From cd4268a9cedf7e858a43b7f1638b503485c67878 Mon Sep 17 00:00:00 2001
From: Quentin Boyer <quentin+dev@familleboyer.net>
Date: Sat, 4 Jan 2025 20:07:25 +0100
Subject: [PATCH] Day 18

---
 src/bin/day18.rs | 211 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 211 insertions(+)
 create mode 100644 src/bin/day18.rs

diff --git a/src/bin/day18.rs b/src/bin/day18.rs
new file mode 100644
index 0000000..76c730e
--- /dev/null
+++ b/src/bin/day18.rs
@@ -0,0 +1,211 @@
+use std::{
+    collections::{BinaryHeap, HashMap, HashSet},
+    time::Instant,
+};
+
+use aoc_2024::{load, print_res};
+use bstr::BString;
+use color_eyre::eyre::OptionExt;
+
+type Parsed = Vec<(usize, usize)>;
+
+#[inline(never)]
+pub fn parsing(input: &BString) -> color_eyre::Result<Parsed> {
+    std::str::from_utf8(input)?
+        .lines()
+        .map(|l| {
+            let (x, y) = l.split_once(',').ok_or_eyre("Missing ,")?;
+
+            Ok((x.parse()?, y.parse()?))
+        })
+        .collect()
+}
+
+fn a_star(grid: &[[bool; GRID_SIZE]; GRID_SIZE]) -> Option<usize> {
+    #[derive(Clone, PartialEq, Eq)]
+    struct Path {
+        x: usize,
+        y: usize,
+        score: usize,
+    }
+
+    impl Path {
+        fn heuristic(&self) -> usize {
+            self.score + (GRID_SIZE - 1 - self.x) + (GRID_SIZE - 1 - self.y)
+        }
+    }
+
+    impl Ord for Path {
+        fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+            other.heuristic().cmp(&self.heuristic())
+        }
+    }
+
+    impl PartialOrd for Path {
+        fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+            Some(self.cmp(other))
+        }
+    }
+
+    let mut prev = HashMap::new();
+
+    let mut visited = HashMap::new();
+    visited.insert((0, 0), 0);
+
+    let mut current = BinaryHeap::new();
+    current.push(Path {
+        x: 0,
+        y: 0,
+        score: 0,
+    });
+
+    while let Some(p) = current.pop() {
+        if p.x == GRID_SIZE - 1 && p.y == GRID_SIZE - 1 {
+            return Some(p.score);
+        }
+
+        let mut try_insert = |n: Path| {
+            if !grid[n.x][n.y] {
+                match visited.get(&(n.x, n.y)) {
+                    Some(&s) if s >= n.score => (),
+                    _ => {
+                        visited.insert((n.x, n.y), n.score);
+                        prev.insert((n.x, n.y), (p.x, p.y));
+                        current.push(n);
+                    }
+                }
+            }
+        };
+
+        let x = p.x;
+        let y = p.y;
+        let score = p.score + 1;
+
+        if x > 0 {
+            let new_path = Path { x: x - 1, y, score };
+            try_insert(new_path);
+        }
+
+        if x < GRID_SIZE - 1 {
+            let new_path = Path { x: x + 1, y, score };
+            try_insert(new_path);
+        }
+
+        if y > 0 {
+            let new_path = Path { x, y: y - 1, score };
+            try_insert(new_path);
+        }
+
+        if y < GRID_SIZE - 1 {
+            let new_path = Path { x, y: y + 1, score };
+            try_insert(new_path);
+        }
+    }
+
+    None
+}
+
+const GRID_SIZE: usize = 70 + 1;
+const CUTTOF: usize = 1024;
+
+#[inline(never)]
+pub fn part1(input: Parsed) {
+    let mut grid = [[false; GRID_SIZE]; GRID_SIZE];
+
+    for &(x, y) in &input[0..CUTTOF] {
+        grid[x][y] = true;
+    }
+
+    let result = a_star(&grid).unwrap();
+    print_res!("Steps to take: {result}");
+}
+
+fn bfs(grid: &[[bool; GRID_SIZE]; GRID_SIZE]) -> Option<HashSet<(usize, usize)>> {
+    let mut seen = HashMap::new();
+    seen.insert((0, 0), (0, 0));
+
+    let mut paths = Vec::new();
+    paths.push((0, 0));
+
+    while let Some((x, y)) = paths.pop() {
+        if x == GRID_SIZE - 1 && y == GRID_SIZE - 1 {
+            let mut paths = HashSet::new();
+            let mut current = (x, y);
+
+            while current != (0, 0) {
+                paths.insert(current);
+                current = seen[&current];
+            }
+
+            return Some(paths);
+        }
+
+        let mut try_insert = |nx: usize, ny: usize| {
+            if !grid[nx][ny] && !seen.contains_key(&(nx, ny)) {
+                seen.insert((nx, ny), (x, y));
+                paths.push((nx, ny));
+            }
+        };
+
+        if x > 0 {
+            try_insert(x - 1, y);
+        }
+
+        if x < GRID_SIZE - 1 {
+            try_insert(x + 1, y);
+        }
+
+        if y > 0 {
+            try_insert(x, y - 1);
+        }
+
+        if y < GRID_SIZE - 1 {
+            try_insert(x, y + 1);
+        }
+    }
+
+    None
+}
+
+#[inline(never)]
+pub fn part2(input: Parsed) {
+    let mut grid = [[false; GRID_SIZE]; GRID_SIZE];
+    let mut seen = bfs(&grid).unwrap();
+
+    for (x, y) in input {
+        grid[x][y] = true;
+
+        if seen.contains(&(x, y)) {
+            match bfs(&grid) {
+                Some(p) => seen = p,
+                None => {
+                    print_res!("Breaking block: {x},{y}");
+                    return;
+                }
+            }
+        }
+    }
+
+    panic!("Did not find a breaking block");
+}
+
+pub fn main() -> color_eyre::Result<()> {
+    let context = load()?;
+
+    let start = Instant::now();
+    let parsed = parsing(&context.input)?;
+    let elapsed = humantime::format_duration(start.elapsed());
+
+    let start = Instant::now();
+    if context.part == 1 {
+        part1(parsed);
+    } else {
+        part2(parsed);
+    }
+    let elapsed_part = humantime::format_duration(start.elapsed());
+
+    println!("  Parsing: {elapsed}");
+    println!("  Solving: {elapsed_part}");
+
+    Ok(())
+}