This project does not contain the puzzle or example inputs as per the [copyright notice of Advent of Code](https://adventofcode.com/about). In order to run the compiled application, the puzzle inputs have to be downloaded from the [Advent of Code 2024](https://adventofcode.com/2024/) puzzle pages, and placed as text files into the `AdventOfCode2024\data` directory, e.g. `AdventOfCode2024\data\historian_hysteria.txt`, or `AdventOfCode2024\data\example\historian_hysteria.txt` for the unit tests. The application will output an error message with details if it cannot find an input file.
## Tests
The solution contains a unit test project to help troubleshoot issues and prevent regressions in the solver class framework. These tests cover the solutions for provided examples and full data inputs. I did not find a definitive source for it, but the full data inputs seem to be user-specific, so adding my solutions as tests does not spoil the solutions.
I'm using a `std::multiset` to collect and sort the values for both lists. This allows to use a single iteration of the left list and two iterations of the right list simultaneously to solve both parts. Nice application of iterators.
Here, we have a few conditionals to determine on the fly which of the numbers would make the report safe if dropped. The amount of cases is actually quite manageable.
A simple [finite state machine](AdventOfCode2024/StringStateMachine.h) crawling along the input character by character solves both parts nicely. The algorithm tracks whether `mul` instructions are enabled or not, but ignores this setting for part 1.
For this puzzle I added a class for [points in two-dimensional space](AdventOfCode2024/Point2.h), so I can use these for simplifying directional computations. With that, the algorithm looks for all `X` and `A` for part 1 and 2, respectively, and tries to find the remaining characters, starting from that `X` or `A`.
My implementation uses an ordering matrix (a two-dimensional boolean array) to track which page combinations are ordered, and then queries that matrix for each ordered combination of pages in a single line. The same matrix can then also be used for a custom sort function for part 2.
The algorithm starts at the trail ends (the `9`'s) and searches all options from there downwards until trailheads are reached. For each visited map location, it tracks unique reachable end points (for part 1) and number of possible paths to end points (for part 2). These values are accumulated, if during the search the same map location is encountered again.
Since the order of the pebbles does not actually matter, we can simply blink from one line of pebbles to the next and count only cardinalities of each number in the current line.
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This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
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