Implement 2023 day 11 part 2

2023/src/day11.rs

@@ -1,7 +1,74 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
+use crate::common::Grid;
-    anyhow::bail!("Not implemented")
+use crate::common::IndexSet;
+
+fn find_doubles(occupied: IndexSet, len: usize) -> Vec<usize> {
+    // TODO: this should iterate over IndexSet but that doesn't work yet and I can't be bothered.
+    (0..len).filter(|&v| !occupied.contains(v)).collect()
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
+fn transform<const S: usize>(pos: usize, doubles: &[usize]) -> usize {
-    anyhow::bail!("Not implemented")
+    let before = doubles.partition_point(|&v| v < pos);
+    pos + before * (S - 1)
+}
+
+fn part_common<const S: usize>(input: &[u8]) -> anyhow::Result<String> {
+    let map = Grid::new(input)?;
+    let mut stars = Vec::new();
+    let mut cols_occupied = IndexSet::with_capacity(map.width());
+    let mut rows_occupied = IndexSet::with_capacity(map.height());
+
+    for (y, row) in map.rows().enumerate() {
+        for (x, &pixel) in row.iter().enumerate() {
+            if pixel == b'#' {
+                cols_occupied.insert(x);
+                rows_occupied.insert(y);
+                stars.push((x, y));
+            }
+        }
+    }
+    let cols_doubled = find_doubles(cols_occupied, map.width());
+    let rows_doubled = find_doubles(rows_occupied, map.height());
+
+    for star in &mut stars {
+        star.0 = transform::<S>(star.0, &cols_doubled);
+        star.1 = transform::<S>(star.1, &rows_doubled);
+    }
+
+    let total: usize = stars
+        .iter()
+        .enumerate()
+        .flat_map(|(i, star)| {
+            stars[i + 1..]
+                .iter()
+                .map(|other| star.0.abs_diff(other.0) + star.1.abs_diff(other.1))
+        })
+        .sum();
+
+    Ok(total.to_string())
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    part_common::<2>(input)
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    part_common::<1_000_000>(input)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/11.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("374", part1(SAMPLE).unwrap());
+    }
+
+    #[test]
+    fn sample_scaled() {
+        assert_eq!("1030", part_common::<10>(SAMPLE).unwrap());
+        assert_eq!("8410", part_common::<100>(SAMPLE).unwrap());
+    }
 }

2023/src/samples/11.txt

@@ -0,0 +1,10 @@
+...#......
+.......#..
+#.........
+..........
+......#...
+.#........
+.........#
+..........
+.......#..
+#...#.....