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