Day 3 part 1.

2020/src/day3.rs

@@ -0,0 +1,117 @@
+//! --- 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?
+
+use std::ops::Index;
+
+use aoc_runner_derive::{aoc, aoc_generator};
+
+#[derive(Debug, PartialEq)]
+struct Bitmap {
+    width: usize,
+    height: usize,
+    bits: Vec<bool>,
+}
+
+impl Index<(usize, usize)> for Bitmap {
+    type Output = bool;
+
+    // Perform 2-dimensional indexing of bits, wrapping the X coordinates if it is larger than the
+    // width of the Bitmap.
+    fn index(&self, (x, y): (usize, usize)) -> &Self::Output {
+        &self.bits[x % self.width + y * self.width]
+    }
+}
+#[aoc_generator(day3)]
+fn parse(input: &str) -> Bitmap {
+    let mut height = 0;
+    let bits: Vec<bool> = input
+        .chars()
+        .filter_map(|c| match c {
+            '.' => Some(false),
+            '#' => Some(true),
+            '\n' => {
+                height += 1;
+                None
+            }
+            _ => panic!("Unexpected character"),
+        })
+        .collect();
+
+    let width = bits.len() / height;
+    Bitmap {
+        width,
+        height,
+        bits,
+    }
+}
+
+#[aoc(day3, part1)]
+fn answer(map: &Bitmap) -> usize {
+    (0..map.height).filter(|y| map[(*y * 3, *y)]).count()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn make_bitmap() {
+        assert_eq!(
+            parse("..##\n##..\n"),
+            Bitmap {
+                width: 4,
+                height: 2,
+                bits: vec![false, false, true, true, true, true, false, false]
+            }
+        );
+    }
+}

2020/src/lib.rs

@@ -1,5 +1,6 @@
 mod day1;
 mod day2;
+mod day3;
 
 use aoc_runner_derive::aoc_lib;