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- --- Day 3: Toboggan Trajectory ---
- 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.
- 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:
- ..##.......
- #...#...#..
- .#....#..#.
- ..#.#...#.#
- .#...##..#.
- ..#.##.....
- .#.#.#....#
- .#........#
- #.##...#...
- #...##....#
- .#..#...#.#
- 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:
- ..##.........##.........##.........##.........##.........##....... --->
- #...#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
- .#....#..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
- ..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
- .#...##..#..#...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
- ..#.##.......#.##.......#.##.......#.##.......#.##.......#.##..... --->
- .#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
- .#........#.#........#.#........#.#........#.#........#.#........#
- #.##...#...#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...
- #...##....##...##....##...##....##...##....##...##....##...##....#
- .#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.# --->
- 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).
- 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:
- 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.
- 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:
- ..##.........##.........##.........##.........##.........##....... --->
- #..O#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
- .#....X..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
- ..#.#...#O#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
- .#...##..#..X...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
- ..#.##.......#.X#.......#.##.......#.##.......#.##.......#.##..... --->
- .#.#.#....#.#.#.#.O..#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
- .#........#.#........X.#........#.#........#.#........#.#........#
- #.##...#...#.##...#...#.X#...#...#.##...#...#.##...#...#.##...#...
- #...##....##...##....##...#X....##...##....##...##....##...##....#
- .#..#...#.#.#..#...#.#.#..#...X.#.#..#...#.#.#..#...#.#.#..#...#.# --->
- In this example, traversing the map using this slope would cause you to encounter 7 trees.
- 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?
- Your puzzle answer was 200.
- --- Part Two ---
- Time to check the rest of the slopes - you need to minimize the probability of a sudden arboreal stop, after all.
- 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:
- Right 1, down 1.
- Right 3, down 1. (This is the slope you already checked.)
- Right 5, down 1.
- Right 7, down 1.
- Right 1, down 2.
- In the above example, these slopes would find 2, 7, 3, 4, and 2 tree(s) respectively; multiplied together, these produce the answer 336.
- What do you get if you multiply together the number of trees encountered on each of the listed slopes?
- Your puzzle answer was 3737923200.
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