|
|
|
|
Thoughts:
|
|
|
|
Not really sure what the missing front/back seats was about. Decided just to just compare the sorted list
|
|
with itself shifted by one (by zipping them), and finding the first ID without a next ID, and assume
|
|
the next ID is my seat.
|
|
That turned out to be the right answer.
|
|
|
|
|
|
+------------------+
|
|
| Overall Thoughts |
|
|
+------------------+
|
|
|
|
Looking forward to seeing how others implemented the seat ID parsing.
|
|
Did a lot of refactoring here but the end result, especially for part 2, is pretty clean I think.
|
|
Even if I decided to refactor the parser, it can just be swapped in for part 2. Zipping was a nice idea.
|
|
|
|
I quite enjoyed this one, maybe because it's a Saturday and I wasn't burnt out from work :-)
|
|
|
|
** Update **
|
|
|
|
After reading the Elixir Forum thread: https://elixirforum.com/t/advent-of-code-2020-day-5/35997/
|
|
|
|
I really had to have a go at just parsing the boarding pass directly into an integer. I said I wouldn't
|
|
commit code to my solutions after reading forums, so I'll add it here instead :-) It converts each
|
|
character to a bit on the fly, and interprets the result as a 10-bit integer. 🦾
|
|
|
|
def char_to_bit(c) when c in 'FL', do: <<0::1>>
|
|
def char_to_bit(c) when c in 'BR', do: <<1::1>>
|
|
|
|
def seat_id_bitstring(boarding_pass \\ "FBFBBFFRLR") do
|
|
<<id::integer-10>> = for <<char <- boarding_pass>>, into: <<>>, do: char_to_bit(char)
|
|
id
|
|
end
|