Implement 2022 day 8 part 2

2022/src/day08.rs

@@ -59,8 +59,65 @@ pub fn part1(input: &[u8]) -> Result<String> {
     Ok(visible.into_iter().filter(|&b| b).count().to_string())
 }
 
-pub fn part2(_input: &[u8]) -> Result<String> {
-    anyhow::bail!("not implemented")
+#[inline]
+fn scenery<'a>(
+    values: impl IntoIterator<Item = &'a u8>,
+    visible: impl IntoIterator<Item = &'a mut usize>,
+) {
+    let mut last_seen = [0; 10];
+
+    for (i, (&val, score)) in values.into_iter().zip(visible).enumerate() {
+        let val = val - b'0';
+        let visible = i - last_seen[val as usize];
+
+        if i > 0 {
+            *score *= visible;
+            for s in 0..=val {
+                last_seen[s as usize] = i;
+            }
+        } else {
+            *score = 0;
+        }
+    }
+}
+
+pub fn part2(input: &[u8]) -> Result<String> {
+    let width = input
+        .iter()
+        .position(|&b| b == b'\n')
+        .context("Single row field")?;
+    let height = input.len() / (width + 1); // Include newlines
+
+    let mut score = vec![1; width * height];
+
+    // Horizontal striping
+    for (y, row) in input.chunks_exact(width + 1).enumerate() {
+        // First, left to right
+        scenery(&row[..width], &mut score[(y * width)..]);
+
+        // Then right to left
+        scenery(
+            row[..width].iter().rev(),
+            score[(y * width)..(y * width + width)].iter_mut().rev(),
+        );
+    }
+
+    // Vertical striping
+    for x in 0..width {
+        // Top to bottom
+        scenery(
+            input[x..].iter().step_by(width + 1),
+            score[x..].iter_mut().step_by(width),
+        );
+
+        // Bottom to top
+        scenery(
+            input[x..].iter().step_by(width + 1).rev(),
+            score[x..].iter_mut().step_by(width).rev(),
+        )
+    }
+
+    Ok(score.into_iter().max().context("empty field")?.to_string())
 }
 
 #[cfg(test)]