Very clunky implementation of 2023 day 14 part 2

2023/src/common.rs

@@ -300,3 +300,98 @@ impl Display for Grid<'_> {
         write!(f, "{}", String::from_utf8_lossy(self.data))
     }
 }
+
+// TODO: merge OwnedGrid and Grid impls so I don't go insane
+pub struct OwnedGrid {
+    width: usize,
+    data: Vec<u8>,
+}
+
+impl OwnedGrid {
+    pub fn new(data: Vec<u8>) -> anyhow::Result<Self> {
+        let width = 1 + data
+            .iter()
+            .position(|&c| c == b'\n')
+            .context("Failed to find end of line in grid")?;
+
+        anyhow::ensure!(
+            data.len() % width == 0,
+            "Grid should divide equally into rows"
+        );
+
+        Ok(Self { width, data })
+    }
+
+    pub fn height(&self) -> usize {
+        self.data.len() / self.width
+    }
+
+    pub fn width(&self) -> usize {
+        self.width - 1
+    }
+
+    pub fn rows(&self) -> impl Iterator<Item = &[u8]> {
+        let width = self.width();
+        self.data
+            .chunks_exact(self.width)
+            .map(move |row| &row[..width])
+    }
+
+    pub fn rows_mut(&mut self) -> impl Iterator<Item = &mut [u8]> {
+        let width = self.width();
+        self.data
+            .chunks_exact_mut(self.width)
+            .map(move |row| &mut row[..width])
+    }
+
+    pub fn find(&self, c: u8) -> Option<(usize, usize)> {
+        let pos = self.data.iter().position(|&d| d == c)?;
+
+        Some((pos % self.width, pos / self.width))
+    }
+}
+
+impl Index<usize> for OwnedGrid {
+    type Output = [u8];
+
+    fn index(&self, y: usize) -> &Self::Output {
+        let offset = y * self.width;
+        &self.data[offset..(offset + self.width())]
+    }
+}
+
+impl IndexMut<usize> for OwnedGrid {
+    fn index_mut(&mut self, y: usize) -> &mut Self::Output {
+        let offset = y * self.width;
+        let width = self.width;
+        &mut self.data[offset..(offset + width)]
+    }
+}
+
+impl Display for OwnedGrid {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", String::from_utf8_lossy(&self.data))
+    }
+}
+
+impl PartialEq<OwnedGrid> for OwnedGrid {
+    fn eq(&self, other: &OwnedGrid) -> bool {
+        // No need to compare width as width is a function of data
+        self.data == other.data
+    }
+}
+
+// Custom Clone impl so we don't allocate in `clone_from`
+impl Clone for OwnedGrid {
+    fn clone(&self) -> Self {
+        Self {
+            width: self.width.clone(),
+            data: self.data.clone(),
+        }
+    }
+
+    fn clone_from(&mut self, source: &Self) {
+        self.width = source.width;
+        self.data.clone_from(&source.data);
+    }
+}

2023/src/day14.rs

@@ -1,4 +1,5 @@
 use crate::common::Grid;
+use crate::common::OwnedGrid;
 
 pub fn part1(input: &[u8]) -> anyhow::Result<String> {
     let grid = Grid::new(input)?;
@@ -22,8 +23,131 @@ pub fn part1(input: &[u8]) -> anyhow::Result<String> {
     Ok(load.to_string())
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+fn advance(grid: &mut OwnedGrid, stack_heights: &mut [usize]) {
+    // Tilt north
+    stack_heights.fill(0);
+    for y in 0..grid.height() {
+        for (x, stack_height) in stack_heights.iter_mut().enumerate() {
+            let c = grid[y][x];
+            match c {
+                b'#' => *stack_height = y + 1,
+                b'O' => {
+                    grid[y][x] = b'.';
+                    grid[*stack_height][x] = b'O';
+
+                    *stack_height += 1;
+                }
+                _ => continue,
+            }
+        }
+    }
+
+    // Tilt west
+    for row in grid.rows_mut() {
+        let mut stack_height = 0;
+        for x in 0..row.len() {
+            let c = row[x];
+            match c {
+                b'#' => stack_height = x + 1,
+                b'O' => {
+                    row[x] = b'.';
+                    row[stack_height] = b'O';
+
+                    stack_height += 1;
+                }
+                _ => continue,
+            }
+        }
+    }
+
+    // Tilt south
+    stack_heights.fill(grid.height() - 1);
+    for y in (0..grid.height()).rev() {
+        for (x, stack_height) in stack_heights.iter_mut().enumerate() {
+            let c = grid[y][x];
+            match c {
+                b'#' => *stack_height = y.saturating_sub(1),
+                b'O' => {
+                    grid[y][x] = b'.';
+                    grid[*stack_height][x] = b'O';
+
+                    // Saturating because possible underflow
+                    *stack_height = stack_height.saturating_sub(1);
+                }
+                _ => continue,
+            }
+        }
+    }
+
+    // Tilt east
+    for row in grid.rows_mut() {
+        let mut stack_height = row.len() - 1;
+        for x in (0..row.len()).rev() {
+            let c = row[x];
+            match c {
+                b'#' => stack_height = x.saturating_sub(1),
+                b'O' => {
+                    row[x] = b'.';
+                    row[stack_height] = b'O';
+
+                    stack_height = stack_height.saturating_sub(1);
+                }
+                _ => continue,
+            }
+        }
+    }
+}
+
+fn find_cycle(
+    it: impl Iterator<Item = usize>,
+    hare: &mut OwnedGrid,
+    stack_heights: &mut [usize],
+) -> Option<usize> {
+    let mut tortoise = hare.clone();
+    let mut last_sync = 0;
+
+    for cycle in it {
+        advance(hare, stack_heights);
+
+        if tortoise == *hare {
+            return Some(cycle - last_sync);
+        } else if cycle.count_ones() == 1 {
+            // New power of two, sync up the tortoise and the hare
+            tortoise.clone_from(&hare);
+            last_sync = cycle;
+        }
+    }
+    None
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    const GOAL: usize = 1000000000;
+    let mut hare = OwnedGrid::new(input.to_owned())?;
+    let mut stack_heights = vec![0; hare.width()];
+
+    let mut it = 1..=GOAL;
+
+    // If there is a cycle found, skip ahead
+    if let Some(len) = find_cycle(&mut it, &mut hare, &mut stack_heights) {
+        let remaining = it.size_hint().0;
+        let steps = remaining % len;
+
+        for _ in 0..steps {
+            advance(&mut hare, &mut stack_heights);
+        }
+    }
+
+    let height = hare.height();
+    let load = hare
+        .rows()
+        .enumerate()
+        .flat_map(|(y, row)| {
+            row.iter()
+                .filter_map(move |&c| (c == b'O').then_some(height - y))
+        })
+        .sum::<usize>();
+
+    Ok(load.to_string())
 }
 
 #[cfg(test)]
@@ -36,4 +160,9 @@ mod tests {
     fn sample_part1() {
         assert_eq!("136", part1(SAMPLE).unwrap());
     }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!("64", part2(SAMPLE).unwrap());
+    }
 }