day 6 part 2 1.5 times faster

src/day06.rs

@@ -64,7 +64,7 @@ fn parse(input: &str) -> (Grid<char>, Coord) {
 }
 
 fn is_loop(m: &Grid<char>, block: Coord, mut pos: Coord, mut dir: char) -> bool {
-    let mut visited = HashSet::new();
+    let mut visited: Grid<[bool; 5]> = Grid::from_default(m.content_width, m.content_height);
     loop {
         let (x, y) = pos;
 
@@ -79,10 +79,11 @@ fn is_loop(m: &Grid<char>, block: Coord, mut pos: Coord, mut dir: char) -> bool
             return false;
         } else if next == block || '#' == m[next] {
             // we only check for loops on a collision to speed things up
-            if visited.contains(&(pos, dir)) {
+            // this is our own little hash function to speed things up
+            if visited[pos][dir as usize % 5] {
                 return true;
             }
-            visited.insert((pos, dir));
+            visited[pos][dir as usize % 5] = true;
 
             dir = match dir {
                 '^' => '>',

src/day14.rs

@@ -69,7 +69,7 @@ fn part2(input: &str) -> i64 {
     let width = 101;
     let height = 103;
     let mut robots = parse(input);
-    let max_seconds = width * height as i64;
+    let max_seconds = width * height;
     for second in 0.. {
         if second > max_seconds {
             panic!("Seen all combinations but no christmas tree. So sad!");

src/utils/grid.rs

@@ -89,6 +89,15 @@ impl<T> Grid<T> {
         })
     }
 
+    pub fn entries_mut(&mut self) -> impl Iterator<Item = (Coord, &mut T)> {
+        self.content.iter_mut().enumerate().map(|(i, val)| {
+            (
+                (i as i64 % self.content_width, i as i64 / self.content_width),
+                val,
+            )
+        })
+    }
+
     pub fn map_values<U>(self, f: fn(T) -> U) -> Grid<U> {
         let new_content = self.content.into_iter().map(f).collect_vec();
         Grid {