day7: optimize away needing a second grid

src/day7.rs

@@ -25,34 +25,36 @@ fn part1(input: &Grid<u8>) -> u64 {
 
 #[aoc(day7, part2)]
 fn part2(input: &Grid<u8>) -> u64 {
-    let mut grid = Grid::same_shape(input, 0u64);
-    grid.set(&input.find(&b'S').unwrap(), 1);
+    let mut col_counts = vec![0; input.width()];
+    col_counts[input.find(&b'S').unwrap().x as usize] = 1;
 
     // Start row is already set up
-    for y in 1..grid.height() {
-        let row_above = grid.row_iter(y as i64 - 1).unwrap().cloned().collect_vec();
-        // for each column with non-zero counts in the row above get its column index and count
-        for (x, n) in row_above.into_iter().enumerate().filter(|(_i, n)| *n > 0) {
+    for y in 1..input.height() {
+        // for each column with non-zero counts
+        for x in 0..input.width() {
+            if col_counts[x] == 0 {
+                continue;
+            }
             match input.get(&(x, y)) {
-                None => panic!("How did we end up outside the grid?"),
                 // if our current position is a caret, add that count to our neighbours
                 Some(b'^') => {
-                    grid.increment(&(x - 1, y), n);
-                    grid.increment(&(x + 1, y), n);
+                    for col in [x as i64 - 1, x as i64 + 1]
+                        .into_iter()
+                        .filter(|x| *x >= 0 && *x < input.width() as i64)
+                    {
+                        col_counts[col as usize] += col_counts[x];
                     }
-                // if our current position is a . add that count here
-                Some(b'.') => {
-                    grid.increment(&(x, y), n);
+                    col_counts[x] = 0;
                 }
-                _ => panic!(),
+                // if our current position is a . do nothing, it's already counted
+                Some(b'.') => continue,
+                Some(c) => panic!("unexpected character {c}"),
+                None => panic!("How did we end up outside the grid?"),
             }
         }
     }
 
-    grid.row(grid.height() as i64 - 1)
-        .unwrap()
-        .iter()
-        .sum::<u64>()
+    col_counts.iter().sum::<u64>()
 }
 
 #[cfg(test)]