day6, part2

day06/src/main.rs

@@ -29,13 +29,30 @@ fn pt2(input: &str) -> usize {
     let guard = board.guard;
 
     let no = [guard.0, guard.1.next(guard.0)];
+    let mut trace = HashSet::new();
+    while let Some((loc, _)) = board.step() {
+        if !no.contains(&loc) {
+            trace.insert(loc);
+        }
+    }
 
-    while let Some(loc) = board.step() {}
+    for loc in trace {
+        board = Board::new(input);
+        let mut steps = HashSet::new();
+        board.set(loc, Cell::Obstacle);
+        steps.insert(board.guard);
+        while let Some(guard) = board.step() {
+            if !steps.insert(guard) {
+                total += 1;
+                break;
+            }
+        }
+    }
 
     total
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, Hash)]
 enum Dir {
     #[default]
     N,
@@ -127,6 +144,16 @@ impl Board {
         parse(input).unwrap()
     }
 
+    fn set(&mut self, loc: Loc, cell: Cell) -> bool {
+        if let Some(row) = self.cells.get_mut(loc.0) {
+            if let Some(c) = row.get_mut(loc.1) {
+                *c = cell;
+                return true;
+            }
+        }
+        false
+    }
+
     fn step(&mut self) -> Option<(Loc, Dir)> {
         let (loc, dir) = &mut self.guard;
         *self