Added Solution for 2020 day 03

2020/day03_toboggan_trajectory/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "day03_toboggan_trajectory"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2020/day03_toboggan_trajectory/challenge.txt

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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.
+
+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?*
+
+To begin, [get your puzzle input](3/input).
+
+Answer:

2020/day03_toboggan_trajectory/src/lib.rs

@@ -0,0 +1,68 @@
+use core::fmt::Display;
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum ParseError {
+    InvalidChar(char)
+}
+
+impl Display for ParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::InvalidChar(c) => write!(f, "Invalid Character encountered: {c}"),
+        }
+    }
+}
+
+struct Grid {
+    trees: Vec<Vec<bool>>,
+}
+
+impl TryFrom<&str> for Grid {
+    type Error = ParseError;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        Ok(Self {
+            trees: value.lines().map(|line| line.chars().map(|c| match c {
+                '.' => Ok(false),
+                '#' => Ok(true),
+                _ => Err(ParseError::InvalidChar(c)),
+            }).collect::<Result<Vec<_>, _>>()).collect::<Result<Vec<_>, _>>()?,
+        })
+    }
+}
+
+impl Grid {
+    fn trees_hit_by_going(&self, (right, down): (usize, usize)) -> usize {
+            self.trees.iter().enumerate().step_by(down).filter(|(idx, row)| row[(right*idx/down)%row.len()]).count()
+    }
+}
+
+pub fn run(input: &str) -> Result<(usize, usize), ParseError> {
+    let grid = Grid::try_from(input)?;
+    let first = grid.trees_hit_by_going((3, 1));
+    let slopes = [(1, 1), (5, 1), (7, 1), (1, 2)];
+    let second = slopes.iter().map(|slope| grid.trees_hit_by_going(*slope)).product::<usize>() * first;
+    Ok((first, second))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs::read_to_string;
+
+    fn read_file(name: &str) -> String {
+        read_to_string(name).expect(&format!("Unable to read file: {name}")[..]).trim().to_string()
+    }
+
+    #[test]
+    fn test_sample() {
+        let sample_input = read_file("tests/sample_input");
+        assert_eq!(run(&sample_input), Ok((7, 336)));
+    }
+
+    #[test]
+    fn test_challenge() {
+        let challenge_input = read_file("tests/challenge_input");
+        assert_eq!(run(&challenge_input), Ok((272, 3898725600)));
+    }
+}

2020/day03_toboggan_trajectory/tests/challenge_input

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+.#...#.......#...#...#.#.#.....
+####.....#.#..#...#...........#
+.....#...........#......#....#.
+......#..#......#.#..#...##.#.#
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+.#.....#........#...##....#..#.
+.....#.......#######.#.#...#...

2020/day03_toboggan_trajectory/tests/sample_input

@@ -0,0 +1,11 @@
+..##.......
+#...#...#..
+.#....#..#.
+..#.#...#.#
+.#...##..#.
+..#.##.....
+.#.#.#....#
+.#........#
+#.##...#...
+#...##....#
+.#..#...#.#