day 10 part 2

1 files changed, 77 insertions, 18 deletions

diff --git a/src/bin/2023/day10.rs b/src/bin/2023/day10.rs
index ef055e5..07e9741 100644
--- a/src/bin/2023/day10.rs
+++ b/src/bin/2023/day10.rs
@@ -68,21 +68,7 @@ impl Tile {
     }
 }
 
-pub fn parse(fh: File) -> Result<Grid<Tile>> {
-    Ok(parse::grid(parse::raw_lines(fh), |c, _, _| match c {
-        b'.' => Tile::Floor,
-        b'S' => Tile::Start,
-        b'|' => Tile::UpDown,
-        b'-' => Tile::LeftRight,
-        b'L' => Tile::UpRight,
-        b'J' => Tile::UpLeft,
-        b'F' => Tile::DownRight,
-        b'7' => Tile::DownLeft,
-        _ => panic!("bad tile {c}"),
-    }))
-}
-
-pub fn part1(map: Grid<Tile>) -> Result<i64> {
+fn find_loop(map: &Grid<Tile>) -> (Vec<(Row, Col)>, Tile) {
     let mut cur = map
         .indexed_cells()
         .find_map(|(pos, tile)| {
@@ -111,11 +97,84 @@ pub fn part1(map: Grid<Tile>) -> Result<i64> {
             }
         }
     }
-    Ok(i64::try_from(pipe_loop.len() - 1).unwrap() / 2)
+    pipe_loop.pop();
+    let start_tile = match (
+        pipe_loop[0].0.cmp(&pipe_loop[1].0),
+        pipe_loop[0].1.cmp(&pipe_loop[1].1),
+        pipe_loop[0].0.cmp(&pipe_loop[pipe_loop.len() - 1].0),
+        pipe_loop[0].1.cmp(&pipe_loop[pipe_loop.len() - 1].1),
+    ) {
+        (Ordering::Less, _, Ordering::Greater, _)
+        | (Ordering::Greater, _, Ordering::Less, _) => Tile::UpDown,
+        (_, Ordering::Less, _, Ordering::Greater)
+        | (_, Ordering::Greater, _, Ordering::Less) => Tile::LeftRight,
+        (Ordering::Greater, _, _, Ordering::Greater)
+        | (_, Ordering::Greater, Ordering::Greater, _) => Tile::UpLeft,
+        (Ordering::Greater, _, _, Ordering::Less)
+        | (_, Ordering::Less, Ordering::Greater, _) => Tile::UpRight,
+        (Ordering::Less, _, _, Ordering::Greater)
+        | (_, Ordering::Greater, Ordering::Less, _) => Tile::DownLeft,
+        (Ordering::Less, _, _, Ordering::Less)
+        | (_, Ordering::Less, Ordering::Less, _) => Tile::DownRight,
+        _ => unreachable!(),
+    };
+    (pipe_loop, start_tile)
 }
 
-pub fn part2(_: Grid<Tile>) -> Result<i64> {
-    todo!()
+pub fn parse(fh: File) -> Result<Grid<Tile>> {
+    Ok(parse::grid(parse::raw_lines(fh), |c, _, _| match c {
+        b'.' => Tile::Floor,
+        b'S' => Tile::Start,
+        b'|' => Tile::UpDown,
+        b'-' => Tile::LeftRight,
+        b'L' => Tile::UpRight,
+        b'J' => Tile::UpLeft,
+        b'F' => Tile::DownRight,
+        b'7' => Tile::DownLeft,
+        _ => panic!("bad tile {c}"),
+    }))
+}
+
+pub fn part1(map: Grid<Tile>) -> Result<i64> {
+    Ok(i64::try_from(find_loop(&map).0.len()).unwrap() / 2)
+}
+
+pub fn part2(map: Grid<Tile>) -> Result<i64> {
+    let (pipe_loop, start_tile) = find_loop(&map);
+    let pipe_loop: HashSet<_> = pipe_loop.into_iter().collect();
+    let mut total = 0;
+    for ((row, col), tile) in map.indexed_cells() {
+        if pipe_loop.contains(&(row, col)) {
+            continue;
+        }
+        let mut count = 0;
+        for offset in 0..=(row.0.min(col.0)) {
+            let check_row = row - offset;
+            let check_col = col - offset;
+            if !pipe_loop.contains(&(check_row, check_col)) {
+                continue;
+            }
+            let check_tile = map[check_row][check_col];
+            let check_tile = if matches!(check_tile, Tile::Start) {
+                start_tile
+            } else {
+                check_tile
+            };
+            if matches!(
+                check_tile,
+                Tile::UpDown
+                    | Tile::LeftRight
+                    | Tile::UpLeft
+                    | Tile::DownRight
+            ) {
+                count += 1;
+            }
+        }
+        if count % 2 == 1 {
+            total += 1;
+        }
+    }
+    Ok(total)
 }
 
 #[test]