From 92a66b3d5f2e1bdf22dedfcc12c9d950dcb7159f Mon Sep 17 00:00:00 2001
From: Quentin Boyer <quentin+dev@familleboyer.net>
Date: Thu, 4 Dec 2025 23:56:53 +0100
Subject: [PATCH] Day 4

---
 src/bin/day4.rs | 157 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 157 insertions(+)
 create mode 100644 src/bin/day4.rs

diff --git a/src/bin/day4.rs b/src/bin/day4.rs
new file mode 100644
index 0000000..05a27d4
--- /dev/null
+++ b/src/bin/day4.rs
@@ -0,0 +1,157 @@
+use std::time::Instant;
+
+use aoc_2025::{load, print_res};
+use bstr::{BString, ByteSlice};
+use color_eyre::eyre::ContextCompat;
+
+type Parsed = Grid;
+
+pub struct Grid {
+    values: Vec<bool>,
+    line_len: usize,
+    col_len: usize,
+}
+
+impl Grid {
+    fn get(&self, line: isize, col: isize) -> Option<bool> {
+        if line < 0 || col < 0 || line as usize >= self.line_len || col as usize >= self.col_len {
+            None
+        } else {
+            Some(self[(line as usize, col as usize)])
+        }
+    }
+
+    fn neighbours(&self, line: usize, col: usize) -> usize {
+        let mut sum = 0;
+        for l in -1..=1 {
+            for c in -1..=1 {
+                if l == 0 && c == 0 {
+                    continue;
+                }
+
+                if self
+                    .get(line as isize + l, col as isize + c)
+                    .unwrap_or(false)
+                {
+                    sum += 1
+                }
+            }
+        }
+        sum
+    }
+}
+
+impl std::ops::Index<(usize, usize)> for Grid {
+    type Output = bool;
+
+    fn index(&self, (line, col): (usize, usize)) -> &Self::Output {
+        &self.values[line * self.line_len + col]
+    }
+}
+
+impl std::ops::IndexMut<(usize, usize)> for Grid {
+    fn index_mut(&mut self, (line, col): (usize, usize)) -> &mut Self::Output {
+        &mut self.values[line * self.line_len + col]
+    }
+}
+
+impl std::fmt::Display for Grid {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        for line in self.values.chunks_exact(self.line_len) {
+            line.iter().try_for_each(|v| {
+                write!(
+                    f,
+                    "{}",
+                    match v {
+                        true => '@',
+                        false => '.',
+                    }
+                )
+            })?;
+            writeln!(f)?;
+        }
+
+        Ok(())
+    }
+}
+
+#[inline(never)]
+pub fn parsing(input: &BString) -> color_eyre::Result<Parsed> {
+    let line_len = input.find(b"\n").with_context(|| "No newline found")?;
+    let values: Vec<_> = input
+        .iter()
+        .filter(|&&v| v != b'\n')
+        .map(|&v| v == b'@')
+        .collect();
+
+    assert_eq!(
+        values.len() % line_len,
+        0,
+        "Line length does not divide the grid size"
+    );
+    Ok(Grid {
+        line_len,
+        col_len: values.len() / line_len,
+        values,
+    })
+}
+
+#[inline(never)]
+pub fn part1(input: Parsed) {
+    let mut count = 0;
+
+    for line in 0..input.line_len {
+        for col in 0..input.line_len {
+            if input[(line, col)] && input.neighbours(line, col) < 4 {
+                count += 1;
+            }
+        }
+    }
+
+    print_res!("Accessible roll count: {count}");
+}
+
+#[inline(never)]
+pub fn part2(mut input: Parsed) {
+    let mut count = 0;
+
+    loop {
+        let mut found = false;
+        for line in 0..input.line_len {
+            for col in 0..input.line_len {
+                if input[(line, col)] && input.neighbours(line, col) < 4 {
+                    input[(line, col)] = false;
+                    found = true;
+                    count += 1;
+                }
+            }
+        }
+
+        if !found {
+            break;
+        }
+    }
+
+    print_res!("Total removed: {count}");
+}
+
+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(())
+}