68 lines
2.2 KiB
Text
68 lines
2.2 KiB
Text
Day 14 Notes
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+--------+
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| Part 1 |
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+--------+
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$ elixir day14part1.exs
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12135523360904
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Thoughts:
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Bitmasking.
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Not sure if I missed something here. Since the mask bits overwrite the value, we could
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just copy the bits. But I decided to implement it with real bitmasking, so I *think*
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it is necessary to split the mask into two seperate masks - one containing the 1s which
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will be ORed to set the bits, and another containing the 0s will will be ANDed to unset
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the bits.
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When parsing the file, convert the mask into the and and or masks, then when
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executing the parsed instructions, apply the masks as necessary.
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+--------+
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| Part 2 |
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+--------+
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$ elixir day14part2.exs
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2741969047858
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Thoughts:
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OK. This requires quite a re-write of part 1. Now the Xs in the mask represent
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floating values. We need to permute all the possible floating values, then substitute
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those into the result.
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I wasted a lot of time misreading the question, by replacing the values in the mask
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*before* applying the mask to the memory address. But the question requires the mask
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to be applied first, and then replace the values in the masked result. Reworking ensues.
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This got a bit long so decided to split the file parsing logic into its own module.
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Parser Module
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Parses the file, and produces the "mask" along with the indices that contained the X bits.
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Can't perform the substitution at this stage, because it has to be applied to the
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masked value.
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Day14Part2 Module
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Runs the main logic. For each mask, applies it to the value, permutes the options
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nd subsitutes the relevant bits.
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I was unsure whether it was better to compute all the permuted bits ahead of time in the
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parsing stage, or do it on demand each time we apply a mask. Decided to do it on demand,
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because it keeps the data that is being passed around simpler - the mask can just
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store the X indices, rather than all the permuted replacement values.
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+------------------+
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| Overall Thoughts |
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+------------------+
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Enjoyed this one. Was great to get the experience of working with binaries as integer
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values rather than strings. Getting used to working with the <<>> syntax, although it
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took me a long time to work it all out.
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Especially for Part 2 - generating permutations by counting in binary was pretty
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fun, but completely unnecessary. :-)
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