From 00000270f67b9ef43168f0397ff2db84c1a72e2e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matej=20Jane=C5=BEi=C4=8D?= <janezic.mj@gmail.com>
Date: Sat, 21 Dec 2024 17:42:27 +0100
Subject: [PATCH] solution: day 20

---
 data/examples/20.txt |  15 +++++
 src/bin/20.rs        | 135 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 150 insertions(+)
 create mode 100644 data/examples/20.txt
 create mode 100644 src/bin/20.rs

diff --git a/data/examples/20.txt b/data/examples/20.txt
new file mode 100644
index 000000000..c097dae
--- /dev/null
+++ b/data/examples/20.txt
@@ -0,0 +1,15 @@
+###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############
diff --git a/src/bin/20.rs b/src/bin/20.rs
new file mode 100644
index 000000000..35c7d32
--- /dev/null
+++ b/src/bin/20.rs
@@ -0,0 +1,135 @@
+use std::collections::VecDeque;
+
+use aoc::grid_vec::{Direction, Grid, Point};
+use hashbrown::HashMap;
+
+fn parse_input(input: &str) -> (Grid, Point, Point) {
+    let mut grid: Grid = input.parse().unwrap();
+    let start = grid.find(b'S').unwrap();
+    let end = grid.find(b'E').unwrap();
+
+    grid[start] = b'.';
+    grid[end] = b'.';
+
+    (grid, start, end)
+}
+fn bfs(
+    grid: &Grid,
+    start: Point,
+    limit: Option<usize>,
+    cheat: bool,
+    visited: &mut HashMap<Point, usize>,
+    queue: &mut VecDeque<(Point, usize)>,
+) {
+    visited.clear();
+    queue.clear();
+
+    queue.push_back((start, 0));
+
+    while let Some((p, l)) = queue.pop_front() {
+        if visited.contains_key(&p) {
+            continue;
+        }
+        visited.insert(p, l);
+
+        for d in Direction::CROSS {
+            let n = p + d;
+
+            let nv = grid.get(&n);
+            let uwnv = *nv.unwrap_or(&b'#');
+            let valid_neigh = (cheat && nv.is_some()) || uwnv == b'.';
+            let valid_length = l < limit.unwrap_or(usize::MAX);
+            let unvisited = !visited.contains_key(&n);
+
+            if valid_neigh && valid_length && unvisited {
+                queue.push_back((n, l + 1));
+            }
+        }
+    }
+}
+
+fn get_cheat_neighbours_and_benchmark(
+    grid: &Grid,
+    start: Point,
+    cheat_limit: usize,
+) -> HashMap<Point, Vec<(Point, usize)>> {
+    let mut map = HashMap::<Point, Vec<(Point, usize)>>::new();
+
+    let mut reuse_queue = VecDeque::new();
+    let mut reuse_visited = HashMap::new();
+
+    let mut valid_points = HashMap::new();
+
+    bfs(
+        grid,
+        start,
+        None,
+        false,
+        &mut valid_points,
+        &mut reuse_queue,
+    );
+
+    for (&s, _) in valid_points.iter() {
+        bfs(
+            grid,
+            s,
+            Some(cheat_limit),
+            true,
+            &mut reuse_visited,
+            &mut reuse_queue,
+        );
+        for (&point, &length) in reuse_visited.iter().filter(|(&p, _)| grid[p] == b'.') {
+            map.entry(s).or_default().push((point, length));
+        }
+    }
+
+    map
+}
+
+fn cheated_bfs(
+    grid: &Grid,
+    start: Point,
+    end: Point,
+    cheat_limit: usize,
+    increase_limit: usize,
+) -> usize {
+    let neighbours = get_cheat_neighbours_and_benchmark(grid, start, cheat_limit);
+
+    let mut reuse_queue = VecDeque::new();
+
+    let mut from_start = HashMap::new();
+    let mut from_end = HashMap::new();
+
+    bfs(grid, start, None, false, &mut from_start, &mut reuse_queue);
+    bfs(grid, end, None, false, &mut from_end, &mut reuse_queue);
+
+    let benchmark = from_start[&end];
+
+    let mut count = 0;
+
+    for node in from_start.keys() {
+        for (neigh, cheat_dist) in neighbours[node].iter() {
+            let start_dist = from_start[node];
+            let end_dist = from_end[neigh];
+
+            let dist = start_dist + cheat_dist + end_dist;
+            if dist < benchmark && dist.abs_diff(benchmark) >= increase_limit {
+                count += 1;
+            }
+        }
+    }
+
+    count
+}
+
+pub fn part_one(input: &str) -> Option<usize> {
+    let (grid, start, end) = parse_input(input);
+    cheated_bfs(&grid, start, end, 2, 100).into()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let (grid, start, end) = parse_input(input);
+    cheated_bfs(&grid, start, end, 20, 100).into()
+}
+
+aoc::solution!(20);