Adding a better solution for Day 23 that runs in 12s rather than 8 hours

johnhurt · Jan 4, 2024 · bc1e585 · bc1e585
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diff --git a/README.md b/README.md
@@ -39,35 +39,35 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 <!--- benchmarking table --->
 ## Benchmarks
 
-|            Day            |  Part 1   |      Part 2      |
-| :-----------------------: | :-------: | :--------------: |
-| [Day 1](./src/bin/01.rs)  | `31.9µs`  |    `375.3µs`     |
-| [Day 2](./src/bin/02.rs)  | `100.6µs` |    `100.3µs`     |
-| [Day 3](./src/bin/03.rs)  |  `8.5ms`  |     `9.4ms`      |
-| [Day 4](./src/bin/04.rs)  | `261.0µs` |    `300.1µs`     |
-| [Day 5](./src/bin/05.rs)  | `26.6µs`  |     `97.9µs`     |
-| [Day 6](./src/bin/06.rs)  | `572.0ns` |     `1.0µs`      |
-| [Day 7](./src/bin/07.rs)  | `636.9µs` |    `660.6µs`     |
-| [Day 8](./src/bin/08.rs)  | `792.2µs` |     `7.0ms`      |
-| [Day 9](./src/bin/09.rs)  | `490.3µs` |    `492.6µs`     |
-| [Day 10](./src/bin/10.rs) | `164.8µs` |     `3.9ms`      |
-| [Day 11](./src/bin/11.rs) |  `5.3ms`  |     `5.4ms`      |
-| [Day 12](./src/bin/12.rs) |  `1.3ms`  |     `24.3ms`     |
-| [Day 13](./src/bin/13.rs) | `194.5µs` |    `196.7µs`     |
-| [Day 14](./src/bin/14.rs) | `397.3µs` |    `332.5ms`     |
-| [Day 15](./src/bin/15.rs) | `67.7µs`  |    `359.8µs`     |
-| [Day 16](./src/bin/16.rs) |  `3.7ms`  |    `989.8ms`     |
-| [Day 17](./src/bin/17.rs) | `110.8ms` |    `318.6ms`     |
-| [Day 18](./src/bin/18.rs) | `687.4µs` |    `701.3µs`     |
-| [Day 19](./src/bin/19.rs) | `327.7µs` |    `318.7µs`     |
-| [Day 20](./src/bin/20.rs) |  `3.9ms`  |     `60.4ms`     |
-| [Day 21](./src/bin/21.rs) |  `1.4ms`  |    `206.2ms`     |
-| [Day 22](./src/bin/22.rs) |  `9.7ms`  |      `7.5s`      |
-| [Day 23](./src/bin/23.rs) | `82.2ms`  | `Like 8 hours 😭` |
-| [Day 24](./src/bin/24.rs) |  `1.1ms`  |     `??????`     |
-| [Day 25](./src/bin/25.rs) |  `5.6s`   |       `-`        |
-
-**Total: 15.3s (+ 8 hours for 23-P2)**
+| Day | Part 1 | Part 2 |
+| :---: | :---: | :---:  |
+| [Day 1](./src/bin/01.rs) | `38.5µs` | `271.4µs` |
+| [Day 2](./src/bin/02.rs) | `60.0µs` | `59.3µs` |
+| [Day 3](./src/bin/03.rs) | `8.1ms` | `8.6ms` |
+| [Day 4](./src/bin/04.rs) | `165.1µs` | `182.6µs` |
+| [Day 5](./src/bin/05.rs) | `24.7µs` | `61.8µs` |
+| [Day 6](./src/bin/06.rs) | `434.0ns` | `705.0ns` |
+| [Day 7](./src/bin/07.rs) | `380.3µs` | `404.8µs` |
+| [Day 8](./src/bin/08.rs) | `671.9µs` | `5.6ms` |
+| [Day 9](./src/bin/09.rs) | `247.9µs` | `247.9µs` |
+| [Day 10](./src/bin/10.rs) | `122.9µs` | `3.1ms` |
+| [Day 11](./src/bin/11.rs) | `4.1ms` | `4.0ms` |
+| [Day 12](./src/bin/12.rs) | `630.5µs` | `11.6ms` |
+| [Day 13](./src/bin/13.rs) | `123.5µs` | `127.2µs` |
+| [Day 14](./src/bin/14.rs) | `223.9µs` | `237.7ms` |
+| [Day 15](./src/bin/15.rs) | `52.3µs` | `210.8µs` |
+| [Day 16](./src/bin/16.rs) | `3.2ms` | `875.6ms` |
+| [Day 17](./src/bin/17.rs) | `80.4ms` | `207.8ms` |
+| [Day 18](./src/bin/18.rs) | `427.1µs` | `441.9µs` |
+| [Day 19](./src/bin/19.rs) | `225.2µs` | `198.7µs` |
+| [Day 20](./src/bin/20.rs) | `2.7ms` | `40.8ms` |
+| [Day 21](./src/bin/21.rs) | `1.1ms` | `168.3ms` |
+| [Day 22](./src/bin/22.rs) | `6.4ms` | `6.0s` |
+| [Day 23](./src/bin/23.rs) | `52.9ms` | `12.4s` |
+| [Day 24](./src/bin/24.rs) | `1.1ms` | `-` |
+| [Day 25](./src/bin/25.rs) | `3.4s` | `-` |
+
+**Total: 23528.70ms**
 <!--- benchmarking table --->
 
 ---

diff --git a/src/bin/23.rs b/src/bin/23.rs
@@ -1,4 +1,4 @@
-use std::collections::HashSet;
+use std::collections::{HashMap, HashSet, VecDeque};
 
 use advent_of_code::{Compass, Grid};
 
@@ -38,7 +38,7 @@ impl From<Hike> for char {
     }
 }
 
-fn max_path(start: usize, end: usize, grid: &Grid<Hike>) -> usize {
+fn max_path(start: usize, end: usize, grid: &mut Grid<Hike>) -> usize {
     let mut stack = vec![];
     stack.push((0, start, HashSet::new()));
 
@@ -93,13 +93,101 @@ fn max_path(start: usize, end: usize, grid: &Grid<Hike>) -> usize {
 }
 
 pub fn part_one(input: &str) -> Option<usize> {
-    let grid = Grid::parse_lines(input);
+    let mut grid = Grid::parse_lines(input);
 
-    let result = max_path(1, grid.data.len() - 2, &grid);
+    let result = max_path(1, grid.data.len() - 2, &mut grid);
 
     Some(result)
 }
 
+fn graph_neighbors(
+    from: usize,
+    grid: &Grid<Hike>,
+    nodes: &HashSet<usize>,
+) -> HashMap<usize, usize> {
+    let mut result = HashMap::new();
+    let mut queue = VecDeque::new();
+
+    queue.push_back((from, 0, from));
+
+    while let Some((curr, dist, prev)) = queue.pop_front() {
+        if curr != from && nodes.contains(&curr) {
+            result.insert(curr, dist);
+        } else {
+            queue.extend(
+                grid.neighbors(curr)
+                    .map(|(_, n)| n)
+                    .filter(|n| grid.data[*n] != Hike::Wall)
+                    .filter(|n| *n != prev)
+                    .map(|n| (n, dist + 1, curr)),
+            )
+        }
+    }
+
+    result
+}
+
+fn extract_graph(
+    grid: &Grid<Hike>,
+    start: usize,
+    end: usize,
+) -> HashMap<usize, HashMap<usize, usize>> {
+    let nodes = [start, end]
+        .into_iter()
+        .chain(
+            grid.data
+                .iter()
+                .enumerate()
+                .filter(|(_, h)| **h != Hike::Wall)
+                .map(|(i, _)| i)
+                .filter(|i| {
+                    grid.neighbors(*i)
+                        .map(|(_, n)| n)
+                        .filter(|n| grid.data[*n] != Hike::Wall)
+                        .count()
+                        > 2
+                }),
+        )
+        .collect::<HashSet<_>>();
+
+    nodes
+        .iter()
+        .map(|n| (*n, graph_neighbors(*n, grid, &nodes)))
+        .collect::<HashMap<_, _>>()
+}
+
+fn search_simplified(start: usize, end: usize, grid: &mut Grid<Hike>) -> usize {
+    let simplified = extract_graph(grid, start, end);
+
+    let mut stack = vec![];
+    stack.push((0, start, HashSet::new()));
+
+    let mut max_dist = 0;
+
+    while let Some((dist, curr, mut hist)) = stack.pop() {
+        if curr == end {
+            if dist > max_dist {
+                max_dist = max_dist.max(dist);
+            }
+            continue;
+        }
+
+        if !hist.insert(curr) {
+            continue;
+        }
+
+        stack.extend(
+            simplified
+                .get(&curr)
+                .unwrap()
+                .iter()
+                .map(|(too, add_dist)| (dist + add_dist, *too, hist.clone())),
+        );
+    }
+
+    max_dist
+}
+
 pub fn part_two(input: &str) -> Option<usize> {
     let mut grid = Grid::parse_lines(input);
 
@@ -109,7 +197,7 @@ pub fn part_two(input: &str) -> Option<usize> {
         }
     });
 
-    let result = max_path(1, grid.data.len() - 2, &grid);
+    let result = search_simplified(1, grid.data.len() - 2, &mut grid);
 
     Some(result)
 }