day 3

2 files changed, 98 insertions, 0 deletions

diff --git a/src/2020/3/mod.rs b/src/2020/3/mod.rs
new file mode 100644
index 0000000..58aabfd
--- /dev/null
+++ b/src/2020/3/mod.rs
@@ -0,0 +1,94 @@
+use anyhow::Context as _;
+use std::io::Read as _;
+
+struct Map {
+    grid: Vec<Vec<bool>>,
+}
+
+impl Map {
+    fn parse(s: &[u8]) -> anyhow::Result<Self> {
+        let mut grid = vec![];
+        let mut current_row = vec![];
+        for c in s {
+            match c {
+                b'#' => {
+                    current_row.push(true);
+                }
+                b'.' => {
+                    current_row.push(false);
+                }
+                b'\n' => {
+                    grid.push(current_row);
+                    current_row = vec![];
+                }
+                _ => {
+                    return Err(anyhow::anyhow!("invalid map char: '{}'", c));
+                }
+            }
+        }
+        if !current_row.is_empty() {
+            grid.push(current_row);
+        }
+        Ok(Self { grid })
+    }
+
+    fn rows(&self) -> usize {
+        self.grid.len()
+    }
+
+    fn tree_at(&self, x: usize, y: usize) -> anyhow::Result<bool> {
+        // unwrap safe because cycle().nth() can never fail
+        Ok(*self
+            .grid
+            .get(y)
+            .context("row too large")?
+            .iter()
+            .cycle()
+            .nth(x)
+            .unwrap())
+    }
+
+    fn trees_for_slope(
+        &self,
+        x_incr: usize,
+        y_incr: usize,
+    ) -> anyhow::Result<usize> {
+        let mut trees = 0;
+        for r in 0..self.rows() / y_incr {
+            let x = r * x_incr;
+            let y = r * y_incr;
+            if self.tree_at(x, y)? {
+                trees += 1;
+            }
+        }
+        Ok(trees)
+    }
+}
+
+pub fn part1() -> anyhow::Result<()> {
+    let map = read_map()?;
+    println!("{}", map.trees_for_slope(3, 1)?);
+    Ok(())
+}
+
+pub fn part2() -> anyhow::Result<()> {
+    let map = read_map()?;
+    println!(
+        "{}",
+        map.trees_for_slope(1, 1)?
+            * map.trees_for_slope(3, 1)?
+            * map.trees_for_slope(5, 1)?
+            * map.trees_for_slope(7, 1)?
+            * map.trees_for_slope(1, 2)?
+    );
+    Ok(())
+}
+
+fn read_map() -> anyhow::Result<Map> {
+    let mut f = std::fs::File::open("data/3.txt")
+        .context("couldn't find data file 3.txt")?;
+    let mut map_str = vec![];
+    f.read_to_end(&mut map_str)
+        .context("failed to read map contents")?;
+    Map::parse(&map_str)
+}
diff --git a/src/2020/mod.rs b/src/2020/mod.rs
index 87ceb55..ff08a7e 100644
--- a/src/2020/mod.rs
+++ b/src/2020/mod.rs
@@ -2,6 +2,8 @@
 mod day1;
 #[path = "2/mod.rs"]
 mod day2;
+#[path = "3/mod.rs"]
+mod day3;
 
 pub fn run(day: u8, puzzle: u8) -> anyhow::Result<()> {
     match (day, puzzle) {
@@ -9,6 +11,8 @@ pub fn run(day: u8, puzzle: u8) -> anyhow::Result<()> {
         (1, 2) => day1::part2(),
         (2, 1) => day2::part1(),
         (2, 2) => day2::part2(),
+        (3, 1) => day3::part1(),
+        (3, 2) => day3::part2(),
         _ => Err(anyhow::anyhow!("unknown puzzle {}-{}", day, puzzle)),
     }
 }