day7: part2 + clippies

README.md

@@ -1,6 +1,6 @@
 <!-- AOC TILES BEGIN -->
 <h1 align="center">
-  2025 - 12 ⭐ - Rust
+  2025 - 14 ⭐ - Rust
 </h1>
 <a href="src/day1.rs">
   <img src=".aoc_tiles/tiles/2025/01.png" width="161px">
@@ -20,4 +20,7 @@
 <a href="src/day6.rs">
   <img src=".aoc_tiles/tiles/2025/06.png" width="161px">
 </a>
+<a href="src/day7.rs">
+  <img src=".aoc_tiles/tiles/2025/07.png" width="161px">
+</a>
 <!-- AOC TILES END -->

src/day7.rs

@@ -1,26 +1,19 @@
-use std::collections::HashSet;
-use std::str::FromStr;
-
 use aoc_runner_derive::{aoc, aoc_generator};
 use grid::{AsCoord2d, Grid};
+use itertools::Itertools;
+use std::str::FromStr;
 
 #[aoc_generator(day7)]
 fn parse(input: &str) -> Grid<u8> {
     Grid::from_str(input).unwrap()
 }
 
-//
 fn emit_beams(grid: &mut Grid<u8>, pos: impl AsCoord2d) -> u64 {
-    match grid.get(&pos) {
+    match grid.set(&pos, b'|') {
         None | Some(b'|') => 0,
-        Some(b'.') | Some(b'S') => {
-            grid.set(&pos, b'|');
-            emit_beams(grid, &(pos.x(), pos.y() + 1))
-        }
+        Some(b'.') | Some(b'S') => emit_beams(grid, (pos.x(), pos.y() + 1)),
         Some(b'^') => {
-            grid.set(&pos, b'|');
-            1 + emit_beams(grid, &(pos.x() - 1, pos.y()))
-                + emit_beams(grid, &(pos.x() + 1, pos.y()))
+            1 + emit_beams(grid, (pos.x() - 1, pos.y())) + emit_beams(grid, (pos.x() + 1, pos.y()))
         }
         _ => panic!(),
     }
@@ -34,13 +27,48 @@ fn part1(input: &Grid<u8>) -> u64 {
 
 #[aoc(day7, part2)]
 fn part2(input: &Grid<u8>) -> u64 {
-    0
+    let mut grid = Grid::same_shape(input, 0u64);
+    grid.set(&input.find(&b'S').unwrap(), 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) {
+            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.set(&(x - 1, y), grid.get(&(x - 1, y)).unwrap() + n);
+                    grid.set(&(x + 1, y), grid.get(&(x + 1, y)).unwrap() + n);
+                }
+                // if our current position is a . add that count here
+                Some(b'.') => {
+                    grid.set(&(x, y), grid.get(&(x, y)).unwrap() + n);
+                }
+                _ => panic!(),
+            }
+        }
+    }
+
+    grid.row(grid.height() as i64 - 1)
+        .unwrap()
+        .iter()
+        .sum::<u64>()
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    const TRIVIAL_EXAMPLE: &str = "...S...
+.......
+...^...
+..^.^.^
+...^.^.
+.......
+.......";
+
     const EXAMPLE: &str = ".......S.......
 ...............
 .......^.......
@@ -65,6 +93,11 @@ mod tests {
 
     #[test]
     fn part2_example() {
-        assert_eq!(part2(&parse(EXAMPLE)), 0);
+        assert_eq!(part2(&parse(EXAMPLE)), 40);
+    }
+
+    #[test]
+    fn part2_trivial_example() {
+        assert_eq!(part2(&parse(TRIVIAL_EXAMPLE)), 7);
     }
 }