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Day 18: Lavaduct Lagoon

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  • C

    Fun and interesting puzzle! In part 1 I fumbled a bit trying to implement even/odd outside/inside tracking before realizing that wouldn't work for this shape and just did the flood fill.

    For part 2 I correctly guessed that like the intersecting cuboids (2021 day 22) it would be about finding a better representation for the grid or avoiding representing it entirely. Long story shorter:

    /*
     * Conceptually: the raw map, which is too large to fit directly in
     * memory for part 2, is made much smaller by collapsing (and counting)
     * identical rows and columns. Another way to look it at is that a grid
     * is fitted to make 'opaque' cells.
     *                                           |   |#|##|#
     * For example:                             -+---+-+--+-
     *                                          #|###|#|  |#
     *       ####               ### 1           -+---+-+--+-
     *   #####  #             ### # 1           #|   | |  |#
     *   #      #   becomes   #   # 2     or:   #|   | |  |#
     *   #      #             ##### 1           -+---+-+--+-
     *   ########             13121             #|###|#|##|#
     *
     * To avoid a lot of complex work, instead of actually collapsing and
     * splitting rows and columns, we first generate the wall rectangles and
     * collect the unique X and Y coordinates. Those are locations of our
     * virtual grid lines.
     */
    

    Despite being quite happy with this solution, I couldn't help but notice the brevity and simplicity of the other solutions here. Gonna have a look what's happening there and see if I can try that approach too.

    (Got bitten by a nasty overflow btw, the list of unique X coordinates was overwriting the list of unique Y coordinates. Oh well, such is the life of a C programmer.)

    https://github.com/sjmulder/aoc/blob/master/2023/c/day18.c

    • Oh, just like day 11! I hadn't thought of that. I was initially about to try something similar by separating into rectangular regions, as in ear-clipping triangulation. But that would require a lot of iterating, and something about "polygon" and "walking the edges" went ping in my memory...

15 comments