From 71ac0a4c35ffe5757b050a0fbf1bcc8d1236a94c Mon Sep 17 00:00:00 2001
From: Kristofers Solo <dev@kristofers.xyz>
Date: Thu, 4 Dec 2025 16:31:31 +0200
Subject: [PATCH] Finish part-2

---
 2025/day-04/src/part2.rs | 141 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 132 insertions(+), 9 deletions(-)

diff --git a/2025/day-04/src/part2.rs b/2025/day-04/src/part2.rs
index ede34a7..451616f 100644
--- a/2025/day-04/src/part2.rs
+++ b/2025/day-04/src/part2.rs
@@ -1,22 +1,145 @@
-use miette::Result;
+use std::{convert::Infallible, str::FromStr};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Space {
+    Roll,
+    Empty,
+}
+
+impl From<char> for Space {
+    fn from(ch: char) -> Self {
+        match ch {
+            '@' => Self::Roll,
+            _ => Self::Empty,
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Row(Vec<Space>);
+
+impl FromStr for Row {
+    type Err = Infallible;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let row = s.chars().map(Space::from).collect();
+        Ok(Self(row))
+    }
+}
+
+#[derive(Debug, Clone)]
+struct Grid(Vec<Row>);
+
+impl FromStr for Grid {
+    type Err = Infallible;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let grid = s
+            .lines()
+            .map(Row::from_str)
+            .collect::<Result<Vec<_>, Infallible>>()
+            .unwrap();
+        Ok(Self(grid))
+    }
+}
+
+#[derive(Debug)]
+struct Coords {
+    x: isize,
+    y: isize,
+}
+
+impl Coords {
+    const fn new(x: isize, y: isize) -> Self {
+        Self { x, y }
+    }
+
+    const fn offset(&self, coords: &Self) -> Self {
+        Self {
+            x: self.x + coords.x,
+            y: self.y + coords.y,
+        }
+    }
+}
+
+impl Grid {
+    fn find_accessible(&mut self) -> usize {
+        let mut total_removed = 0;
+        loop {
+            let accessible = self
+                .0
+                .iter()
+                .enumerate()
+                .flat_map(|(x, row)| row.0.iter().enumerate().map(move |(y, _)| (x, y)))
+                .filter(|(x, y)| {
+                    let coords = Coords::new((*x).try_into().unwrap(), (*y).try_into().unwrap());
+                    let adjacent = self.get_adjacent(&coords);
+                    adjacent < 4 && matches!(self.get(&coords), Space::Roll)
+                })
+                .collect::<Vec<_>>();
+            if accessible.is_empty() {
+                break;
+            }
+            for (x, y) in &accessible {
+                self.0[*x].0[*y] = Space::Empty;
+            }
+            total_removed += accessible.len();
+        }
+        total_removed
+    }
+
+    fn get_adjacent(&self, coords: &Coords) -> usize {
+        let max_x = self.0.len() - 1;
+        let max_y = self.0[0].0.len() - 1;
+
+        (-1..=1)
+            .flat_map(|x| (-1..=1).map(move |y| (x, y)))
+            .filter(|(x, y)| {
+                !(*x == 0 && *y == 0)
+                    && !((coords.x == 0 && *x == -1)
+                        || (coords.y == 0 && *y == -1)
+                        || (usize::try_from(coords.x).unwrap() == max_x && *x == 1)
+                        || (usize::try_from(coords.y).unwrap() == max_y && *y == 1))
+            })
+            .filter(|(x, y)| {
+                let coord = coords.offset(&Coords::new(
+                    (*x).try_into().unwrap(),
+                    (*y).try_into().unwrap(),
+                ));
+
+                matches!(self.get(&coord), Space::Roll)
+            })
+            .count()
+    }
+
+    fn get(&self, coords: &Coords) -> Space {
+        self.0[usize::try_from(coords.x).unwrap()].0[usize::try_from(coords.y).unwrap()]
+    }
+}
 
 #[tracing::instrument]
 #[allow(clippy::missing_panics_doc)]
 #[allow(clippy::missing_errors_doc)]
-pub fn process(input: &str) -> Result<usize> {
-    todo!("day xx - part 2");
-    Ok(0)
+pub fn process(input: &str) -> miette::Result<usize> {
+    let mut grid = Grid::from_str(input)?;
+    Ok(grid.find_accessible())
 }
-
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
-    fn test_process() -> Result<()> {
-        let input = "";
-        todo!("haven't built test yet");
-        let result = 0;
+    fn test_process() -> miette::Result<()> {
+        let input = "..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
+";
+        let result = 43;
         assert_eq!(process(input)?, result);
         Ok(())
     }