From 3ec90894bdf0d715a0c94b610abaa60b85d90c69 Mon Sep 17 00:00:00 2001
From: Himangshu Saikia <himangshu.saikia.iitg@gmail.com>
Date: Fri, 20 Dec 2024 19:53:11 +0100
Subject: [PATCH] 2024_20

---
 src/bin/2024_20/main.rs | 80 +++++++++++++++++++++++++++++++++++++++++
 src/grid.rs             | 16 +++++++++
 2 files changed, 96 insertions(+)
 create mode 100644 src/bin/2024_20/main.rs

diff --git a/src/bin/2024_20/main.rs b/src/bin/2024_20/main.rs
new file mode 100644
index 0000000..1c18d9f
--- /dev/null
+++ b/src/bin/2024_20/main.rs
@@ -0,0 +1,80 @@
+use std::collections::VecDeque;
+
+use aoc::{common, grid::Grid};
+
+fn shortest_path(map: &Grid<char>) -> Vec<(usize, usize)> {
+    let ans = Vec::new();
+    let pos = map.positions('S')[0];
+    let mut q = VecDeque::new();
+    q.push_back(vec![pos]);
+    let mut visited = Vec::new();
+
+    while let Some(tp) = q.pop_front() {
+        let lst = tp.last().unwrap();
+        if visited.contains(lst) {
+            continue;
+        }
+        visited.push(*lst);
+
+        if map.get(lst) == 'E' {
+            return tp;
+        }
+
+        let nx: Vec<(usize, usize)> = map
+            .adjacent_4(lst)
+            .into_iter()
+            .filter(|x| map.get(x) == '.' || map.get(x) == 'E')
+            .collect();
+
+        for n in nx.iter() {
+            let mut tp_tmp = tp.clone();
+            tp_tmp.push(*n);
+            q.push_back(tp_tmp);
+        }
+    }
+    ans
+}
+
+fn solve<const PICOSECONDS: usize, const CHEAT_PICOSECONDS: usize>(input: &str) -> usize {
+    let map = Grid::from_str(input, |c| c);
+    let path = shortest_path(&map);
+    let path_d: Vec<((usize, usize), usize)> =
+        path.iter().enumerate().map(|(i, v)| (*v, i)).collect();
+
+    let mut res: [usize; 10000] = [0; 10000];
+
+    for (v1, l1) in path_d.iter() {
+        for (v2, l2) in path_d.iter().skip(PICOSECONDS) {
+            let dist = map.l1_distance(v1, v2);
+            if dist <= CHEAT_PICOSECONDS && *l2 > *l1 + dist {
+                let l = *l2 - *l1 - dist;
+                res[l] += 1;
+            }
+        }
+    }
+
+    res.iter()
+        .enumerate()
+        .filter(|(k, _)| *k >= PICOSECONDS)
+        .map(|(_, v)| v)
+        .sum()
+}
+
+fn main() {
+    let input = common::get_input();
+    println!("{input:?}");
+    common::timed(&input, solve::<100, 2>, true);
+    common::timed(&input, solve::<100, 20>, false);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_samples() {
+        let sample_input = "###############\n#...#...#.....#\n#.#.#.#.#.###.#\n#S#...#.#.#...#\n#######.#.#.###\n#######.#.#...#\n#######.#.###.#\n###..E#...#...#\n###.#######.###\n#...###...#...#\n#.#####.#.###.#\n#.#...#.#.#...#\n#.#.#.#.#.#.###\n#...#...#...###\n###############";
+        assert_eq!(solve::<2, 2>(sample_input), 44);
+        assert_eq!(solve::<2, 20>(sample_input), 3081);
+    }
+}
diff --git a/src/grid.rs b/src/grid.rs
index 5a77b4d..cc9748c 100644
--- a/src/grid.rs
+++ b/src/grid.rs
@@ -233,6 +233,22 @@ impl<T: std::fmt::Debug + Clone + Default + PartialEq + Hash> Grid<T> {
         ret
     }
 
+    pub fn l1_distance(&self, idx1: &CellIndex, idx2: &CellIndex) -> usize {
+        let dx = if idx1.0 > idx2.0 {
+            idx1.0 - idx2.0
+        } else {
+            idx2.0 - idx1.0
+        };
+
+        let dy = if idx1.1 > idx2.1 {
+            idx1.1 - idx2.1
+        } else {
+            idx2.1 - idx1.1
+        };
+
+        dx + dy
+    }
+
     pub fn adjacent_2_row(&self, idx: &CellIndex) -> Vec<(usize, usize)> {
         self.adjacent_in_dir(idx, &[(0, 1), (0, -1)])
     }