Year 2024, Day 20, part 2

kurisuke · Dec 20, 2024 · 693ecab · 693ecab
1 parent 66002cb
commit 693ecab
Showing 1 changed file with 47 additions and 23 deletions.
diff --git a/year2024/src/day20.rs b/year2024/src/day20.rs
@@ -1,15 +1,15 @@
 use std::collections::HashMap;
 
 use common::day::Day;
-use util::grid2d::{Coords, Direction, Grid2D};
+use util::grid2d::{Coords, Grid2D};
 
 pub struct Day20 {}
 
 impl Day for Day20 {
     fn star1(&self, input: &str) -> String {
         let grid = parse_input(input);
         let track = find_track(&grid);
-        let shortcuts = find_shortcuts(&grid, &track);
+        let shortcuts = find_shortcuts(&track, 2);
         let result = shortcuts
             .iter()
             .filter(|(&key, _)| key >= 100)
@@ -19,8 +19,17 @@ impl Day for Day20 {
         result.to_string()
     }
 
-    fn star2(&self, _input: &str) -> String {
-        String::from("not implemented")
+    fn star2(&self, input: &str) -> String {
+        let grid = parse_input(input);
+        let track = find_track(&grid);
+        let shortcuts = find_shortcuts(&track, 20);
+        let result = shortcuts
+            .iter()
+            .filter(|(&key, _)| key >= 100)
+            .map(|(_, value)| value)
+            .sum::<usize>();
+
+        result.to_string()
     }
 }
 
@@ -70,25 +79,17 @@ fn find_track(grid: &Grid2D<Field>) -> Vec<Coords> {
     track
 }
 
-fn find_shortcuts(grid: &Grid2D<Field>, track: &[Coords]) -> HashMap<usize, usize> {
+fn find_shortcuts(track: &[Coords], max_len: usize) -> HashMap<usize, usize> {
     let mut shortcuts = HashMap::new();
     for (pos_index, pos) in track.iter().enumerate() {
-        for d in [Direction::N, Direction::E, Direction::S, Direction::W] {
-            let skip_pos = pos.mov(d);
-            let next_pos = skip_pos.mov(d);
-            if let (Some(skip_field), Some(next_field)) = (grid.at(&skip_pos), grid.at(&next_pos)) {
-                if skip_field == &Field::Wall && next_field != &Field::Wall {
-                    let next_index = track
-                        .iter()
-                        .position(|&track_pos| track_pos == next_pos)
-                        .unwrap();
-
-                    if pos_index < next_index {
-                        let saved_time = next_index - pos_index - 2;
-                        let e = shortcuts.entry(saved_time).or_insert(0);
-                        *e += 1;
-                    }
-                }
+        for (next_index, next_pos) in track.iter().enumerate().skip(pos_index + max_len + 1) {
+            let regular_distance = next_index - pos_index;
+            let shortcut_distance = pos.manhattan(next_pos) as usize;
+            if shortcut_distance <= max_len && shortcut_distance < regular_distance {
+                let e = shortcuts
+                    .entry(regular_distance - shortcut_distance)
+                    .or_insert(0);
+                *e += 1;
             }
         }
     }
@@ -116,12 +117,12 @@ mod tests {
 ###############"#;
 
     #[test]
-    fn ex1() {
+    fn star1() {
         let grid = parse_input(INPUT);
         let track = find_track(&grid);
         assert_eq!(track.len(), 85);
 
-        let shortcuts = find_shortcuts(&grid, &track);
+        let shortcuts = find_shortcuts(&track, 2);
         assert_eq!(shortcuts.get(&2), Some(&14));
         assert_eq!(shortcuts.get(&4), Some(&14));
         assert_eq!(shortcuts.get(&6), Some(&2));
@@ -134,4 +135,27 @@ mod tests {
         assert_eq!(shortcuts.get(&40), Some(&1));
         assert_eq!(shortcuts.get(&64), Some(&1));
     }
+
+    #[test]
+    fn star2() {
+        let grid = parse_input(INPUT);
+        let track = find_track(&grid);
+        assert_eq!(track.len(), 85);
+
+        let shortcuts = find_shortcuts(&track, 20);
+        assert_eq!(shortcuts.get(&50), Some(&32));
+        assert_eq!(shortcuts.get(&52), Some(&31));
+        assert_eq!(shortcuts.get(&54), Some(&29));
+        assert_eq!(shortcuts.get(&56), Some(&39));
+        assert_eq!(shortcuts.get(&58), Some(&25));
+        assert_eq!(shortcuts.get(&60), Some(&23));
+        assert_eq!(shortcuts.get(&62), Some(&20));
+        assert_eq!(shortcuts.get(&64), Some(&19));
+        assert_eq!(shortcuts.get(&66), Some(&12));
+        assert_eq!(shortcuts.get(&68), Some(&14));
+        assert_eq!(shortcuts.get(&70), Some(&12));
+        assert_eq!(shortcuts.get(&72), Some(&22));
+        assert_eq!(shortcuts.get(&74), Some(&4));
+        assert_eq!(shortcuts.get(&76), Some(&3));
+    }
 }