2020-12-05 13:29:55 +00:00
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Day 5 Notes
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+--------+
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| Part 1 |
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+--------+
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$ elixir day5part1.exs
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906
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Thoughts:
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Could do something clever with binaries or the Bitwise module?
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The number of rows is even (128), but we're zero-indexed (0-127), so how to deal with calculating half?
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- in binary, can just shift one bit. But again, we need to halve a range (e.g. 32 to 63) so not sure
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how to do that.
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Instead of storing the range, I decided to store the current "front" seat, and how many rows back to count.
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Then we can work with 128 which divides by two without worrying about rounding problems.
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When there are only two seats left, just return the correct one based on the flag.
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+--------+
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| Part 2 |
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+--------+
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$ elixir day5part2.exs
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519
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Thoughts:
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Not really sure what the missing front/back seats was about. Decided just to just compare the sorted list
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with itself shifted by one (by zipping them), and finding the first ID without a next ID, and assume
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the next ID is my seat.
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That turned out to be the right answer.
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+------------------+
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| Overall Thoughts |
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+------------------+
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Looking forward to seeing how others implemented the seat ID parsing.
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Did a lot of refactoring here but the end result, especially for part 2, is pretty clean I think.
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Even if I decided to refactor the parser, it can just be swapped in for part 2. Zipping was a nice idea.
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I quite enjoyed this one, maybe because it's a Saturday and I wasn't burnt out from work :-)
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2020-12-05 16:48:26 +00:00
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** Update **
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2020-12-06 02:42:48 +00:00
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After reading the Elixir Forum thread (https://elixirforum.com/t/advent-of-code-2020-day-5/35997/),
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I really had to have a go at just parsing the boarding pass directly into an integer, so I added
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simplified versions as day5part1-bitstring.exs and day5part2-bitstring.exs
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2020-12-05 16:48:26 +00:00
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2020-12-06 02:42:48 +00:00
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They use this bitstring-comprehension:
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<<id::10>> = for <<char <- boarding_pass>>, into: <<>>, do: char_to_bit(char)
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def char_to_bit(c) when c in 'FL', do: <<0::1>>
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def char_to_bit(c) when c in 'BR', do: <<1::1>>
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2020-12-05 16:48:26 +00:00
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2020-12-05 17:05:06 +00:00
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It converts each character to a bit on the fly, and interprets the result as a 10-bit integer. 🦾
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2020-12-05 16:48:26 +00:00
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