Implement day 9

2021/src/day09.rs

@@ -1,9 +1,143 @@
+use std::cmp::Reverse;
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+struct LowPointsIter<'a> {
+    lines: &'a [&'a [u8]],
+    x: usize,
+    y: usize,
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+impl<'a> LowPointsIter<'a> {
+    pub fn new(lines: &'a [&'a [u8]]) -> Self {
+        Self { lines, x: 0, y: 0 }
+    }
+}
+
+impl Iterator for LowPointsIter<'_> {
+    type Item = (usize, usize);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let lines = self.lines;
+
+        for y in self.y..self.lines.len() {
+            for x in self.x..self.lines[y].len() {
+                if x > 0 && lines[y][x - 1] <= lines[y][x] {
+                    continue;
+                }
+
+                if x + 1 < lines[y].len() && lines[y][x + 1] <= lines[y][x] {
+                    continue;
+                }
+
+                if y > 0 && lines[y - 1][x] <= lines[y][x] {
+                    continue;
+                }
+
+                if y + 1 < lines.len() && lines[y + 1][x] <= lines[y][x] {
+                    continue;
+                }
+
+                self.x = x + 1;
+                self.y = y;
+
+                return Some((x, y));
+            }
+            self.x = 0;
+        }
+
+        self.y = lines.len();
+
+        None
+    }
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    let mut buffer = Vec::new();
+    input.read_to_end(&mut buffer).unwrap();
+
+    let lines: Vec<_> = buffer
+        .split(|&s| s == b'\n')
+        .filter(|s| !s.is_empty())
+        .collect();
+
+    LowPointsIter::new(&lines)
+        .map(|(x, y)| 1 + (lines[y][x] - b'0') as i32)
+        .sum::<i32>()
+        .to_string()
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    let mut buffer = Vec::new();
+    input.read_to_end(&mut buffer).unwrap();
+
+    let mut visited_buffer = vec![false; buffer.len()];
+
+    let lines: Vec<_> = buffer
+        .split(|&s| s == b'\n')
+        .filter(|s| !s.is_empty())
+        .collect();
+
+    let mut visited: Vec<_> = visited_buffer.chunks_exact_mut(lines[0].len()).collect();
+
+    let mut todo = Vec::new();
+
+    let mut sizes = Vec::with_capacity(4);
+
+    for (x, y) in LowPointsIter::new(&lines) {
+        todo.push((x, y));
+
+        let mut size = 1;
+        visited[y][x] = true;
+
+        while let Some((x, y)) = todo.pop() {
+            let mut add = |x: usize, y: usize| {
+                if lines[y][x] != b'9' && !visited[y][x] {
+                    size += 1;
+                    visited[y][x] = true;
+                    todo.push((x, y));
+                }
+            };
+
+            if x > 0 {
+                add(x - 1, y);
+            }
+
+            if x + 1 < lines[y].len() {
+                add(x + 1, y);
+            }
+
+            if y > 0 {
+                add(x, y - 1)
+            }
+
+            if y + 1 < lines.len() {
+                add(x, y + 1);
+            }
+        }
+
+        sizes.push(Reverse(size));
+        sizes.sort_unstable();
+        sizes.truncate(3);
+    }
+
+    sizes.into_iter().fold(1, |a, Reverse(b)| a * b).to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::test_implementation;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/09.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 15);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation(part2, SAMPLE, 1134);
+    }
 }

2021/src/samples/09.txt

@@ -0,0 +1,5 @@
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678