this post was submitted on 05 Dec 2023
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Advent Of Code
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An unofficial home for the advent of code community on programming.dev!
Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.
AoC 2023
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I'm a little confused about this one. The mappings are total, that is any number that is not defined explicitly gets mapped to itself. So it's easy to create an example where the lowest number does not get mentioned within a range:
Here, we have seeds 0, 1 and 2. seed 0 gets mapped to location 10, seed 1 gets mapped to location 11 and seed 2 gets mapped to location 2. That means location 2 would be the answer, but it's not a start of any range. I guess this just doesn't happen in any of the inputs?
EDIT: actually it's double weird. If you implemented a backwards search, that is you create reverse mappings and then try out all locations (which is what I and many others did), the result of the above example is location 0, whereas if you create a forwards brute force of all seeds, the result is 2. For the reverse approach to work in all cases, the mappings would have to be bijective.
Indeed, my solution fails on this input (returns 10, which is the location to seed 0), but it can be easily solved by also adding the ends of each range as well.
Maybe the input was quite forgiving. Thinking about it more, reversing the mapping can get quite involved, because it is neither surjective nor injective, so the inverse can actually have any number of results.
In your example there is no input that maps to 0, but there are two inputs that map to 11 (1 and 11). If the seed-to-soil map also included
10 20 2
, 21 would also map to 11.