72 lines
3.8 KiB
Plaintext
72 lines
3.8 KiB
Plaintext
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--- Day 3: Toboggan Trajectory ---
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With the toboggan login problems resolved, you set off toward the airport. While travel by toboggan might be easy, it's certainly not safe: there's very minimal steering and the area is covered in trees. You'll need to see which angles will take you near the fewest trees.
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Due to the local geology, trees in this area only grow on exact integer coordinates in a grid. You make a map (your puzzle input) of the open squares (.) and trees (#) you can see. For example:
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..##.......
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#...#...#..
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.#....#..#.
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..#.#...#.#
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.#...##..#.
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..#.##.....
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.#.#.#....#
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.#........#
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#.##...#...
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#...##....#
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.#..#...#.#
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These aren't the only trees, though; due to something you read about once involving arboreal genetics and biome stability, the same pattern repeats to the right many times:
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..##.........##.........##.........##.........##.........##....... --->
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#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
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.#....#..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
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..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
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.#...##..#..#...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
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..#.##.......#.##.......#.##.......#.##.......#.##.......#.##..... --->
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.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
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.#........#.#........#.#........#.#........#.#........#.#........#
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#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...
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#...##....##...##....##...##....##...##....##...##....##...##....#
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.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.# --->
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You start on the open square (.) in the top-left corner and need to reach the bottom (below the bottom-most row on your map).
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The toboggan can only follow a few specific slopes (you opted for a cheaper model that prefers rational numbers); start by counting all the trees you would encounter for the slope right 3, down 1:
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From your starting position at the top-left, check the position that is right 3 and down 1. Then, check the position that is right 3 and down 1 from there, and so on until you go past the bottom of the map.
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The locations you'd check in the above example are marked here with O where there was an open square and X where there was a tree:
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..##.........##.........##.........##.........##.........##....... --->
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#..O#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
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.#....X..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
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..#.#...#O#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
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.#...##..#..X...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
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..#.##.......#.X#.......#.##.......#.##.......#.##.......#.##..... --->
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.#.#.#....#.#.#.#.O..#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
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.#........#.#........X.#........#.#........#.#........#.#........#
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#.##...#...#.##...#...#.X#...#...#.##...#...#.##...#...#.##...#...
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#...##....##...##....##...#X....##...##....##...##....##...##....#
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.#..#...#.#.#..#...#.#.#..#...X.#.#..#...#.#.#..#...#.#.#..#...#.# --->
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In this example, traversing the map using this slope would cause you to encounter 7 trees.
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Starting at the top-left corner of your map and following a slope of right 3 and down 1, how many trees would you encounter?
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--- Part Two ---
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Time to check the rest of the slopes - you need to minimize the probability of a sudden arboreal stop, after all.
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Determine the number of trees you would encounter if, for each of the following slopes, you start at the top-left corner and traverse the map all the way to the bottom:
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Right 1, down 1.
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Right 3, down 1. (This is the slope you already checked.)
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Right 5, down 1.
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Right 7, down 1.
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Right 1, down 2.
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In the above example, these slopes would find 2, 7, 3, 4, and 2 tree(s) respectively; multiplied together, these produce the answer 336.
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What do you get if you multiply together the number of trees encountered on each of the listed slopes?
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