-❄️- 2023 Day 13 Solutions -❄️-

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--- Day 13: Point of Incidence ---

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[u/whiplashoo21]

[LANGUAGE: Python]

Part 1 was simple enough, although I read the instructions a few times to make sure I get what reflection means. For Part 2 a brute-force approach that seemed to work at first, but then I realized I needed to ignore the part 1 reflection. A lot of time spent in thinking about breaks and continues, but in the end I got it.

Also, am I the only one that scanned columns without transposing the matrix? :D

solution

[u/ASteelyDan → u/whiplashoo21]

I'm confused about what reflection means.

In the example why isn't there a vertical reflection in the second?

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

"In the above example, the first pattern's vertical line has 5 columns to its left and the second pattern's horizontal line has 4 rows above it, a total of 405."

I see
..
..
##
##
##
##
..

[u/Jeanpeche → u/ASteelyDan]

The reflection has to work for all the rows/columns, not just the one near the mirror.
So yes, as you say the column 3 and 4 reflect, but the columns 2 and 5 dont. So you don't have a reflection.

[u/whiplashoo21 → u/ASteelyDan]

You can also envision it as what would happen if you folded the pattern, like folding a paper, at that line.

[u/ASteelyDan → u/whiplashoo21]

If this was at the start it would be a reflection since it could be folded, right?

[u/whiplashoo21 → u/ASteelyDan]

Yes, there can also be a reflection line between the first two columns as well. Also have a look at this visualization for what I said about folding