From 44c1178c72b71ce23ad6c63cd8993f40dcbbc6f4 Mon Sep 17 00:00:00 2001
From: Antoine Martin <antoine97.martin@gmail.com>
Date: Thu, 17 Dec 2020 14:47:54 +0100
Subject: [PATCH] 2020: day17: part 2

---
 aoc2020/src/day17.rs | 91 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)

diff --git a/aoc2020/src/day17.rs b/aoc2020/src/day17.rs
index baa726f..e518242 100644
--- a/aoc2020/src/day17.rs
+++ b/aoc2020/src/day17.rs
@@ -11,6 +11,7 @@ pub fn run() -> Result<String> {
     let mut res = String::with_capacity(128);
 
     writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    writeln!(res, "part 2: {}", part2(INPUT)?)?;
 
     Ok(res)
 }
@@ -25,6 +26,16 @@ fn part1(input: &str) -> Result<usize> {
     Ok(pocket_dim.points.len())
 }
 
+fn part2(input: &str) -> Result<usize> {
+    let mut pocket_dim: PocketDimension4D = input.parse()?;
+
+    for _ in 0..6 {
+        pocket_dim.update();
+    }
+
+    Ok(pocket_dim.points.len())
+}
+
 type Point = (i64, i64, i64);
 
 struct PocketDimension {
@@ -92,6 +103,74 @@ impl std::str::FromStr for PocketDimension {
     }
 }
 
+// TODO: see how much of this can be factorized once min_const_generics is stabilized
+type Point4D = (i64, i64, i64, i64);
+
+struct PocketDimension4D {
+    points: HashSet<Point4D>,
+}
+
+impl PocketDimension4D {
+    fn neighbours(point: Point4D) -> impl Iterator<Item = Point4D> {
+        iproduct!(-1..=1, -1..=1, -1..=1, -1..=1)
+            .filter(|(x, y, z, w)| *x != 0 || *y != 0 || *z != 0 || *w != 0)
+            .map(move |(dx, dy, dz, dw)| (point.0 + dx, point.1 + dy, point.2 + dz, point.3 + dw))
+    }
+
+    fn active_neighbours(&self, point: Point4D) -> usize {
+        Self::neighbours(point)
+            .filter(|p| self.points.contains(p))
+            .count()
+    }
+
+    fn update(&mut self) {
+        let mut processed: HashSet<Point4D> = 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 PocketDimension4D {
+    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, 0))),
+                    '.' => None,
+                    _ => Some(Err(anyhow!("unexpected char: `{}`", c))),
+                })
+            })
+            .collect::<Result<_>>()?;
+
+        Ok(Self { points })
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -107,4 +186,16 @@ mod tests {
     fn part1_real() {
         assert_eq!(part1(INPUT).unwrap(), 336);
     }
+
+    #[test]
+    #[ignore]
+    fn part2_provided() {
+        assert_eq!(part2(PROVIDED).unwrap(), 848);
+    }
+
+    #[test]
+    #[ignore]
+    fn part2_real() {
+        assert_eq!(part2(INPUT).unwrap(), 2620);
+    }
 }