adventofcode/2023/day16/problem

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   br0xen [7](AoC++) 32*

                                      λy.[8]2023

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--- Day 16: The Floor Will Be Lava ---

   With the beam of light completely focused somewhere, the reindeer leads
   you deeper still into the Lava Production Facility. At some point, you
   realize that the steel facility walls have been replaced with cave, and
   the doorways are just cave, and the floor is cave, and you're pretty sure
   this is actually just a giant cave.

   Finally, as you approach what must be the heart of the mountain, you see a
   bright light in a cavern up ahead. There, you discover that the beam of
   light you so carefully focused is emerging from the cavern wall closest to
   the facility and pouring all of its energy into a contraption on the
   opposite side.

   Upon closer inspection, the contraption appears to be a flat,
   two-dimensional square grid containing empty space (.), mirrors (/ and \),
   and splitters (| and -).

   The contraption is aligned so that most of the beam bounces around the
   grid, but each tile on the grid converts some of the beam's light into
   heat to melt the rock in the cavern.

   You note the layout of the contraption (your puzzle input). For example:

 .|...\....
 |.-.\.....
 .....|-...
 ........|.
 ..........
 .........\
 ..../.\\..
 .-.-/..|..
 .|....-|.\
 ..//.|....

   The beam enters in the top-left corner from the left and heading to the
   right. Then, its behavior depends on what it encounters as it moves:

     • If the beam encounters empty space (.), it continues in the same
       direction.
     • If the beam encounters a mirror (/ or \), the beam is reflected 90
       degrees depending on the angle of the mirror. For instance, a
       rightward-moving beam that encounters a / mirror would continue upward
       in the mirror's column, while a rightward-moving beam that encounters
       a \ mirror would continue downward from the mirror's column.
     • If the beam encounters the pointy end of a splitter (| or -), the beam
       passes through the splitter as if the splitter were empty space. For
       instance, a rightward-moving beam that encounters a - splitter would
       continue in the same direction.
     • If the beam encounters the flat side of a splitter (| or -), the beam
       is split into two beams going in each of the two directions the
       splitter's pointy ends are pointing. For instance, a rightward-moving
       beam that encounters a | splitter would split into two beams: one that
       continues upward from the splitter's column and one that continues
       downward from the splitter's column.

   Beams do not interact with other beams; a tile can have many beams passing
   through it at the same time. A tile is energized if that tile has at least
   one beam pass through it, reflect in it, or split in it.

   In the above example, here is how the beam of light bounces around the
   contraption:

 >|<<<\....
 |v-.\^....
 .v...|->>>
 .v...v^.|.
 .v...v^...
 .v...v^..\
 .v../2\\..
 <->-/vv|..
 .|<<<2-|.\
 .v//.|.v..

   Beams are only shown on empty tiles; arrows indicate the direction of the
   beams. If a tile contains beams moving in multiple directions, the number
   of distinct directions is shown instead. Here is the same diagram but
   instead only showing whether a tile is energized (#) or not (.):

 ######....
 .#...#....
 .#...#####
 .#...##...
 .#...##...
 .#...##...
 .#..####..
 ########..
 .#######..
 .#...#.#..

   Ultimately, in this example, 46 tiles become energized.

   The light isn't energizing enough tiles to produce lava; to debug the
   contraption, you need to start by analyzing the current situation. With
   the beam starting in the top-left heading right, how many tiles end up
   being energized?

   Your puzzle answer was 6816.

--- Part Two ---

   As you try to work out what might be wrong, the reindeer tugs on your
   shirt and leads you to a nearby control panel. There, a collection of
   buttons lets you align the contraption so that the beam enters from any
   edge tile and heading away from that edge. (You can choose either of two
   directions for the beam if it starts on a corner; for instance, if the
   beam starts in the bottom-right corner, it can start heading either left
   or upward.)

   So, the beam could start on any tile in the top row (heading downward),
   any tile in the bottom row (heading upward), any tile in the leftmost
   column (heading right), or any tile in the rightmost column (heading
   left). To produce lava, you need to find the configuration that energizes
   as many tiles as possible.

   In the above example, this can be achieved by starting the beam in the
   fourth tile from the left in the top row:

 .|<2<\....
 |v-v\^....
 .v.v.|->>>
 .v.v.v^.|.
 .v.v.v^...
 .v.v.v^..\
 .v.v/2\\..
 <-2-/vv|..
 .|<<<2-|.\
 .v//.|.v..

   Using this configuration, 51 tiles are energized:

 .#####....
 .#.#.#....
 .#.#.#####
 .#.#.##...
 .#.#.##...
 .#.#.##...
 .#.#####..
 ########..
 .#######..
 .#...#.#..

   Find the initial beam configuration that energizes the largest number of
   tiles; how many tiles are energized in that configuration?

   Your puzzle answer was 8163.

   Both parts of this puzzle are complete! They provide two gold stars: **

   At this point, you should [16]return to your Advent calendar and try
   another puzzle.

   If you still want to see it, you can [17]get your puzzle input.

   You can also [Shareon [18]Twitter [19]Mastodon] this puzzle.

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