Using Rayon gets 6b runtime down to only five seconds

2024/src/day6.rs

@@ -1,6 +1,7 @@
-use std::collections::HashSet;
+use std::{collections::HashSet, sync::{Arc, RwLock}};
 
 use crate::grid::{Direction, Grid};
+use rayon::prelude::*;
 
 const INPUT: &'static str = include_str!("../data/day6.txt");
 
@@ -23,31 +24,42 @@ struct Guard {
 fn distinct_locations(grid: &Grid) -> usize {
     let guard = find_guard(grid);
     let (_, path) = simulate_path(grid, guard.clone());
-    path.into_iter().map(|guard| guard.location).collect::<HashSet<(usize, usize)>>().len()
+    path.into_iter()
+        .map(|guard| guard.location)
+        .collect::<HashSet<(usize, usize)>>()
+        .len()
 }
 
-fn looping_obstructions(mut grid: Grid) -> HashSet<(usize, usize)> {
+fn looping_obstructions(grid: Grid) -> HashSet<(usize, usize)> {
     let guard = find_guard(&grid);
     let (_, path) = simulate_path(&grid, guard.clone());
-    let mut obstructions = HashSet::new();
+    let obstructions = Arc::new(RwLock::new(HashSet::new()));
 
     let start = path.first().unwrap().clone();
-    let mut path = path.into_iter().map(|g| g.location).collect::<HashSet<(usize, usize)>>();
+    let mut path = path
+        .into_iter()
+        .map(|g| g.location)
+        .collect::<HashSet<(usize, usize)>>();
     path.remove(&start.location);
     // Now that we know the guard's original path, we can traverse all of the locations she would
     // step through and consider that location as a potential new obstacle. Place and obstacle
     // there and re-run the simulation.
 
-    for location in path.into_iter() {
-        *grid.elem_mut(location.0, location.1) = '#';
-        let (looping, _) = simulate_path(&grid, guard.clone());
-        *grid.elem_mut(location.0, location.1) = '.';
-        if looping {
-            obstructions.insert(location);
+    path.into_par_iter().for_each({
+        let obstructions = obstructions.clone();
+        move |location| {
+            let mut grid = grid.clone();
+            *grid.elem_mut(location.0, location.1) = '#';
+            let (looping, _) = simulate_path(&grid, guard.clone());
+            *grid.elem_mut(location.0, location.1) = '.';
+            if looping {
+                obstructions.write().unwrap().insert(location);
+            }
         }
-    }
+    });
 
-    obstructions
+    let x = obstructions.read().unwrap().clone();
+    x
 }
 
 fn find_guard(grid: &Grid) -> Guard {
@@ -62,7 +74,7 @@ fn find_guard(grid: &Grid) -> Guard {
             };
 
             if let Some(orientation) = orientation {
-                return Guard{
+                return Guard {
                     location: (x, y),
                     orientation,
                 };
@@ -129,7 +141,13 @@ mod test {
     #[test]
     fn it_finds_the_guard() {
         let grid = Grid::from(TEST_INPUT);
-        assert_eq!(find_guard(&grid), Guard{ location: (4, 6), orientation: Direction::North });
+        assert_eq!(
+            find_guard(&grid),
+            Guard {
+                location: (4, 6),
+                orientation: Direction::North
+            }
+        );
     }
 
     #[test]
@@ -156,7 +174,9 @@ mod test {
         assert!(obs.contains(&(3, 8)));
         assert!(obs.contains(&(7, 9)));
 
+        /* Even with par_iter, this is still too slow to run on every test
         let grid = Grid::from(INPUT);
         assert_eq!(looping_obstructions(grid).len(), 1703);
+        */
     }
 }