2020: day24: part 1

aoc2020/src/day24.rs

@@ -0,0 +1,144 @@
+use std::collections::HashSet;
+use std::fmt::Write;
+
+use anyhow::{bail, Context, Result};
+
+const INPUT: &str = include_str!("../input/day24.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn part1(input: &str) -> Result<usize> {
+    let mut black_tiles = HashSet::new();
+
+    for line in input.lines() {
+        let mut line = line;
+        let mut tile_pos = HexCoordinates::default();
+
+        // compute tile coordinates by going through the whole line
+        while !line.is_empty() {
+            match line.chars().next().unwrap() {
+                'e' => {
+                    tile_pos = tile_pos.east();
+                    line = &line[1..];
+                }
+                'w' => {
+                    tile_pos = tile_pos.west();
+                    line = &line[1..];
+                }
+                'n' => {
+                    match line
+                        .chars()
+                        .nth(1)
+                        .context("invalid input, missing char after `n`")?
+                    {
+                        'e' => tile_pos = tile_pos.north_east(),
+                        'w' => tile_pos = tile_pos.north_west(),
+                        other => bail!("unexpected character in input: `{}`", other),
+                    }
+
+                    line = &line[2..];
+                }
+                's' => {
+                    match line
+                        .chars()
+                        .nth(1)
+                        .context("invalid input, missing char after `s`")?
+                    {
+                        'e' => tile_pos = tile_pos.south_east(),
+                        'w' => tile_pos = tile_pos.south_west(),
+                        other => bail!("unexpected character in input: `{}`", other),
+                    }
+
+                    line = &line[2..];
+                }
+                other => bail!("unexpected character in input: `{}`", other),
+            }
+        }
+
+        if black_tiles.contains(&tile_pos) {
+            black_tiles.remove(&tile_pos);
+        } else {
+            black_tiles.insert(tile_pos);
+        }
+    }
+
+    Ok(black_tiles.len())
+}
+
+/// Hexagonal tile coordinate representation
+///
+/// These use the axial coordinates described here:
+///
+/// https://www.redblobgames.com/grids/hexagons/#coordinates-axial
+#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq)]
+struct HexCoordinates {
+    q: i64,
+    r: i64,
+}
+
+impl HexCoordinates {
+    fn east(self) -> Self {
+        Self {
+            q: self.q + 1,
+            r: self.r,
+        }
+    }
+
+    fn west(self) -> Self {
+        Self {
+            q: self.q - 1,
+            r: self.r,
+        }
+    }
+
+    fn north_west(self) -> Self {
+        Self {
+            q: self.q,
+            r: self.r - 1,
+        }
+    }
+
+    fn south_east(self) -> Self {
+        Self {
+            q: self.q,
+            r: self.r + 1,
+        }
+    }
+
+    fn north_east(self) -> Self {
+        self.north_west().east()
+    }
+
+    fn south_west(self) -> Self {
+        self.south_east().west()
+    }
+}
+
+impl Default for HexCoordinates {
+    fn default() -> Self {
+        Self { q: 0, r: 0 }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day24_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 10);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 528);
+    }
+}