2020: day17: part 1

aoc2020/src/day17.rs

@@ -0,0 +1,110 @@
+use std::collections::HashSet;
+use std::fmt::Write;
+
+use anyhow::{anyhow, Result};
+
+use itertools::iproduct;
+
+const INPUT: &str = include_str!("../input/day17.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 pocket_dim: PocketDimension = input.parse()?;
+
+    for _ in 0..6 {
+        pocket_dim.update();
+    }
+
+    Ok(pocket_dim.points.len())
+}
+
+type Point = (i64, i64, i64);
+
+struct PocketDimension {
+    points: HashSet<Point>,
+}
+
+impl PocketDimension {
+    fn neighbours(point: Point) -> impl Iterator<Item = Point> {
+        iproduct!(-1..=1, -1..=1, -1..=1)
+            .filter(|(x, y, z)| *x != 0 || *y != 0 || *z != 0)
+            .map(move |(dx, dy, dz)| (point.0 + dx, point.1 + dy, point.2 + dz))
+    }
+
+    fn active_neighbours(&self, point: Point) -> usize {
+        Self::neighbours(point)
+            .filter(|p| self.points.contains(p))
+            .count()
+    }
+
+    fn update(&mut self) {
+        let mut processed: HashSet<Point> = HashSet::new();
+        let mut new = self.points.clone();
+
+        for point in &self.points {
+            for neighbour in Self::neighbours(*point).chain(std::iter::once(*point)) {
+                if processed.contains(&neighbour) {
+                    continue;
+                }
+
+                processed.insert(neighbour);
+
+                // if point is active
+                if self.points.contains(&neighbour) {
+                    if !(2..=3).contains(&self.active_neighbours(neighbour)) {
+                        // now inactive
+                        new.remove(&neighbour);
+                    }
+                } else if self.active_neighbours(neighbour) == 3 {
+                    new.insert(neighbour);
+                }
+            }
+        }
+
+        self.points = new;
+    }
+}
+
+impl std::str::FromStr for PocketDimension {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        let points = s
+            .lines()
+            .enumerate()
+            .flat_map(|(i, line)| {
+                line.chars().enumerate().filter_map(move |(j, c)| match c {
+                    '#' => Some(Ok((i as i64, j as i64, 0))),
+                    '.' => None,
+                    _ => Some(Err(anyhow!("unexpected char: `{}`", c))),
+                })
+            })
+            .collect::<Result<_>>()?;
+
+        Ok(Self { points })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day17_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 112);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 336);
+    }
+}

aoc2020/src/lib.rs

@@ -16,3 +16,4 @@ pub mod day13;
 pub mod day14;
 pub mod day15;
 pub mod day16;
+pub mod day17;

aoc2020/src/main.rs

@@ -18,6 +18,7 @@ use aoc2020::day13;
 use aoc2020::day14;
 use aoc2020::day15;
 use aoc2020::day16;
+use aoc2020::day17;
 
 fn main() -> Result<()> {
     let days: &[DayFunc] = &[
@@ -37,6 +38,7 @@ fn main() -> Result<()> {
         day14::run,
         day15::run,
         day16::run,
+        day17::run,
     ];
 
     aoc::run(days)