From 9a5d81479211f1295850465fb5b3f4d10f5f74b1 Mon Sep 17 00:00:00 2001
From: Matej Focko <me@mfocko.xyz>
Date: Mon, 18 Dec 2023 19:21:41 +0100
Subject: [PATCH] day(18): use tricks from day(10) to solve part 2

Signed-off-by: Matej Focko <me@mfocko.xyz>
---
 src/bin/day18.rs | 87 +++++++++++++++++-------------------------------
 1 file changed, 31 insertions(+), 56 deletions(-)

diff --git a/src/bin/day18.rs b/src/bin/day18.rs
index e98ed73..28f49a3 100644
--- a/src/bin/day18.rs
+++ b/src/bin/day18.rs
@@ -1,6 +1,7 @@
-use std::{collections::VecDeque, str::FromStr};
-
-use itertools::iproduct;
+use std::{
+    collections::BTreeMap,
+    str::FromStr,
+};
 
 use aoc_2023::*;
 
@@ -48,73 +49,47 @@ impl Step {
     }
 }
 
-fn flood_fill(m: &mut [Vec<char>], start: Vector2D<isize>, original: char, fill: char) -> usize {
-    let mut filled_in = 0;
-
-    let mut q: VecDeque<Vector2D<isize>> = VecDeque::from([start]);
-    m[start] = fill;
-    filled_in += 1;
-
-    while let Some(p) = q.pop_front() {
-        for (dx, dy) in [(0, 1), (0, -1), (1, 0), (-1, 0)] {
-            let neighbour = p + Vector2D::new(dx, dy);
-            if !in_range(m, &neighbour) || m[neighbour] != original {
-                continue;
-            }
-
-            q.push_back(neighbour);
-            m[neighbour] = fill;
-            filled_in += 1;
-        }
+fn dir_to_u8(u: (isize, isize)) -> u8 {
+    match u {
+        (0, 1) => 1,
+        (0, -1) => 2,
+        (1, 0) => 4,
+        (-1, 0) => 8,
+        _ => unreachable!(),
     }
+}
 
-    filled_in
+fn opposite(u: (isize, isize)) -> (isize, isize) {
+    (-u.0, -u.1)
 }
 
 fn solve(plan: &[Step]) -> usize {
-    let (min_x, max_x, min_y, max_y) = plan.iter().fold((0, 0, 0, 0), |dimensions, s| {
-        let (mut lx, mut ux, mut ly, mut uy) = dimensions;
-
-        match s._direction {
-            (0, 1) => uy += s.count,
-            (0, -1) => ly += s.count,
-            (1, 0) => ux += s.count,
-            (-1, 0) => lx += s.count,
-            _ => unreachable!(),
-        }
-
-        (lx, ux, ly, uy)
-    });
-    let (width, height) = (min_x + max_x + 1, min_y + max_y + 1);
-    debug!("Dimensions: {}×{}", width, height);
-
-    let mut map = vec![vec!['.'; width as usize]; height as usize];
-    let mut position = Vector2D::new(min_x, min_y);
+    let mut hole: BTreeMap<Vector2D<isize>, u8> = BTreeMap::new();
 
+    let mut position = Vector2D::new(0, 0);
     for s in plan.iter() {
         let direction = s.direction();
+
         for _ in 0..s.count {
-            map[position] = '#';
+            *hole.entry(position).or_insert(0) |= dir_to_u8(s._direction);
             position = position + direction;
+            *hole.entry(position).or_insert(0) |= dir_to_u8(opposite(s._direction));
         }
     }
 
-    flood_fill(&mut map, Vector2D::new(0, 0), '.', '+');
-
-    while let Some((y, x)) =
-        iproduct!(0..height as usize, 0..width as usize).find(|&(y, x)| map[y][x] == '.')
-    {
-        flood_fill(&mut map, Vector2D::new(x as isize, y as isize), '.', '#');
+    let mut counter = 0;
+    let mut in_between = 0;
+    let mut x_prev = 0;
+    for (v, directions) in hole.iter() {
+        if in_between != 0 && x_prev < v.x() {
+            // debug!("{} {}", v.x(), x_prev);
+            counter += (v.x() - x_prev) as usize - 1;
+        }
+        in_between ^= directions & 3;
+        x_prev = v.x();
     }
 
-    // debug!(
-    //     "Map:\n{}",
-    //     map.iter().map(|r| r.iter().collect::<String>()).join("\n")
-    // );
-
-    map.iter()
-        .map(|r| r.iter().filter(|&&c| c == '#').count())
-        .sum()
+    counter + hole.len()
 }
 
 struct Day18 {
@@ -150,7 +125,7 @@ impl Solution<Output1, Output2> for Day18 {
 
                     Step {
                         _direction,
-                        count: number >> 8,
+                        count: number >> 4,
                         color: String::new(),
                     }
                 })