Solution1.java

import java.io.BufferedReader;
import java.io.FileReader;
import java.util.ArrayList;
import java.util.List;

// --- 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?

public class Solution1 {

    public static void main(String[] args) {
        List<String> input = getInput();
        System.out.println(checkTrees(input));
    }

    private static List<String> getInput() {
        List<String> input = new ArrayList<>();
        try (BufferedReader br = new BufferedReader(new FileReader("input.txt"))) {
            br.lines().forEach(line -> input.add(line));
            br.close();
        } catch (Exception e) {
            System.out.println(e);
        }
        return input;
    }

    private static Integer checkTrees(List<String> input) {
        int trees = 0;
        int column = 0;
        final int WIDTH = input.get(0).length();
        for (int row = 1; row < input.size(); row++) {
            column = column + 3 <= WIDTH - 1 ? column + 3 : column + 3 - WIDTH;
            if (input.get(row).charAt(column) == '#')
                trees++;
        }
        return trees;
    }
}